Elena Gómez Alvarez

Nicola Carslaw

Sébastien Dusanter

Pete Edwards

Viktor Gábor Mihucz

Dwayne Heard

Jörg Kleffmann

Sascha Nehr

Coralie Schoemacker

Dean Venables
email: d.venables@ucc.ie

Viktor Gábor Mihucz

Techniques for measuring indoor radicals and radical precursors

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

A small group of highly reactive species, often radicals and their precursors, play a central role in the chemical transformations occurring in indoor air and on indoor surfaces. Reactions involving these species affect indoor air quality through the loss of reactants and the formation of new chemical species whose toxicity could differ substantially from their precursors. Since reaction products are often of lower volatility than their parent compounds, these reactions can also lead to the growth or formation of particulate matter.

Measuring the concentrations of highly reactive species is challenging for several reasons:  mixing ratios in indoor air are often extremely low  inlet losses can be problematic. Instrument residence times and material selection are therefore important considerations  pre-concentration is not generally viable as a measurement strategy as it is with stable organic compounds  instrument calibration and validation studies are complicated by the absence of commercially available reference materials Moreover, practical issues like instrument size, noise, and hazards associated with lasers and electrical cables need to be considered in occupied indoor spaces.

Based on the assumption that indoor radical concentrations were much lower than outdoors, the study of reactive indoor chemistry was initially restricted to ozone chemistry. However, first modelling and then experimental studies began to question this assumption and indicated that concentrations of radicals can be on the same order of magnitude as that observed outdoors, or even larger for chlorine radicals under circumstances such as cleaning. Indoor radicals can derive from sources indoors (such as cleaning or air-freshener use) or through infiltration of radical precursors (VOC, O 3 and NO X ) from outside air.

Transmission of UV and visible light into buildings plays a fundamental role in the generation of radicals. Outdoors, oxidation is driven by photochemistry, often initiated by sunlight at wavelengths shorter than 320 nm. Such high energy photons are not generally available indoors and the oxidizing capacity can be dominated by infiltration of reactant species from outdoor air and by non-photochemical reactions (also known as "dark chemistry") [START_REF] Young | Illuminating the dark side of indoor oxidants[END_REF]. This results in different oxidizing atmospheres indoors and outdoors.

Recent studies suggest that photolysis of molecules that absorb light at wavelengths longer than 320 nm, such as nitrous acid (HONO) and formaldehyde (HCHO), may lead to higher indoor hydroxyl radical (OH) concentrations than previously expected. Gomez [START_REF] Alvarez | Unexpectedly high indoor hydroxyl radical concentrations associated with nitrous acid[END_REF] provided the first direct experimental confirmation that the indoor levels of OH radicals are comparable to outdoor concentrations. Particularly noteworthy are activities that lead to high indoor concentrations of these species, such as of HONO following combustion [START_REF] Brauer | Measurements of nitrous acid inside two research houses[END_REF]. HONO photolysis can compensate for the missing OH formation by ozone photolysis, which is normally the dominant source of OH radicals outdoors.

Indoor cleaning can also lead to radical production. [START_REF] Carslaw | Significant OH production under surface cleaning and air cleaning conditions: Impact on indoor air quality[END_REF] reported measurements of OH and hydroperoxy (HO 2 ) radicals made by laser-induced fluorescence spectroscopy in a computer classroom, (i) in the absence of indoor activities, (ii) during desk cleaning with a limonene-containing cleaner and (iii) during operation of a commercially available "air cleaning" device. In the unmanipulated environment, the one-minute averaged OH concentration remained close to or below the limit of detection (6.5×10 5 molecules cm -3 ), while HO 2 was 1.3×10 7 molecules cm -3 . These concentrations increased to ~4×10 6 and 4×10 8 molecules cm -3 , respectively during desk cleaning. During operation of the air cleaning device, OH and HO 2 concentrations reached ~2×10 7 and ~6×10 8 molecules cm -3 , respectively.

Owing to the presence of O 3 and nitrogen dioxide (NO 2 ) indoors and the relatively low light levels, it might be expected that the nitrate radical (NO 3 ) could attain significant indoor concentrations. [START_REF] Arata | Measurement of NO 3 and N 2 O 5 in a residential kitchen[END_REF] conducted direct indoor measurements of the nitrate radical (NO 3 ) and dinitrogen pentoxide (N 2 O 5 ) produced following combustion cooking emissions in a residential kitchen. They concluded that indoor combustion suppresses nitrate radical chemistry when O 3 concentrations are low, but at moderate O 3 levels (∼40 ppbv), measured NO 3 concentrations reached 3-4 pptv. A box model showed that moderate O 3 levels led to a nitrate radical production rate of 7 ppbv h -1 .

It has also recently been shown that cleaning activities using chlorine-based cleaners including use of bleach can lead to the formation of numerous reactive chlorine (Cl) species, many of which are photo-labile and could generate atomic Cl indoors. [START_REF] Wong | Observations and impacts of bleach washing on indoor chlorine chemistry[END_REF] measured concentrations of chlorinated gases and aerosol components using online chemical ionization and aerosol mass spectrometers after an indoor floor was repeatedly washed with a commercial bleach solution. Gaseous chlorine (Cl 2 ) at mixing ratios of tens of ppbv and hypochlorous acid (HOCl) at mixing ratios of hundreds of ppbv were observed after floor washing. Nitryl chloride (ClNO 2 ), dichlorine monoxide (Cl 2 O), and chloramines (NHCl 2 , NCl 3 ) were also formed under these conditions. Photochemical box models indicated that OH, Cl, and chlorine monoxide (ClO) gas-phase radical concentrations could be greatly enhanced (>10 6 and 10 5 molecules cm -3 for OH and Cl, respectively) in such bleach cleaning conditions, depending on the amount of indoor illumination [START_REF] Wong | Observations and impacts of bleach washing on indoor chlorine chemistry[END_REF]. Dark chemistry during bleach cleaning and use of surface disinfectants also affects indoor air quality by enhancing oxidation of organics and production of secondary organic aerosols indoors (Mattila et al., 2020;[START_REF] Rosales | Chemistry and human exposure implications of secondary organic aerosol production from indoor terpene ozonolysis[END_REF]. [START_REF] Gandolfo | Spectrally resolved actinic flux and photolysis frequencies of key species within an indoor environment[END_REF] provided direct measurements and modelling of the actinic flux and photolysis frequencies of species relevant for indoor atmospheres (HONO, NO 2 and NO 3 ) during summer and winter periods. [START_REF] Zhou | Factors affecting wavelength-resolved ultraviolet irradiance indoors and their impacts on indoor photochemistry[END_REF] measured wavelength-resolved ultraviolet (UV) irradiance in multiple indoor environments and quantified the effects of variables such as light source, solar angles, cloud cover, window type, and electric light colour temperature on indoor photon fluxes. They reported mathematical relationships that predict indoor solar UV irradiance as a function of solar zenith angle, incident angle of sunlight on windows, and distance from windows and surfaces for direct and diffuse sunlight. Using these relationships, they predict elevated indoor steady-state OH concentrations (0.80-7.4 × 10 6 molecules cm -3 ) under illumination by direct and diffuse sunlight and fluorescent tubes near windows or light sources. In a later study, [START_REF] Zhou | Factors affecting wavelength-resolved ultraviolet irradiance indoors and their impacts on indoor photochemistry[END_REF] made intensive measurements of wavelength-resolved spectral irradiance in a test house during the HOMEChem campaign and reported diurnal profiles and two-dimensional spatial distribution of photolysis rates (J) of several important indoor photolabile gases. Their results showed that sunlight transmitted through windows, which was the dominant source of ultraviolet (UV) light in the test house, led to clear diurnal cycles and large timeand location-dependent variations in local gas-phase photochemical activity. Compared to diffuse sunlight, local J values of several key indoor gases under direct solar illumination were 1.8-7.4 times larger and strongly dependent on time, solar zenith angle, and incident angle of sunlight relative to the window. Photolysis rates were highly spatially heterogeneous and fast photochemical reactions in the gas phase were generally confined to within tens of centimetres of the area exposed to direct sunlight. Opening windows increased UV photon fluxes by a factor of 3 and increased predicted local hydroxyl radical (OH) concentrations in the sunlit region by a factor of 4.5, leading to a concentration of 3.2 × 10 7 molecules cm -3 as a result of higher J values and an increased contribution from O 3 photolysis. Figure 1 displays photon fluxes of indoor light sources and the product of absorption cross section and photolysis quantum yield of likely indoor oxidants. The above overview of recent measurements and modelling studies demonstrates the need for a wider range of measurements of radical species and their precursors in indoor air. This paper therefore focuses on reviewing measurement techniques and provides considerations for indoor measurements of OH, HO 2 , and peroxy radicals, OH reactivity, the OH radical precursor, nitrous acid, reactive chlorine species, NO 3 and its atmospheric reservoir N 2 O 5 , Criegee intermediates, and the bicarbonyl species glyoxal and methylglyoxal indoors. Techniques to measure the actinic flux are also presented owing to the relevance of photolytically-initiated processes indoors. The inorganic species NO, NO 2 , and O 3 were omitted as they are routinely measured relatively straightforwardly.

This review of indoor radicals and radical precursor measurement techniques is intended to provide some pointers for those wanting to learn more about radical concentrations indoors. It is based on recent innovations for radical measurements and connected species/parameters. It is not an exhaustive review of all possible techniques. For instance, Matrix Isolation Electron Spin Resonance (MIESR) has been superseded by other methods and long physical path methods like multi-pass Differential Optical Absorption Spectroscopy (DOAS) are normally too bulky for indoor measurements.

Measurements techniques of radicals and radical precursors indoors

1.

Hydroxyl and hydroperoxy radicals Oxidation of volatile organic compounds produces radical species indoors. These gas-phase oxidation reactions transform primary VOC emissions into secondary species (including into harmful pollutants), and involve the OH and HO 2 radicals, collectively known as HO x . There is a need to better understand the chemical transformations of multiple VOC species that are present indoors and the potential impact of their oxidation on indoor air quality and human health. Doing so requires the measurement of radical concentrations and the secondary species which derive from them in real-life environments. Such measurements will permit the further development of chemical mechanisms for models and to gain further insight.

Owing to the short-lived nature of radical species (for example, <1 second for OH), their local concentrations are controlled by the rate at which they are chemically formed and removed, and not by transport processes. As a result, comparing measured and modelled concentration profiles of oxidants such as OH radicals is an ideal test of how well we understand the details of these chemical transformations. Another parameter linked to the OH radical is the pseudo-first-order rate coefficient at which OH is removed by its sinks, known as the OH reactivity. This parameter is representative of all the chemical losses of OH. The OH reactivity can be measured in ambient air by coupling a reactor in which OH is generated to instruments able to quantify OH or OH reactivity.

Peroxy radicals, which comprise both HO 2 and organic RO 2 (where R is an organic group), are key intermediate species in the oxidation process. These radicals facilitate fast chemical cycles that reform OH and, in the presence of NO, lead to the formation of NO 2 which can photolyze to generate ozone. The central role of RO 2 and their short lifetimes make measurements of their concentrations useful for comparison with model predictions. However, the measurement of these radicals is an experimental challenge owing to their high reactivity, short lifetimes, and low concentrations. Table 1 summarises typical concentrations of radicals in the atmosphere, including indoor environments.

Radical(s)

Atmospheric concentration (molecules cm -3 ) OH 10 5 -10 7 HO 2 10 6 -10 9 CH 3 O 2 10 6 -10 8 sum RO 2 10 6 -10 9

A range of instruments have been developed to measure HO x radicals in the atmosphere using optical and mass spectrometric techniques (Dusanter and Stevens, 2016;[START_REF] Heard | Measurement of OH and HO 2 in the Troposphere[END_REF][START_REF] Stone | Tropospheric OH and HO 2 radicals: field measurements and model comparisons[END_REF].

OH quantification

The techniques currently used for OH are Fluorescence Assay by Gas Expansion (FAGE), Chemical Ionisation Mass Spectrometry (CIMS) and Differential Optical Absorption Spectroscopy (DOAS), although DOAS is now retired from field use and is only used at one outdoor atmospheric simulation chamber (SAPHIR at Jülich). Figure 2 shows a range of methods for quantifying OH across a range of concentrations. Techniques based on the measurement of a tracer have been used indoors and in atmospheric simulation chambers (e.g. HIRAC, EUPHORE and SAPHIR). [START_REF] Faloona | A Laser-induced Fluorescence Instrument for Detecting Tropospheric OH and HO 2 : Characteristics and Calibration[END_REF], DOAS 2 [START_REF] Dorn | Investigation of OH absorption cross sections of rotational transitions in the A 2 Σ + , v′=0 ←X 2 Π, v″=0 band under atmospheric conditions: implications for tropospheric long-path absorption measurements[END_REF], Cavity Ringdown Spectroscopy (CRDS) 3 [START_REF] Mercier | Cavity ring-down measurements of OH radical in atmospheric premixed and diffusion flames.: A comparison with laserinduced fluorescence and direct laser absorption[END_REF], Diode-laser-based UV absorption 4 [START_REF] Meyer | Measurements of OH mole fraction and temperature up to 20 kHz by using a diodelaser-based UV absorption sensor[END_REF], cw-CRDS 5 [START_REF] Assaf | Cross Section of OH Radical Overtone Transition near 7028 cm -1 and Measurement of the Rate Constant of the Reaction of OH with HO 2 Radicals[END_REF][START_REF] Guoli | Measurement of OH Radicals in Dielectric Barrier Discharge Plasmas by Cavity Ring-Down Spectroscopy[END_REF], Cavity-Enhanced Absorption Spectroscopy (CEAS) 6 [START_REF] Peeters | Cavity enhanced absorption spectroscopy in the 10 μm region using a waveguide CO 2 laser[END_REF], Diode-Laser wavelength Wavelength Modulation Absorption Spectroscopy (DL-WMAS) 7 [START_REF] Aizawa | Diode-laser wavelength-modulation absorption spectroscopy for quantitative in situ measurements of temperature and OH radical concentration in combustion gases[END_REF], Laser-Induced Fluorescence (LIF) (FAGE) 8 [START_REF] Blitz | An Experimental Study of the Kinetics of OH/OD(v = 1,2,3) + SO 2 : The Limiting High-Pressure Rate Coefficients as a Function of Temperature[END_REF], diode laser absorption 9 [START_REF] Pesce | Detection and spectroscopy of OH fundamental vibrational band based on a difference frequency generator at 3 μm[END_REF], Faraday Rotation Spectroscopy (FRS) 10 (Zhao et al., 2011), CIMS 11 [START_REF] Tanner | Selected ion chemical ionization mass spectrometric measurement of OH[END_REF].

FAGE technique principle

The FAGE (Fluorescence Assay by Gas Expansion) technique is based on detection of Laser Induced Fluorescence (LIF) detection of OH after excitation at 308 nm. The sample is at low pressure, after a gas expansion formed by sampling the atmosphere through a small pinhole. As of 2021, the FAGE technique is the approach most commonly used in field campaigns. Eleven groups in the world have developed instruments: University of Leeds [START_REF] Smith | Concentrations of OH and HO2 radicals during NAMBLEX: measurements and steady state analysis[END_REF], Max Planck Institute in Mainz [START_REF] Novelli | Characterisation of an inlet pre-injector laser-induced fluorescence instrument for the measurement of atmospheric hydroxyl radicals[END_REF], Forschungszentrum Jülich [START_REF] Holland | In Situ Measurement of Tropospheric OH Radicals by Laser-Induced Fluorescence-A Description of the KFA Instrument[END_REF], in the EUPHORE chamber in Valencia (http://www.ceam.es/wwweuphore/), Pennsylvania State University [START_REF] Faloona | A Laser-induced Fluorescence Instrument for Detecting Tropospheric OH and HO 2 : Characteristics and Calibration[END_REF], University of Indiana at Bloomington [START_REF] Dusanter | Measurements of OH and HO 2 concentrations during the MCMA-2006 field campaign -Part 1: Deployment of the Indiana University laser-induced fluorescence instrument[END_REF], University of Lille (Amedro, 2012), Peking University in collaboration with the Forschungszentrum Jülich [START_REF] Tan | Radical chemistry at a rural site (Wangdu) in the North China Plain: observation and model calculations of OH, HO 2 and RO 2 radicals[END_REF], and Hefei (Wang et al., 2019).

Recently, a modification to the FAGE sampling methodology has been made which uses an inlet pre-injector (IPI) to add a chemical scavenger to remove any ambient OH prior to sampling by the pinhole and entering the fluorescence cell [START_REF] Cho | Characterization of a chemical modulation reactor (CMR) for the measurement of atmospheric concentrations of hydroxyl radicals with a laser-induced fluorescence instrument[END_REF][START_REF] Mao | Insights into hydroxyl measurements and atmospheric oxidation in a California forest[END_REF][START_REF] Novelli | Characterisation of an inlet pre-injector laser-induced fluorescence instrument for the measurement of atmospheric hydroxyl radicals[END_REF][START_REF] Woodward-Massey | Implementation of a chemical background method for atmospheric OH measurements by laser-induced fluorescence: characterisation and observations from the UK and China[END_REF]. A measurement using the IPI is known as OH-Chem, and is different to OH-Wave, which uses wavelength modulation of the laser to measure the background signal. Through the use of the IPI, any remaining LIF signal from OH must have originated from artificially generated OH from an interference present within the fluorescence cell itself, and enables removal of this interference signal. OH-Chem and OH-Wave are similar for most conditions.

CIMS instrument principle

OH can be converted to H 2 SO 4 by titration with SO 2 , with subsequent detection as H 2 SO 4 using CIMS and ionization based on its reaction with NO 3 -ions. As the concentration of OH is low, isotopically labeled 34 SO 2 is used to reduce interferences from naturally present H 2 SO 4 . At present, three groups use CIMS instruments for the quantification of OH in the field [START_REF] Aufmhoff | An ion trap CIMS instrument for combined measurements of atmospheric OH and H 2 SO 4 : First test measurements above and inside the planetary boundary layer[END_REF][START_REF] Kukui | Chemical ionisation mass spectrometer for measurements of OH and Peroxy radical concentrations in moderately polluted atmospheres[END_REF][START_REF] Mauldin | Measurements of OH, HO 2 +RO 2 , H 2 SO 4 , and MSA at the South Pole during ISCAT 2000[END_REF]. Isotopically labelled 34 SO 2 is very expensive and shortages of it have been known. However, it increases the precision of the measurement as background levels of H 2 SO 4 can be significant.

DOAS instrument principle

DOAS measurement of OH has been developed in the UV (at 308 nm), where the absorption cross sections for OH are large and well known [START_REF] Dorn | Investigation of OH absorption cross sections of rotational transitions in the A 2 Σ + , v′=0 ←X 2 Π, v″=0 band under atmospheric conditions: implications for tropospheric long-path absorption measurements[END_REF]. DOAS has the advantage of providing an absolute OH concentration but requires a long absorption path (several km) to reach a sensitivity high enough for atmospheric applications [START_REF] Hausmann | Detection of tropospheric OH radicals by long-path differential-optical-absorption spectroscopy: Experimental setup, accuracy, and precision[END_REF]. The only current operational instrument for OH measurements is permanently installed in the large SAPHIR chamber in Jülich. The SAPHIR instrument has a multipass cell with a base length of 20 m and a total path length of 2.24 km, giving a limit of detection (LoD) of 7.3 × 10 5 molecules cm -3 for 200 s integration time (Fuchs et al., 2012).

OH reactivity

The measurement of OH reactivity has been developed over the last 2 decades for atmospheric applications. This parameter corresponds to the sum of all losses of OH and complements other measurements characterizing the oxidation processes linked to OH. It measures the first order rate coefficient of the OH loss in a single measurement. When compared with model calculations it allows evaluation of whether all the sinks of OH are accurately captured in the chemical mechanism. For this, the directly measured OH destruction rate is compared against the destruction rate inferred from other field measurements. When coupled to the measured OH concentration and to measurements characterizing the major OH-producing pathways (HO 2 + NO, HONO-photolysis and O 3photolysis), it also allows to analyse OH budgets exclusively on experimental data. The overall OH production rate is given by OH-recycling via the HO 2 + NO reaction plus the primary production rates mainly resulting from photolysis of HONO and O 3 . The overall OH destruction rate is given by the product of total OH reactivity and the OH concentration. The FT-LIF method is based on the continuous generation of OH radicals in a flow tube by photolysis of H 2 O vapour by a mercury lamp placed in a moving injector, allowing measurement of OH at different reaction times corresponding to different positions of the injector, with the detection of OH by FAGE [START_REF] Kovacs | Total OH Loss Rate Measurement[END_REF][START_REF] Lee | Measurement and calculation of OH reactivity at a United Kingdom coastal site[END_REF]. Corrections are needed for the recycling of HO 2 (generated simultaneously with OH) into OH in the presence of NO in ambient air.

The LP-LIF is based on detection of OH by FAGE, similarly to FT-LIF, but OH is generated in a reactor by pulsed photolysis of ozone at 266 nm in the presence of water vapour [START_REF] Sadanaga | Development of a measurement system of OH reactivity in the atmosphere by using a laser-induced pump and probe technique[END_REF]. In contrast to FT-LIF, this approach has the advantage of generating only OH, limiting recycling of ambient HO 2 to OH only to high NO conditions. Another advantage is that the OH is directly monitored as a function of time, and conversion from a physical parameter (distance) into time is not required.

The FT-CIMS uses the same OH generation method to FT-LIF but OH is converted first into H 2 SO 4 and detected by CIMS. The measurement is made at one reaction time by alternatively injecting sulfur dioxide in the reactor at the entrance or at a position corresponding to a detection time of about 75 ms [START_REF] Muller | A novel semi-direct method to measure OH reactivity by chemical ionization mass spectrometry (CIMS)[END_REF]. Corrections are also needed for recycling of ambient HO 2 into OH with NO.

The CRM technique does not directly measure OH but rather measures a tracer reacting with it. It is based on the competition between the reactions of OH with all species present in ambient air and with the tracer molecule added to the flow. OH is produced in a reactor by continuous photolysis of H 2 O vapour using a mercury lamp and the tracer is quantified by a suitable detector at the exit of the reactor. Pyrrole (C 4 H 5 N) is currently used as the tracer.

The methods used to measure OH reactivity, together with their performance details, are summarised in Table 2. The data in Table 2 is from two intercomparison campaigns of OH reactivity that took place in the atmospheric simulation chamber SAPHIR in Forschungszentrum Jülich in October 2015 and April 2016 [START_REF] Fuchs | Comparison of OH reactivity measurements in the atmospheric simulation chamber SAPHIR[END_REF]. The intercomparison gathered all the instruments then used for the determination of OH radicals. 2 is what is generated inside the instrument to perform the OH reactivity measurement. For example, in the LP-LIF method, OH is generated by photolysis of ozone at 266 nm followed by the reaction of O( 1 D) with H 2 O vapour at a certain laser power. The O 3 and H 2 O concentrations vary between instruments and therefore affect the initial OH generated (column [OH] in Table 2).

The column corresponding to OH concentration in Table

Table 2. Performance characteristics of instruments used to measure OH reactivity from two intercomparison campaigns [START_REF] Fuchs | Comparison of OH reactivity measurements in the atmospheric simulation chamber SAPHIR[END_REF].

Instrument

Technique Time res. f Derived from the difference in the C1 and C2 measurement (C1 and C2 are specific to the CRM instruments and represent two different conditions within the instrument depending on whether ambient air is introduced or not).

g April 2016.

h Laser flash photolysis and laser induced fluorescence.

i Flow-tube and laser-induced fluorescence. j Limit of detection without the dilution, which amplifies this number by a factor of 5.

k Peak value produced by flash ozone photolysis at 266 nm. l Flow tube and chemical ionisation mass spectrometry.

m Sampling rate from the chamber.

HO 2 quantification

HO 2 is most frequently quantified by indirect detection based on converting HO 2 to OH and subsequent detection as OH using FAGE or CIMS instruments. However, titration by NO can also convert some RO 2 to HO 2 . In FAGE instruments, modulation of NO is used to selectively detect HO 2 [START_REF] Fuchs | Detection of HO 2 by laser-induced fluorescence: calibration and interferences from RO 2 radicals[END_REF][START_REF] Tan | Radical chemistry at a rural site (Wangdu) in the North China Plain: observation and model calculations of OH, HO 2 and RO 2 radicals[END_REF][START_REF] Wang | Measurement of tropospheric HO 2 radical using fluorescence assay by gas expansion with low interferences[END_REF][START_REF] Whalley | Reporting the sensitivity of laser-induced fluorescence instruments used for HO 2 detection to an interference from RO 2 radicals and introducing a novel approach that enables HO 2 and certain RO 2 types to be selectively measured[END_REF]. With PeRCIMS instruments (Peroxy Radical Chemical Ionization Mass Spectrometry), depending on the concentration of O 2 , NO and SO 2 and diluting the air sampled with either O 2 or N 2 , it is possible to speciate measurements of either RO 2 or HO 2 [START_REF] Edwards | Chemical Ionization Mass Spectrometer Instrument for the Measurement of Tropospheric HO 2 and RO 2[END_REF][START_REF] Hanke | Atmospheric peroxy radicals: ROXMAS, a new mass-spectrometric methodology for speciated measurements of HO 2 and ∑RO 2 and first results[END_REF][START_REF] Heard | Measurement of OH and HO 2 in the Troposphere[END_REF][START_REF] Wolfe | Missing peroxy radical sources within a summertime ponderosa pine forest[END_REF] but only if CH 3 O 2 is the major contribution to total RO 2 . A modified PERCA (peroxy radical chemical amplification) instrument dedicated to the sum of RO 2 measurement, using detection of NO 2 by LIF, has been tested to selectively quantify HO 2 using a denuder with heterogeneous losses higher for HO 2 than for RO 2 [START_REF] Miyazaki | A new technique for the selective measurement of atmospheric peroxy radical concentrations of HO 2 and RO 2 using a denuding method[END_REF]. Bromide has recently been shown to be a useful reagent ion for CIMS detection of HO 2 with high selectivity and at concentrations relevant to many ambient conditions (Sanchez et al., 2015;[START_REF] Albrecht | Measurements of hydroperoxy radicals (HO 2 ) at atmospheric concentrations using bromide chemical ionisation mass spectrometry[END_REF]. Approaches to measure HO 2 are shown in Fig. 3. [START_REF] Hong | A shock tube study of the rate constants of HO 2 and CH 3 reactions[END_REF], 2 Flash Photolysis UV Absorption (Tomas et al., 2001), 3 Near-Infrared Wavelength Modulation Spectroscopy&UV Absorption Spectroscopy [START_REF] Christensen | Measurements of the Rate Constant of HO 2 + NO 2 + N 2 → HO 2 NO 2 + N 2 Using Near-Infrared Wavelength-Modulation Spectroscopy and UV-Visible Absorption Spectroscopy[END_REF]), 4 cw-CRDS [START_REF] Thiébaud | Near infrared cw-CRDS coupled to laser photolysis: Spectroscopy and kinetics of the HO 2 radical[END_REF], 5 Optical Feedback Cavity Enhanced Absorption Spectroscopy (Gianella et al., 2016), 6 Noise-Inmune Cavity Enhanced Optical Heterodyne Detection (Bell et al., 2012), 7 Mid-Infrared Faraday Rotation Spectrometry (Brumfield et al., 2013), 8 Bromide Chemical Ionisation Mass Spectrometry (Albrecht et al., 2019), 9 Peroxy Radical Chemical Ionization Mass Spectrometry [START_REF] Edwards | Chemical Ionization Mass Spectrometer Instrument for the Measurement of Tropospheric HO 2 and RO 2[END_REF], 10 FAGE [START_REF] Whalley | Reporting the sensitivity of laser-induced fluorescence instruments used for HO 2 detection to an interference from RO 2 radicals and introducing a novel approach that enables HO 2 and certain RO 2 types to be selectively measured[END_REF]), 11 PERCA-denuder [START_REF] Miyazaki | A new technique for the selective measurement of atmospheric peroxy radical concentrations of HO 2 and RO 2 using a denuding method[END_REF].

Peroxy (RO 2 ) radicals

Quantification of RO 2 radicals is more complex than for OH and HO 2 , because the peroxy radicals comprise a family of radicals and not an individual species. As a result, most techniques only allow measurement of the sum of all RO 2 or of a sub-group of RO 2 . Efforts have been made to selectively measure CH 3 O 2 , the main RO 2 radical in the atmosphere. Selective quantification of CH 3 O 2 is possible by titrating with NO and detection as CH 3 O with laser-induced fluorescence. This approach has been demonstrated in the HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry) chamber [START_REF] Onel | A new method for atmospheric detection of the CH 3 O 2 radical[END_REF], where an intercomparison was also performed with the less sensitive cavity-ring down method [START_REF] Onel | An intercomparison of CH 3 O 2 measurements by fluorescence assay by gas expansion and cavity ring-down spectroscopy within HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry)[END_REF].

Other techniques for unspeciated RO 2 measurements are based on similar detection methods to those used for HO 2 , after converting RO 2 into HO 2 :

1. ROxLIF is derived from the FAGE technique, coupling it to a conversion cell held at ~30 Torr (addition of CO and NO) to convert RO x (RO 2 +RO+HO 2 +OH) into HO 2 .

Sufficient CO is added so that when OH is formed it is immediately converted to HO 2 , which is detected in the FAGE after titration with NO [START_REF] Fuchs | Measurement of tropospheric RO 2 and HO 2 radicals by a laser-induced fluorescence instrument[END_REF][START_REF] Whalley | Understanding in situ ozone production in the summertime through radical observations and modelling studies during the Clean air for London project (ClearfLo)[END_REF][START_REF] Whalley | Reporting the sensitivity of laser-induced fluorescence instruments used for HO 2 detection to an interference from RO 2 radicals and introducing a novel approach that enables HO 2 and certain RO 2 types to be selectively measured[END_REF]. Some speciation of RO 2 is possible by measuring both the total RO 2 and the total RO 2 which gives an interference in the measurement of HO 2 (mostly larger alkane RO 2 and RO 2 derived from unsaturated and aromatic species) [START_REF] Whalley | Reporting the sensitivity of laser-induced fluorescence instruments used for HO 2 detection to an interference from RO 2 radicals and introducing a novel approach that enables HO 2 and certain RO 2 types to be selectively measured[END_REF]. 2. PerCIMS is based on the conversion of RO 2 into HO 2 and HO 2 into OH with the detection of OH by CIMS [START_REF] Hornbrook | Measurements of tropospheric HO 2 and RO 2 by oxygen dilution modulation and chemical ionization mass spectrometry[END_REF]. 3. An alternative approach has been developed based on amplification/conversion (PERCA, peroxy radical chemical amplifier). PERCA is based on the conversion of RO 2 into HO 2 in the presence of a large concentration of CO and NO. HO 2 is then involved in a chain reaction in which HO 2 and OH are interconverted leading to the oxidation of NO to NO 2 , which is then quantified by LIF [START_REF] Sadanaga | Development of a measurement system of OH reactivity in the atmosphere by using a laser-induced pump and probe technique[END_REF], chemiluminescence [START_REF] Parker | Peroxy radicals in the summer free troposphere: seasonality and potential for heterogeneous loss[END_REF], CRDS [START_REF] Liu | Measurements of peroxy radicals using chemical amplification-cavity ringdown spectroscopy[END_REF], or CAPS (cavity attenuated phase shift spectroscopy) [START_REF] Lahib | Analytical Developments for Measuring Atmospheric Peroxy Radicals[END_REF][START_REF] Wood | Chemical Amplification -Cavity Attenuated Phase Shift Spectroscopy Measurements of Atmospheric Peroxy Radicals[END_REF]. Some more recent versions of a PERCA use ethane rather than CO for the chain amplification chemistry [START_REF] Duncianu | Characterization of a chemical amplifier for peroxy radical measurements in the atmosphere[END_REF][START_REF] Wood | Chemical Amplification -Cavity Attenuated Phase Shift Spectroscopy Measurements of Atmospheric Peroxy Radicals[END_REF].

Figure 4 summarises the specifications of these methods. 1 Laser Flash photolysis coupled with UV-time resolved absorption detection [START_REF] English | The Effects of Water Vapor on the CH 3 O 2 Self-Reaction and Reaction with HO 2[END_REF]) , 2 Time-resolved continuous-wave Cavity Ringdown Spectroscopy [START_REF] Faragó | Absorption Spectrum and Absolute Absorption Cross Sections of CH 3 O 2 Radicals and CH 3 I Molecules in the Wavelength Range 7473-7497 cm -1[END_REF], 3 Vacuum ultraviolet radiation of combined with time-resolved mass spectrometry (Meloni et al., 2006), 4 Laser-Induced Fluorescence (LIF) [START_REF] Onel | A new method for atmospheric detection of the CH 3 O 2 radical[END_REF]. Techniques for sum of RO 2 : 5 CIMS (Berndt et al., 2018), 6 PerCIMS [START_REF] Hornbrook | Measurements of tropospheric HO 2 and RO 2 by oxygen dilution modulation and chemical ionization mass spectrometry[END_REF]), 7 LIF [START_REF] Fuchs | Measurement of tropospheric RO 2 and HO 2 radicals by a laser-induced fluorescence instrument[END_REF] , 8 PERCA [START_REF] Wood | Chemical Amplification -Cavity Attenuated Phase Shift Spectroscopy Measurements of Atmospheric Peroxy Radicals[END_REF].

2.

Nitrous acid, HONO Nitrous Acid (HONO) is a respirable pollutant known to convert secondary amines in vitro to carcinogenic nitrosamines [START_REF] Pitts | Photooxidation of aliphatic amines under simulated atmospheric conditions: formation of nitrosamines, nitramines, amides and photochemical oxidant[END_REF]Sleinman et al., 2010). In addition, it can directly initiate DNA crosslinks and is expected to have mutagenic properties. Indoors, HONO is formed following combustion, as well as by heterogeneous dark and photochemical reaction of NO 2 and is an important radical source indoors. Gómez [START_REF] Alvarez | Unexpectedly high indoor hydroxyl radical concentrations associated with nitrous acid[END_REF] experimentally proved for the first time that photolysis of HONO was possible indoors, i.e., that the range of wavelengths that penetrate through windows could photolysis HONO yielding concentrations of OH radicals that were comparable to outdoor levels. Uptake of NO 2 to indoor surfaces can result in the heterogeneous formation of nitrous acid (HONO) by various mechanisms [START_REF] Finlayson-Pitts | The heterogeneous hydrolysis of NO 2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism[END_REF][START_REF] Grassian | Heterogeneous uptake and reaction of nitrogen oxides and volatile organic compounds on the surface of atmospheric particles including oxides, carbonates, soot and mineral dust: Implications for the chemical balance of the troposphere[END_REF][START_REF] Ammann | Nitrogen dioxide multiphase chemistry: Uptake kinetics on aqueous solutions containing phenolic compounds[END_REF][START_REF] George | Photoenhanced uptake of gaseous NO 2 on solid organic compounds: a photochemical source of HONO?[END_REF].

HONO mixing ratios of up to 100 ppb have been observed indoors [START_REF] Allen | Indoor organic and inorganic pollutants: In-situ formation and dry deposition in Southeastern Brazil[END_REF][START_REF] Brauer | Measurements of nitrous acid inside two research houses[END_REF][START_REF] Collins | Evidence for Gas-Surface Equilibrium Control of Indoor Nitrous Acid[END_REF][START_REF] De Santis | Determination of some atmospheric pollutants inside a museum: relationship with the concentration outside[END_REF]1999;Gomez Alvarez et al., 2013;[START_REF] Khoder | Nitrous acid concentrations in homes and offices in residential areas in Greater Cairo[END_REF][START_REF] Leaderer | Indoor, outdoor, and regional summer and winter concentrations of PM 10[END_REF][START_REF] Lee | Nitrous Acid, Nitrogen Dioxide, and Ozone Concentrations in Residential Environments[END_REF][START_REF] Pitts | Identification and measurement of nitrous acid in an indoor environment[END_REF][START_REF] Spengler | Nitrous Acid in Albuquerque, New Mexico, Homes[END_REF][START_REF] Spicer | Transformations, Lifetimes, and Sources of NO 2 , HONO, and HNO 3 in Indoor Environments[END_REF]Vecera andDasgupta, 1991, 1994;[START_REF] Vichi | Simultaneous measurement of nitrous acid, nitric acid, and nitrogen dioxide by means of a novel multipollutant diffusive sampler in libraries and archives[END_REF][START_REF] Zhou | Time-Resolved Measurements of Nitric Oxide, Nitrogen Dioxide, and Nitrous Acid in an Occupied New York Home[END_REF]. The highest values measured are related to combustion processes. Under conditions where a direct source of HONO, such as a combustion appliance, is not active, mixing ratios are typically less than 10 ppb, which is still much higher than in the ambient atmosphere [START_REF] Simon | Continuous Automated Measurement of Gaseous Nitrous and Nitric Acids and Particulate Nitrite and Nitrate[END_REF]Gomez Alvarez et al., 2013).

Due to the wide range of reported HONO levels (<1-100 ppb), instruments used for indoor studies should have a large dynamic range. In addition, since the indoor atmosphere can be a very complex reaction mixture including ambient pollutants (NO x , O 3 , VOCs), specific indoor pollutants (e.g., cleaning products, air fresheners, cooking emissions) and their degradation products, HONO instruments should be also selective and almost free of interferences. Furthermore, to study fast variations of indoor HONO levels, such as during cooking events, the instruments should have a time resolution at least in the low minutes range. Finally, HONO instruments should be compact to minimize disruption in indoor spaces.

Early studies using simple carbonate denuders or filter techniques did not fulfil all these requirements (e.g. De [START_REF] De Santis | Determination of some atmospheric pollutants inside a museum: relationship with the concentration outside[END_REF]1999;[START_REF] Febo | Prediction and experimental evidence for high air concentration of nitrous acid in indoor environments[END_REF][START_REF] Khoder | Nitrous acid concentrations in homes and offices in residential areas in Greater Cairo[END_REF][START_REF] Leaderer | Indoor, outdoor, and regional summer and winter concentrations of PM 10[END_REF][START_REF] Lee | Nitrous Acid, Nitrogen Dioxide, and Ozone Concentrations in Residential Environments[END_REF][START_REF] Spengler | Nitrous Acid in Albuquerque, New Mexico, Homes[END_REF][START_REF] Vichi | Simultaneous measurement of nitrous acid, nitric acid, and nitrogen dioxide by means of a novel multipollutant diffusive sampler in libraries and archives[END_REF]. These approaches have a low time resolution and are affected by interferences like NO 2 .

Other indoor HONO studies used a modified NO x analyzer to allow simultaneous quantitative measurements of HONO [START_REF] Brauer | Measurements of nitrous acid inside two research houses[END_REF][START_REF] Febo | Prediction and experimental evidence for high air concentration of nitrous acid in indoor environments[END_REF][START_REF] Leaderer | Indoor, outdoor, and regional summer and winter concentrations of PM 10[END_REF][START_REF] Zhou | Time-Resolved Measurements of Nitric Oxide, Nitrogen Dioxide, and Nitrous Acid in an Occupied New York Home[END_REF]. The HONO inlet of the instrument consists of a gas denuder (GD) channel and a bypass (BP) channel inserted upstream of the NO x analyzer. The GD removes gaseous acids, including HONO, using a sodium carbonate coated annular denuder which does not remove NO and NO 2 . Air sampled through the BP channel contains NO 2 and HONO; both molecules are converted to NO by the molybdenum catalyst [START_REF] Brauer | Measurements of nitrous acid inside two research houses[END_REF][START_REF] Dunlea | Evaluation of nitrogen dioxide chemiluminescence monitors in a polluted urban environment[END_REF][START_REF] Spicer | Transformations, Lifetimes, and Sources of NO 2 , HONO, and HNO 3 in Indoor Environments[END_REF][START_REF] Villena | Interferences of commercial NO 2 instruments in the urban atmosphere and in a smog chamber[END_REF]. A Teflon solenoid threeway valve enabled the sampling of GD (NO and NO 2 only) and BP (sum of NO, NO 2 and HONO, ∑(NO X + HONO)) air alternating at 5 min intervals. However, this technique is not very specific since other acidic NO y species (e.g., HNO 3 ) besides HONO are also removed by the carbonate denuder. [START_REF] Večeřa | Measurement of atmospheric nitric and nitrous acids with a wet effluent diffusion denuder and low-pressure ion chromatography-postcolumn reaction detection[END_REF] used a wetted wall effluent diffusion denuder, in which water flows down on a modified interior wall of a borosilicate glass tube while sample air moves upward in countercurrent flow. The effluent liquid was debubbled and the anionic constituents were preconcentrated. The separated ions flowed through a cadmium cell to reduce NO 3 -to NO 2 -, which was detected by the Griess-Saltzman reaction. Following a 3minute reaction time, the product dye was detected based on its absorption at 555 nm. For a 50 cm wet effluent diffusion denuder, essentially quantitative collection efficiency was observed for both HONO and HNO 3 . Differential Optical Absorption Spectroscopy (DOAS) was used in the first HONO indoor studies [START_REF] Pitts | Identification and measurement of nitrous acid in an indoor environment[END_REF]. DOAS is fast, sensitive and selective, but indoor set-ups are cumbersome since a long path absorption system (like a White cell) is needed for high sensitivity. Thus, this technique has not been used in more recent studies.

High time resolution based on incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) was used for the simultaneous measurements of HONO and NO 2 [START_REF] Gherman | Incoherent broadband cavity-enhanced absorption spectroscopy in the near-ultraviolet: Application to HONO and NO 2[END_REF]. [START_REF] Duan | Development of an incoherent broadband cavityenhanced absorption spectrometer for in situ measurements of HONO and NO 2[END_REF] found measurement precisions (2σ) for HONO and NO 2 of about 180 and 340 ppt in 30 s, respectively. A field inter-comparison was carried out at a rural suburban site in Wangdu, Hebei Province, China. The concentrations of HONO and NO 2 measured by IBBCEAS were compared with a long optical path absorption photometer (LOPAP) and a NO x analyzer (Thermo Fisher Electron Model 42i), and the results showed very good agreement, and good correlation with slopes of 0.941±00,0069 and 0.964±0,0042 for HONO and NO 2 respectively and correlation coefficients (R 2 ) of HONO and NO 2 being ∼0.89 and ∼0.95, respectively. In addition, vehicle deployments were also tested to enable mobile measurements of HONO and NO 2 , demonstrating the promising potential of using IBBCEAS for sensitive, accurate and fast simultaneous measurements of HONO and NO 2 in the future.

Laser-photofragmentation laser-induced fluorescence (LP-LIF) has recently been applied to the indirect measurement of HONO with good sensitivity. The approach taken is to photolyze HONO and detect its OH photofragments. [START_REF] Bottorff | Development of a laserphotofragmentation laser-induced fluorescence instrument for the detection of nitrous acid and hydroxyl radicals in the atmosphere[END_REF] have used this approach to measure HONO and OH in forests, urban settings, and indoors.

Mass spectroscopic techniques have been used in a few HONO indoor studies. For example, HONO can be measured with Atmospheric Pressure Interface Chemical Ionization Mass Spectrometry (API-CIMS) using chloride reagent ion chemistry [START_REF] Spicer | Transformations, Lifetimes, and Sources of NO 2 , HONO, and HNO 3 in Indoor Environments[END_REF]. In a study by [START_REF] Collins | Evidence for Gas-Surface Equilibrium Control of Indoor Nitrous Acid[END_REF], HONO was detected indoors using a High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (HR-ToF-CIMS) after reaction with the acetate (CH 3 COO -) reagent ion. MS techniques are fast, sensitive, and can be selective when welldesigned. The humidity dependence of the sensitivity by formation of water clusters has to be carefully calibrated for the CIMS technique [START_REF] Jurkat | Measurements of HONO, NO, NO y and SO 2 in aircraft exhaust plumes at cruise[END_REF]. MS instruments are not compact but still have the potential for standard use in indoor studies.

In more recent indoors studies, the LOPAP (Long-Path Absorption Photometer) has frequently been used for real time measurements of HONO [START_REF] Bartolomei | Combustion processes as a source of high levels of indoor hydroxyl radicals (OH) through the photolysis of nitrous acid (HONO)[END_REF]2014;[START_REF] Gandolfo | The effectiveness of indoor photocatalytic paints on NO x and HONO levels[END_REF][START_REF] Liu | Unexpectedly High Indoor HONO Concentrations Associated with Photochemical NO 2 Transformation on Glass Windows[END_REF][START_REF] Lahib | Analytical Developments for Measuring Atmospheric Peroxy Radicals[END_REF]. This instrument fulfils the criteria mentioned above: a time resolution of a few minutes, high sensitivity and high dynamic range (from low ppt to ppm level), selective detection and a compact design. Since the instrument uses liquid reagents, it is not well-suited for very long-time network measurements. Table 3 summarizes results from some indoor HONO studies performed in different regions with different techniques and sample integration times (concentration levels, instrumentation used, type of indoor environment and location) using either online measurement techniques or offline analysis of collected samples.

HONO intercomparison campaigns

There have been several intercomparison campaigns of HONO measurements and instrumentation and the main findings are summarized in Table 4. This section is to provide general performance parameters for the most relevant instrumental techniques that have been used to quantify HONO concentrations, to aid in the selection of suitable techniques for future campaigns. LOPAP is particularly suitable for the determination of HONO in the indoor environment. It has been subjected to extensive tests through intercomparison exercises and has been compared to absolute techniques, i.e., spectroscopic instrumentation like DOAS, for which deployment in the indoor environment can pose significant hurdles. For higher indoor levels (>several hundred ppts) the IBBCEAS technique is also a suitable candidate, since these instruments are also compact, fast and selective. 3.

Reactive chlorine and precursors

The highly reactive Cl atom is a powerful oxidant of both organic and inorganic compounds and can be an important driver of atmospheric chemistry in both indoor and outdoor environments. The reactive nature of atomic Cl means that its concentrations are too low to be directly measured in the lower atmosphere. Measurements of chlorinecontaining species, however, can give us significant insight into both the scale of atomic Cl oxidation and the production of known respiratory irritants (e.g., NCl 3 ). These chlorinated gas phase compounds can be broadly classified into two groupseither organic/amine or inorganic compounds. Most chlorinated-organic compounds are stable with respect to tropospheric degradation processes (e.g., photolysis and OH reaction), and are either directly emitted or formed as oxidation products of Cl atom reactions with hydrocarbons. In contrast, inorganic chlorine compounds (e.g., HCl, HOCl, ClNO 2 ) can release Cl atoms during their tropospheric cycling and can be considered Cl atom reservoirs. These inorganic compounds are often formed via heterogeneous processes, which are thought to be important drivers of indoor chlorine chemistry (Mattila et al., 2020;[START_REF] Odabasi | Halogenated Volatile Organic Compounds from the Use of Chlorine-Bleach-Containing Household Products[END_REF][START_REF] Wong | Observations and impacts of bleach washing on indoor chlorine chemistry[END_REF].

Observations of chlorinated organic compounds provide useful information as tracers of Cl atom oxidation of hydrocarbons, and the detection of chloramines is useful both as an indicator of their production mechanisms and their direct health impacts. Measurements of inorganic chlorine compounds are key to improving our understanding of the dominant chlorine production and cycling mechanisms. Much of our understanding of chlorine chemistry comes from the study of stratospheric ozone chemistry, but many of the observational techniques used to study the stratosphere (e.g., satellite differential optical absorption) are not suitable for the indoor environment. However, recent developments in both mass-spectrometry and optical techniques make possible the in-situ study of complex chlorine chemical cycles. Table 5 summarises in-situ observational techniques that have been, or could be, used for the study of chlorine chemistry in the indoor environment. NO 3 is a major nocturnal oxidant in outdoor air formed through the relatively slow reaction between NO 2 and O 3 [START_REF] Brown | Nighttime radical observations and chemistry[END_REF]:

Further reaction with NO 2 reversibly produces N 2 O 5 , which acts as a reservoir for NO 3 : Rapid photolysis by visible radiation (photolysis lifetime of a few seconds in direct sunlight) suppresses NO 3 concentrations under daylight conditions. NO 3 reacts rapidly with unsaturated VOCs and can lead to SOA formation. Heterogeneous hydrolysis of N 2 O 5 on surfaces of solids or particles is an important loss route for nitrogen oxides, producing nitric acid. Heterogenous N 2 O 5 reactions with aerosol and surface chloride produce ClNO 2 , a photolabile source of Cl and NO 2 .

Absorption spectroscopy has been the most commonly used approach for detecting NO 3 based on its intense absorption band at 662 nm, allowing sensitive real-time measurements [START_REF] Yokelson | Temperature dependence of the NO 3 absorption spectrum[END_REF]. For in situ measurements, optical cavities are applied using ringdown or cavity-enhanced absorption spectroscopy. Monochromatic (laser) or broadband light sources may be used. Laser-based methods require additional measures to account for absorption by NO 2 and O 3 and scattering by particles. Inlet filters can be used to remove particle extinction, but must be changed regularly to avoid variable NO 3 losses with aging (Dubá et al., 2006). The rapid titration of NO 3 with NO: is often used to establish a baseline extinction.

Broadband methods use spectral analysis over tens of nanometres to retrieve the absorption and hence concentration of NO 3 . Interfering absorptions are identified through spectral fitting; however, water can complicate spectral retrieval because water absorption lines are too narrow to be resolved by a spectrograph and measured spectra diverge from the Beer-Lambert law [START_REF] Bitter | A broadband cavity ringdown spectrometer for in-situ measurements of atmospheric trace gases[END_REF]. Calibrating the response of broadband instruments is also more complex than for CRDS systems. Many outdoor (often airborne) studies now use the optical cavity methodology. N 2 O 5 is indirectly detected as NO 3 via thermal decomposition at elevated temperatures in the sample inlet. A single, extractive instrument with heated and unheated inlet channels can therefore be used to detect both NO 3 and N 2 O 5 . Although most optical cavity instruments draw the air sample through an enclosed optical cavity, broadband open path systems based on CEAS may have potential for indoor measurements, although N 2 O 5 quantification is not possible with this configuration [START_REF] Venables | High Sensitivity in Situ Monitoring of NO 3 in an Atmospheric Simulation Chamber Using Incoherent Broadband Spectroscopy[END_REF][START_REF] Wang | Monitoring Ambient Nitrate Radical by Open-Path Cavity-Enhanced Absorption Spectroscopy[END_REF].

Alternative methods include thermal dissociation-chemical ionization mass spectrometry (TD-CIMS) to quantify the sum of NO 3 and N 2 O 5 using ionisation by the I -ion [START_REF] Slusher | A thermal dissociation-chemical ionization mass spectrometry (TD-CIMS) technique for the simultaneous measurement of peroxyacyl nitrates and dinitrogen pentoxide[END_REF][START_REF] Yan | Fast heterogeneous loss of N 2 O 5 leads to significant nighttime NO x removal and nitrate aerosol formation at a coastal background environment of southern China[END_REF]. Laser-induced fluorescence detection of NO 3 has been demonstrated to a 1 sensitivity of about 4 pptv over a 10 min period [START_REF] Matsumoto | Development of a measurement system for nitrate radical and dinitrogen pentoxide using a thermal conversion/laser-induced fluorescence technique[END_REF][START_REF] Wood | Prototype for In Situ Detection of Atmospheric NO 3 and N 2 O 5 via Laser-Induced Fluorescence[END_REF]. LIF is less sensitive than absorption-based methods and has not been extensively pursued for detecting NO 3 /N 2 O 5 . A flow tube reactor combined with adsorbent tube sampling and analysis using thermal desorption-gas chromatography was used for the first experimental measurements of indoor NO 3 /N 2 O 5 chemistry but the system is complex and time resolution is poor [START_REF] Nøjgaard | Indoor measurements of the sum of the nitrate radical, NO 3 , and nitrogen pentoxide, N 2 O 5 in Denmark[END_REF].

Performance characteristics of instruments measuring NO 3 and N 2 O 5 are given in Table 6. Instrument intercomparison studies have shown good agreement between the different absorption methods [START_REF] Dorn | Intercomparison of NO 3 radical detection instruments in the atmosphere simulation chamber SAPHIR[END_REF]Fuchs et al., 2012). (2005) 5.

Criegee intermediates

Criegee Intermediates (CIs), also known as Zwitter ionic intermediates or carbonyl oxides, have been the centre of growing interest over the past decade due to their potential role in the oxidative capacity of the atmosphere and specifically in SO 2 oxidation and secondary aerosol formation.

Outdoor concentrations ranging from 10 4 -10 5 molecules cm -3 have been estimated from modelling studies for the sum of atmospheric Criegees [START_REF] Khan | Criegee intermediates and their impacts on the troposphere[END_REF]. This range of values is consistent with indirect observations indicating a mean concentration of approximately 5×10 4 molecules cm -3 during the HUMPPA-COPEC 2010 (forested area) and HOPE 2012 (rural area) field campaigns [START_REF] Novelli | Estimating the atmospheric concentration of Criegee intermediates and their possible interference in a FAGE-LIF instrument[END_REF]. The pool of atmospheric CIs is complex due to (i) the multitude of unsaturated VOCs that can react with ozone in the atmosphere and (ii) the potential formation of syn-and anti-conformers. Both the complexity of the CI mixture and their presence at low concentrations put stringent demands on analytical techniques in terms of sensitivity and selectivity.

While no technique is currently available for atmospheric measurements of CI, some direct and indirect approaches have been developed in the laboratory to characterize the chemical structure of small CIs (C 1 -C 6 ) and to study their kinetics. Techniques for direct detection of specific CIs include photoionization mass spectrometry, spectroscopic approaches in the ultraviolet, infrared and microwave regions, and chemical ionization mass spectrometry [START_REF] Chhantyal-Pun | Criegee intermediates: production, detection and reactivity[END_REF][START_REF] Taatjes | Criegee Intermediates: What Direct Production and Detection Can Teach Us About Reactions of Carbonyl Oxides[END_REF]. An indirect technique relying on the stabilization of CIs using spin trap molecules and their subsequent detection by mass spectrometry was also proposed (Giorio et al., 2017b;2017a;[START_REF] Zaytsev | Application of chemical derivatization techniques combined with chemical ionization mass spectrometry to detect stabilized Criegee intermediates and peroxy radicals in the gas phase[END_REF]. In addition to these laboratory techniques, a few instruments designed for hydroxyl radical measurements in the atmosphere have shown potential to detect unspeciated CIs [START_REF] Mauldin | A new atmospherically relevant oxidant of sulphur dioxide[END_REF][START_REF] Novelli | Estimating the atmospheric concentration of Criegee intermediates and their possible interference in a FAGE-LIF instrument[END_REF]. First, we briefly describe techniques designed to investigate the structure and the kinetics of CIs in the laboratory and we then discuss OH field instruments that may be capable of detecting CIs in the atmosphere.

Detection techniques employed in the laboratory for fundamental research:

1. Photoionization mass spectrometry (PIMS): [START_REF] Taatjes | Direct Observation of the Gas-Phase Criegee Intermediate (CH 2 OO)[END_REF] and [START_REF] Welz | Direct Kinetic Measurements of Criegee Intermediate (CH 2 OO) Formed by Reaction of CH 2 I with O 2[END_REF] demonstrated that PIMS can be used for detecting individual CIs in simple chemical mixtures. The authors employed the tunable vacuum UV light from the ALS synchrotron at Berkeley National Laboratory to perform kinetic experiments on the simplest CI, formaldehyde oxide (CH 2 OO). A subsequent study from the same group highlighted that some selectivity can also be achieved between anti-and synconformers of acetaldehyde-oxide (CH 3 COO) on the basis of their difference in ionization energy [START_REF] Taatjes | Direct measurements of conformer-dependent reactivity of the Criegee intermediate CH 3 CHOO[END_REF]. This technique was extended to the detection of acetone-oxide ((CH 3 ) 2 COO) (Chhantyal-Pun et al., 2017).

UV-Vis absorption spectroscopy:

This technique was used by several groups for the detection of formaldehyde-oxide, acetaldehyde-oxide, methylvinylketone-oxide, and methacrolein-oxide, taking advantage of their specific absorption features [START_REF] Caravan | Direct kinetic measurements and theoretical predictions of an isoprene-derived Criegee intermediate[END_REF][START_REF] Chhantyal-Pun | Criegee intermediates: production, detection and reactivity[END_REF]. [START_REF] Sheps | UV absorption probing of the conformer-dependent reactivity of a Criegee intermediate CH 3 CHOO[END_REF] [START_REF] Chhantyal-Pun | Criegee intermediates: production, detection and reactivity[END_REF][START_REF] Chung | Infrared characterization of formation and resonance stabilization of the Criegee intermediate methyl vinyl ketone oxide[END_REF][START_REF] Lin | Infrared identification of the Criegee intermediates syn-and anti-CH 3 CHOO, and their distinct conformation-dependent reactivity[END_REF]. A higher degree of selectivity than in the UV region can be achieved if high-resolution IR sources are used. The detection of CIs is performed using broadband FTIR, high-resolution CW-QCL, and action spectroscopy [START_REF] Chhantyal-Pun | Criegee intermediates: production, detection and reactivity[END_REF]. 4. Microwave spectroscopy: This technique was used under molecular beam conditions to investigate the molecular structure of CIs and their kinetics, including various isotopologues of formaldehyde oxide, syn-and anti-conformers of acetaldehydeoxide, and acetone-oxide (Chhantyal-Pun et al., 2020). 5. Spin trap / Chemical Ionization Mass Spectrometry: Gioro et al. (Giorio et al., 2017a;2017b) proposed an online method relying on the stabilization of CIs using a spin trap molecule (DMPO -5,5-dimethylpyrroline-N-oxide) and the quantification of the CI-spin trap adducts by Proton Transfer Reaction-Mass Spectrometry (PTR-MS) using H 3 O + reagent ions. Combining high resolution mass spectrometry with the selective nature of the DMPO-CI reaction has the potential to provide the discrimination needed to speciate non-isobaric CIs in complex chemical mixtures. The authors reported detection of CIs generated from ozonolysis of biogenic and anthropogenic alkenes with a detection limit of approximately 7×10 8 molecules cm -3 (10 s integration time) for α-pinene-derived CIs. More recently, Zaytsev et al. ( 2021) also reported the use of DMPO and other spin trap species (HFA: Hexafluoroacetone, TEMPO: (2,2,6,6-tetramethylpiperidin-1-yl)oxyl) with PTR-MS using H 3 O + and NH 4 + reagent ions. The authors reported detection limits for acetone-oxide of 1.4×10 7 molecules cm -3 and 3.4×10 7 molecules cm -3 at a time resolution of 30 seconds using HFA/H 3 O + and DMPO/NH 4 + , respectively. 6. Chemical Ionization Mass Spectrometry: [START_REF] Berndt | Direct Probing of Criegee Intermediates from Gas-Phase Ozonolysis Using Chemical Ionization Mass Spectrometry[END_REF] demonstrated that CIs can also be directly detected by CIMS as adducts using reagent ions such as protonated ethers (tetrahydrofuran, diethyl ether) for formaldehyde-oxide and protonated amines (n-and tert-butylamine, diethylamine) for cyclohexene-derived C6-CIs. The authors stressed that selective detection of CIs from other isomeric compounds is possible through careful selection of reagent ions. However, selective detection of CIs in the complex mixture of ambient trace gases has yet to be demonstrated. Detection limits ranging from 10 4 -10 5 molecules cm -3 were observed during kinetic experiments using a CI-APi-TOF (Chemical Ionization -Atmospheric Pressure interface -Time Of Flight) instrument at a time resolution of 10 min.

Field deployable instruments and atmospheric measurements of CIs

The deployment of instruments capable of measuring the atmospheric hydroxyl radical (OH) has provided surprising observations over the past decades, indicating that species other than OH were detected during specific steps of the OH measurement sequence. Indeed, zeroing approaches implemented by CIMS, [START_REF] Mauldin | A new atmospherically relevant oxidant of sulphur dioxide[END_REF] and FAGE, [START_REF] Novelli | Characterisation of an inlet pre-injector laser-induced fluorescence instrument for the measurement of atmospheric hydroxyl radicals[END_REF][START_REF] Taatjes | Criegee Intermediates: What Direct Production and Detection Can Teach Us About Reactions of Carbonyl Oxides[END_REF]) instruments have highlighted the presence of reactive species capable of oxidizing SO 2 in CIMS instruments or capable of producing OH in the low pressure sampling cell of FAGE instruments. Further investigations indicated that the behaviour of these unidentified species is consistent with the known CI chemistry, which suggests that a substantial fraction of these unidentified species could be CIs. The extent to which "background" signals from both CIMS and FAGE instruments are connected to ambient CIs is not clear and further investigation is necessary to determine whether these techniques could be used to provide integrated measurements of the pool of ambient CIs.

While a large number of techniques have been used in the laboratory to characterize the molecular structure of CIs and their kinetics, it will be challenging to transfer these techniques for indoor environments due to the sensitivity and selectivity required. Indeed, these techniques were employed on experimental systems where large concentrations of CIs (likely higher by several orders of magnitude than in the atmosphere) can be generated in simple chemical mixtures, with a lower number of potentially interfering species than in the atmosphere. The most promising approach, which was shown to reach detection limits that are close to estimated ambient CI concentrations, is CIMS [START_REF] Berndt | Direct Probing of Criegee Intermediates from Gas-Phase Ozonolysis Using Chemical Ionization Mass Spectrometry[END_REF], which has already been used for monitoring other reactive species such as the hydroperoxyl radical (HO 2 ) [START_REF] Sanchez | A new technique for the direct detection of HO 2 radicals using bromide chemical ionization mass spectrometry (Br-CIMS): initial characterization[END_REF] and HOMs [START_REF] Bianchi | Highly Oxygenated Organic Molecules (HOM) from Gas-Phase Autoxidation Involving Peroxy Radicals: A Key Contributor to Atmospheric Aerosol[END_REF]. The quantification of atmospheric CIs will also require the development of calibration apparatus capable of generating known concentrations of specific CIs [START_REF] Novelli | Estimating the atmospheric concentration of Criegee intermediates and their possible interference in a FAGE-LIF instrument[END_REF].

6.

Glyoxal and methylglyoxal

Photolysis of dicarbonyl compounds, specifically glyoxal (CHOCHO, GLY) and methyl glyoxal (CH 3 C(O)CHO, MGLY), is the main initiation route that produces HO 2 and RO 2 [START_REF] Emmerson | Free radical modelling studies during the UK TORCH Campaign in Summer 2003[END_REF]. This route has been verified several times in the outdoor environment. It also operates efficiently in the indoor environment and can even be enhanced under certain circumstances such as cleaning or use of air cleaners [START_REF] Carslaw | Significant OH production under surface cleaning and air cleaning conditions: Impact on indoor air quality[END_REF]. GLY and MGLY are produced in the atmosphere by several processes, including oxidation of VOCs (particularly biogenic species) [START_REF] Miller | Glyoxal yield from isoprene oxidation and relation to formaldehyde: chemical mechanism, constraints from SENEX aircraft observations, and interpretation of OMI satellite data[END_REF], anthropogenic photo-oxidation of aromatics in the presence or absence of NO x [START_REF] Bierbach | Atmospheric Chemistry of Unsaturated Carbonyls: Butenedial, 4-Oxo-2-pentenal, 3-Hexene-2,5-dione, Maleic Anhydride, 3H-Furan-2-one, and 5-Methyl-3H-furan-2-one[END_REF][START_REF] Alvarez | Unsaturated dicarbonyl products from the OH-initiated photo-oxidation of furan, 2-methylfuran and 3-methylfuran[END_REF], 2007;[START_REF] Volkamer | Primary and Secondary Glyoxal Formation from Aromatics: Experimental Evidence for the Bicycloalkyl-Radical Pathway from Benzene, Toluene, and p-Xylene[END_REF][START_REF] Wang | Dicarbonyl Products of the OH Radical-Initiated Reactions of Naphthalene and the C 1 -and C 2 -Alkylnaphthalenes[END_REF][START_REF] Yu | Atmospheric photooxidation of alkylbenzenes-I. Carbonyl product analyses[END_REF], pyrogenic sources [START_REF] Fu | Global budgets of atmospheric glyoxal and methylglyoxal, and implications for formation of secondary organic aerosols[END_REF][START_REF] Myriokefalitakis | The influence of natural and anthropogenic secondary sources on the glyoxal global distribution[END_REF][START_REF] Stavrakou | The continental source of glyoxal estimated by the synergistic use of spaceborne measurements and inverse modelling[END_REF][START_REF] Volkamer | A missing sink for gas-phase glyoxal in Mexico City: Formation of secondary organic aerosol[END_REF][START_REF] Washenfelder | The glyoxal budget and its contribution to organic aerosol for Los Angeles, California, during CalNex 2010[END_REF], and via chemical routes such as dicarbonyl formation from Criegee biradicals [START_REF] Grosjean | Air Quality Model Evaluation Data for Organics. 2. C 1 -C 14 Carbonyls in Los Angeles Air[END_REF].

The main sources of GLY and MGLY in the indoor environment are diverse, and most of them are also observed outdoors (e.g., oxidation of aromatic species). Other routes more specific to the indoor environment would be formation via metabolic processes since they are formed endogenously in numerous enzymatic and nonenzymatic reactions [START_REF] Kulkarni | ALKBH7 mediates necrosis via rewiring of glyoxal metabolism[END_REF][START_REF] Lange | Glyoxal Formation and Its Role in Endogenous Oxalate Synthesis[END_REF][START_REF] Lee | Transcriptional activation of the aldehyde reductase YqhD by YqhC and its implication in glyoxal metabolism of Escherichia coli K-12[END_REF][START_REF] Nemet | Methylglyoxal in food and living organisms[END_REF][START_REF] Nigro | Methylglyoxal-Glyoxalase 1 Balance: The Root of Vascular Damage[END_REF][START_REF] Shangari | The cytotoxic mechanism of glyoxal involves oxidative stress[END_REF]. Particularly important to the indoor environment is their formation by ozonolysis of terpenes, found in cleaning products and fresheners. The ozonolysis of squalene is closely related to the presence of occupants because squalene is an important component of human skin oil. Given the high degree of unsaturation in squalene, tertiary and higher-order reaction products can be generated and sequential ozone attack generates bifunctional secondary products that contain carbonyl, carboxyl, or α-hydroxy ketone terminal groups [START_REF] Wisthaler | Reactions of ozone with human skin lipids: Sources of carbonyls, dicarbonyls, and hydroxycarbonyls in indoor air[END_REF]. The major bifunctional products are the four dicarbonyls: 4-oxopentanal (4-OPA), 1,4-butanedial (succinic dialdehyde), 4-methyl-8oxo-4-nonenal (4-MON), and 4-methyl-4-octene-1,8-dial (4-MOD) produced by attack at the bonds.

Measurement of indoor glyoxal and methylglyoxal

GLY and MGLY can be measured by various techniques: infrared (IR) absorption spectroscopy [START_REF] Profeta | Quantitative Infrared Intensity Studies of Vapor-Phase Glyoxal, Methylglyoxal, and 2,3-Butanedione (Diacetyl) with Vibrational Assignments[END_REF][START_REF] Tuazon | A product study of the gas-phase reaction of Methacrolein with the OH radical in the presence of NO x[END_REF], ultraviolet-visible (UV-Vis) absorption spectroscopy [START_REF] Sinreich | MAX-DOAS detection of glyoxal during ICARTT 2004[END_REF][START_REF] Thalman | Inherent calibration of a blue LED-CE-DOAS instrument to measure iodine oxide, glyoxal, methyl glyoxal, nitrogen dioxide, water vapour and aerosol extinction in open cavity mode[END_REF][START_REF] Volkamer | DOAS measurement of glyoxal as an indicator for fast VOC chemistry in urban air[END_REF][START_REF] Washenfelder | Measurement of glyoxal using an incoherent broadband cavity enhanced absorption spectrometer[END_REF], chromatographic analysis of derivatization by O- (2,3,4,5,6-pentafluorobenzyl)-hydroxylamine (PFBHA) or dinitrophenylhidrazyne (DNPH) followed by mass spectrometry or flame ionization, and phosphorescence [START_REF] Henry | Quantification of gas-phase glyoxal and methylglyoxal via the Laser-Induced Phosphorescence of (methyl)GLyOxal Spectrometry (LIPGLOS) Method[END_REF][START_REF] Huisman | Laser-Induced Phosphorescence for the in Situ Detection of Glyoxal at Part per Trillion Mixing Ratios[END_REF]. CIMS (using H 3 O + , O 2 + or NO + reagent ions) has also been used to detect MGLY (de Gouw et al., 2003;[START_REF] Guimbaud | Kinetics of the reactions of acetone and glyoxal with O 2 + and NO + ions and application to the detection of oxygenated volatile organic compounds in the atmosphere by chemical ionization mass spectrometry[END_REF][START_REF] Karl | Rapid formation of isoprene photo-oxidation products observed in Amazonia[END_REF][START_REF] Michel | A selected ion flow tube study of the reactions of H 3 O + , NO + and O 2 + with methyl vinyl ketone and some atmospherically important aldehydes[END_REF].

Most measurements of GLY and MGLY have used derivatisation techniques and a variety of sampling methods with off-line GC analysis with different detectors, GC/MS or HPLC. The most extensively investigated source of GLY and MGLY and other dicarbonyls indoors is from ozonolysis of terpenes in cleaning products, air fresheners, and ventilation systems. Derivatisation techniques have mainly been used for this analysis, and hundreds of papers can be found in the literature on this topic. Table 7 lists example studies that applied derivatization with off-line chromatographic analysis to illustrate the most common sources of GLY, MGLY (and in some studies, several other dicarbonyls) that have been studied in the indoor environment. [START_REF] Szulejko | Derivatization techniques for determination of carbonyls in air[END_REF] authored a general review of derivatization techniques for determination of carbonyls in air and provided a list of selected derivatising agents. The most common are O- (2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride (PFBHA) 2,4-dinitrophenylhydrazine (DNPH). Studies by [START_REF] Ham | Gas-phase reaction products and yields of terpinolene with ozone and nitric oxide using a new derivatization agent[END_REF] and [START_REF] Jackson | Identification and quantification of carbonyl-containing α-pinene ozonolysis products using O-tert-butylhydroxylamine hydrochloride[END_REF] used TBOX, o-tert-butylhydroxylamine hydrochloride as derivatizing agent. A fluorogenic reagent, dansylhydrazine (DNSH) provide hydrazones that can be analysed by fluorescence, which enhances the sensitivity and selectivity of the technique. [START_REF] Zhang | Development of the Personal Aldehydes and Ketones Sampler Based upon DNSH Derivatization on Solid Sorbent[END_REF] used this method to develop a Personal Aldehydes and Ketones Sampler (PAKS). Other applications have been detection of GLY (and other toxic carbonyl compounds) in cigarette smoke [START_REF] Fujioka | Determination of toxic carbonyl compounds in cigarette smoke[END_REF] by derivatizing reactive carbonyl compounds into stable nitrogen-containing compounds (pyrazoles for -dicarbonyl and -unsaturated aldehyde; and quinoxalines for -dicarbonyls), followed by quantitative detection using gas chromatography with a nitrogen phosphorus detector.

Indoor environments have access obstacles and space constraints that do not influence outdoor measurements. Techniques that have been used for indoor measurements of GLY and MGLY are mostly easy to deploy techniques that do not require bulky equipment or that involve only the sampling of gases or particles followed by offline analysis. There are a few exceptions like the HOMECHEM campaign [START_REF] Farmer | Overview of HOMEChem: House Observations of Microbial and Environmental Chemistry[END_REF] or the SURFIN campaign in a school classroom in Marseille (Gomez [START_REF] Alvarez | Unexpectedly high indoor hydroxyl radical concentrations associated with nitrous acid[END_REF]. For these reasons, the intercomparison exercises carried out in simulation chambers constitute a useful basis to discuss the different measurement techniques for GLY and MGLY. [START_REF] Thalman | Instrument intercomparison of glyoxal, methyl glyoxal and NO 2 under simulated atmospheric conditions[END_REF] carried out an intercomparison of different techniques for the measurement of GLY, MGLY and NO 2 in the NCAR and EUPHORE chambers (see Table 8). Excellent linearity was attained by all instruments/techniques under idealized conditions (pure GLY or MGLY, R 2 > 0.96) and in complex gas mixtures characteristic of dry photochemical smog systems (o-xylene/NOx and isoprene/NOx, R 2 > 0:95; R 2 ~0.65 for offline Solid Phase Micro Extraction, SPME, measurement of MGLY). The correlations were more variable at higher humidity (RH>45 %) for MGLY (0.58 < R 2 < 0.68) than for GLY (0.79 < R 2 < 0.99). Intercepts of correlations were generally below the instruments' experimentally determined detection limits and slopes varied by less than 5% for instruments that could also simultaneously measure NO 2 . For GLY and MGLY the slopes varied by less than 12 and 17% (both 3) between direct absorption techniques (i.e., calibration from knowledge of the absorption cross section). Larger variability was found among in situ techniques that employed external calibration sources (75-90%, 3), and/or techniques that used offline analysis. The main conclusion was that techniques now exist to conduct fast and accurate measurements of GLY at ambient concentrations, and MGLY under simulated conditions. Measuring ambient concentrations of MGLY would be desirable but remains a challenge. Recent work by [START_REF] Michoud | Field measurements of methylglyoxal using proton transfer reaction time-of-flight mass spectrometry and comparison to the DNPH-HPLC-UV method[END_REF] have shown that MGLY can be measured using PTR-ToF-MS and shows reasonable agreement with the DNPH-HPLC-UV method.

In most of the reactions generating GLY and MGLY in the indoor environment other dicarbonylic products are also expected some unsaturated, highly photo-labile and short-lived (e.g., butenedial, 4-oxo-2-pentenal), and hence also powerful sources of radicals in several relevant reactions in the indoor environment for which analytical determination could be even more challenging.

G,M,N G,M,N M G,M G,M,N G,M G,M

Photolysis rates (J values)

To accurately determine the role of photolysis processes in indoors atmospheres, it is necessary to characterize both the natural light transmitted through windows as well as light from artificial sources, in terms of wavelength and intensity. Indeed, determining the production rate of radicals or other species by photolysis requires the photolysis rate J (in s - 1 ) of the species being photolyzed, which is given by: Here F(λ) is the actinic flux (photons cm -2 nm -1 s -1 ), σ(λ) is the cross section of the molecule (cm 2 ) at the wavelength λ (nm) and φ(λ) is the dimensionless quantum efficiency, i.e., the probability of photodissociation of the molecule after absorption of a photon at wavelength λ. λ min and λ max represent the range of wavelengths that need to be considered to cover the absorption spectrum of the molecule. σ(λ) and φ(λ) are spectroscopic parameters, which have been measured in the laboratory for many molecules of atmospheric interest, often as a function of pressure and temperature, and are provided by databases such as the MPI Spectral Atlas (http://satellite.mpic.de/spectral_atlas/index.html). The production rate of the species generated by photolysis can then be calculated multiplying J with the concentration of the photolyzed species.

Spectroradiometers are able to measure actinic fluxes as a function of wavelength, which can then be combined with literature data to calculate photolysis frequencies [START_REF] Hofzumahaus | Measurement of Photolysis Frequencies in the Atmosphere[END_REF]. For the measurement of photolysis rates specifically of O 3 to form O( 1 D), and NO 2 to form O( 3 P), fixed bandwidth filter radiometers can be also used [START_REF] Bohn | Characterisation and improvement of j(O 1 D) filter radiometers[END_REF]. A comprehensive account of the different methods is provided by [START_REF] Hofzumahaus | Measurement of Photolysis Frequencies in the Atmosphere[END_REF]. Some of the different types of spectroradiometers (providing spectrally resolved data) include:

1. LICOR or Ocean optics spectrometer [START_REF] Blocquet | Impact of the spectral and spatial properties of natural light on indoor gas-phase chemistry: Experimental and modeling study[END_REF][START_REF] Kowal | Wavelength-Resolved Photon Fluxes of Indoor Light Sources: Implications for HO x Production[END_REF][START_REF] Zhou | Factors affecting wavelength-resolved ultraviolet irradiance indoors and their impacts on indoor photochemistry[END_REF].

Both instruments have a cosine receptor and measure irradiance. From the irradiance measurement I(λ) at the wavelength λ (in nm), the actinic flux F(λ) is calculated by dividing I by cosine θ, θ being the azimuth angle under conditions of direct sunshine. For conditions of indirect sunlight it is considered that the light comes from all directions (through the window and by reflection off surfaces) and is isotropic, the actinic flux is obtained by multiplying the irradiance by a factor of 2. 2. METCON spectroradiometers [START_REF] Gandolfo | Spectrally resolved actinic flux and photolysis frequencies of key species within an indoor environment[END_REF]Kleffman, 2018). 3. PAR (photosynthetically active radiation) sensors provide integrated light intensity (400-750 nm). When calibrated by a spectrometer, these sensors can provide an integrated photon flux (irradiance) over the wavelength range 400-750 nm. As PAR sensors are compact and cheap, they can be used to map the light distribution in a room [START_REF] Blocquet | Impact of the spectral and spatial properties of natural light on indoor gas-phase chemistry: Experimental and modeling study[END_REF].

Future indoor measurements of radicals and radical precursors

The techniques described above for quantifying the concentrations of radicals and radical precursors illustrate the challenge of measuring highly reactive species. Below we consider the analytical and operational characteristics needed for quantifying indoor air components.

Occupied indoor spaces pose particular measurement challenges, and it may be necessary to adapt instrumentation developed for outdoor measurements to the indoor environment. Instruments must operate in relatively confined indoor spaces that might be occupied by people during the measurements. Instrumentation therefore should be as unobtrusive as possible; this includes considerations of instrumentation size and minimising the generation of heat and noise (for example, from instrumentation pumps). Emissions of instrument chemicals and exhaust gases must also be considered and minimised.

Indoor concentrations of radicals and their precursors can differ considerably from outdoors. For example, indoor combustion sources like cooking can produce high local HONO levels, and cleaning activities can release much higher levels of chlorinated compounds than would occur outdoors. In studies where episodes of high concentration are the primary focus, this could allow instrumentation sensitivity to be slightly relaxed. In other cases, instrumentation may need to have a wider dynamic range to handle both low and high concentrations.

The temporal resolution of radical measurements must be sufficiently short to capture changes in the concentrations of reactive species on the timescale of indoor activities such as cooking or cleaning. Real-time measurements with a measurement interval of a few minutes are adequate for many purposes. However, some of the techniques discussed above (such as derivatisation, followed by off-line analysis) have much longer sample acquisition times. Besides the extra effort and time required for off-line analysis, such approaches would be too slow to capture fast changes in indoor air composition and transformation.

The spatial variability of reactive species must also be evaluated. The high reactivity of some radical species and the location of different sources and sinks can produce significant variation in radical concentrations across an indoor space. It would therefore be advisable to compare fast radical measurements with integrated measurements (using for example, OH tracers). Comparative measurements in direct sunlight, and in the shade or dark zones, are needed to characterize and quantify the contribution of reactive chemistry in different locations in an indoor space. Such measurements would help to establish the relative importance of ozonolysis of alkenes versus photolytic sources, for example. The contribution of direct emissions (such as HONO from candles, gas fires, and stoves) is also unclear and could be extremely large; measurements around such sources are needed. This is an important question because the health impacts of combustion sources in confined indoor environments are of special concern, both from direct emissions and through their influence on indoor air chemistry.

In addition to chemical measurements, spatial differences in the actinic flux also influences the spatial variation in the concentrations and chemical processes taking place indoors. The transmission of light to the indoor environment affects HONO photolysis, to produce OH radicals, and other processes, such as chlorine chemistry and photosensitization processes mediated by several household chemicals. The actinic flux is therefore an important parameter in indoor photochemistry and should be measured at difference locations in an indoor space, in parallel with chemical observations. Measurements are also needed in a range of locations and conditions (including types of windows and building orientation) representative of the indoor environment.

Except for the Criegee intermediates and some chlorine species, techniques are now available which are sufficiently sensitive for exploring the contribution of radicals and their precursors to oxidation processes within the indoor environment. Previous studies of these species have already revealed surprises about their role in the indoor environment. However, comprehensive measurements of these species will not be routinely possible because monitoring atmospheric radicals and their precursors requires custom-built, highly specialized, and specific instrumentation. The most important exceptions in this regard are CIMS instruments (which can measure several of the above species, depending on the ionisation source) and LOPAP (for HONO measurements). For other species requiring custom instruments, the instruments either have to be developed in-house -a formidable challenge -or measured in collaboration with research groups that possess the necessary instrumentation. In this case, the technique and instrument used will be dictated by instrument (and collaborator) availability. Nevertheless, understanding indoor air chemistry is an important scientific objective, and some indoor air measurement campaigns will need to combine the instrumentation and collaborations needed to explore atmospheric chemistry in different indoor environments.

Continued improvement of existing techniques for outdoor measurements, and their adaptation for use in occupied indoor spaces, would also be beneficial. Sensitive and accurate measurements remain vital, as does extending measurements to other species of potential interest, like dicarbonylic products besides GLY and MGLY. Large radical campaigns including NO y /VOC/radical/chlorine compound concentrations, actinic flux and the air exchange rate are also recommended.

To date, standardized measurement methods for the assessment of indoor air quality have been mainly based on discontinuous measurements of long-lived trace substances. The presence of short-lived indoor oxidants and their effect on the abundance of oxidation products that building occupants inhale, ingest or dermally absorb is not tackled by the internationally standardized assessment of indoor air quality. Future research efforts and corresponding standardization activities should focus on expanding our current understanding of the conditions and underlying chemistry that produce high concentrations of pollutants indoors.

Finally, complementary measurements of OH reactivity during standardized material emission tests can help to investigate potential uncertainties in material emissions with special regard to standardized test protocols and material labels. Such measurements will help derive estimates for the formation rate of secondary indoor pollutants resulting from OH-initiated oxidation processes.

Figure 1 :

 1 Figure 1: Photon flux of common indoor light sources (right axis, arbitrary scale) shown with the product of absorption cross section and photolysis quantum yield of likely indoor oxidants (left axis). Actinic fluxes of the indoor lamps were measured at a short distance. Reproduced from Young et al., 2019, with permission from the Royal Society of Chemistry.

Figure 2 .

 2 Figure 2. Techniques currently used to quantify OH in reactive systems: FAGE 1 (Faloona et

a

  Limit of detection/accuracy as stated by the operator. b October 2015. c Comparative reactivity measurement. d Faster flow of 1 L min -1 in inlet line. e Produced continuously by water photolysis by a PenRay lamp.

Figure 3 .

 3 Figure3. Techniques currently used to quantify HO 2 in reactive systems: 1 Absorption Spectroscopy[START_REF] Hong | A shock tube study of the rate constants of HO 2 and CH 3 reactions[END_REF],2 Flash Photolysis UV Absorption(Tomas et al., 2001), 3 Near-Infrared Wavelength Modulation Spectroscopy&UV Absorption Spectroscopy[START_REF] Christensen | Measurements of the Rate Constant of HO 2 + NO 2 + N 2 → HO 2 NO 2 + N 2 Using Near-Infrared Wavelength-Modulation Spectroscopy and UV-Visible Absorption Spectroscopy[END_REF]), 4 cw-CRDS[START_REF] Thiébaud | Near infrared cw-CRDS coupled to laser photolysis: Spectroscopy and kinetics of the HO 2 radical[END_REF],5 Optical Feedback Cavity Enhanced Absorption Spectroscopy(Gianella et al., 2016), 6 Noise-Inmune Cavity Enhanced Optical Heterodyne Detection(Bell et al., 2012), 7 Mid-Infrared Faraday Rotation Spectrometry(Brumfield et al., 2013), 8 Bromide Chemical Ionisation Mass Spectrometry(Albrecht et al., 2019), 9 Peroxy Radical Chemical Ionization Mass Spectrometry[START_REF] Edwards | Chemical Ionization Mass Spectrometer Instrument for the Measurement of Tropospheric HO 2 and RO 2[END_REF], 10 FAGE[START_REF] Whalley | Reporting the sensitivity of laser-induced fluorescence instruments used for HO 2 detection to an interference from RO 2 radicals and introducing a novel approach that enables HO 2 and certain RO 2 types to be selectively measured[END_REF]), 11 PERCA-denuder[START_REF] Miyazaki | A new technique for the selective measurement of atmospheric peroxy radical concentrations of HO 2 and RO 2 using a denuding method[END_REF].

Figure 4 .

 4 Figure 4. Techniques currently used to quantify individual or sum of RO 2 in reactive systems. Techniques for CH 3 O 2 : 1 Laser Flash photolysis coupled with UV-time resolved

Table 1 .

 1 Concentration range of the radicals in the atmosphere. Note that for CH 3 O 2 this represents a model estimate only.

Table 3 .

 3 Examples of indoor HONO studies.

	Location	Building Type	Avg±SD [ppb]	Range [ppb]	Technique	Reference
	Reactions of NO 2 with water vapor in indoor environment	Chamber study: Reactivity and kinetics study of NO 2 with water vapor	(0.25 ±0.04) ppb HONO min -1 per ppm NO 2 (first order rate kinetics)	15 ppb HONO per ppm NO 2 (Steady state)	DOAS	Pitts et al. (1985)
	Chicago and Meriland	Two research houses: Residential with unvented gas space heaters	40 (24 h average)	0-100	Chemiluminescent NO x monitor+carbonate denuder	Brauer et al. (1990)
	Rome, Italy	Residential	Night: 14.3	9.9-20.8	Chemiluminiscence with switchable Na 2 CO 3 scrubber	Febo and Perrino (1991)
	Lubbock, TX (USA)	University Laboratory	N/A	0-3	Wet effluent Diffusion Denuder (WEDD) with Low-pressure Ion Chromatography-Postcolumn Reaction Detection	Večeřa and Dasgupta (1991)
	Alburquerque, NM (USA)	Residential	4.7±2.3	1.8-8.1	Carbonate denuder+filter pack	Spengler et al. (1993)
	Ohio (USA)	Residential (combustion sources)	N/A	0-18	Atmospheric Pressure Chemical Ionisation Mass Spectrometry (APCI MS)	Spicer et al. (1993)
	Lubbock, TX (USA)	Residential (combustion processes)	N/A	20-90	WEDD	Večeřa and Dasgupta (1994)
	Brazil	Commercial	4.3±2.8	1.0-8.8	Carbonate-impregnated filter	Allen and Miguel (1995)
	Eastern USA	Residential	4.0 ± 2.8 (summer) 5.45 ± 3.75 (winter)	0-11.3	Carbonate/glycerol denuder	Leaderer et al. (1999)
	Florence (Italy)	Galleria degli Uffizi	4.8	2.6-7.3	Carbonate denuder	De Santis et al. (1999)
		Investigation on HONO		Up to 12 ppb for 140
	Chamber experiment	formation on Teflon, wallpaper, carpet (50, 70% RH)	N/A	ppb for 100 ppb NO 2 0.53 h ) -1 (air exchange rate=	Chemiluminescence+carbonate denuder	Wainman et al. (2001)
	California, USA	Residential	4.6	0-21	Passive sampler; carbonate-impregnated filter	Lee et al. (2002)
		Residential	3.67 (summer)	1.3-7.3
	Cairo, Egypt	Commercial	6.8 (winter) 1.24 (summer)	1.6-12.5 0.52-2.13	Carbonate denuder	Khoder et al. (2002)
			1.42 (winter)	0.61-2.21
	Gwangiu, Korea	Residential	1.4	0.3-9	Online liquid trap/luminol detection	Jarvis et al. (2005)

Table 4 .

 4 Intercomparison of analytical techniques/instrumentation for the determination of HONO.

	Techniques/Instruments intercompared	Summary of results	Reference
	DOAS/Annular Denuder	Good agreement during night, AD >>DOAS during day	Appel et al. (1990)
	DOAS/Annular Denuder	Good agreement and correlation, but ca. 0.7 ppb offset	Febo et al. (1996)
	DOAS/Chemiluminescence	Good agreement	Coe et al. (1997)
	DOAS/RWAD	Good agreement during night, AD >>DOAS during day	Müller et al. (1999)
	LOPAP/DOAS (EUPHORE)	Good agreement for complex conditions (Diesel exhaust/dark)	Heland et al. (2001) Kleffmann et al. (2002)
	Parallel Plate Wet Denuder (PPD) (pH = neutral), Rotated Wet Annular Denuder (RWAD) (pH =9)	Reasonable agreement, often higher HONO for the RWAD. Higher interferences of the RWAD at high pH and /or incomplete sampling of the PPD at lower pH	Gen-fa et al. (2003)
	Air-dragged Aqua-Membrane-Type Denuder with Fluorescence Detection/dry denuder/DOAS	Reasonable agreement between the fluorescence detection and the denuder, good agreement of fluorescence and DOAS during night, but higher than DOAS during day	Takenaka et al. (2004)
	DOAS/LOPAP	Excellent agreement in a smog chamber under complex conditions and in a field campaign (urban background, Milan)	Kleffmann et al. (2006)
	DOAS/Wet Effluent Diffusion Denuder (WEDD)( PPD)	Reasonable agreement between both instrument, higher HONO by the WEDD during daytime	Acker et al. (2006)
	DOAS/Mistchamber-(IC)	Good agreement during night, Mistchamber >>DOAS during day	Stutz et al. (2010)
	Stripping coil-Ion Chromatography (IC)/LOPAP	Good agreement	Cheng et al. (2013)
	DOAS/Stripping coil-absorption photometry (SC-AP)/LOPAP/MC-IC/Quantum Cascade Tuneable Diode Laser (QC-TILDAS)/Ion Drift-Chemical Ionization Mass Spectrometry (ID-CIMS)	Reasonable agreement between the instruments, except for the ID-CIMS which shows significant higher HONO	Pinto et al. (2014)
	Modified LOPAP (selenoid pumps instead of peristaltic pump)/commercial LOPAP,	Modified LOPAP > commercial LOPAP, potential reason 5 m long inlet of the modified LOPAP	Liu et al. (2016)
	NitroMAC/LOPAP	Excellent agreement in a smog chamber, NitroMAC>LOPAP in the atmosphere	Afif et al. (2016)
	Incoherent Broadband Cavity-Enhanced Absorption Spectrometer (IBBCEAS)/LOPAP	Excellent agreement	Duan et al. (2018)
	Stripping coil-IC/LOPAP/CEAS	Good agreement between all techniques	Xue et al. (2019)
	RWAD (MARGA)/LOPAP	MARGA>LOPAP	Xu et al. (2019)

Table 5 .

 5 Techniques and measurement characteristics for chlorine species.

	Technique	Compound/s Typical Limit	Time resolution	Practical considerations	Example reference
			of Detection	(indicative)
			[pptv]
		Chlorinated			Majority of studies use sampling followed
	Gas chromatography-mass spectrometry	organics (e.g., CFCs,	> 0.5	10 -30 min	by pre-concentration and off-line analysis, although on-line detection is possible with	Odabasi (2008)
		CHCl3, CCl4)			low time resolution (10 min -1 h)
					Highly sensitive, high time resolution
					instrument that can simultaneously
					measure multiple compounds. But
					instrument is expensive and complicated
	Proton-transfer reaction time-of-flight mass spectrometer	Chloramines NCl3) (e.g, NHCl2,	1	1 s	(2020) NCl3 by I-CIMS and PTR by Mattila et al. reported concentration to calibrate/operate. Note that simultaneous measurements of NHCl2 and	Mattila et al. (2020)
					discrepancies of several orders of
					magnitude
			< 10	1 s	Highly sensitive, high time resolution
	Iodide chemical ionisation mass spectrometry (I-CIMS) / time-of-flight CMIS (I-tof-CIMS)	Chlorinated oxygenated organics HOCl ClNO2 Cl2	1 30 1 3	1 s 1 s 1 s 1 s	instrument that can simultaneously measure multiple compounds of interest, but instrument is expensive and complicated to calibrate/operate. Also requires radioactive source. Interferences can be issues, especially from small molecules, which can be formed via fragmentation within the instrument. This has been specifically noted for ClO	Mattila et al. (2020); Wong et al. (2017)
		Cl2O	0.2	1 s	(Mattila et al., 2020).
			3	1 s
	Chemical conversion resonance fluorescence	ClO	4	16 s	Complex custom instrumentation that is no longer in frequent use.	Avallone and Toohey (2001)
					Relatively simple methodology and optical
					nature makes it selective. HCl is difficult to
	Laser Absorption spectroscopy		10	1 s	sample due to wall effects. Specifications are for instrument presented by Halfacre	Harris et al. (1992)
					et al. at EGU 2020. Citation is for an older
					instrument with significant inlet effects.
	Cavity ring-down spectroscopy		18	30 s	Compact instrument. Humidity dependant response time due to wall interactions	Furlani et al. (2021)
					Compact and lightweight instrument. Has
	Off-axis integrated cavity output spectrometer (OA-ICOS)		78	30 s	been deployed as part of a stratospheric balloon payload. Has not been demonstrated for tropospheric humidities	Wilkerson et al. (2021)
					and artifacts
	Denuder or filter collection followed by offline ion-chromatography	HCl	200	30 min-24 h	Offline analysis method. Poor time instrumentation resolution but relatively simple sampling, without the need for large complex in-situ	Keuken et al. (1988)
	Iodide chemical ionisation
	mass spectrometry (I-CIMS) / time-of-flight CMIS (I-tof-		30	30 s	See above for I -CIMS	Lee et al. (2018)
	CIMS)
					Highly sensitive, high time resolution
	Acetate chemical ionisation				instrument,	but	expensive	and
	time-of-flight mass		2	1 min	complicated to calibrate / operate.	Crisp et al. (2014)
	spectrometry				Acetate ion inlet chemistry also less widely
					used than I -
	SF6 -chemical ionisation mass spectrometry		2	30 s	Highly sensitive, high time resolution instrument, but expensive and	Kim et al. (2008)

Table 6 .

 6 Measurement type and performance characteristics of instrumentation to measure NO 3 and N 2 O 5 .

	Technique Compound	Typical Limit of	Time	Description	Reference
			Detection (2)	resolution
			[pptv]	[s]
	CRDS	NO 3					King et al. (2000)
	CRDS	NO 3 N 2 O 5	0.5 0.5	5 5			Brown et al. (2002)
	CW-CRDS	N 2 O 5 NO 3	2.4 1.7	24 24	CW-CRDS system with heated Estimated for NO 3 inlet	Simpson (2003)
	BB-CRDS	NO 3 N 2 O 5	1 1	100 100			Bitter et al. (2005)
	BBCEAS	NO 3	0.5	10			Langridge (2008)
					Open path across chamber,
	IBBCEAS	NO 3 NO 3	2.7 3	60 30	4m Open path, 0.84m cavity	Venables et al. (2006)
					length
	IBBCEAS	NO 3	4	10	Open path across chamber, 0.82cm cavity length	Fouqueau et al. (2020)
	CRDS	NO 3 N 2 O 5	4.6 6.2	2.5 2.5	Compact, inexpensive, 2 channels for NO 3 & N 2 O 5	Li et al. (2018)
					Compact and low-cost
					instrument. NO 2 & NO 3
	IBBCEAS	NO 3	3	100	detection at 625 nm. LoD is stated as upper limit,	Ventrillard-Courtillot et al. (2010)
					but unclear whether LoD
					defined as 1, 2, 3
	BBCEAS	N 2 O 5 NO 3	1.4	1	For airborne measurements. Two channel instrument	Kennedy et al. (2011)
	Open path IBBCEAS	NO 3	3	30	Open path system	Wang and Lu (Wang and Lu, 2019)
	TD-CIMS	NO 3 + N 2 O 5	8	1	Main focus on peroxyacyl nitrates	Slusher et al. (2004)
	TD-CIMS	NO 3 + N 2 O 5	7	60	Use of I field	-ionisation. Used in	Yan et al. (2019)
	LIF	NO 3 N 2 O 5	76 76	60 60			Wood et al. (2002)
	LIF	N 2 O 5 NO 3	12 8	600 600			Matsumoto et al.

  reported that syn-and anti-conformers of acetaldehyde-oxide can be distinguished in this range of wavelengths. Instrumentation used to monitor CIs during laboratory experiments are single pass cell/LED, multipass cell/Ti:Saphire laser, CEAS, and CRDS[START_REF] Chhantyal-Pun | Criegee intermediates: production, detection and reactivity[END_REF]. 3. IR absorption spectroscopy: Several studies used IR absorption spectroscopy to

	detect	different	conformers	of	acetaldehyde-oxide,	acetone-oxide,
	methylvinylketone-oxide, and methyl-ethyl Criegee intermediate (MECI)

Table 7 :

 7 Synopsis of some of the most relevant indoor studies using derivatization followed by off-line chromatographic analysis that have determined glyoxal and/or methylglyoxal as a result of representative indoor activities.

		Sampling method	Indoor	Precursor	Instrument	Carbonyls/Dicarbonyls detected	Reference
	agent		activity
	PFBHA	FLEC Automation and	Cleaning	α-terpineol + O3	GC/MS	GLY, MGLY	Ham and Wells,
		Control System (FACS)				4-oxopentanal, 4-methylcyclohex-3-en-1-one, 6-hydroxyhept-en-2-one, 3-(1-hydroxy-1-methylethyl)-6-methylcyclohex-2-en-1-one), 5-(1-hydroxy-1-	2011
						methylethyl)-2-methylcyclohex-2-en-1-one
	PFBHA	Captured in solution) on	Skin	Squalene + O3	GC-Ion Trap Mass	GLY	Wells et al.,
		glass plate surface)	(emissions due to		Spectrometry	6-methyl-5-hepten-2-one (6MHO), 4-oxopentanal (4OPA), geranyl acetone, 5,9,13-trimethyltetradeca-4,8,12-trienal	2008
			dwellers)
	PFBHA	Samples drawn through	Cleaning	α-Terpineol + NO3 -	GC/MS	GLY, MGLY	Jones and Ham,
		a 6.4 mm Teflon Swagelok fitting				acetone, 6-hydroxyhept-5-en-2-one, 4-(1-hydroxy-1-methylethyl)-1-methyl-2-oxocyclohexyl nitrate, 5-(1-hydroxy-1-methylethyl)-2-oxocyclohexyl nitrate, 1-	2008
		attached to a 65 L Teflon				formyl-5-hydroxy-4-(hydroxymethyl)-1,5-dimethylhexyl nitrate, 1,4-diformyl-
		chamber				5-hydroxy-1,5-dimethylhexyl nitrate
	PFBHA	PDMS/DVB SPME	Cleaning	Citronellol + O3	GC/MS	GLY, MGLY	Ham et al. 2006
						acetone, 6-hydroxy-4-methylhexanal
	PFBHA	four bubblers with frits,	Ventilation	α-pinene (heterogeneous	GC/MS in the positive	GLY, MGLY	Fick et al., 2004
		containing 500 ml		reactions)	chemical ionisation	Several other carbonyls
		acetonitrile each			mode using CH4 as
					reagent gas
	PFBHA	SPME	Cleaning	terpinolene + O3, terpinolene + NO3 -	GC/MS	GLY, MGLY, other dicarbonyls (including some very photolabile unsaturated ones)	Harrison et al., 2013
	DNPH	Silica cartridges	Cleaning	a-pinene, camphene, b-pinene,	HPLC/MS	GLY, MGLY	Huang et al.,
				myrcene, 3-carene, p-cymene, d-limonene, eucalyptol, g-terpinene,		formaldehyde, acetaldehyde, acetone, propionaldehyde, 2-butanone (MEK), iso + n-butyraldehyde, benzaldehyde, iso-valeraldehyde, n-valeraldehyde, o-	2011
				terpinolene, linalool, a-terpineol		tolualdehyde, m-tolualdehyde, p-tolualdehyde, hexanal, 2,5-
						dimethylbenzaldehyde
						Unsaturated carbonyls: acrolein, crotonaldehyde
	O-tert-butyl-	Water in Impinger	Cleaning,	terpinolene+O3, terpinolene+NO	GC/MS	MGLY	Ham et al., 2015
	hydroxyl-amine					4-methylcyclohex-3-en-1-one, (4MCH), 6-oxo-3-(propan-2-ylidene) heptanal
	hydrochloride					(6OPH), 3,6-dioxoheptanal (36DOH). The tricarbonyl 36DOH has not been
	(TBOX)					previously observed.
	TBOX	Bubbler	3D printer	acrylonitrile butadiene styrene	GC/MS	GLY, MGLY	Stefaniak et al.,
				(ABS) or polylactic acid (PLA) filaments		Benzaldehyde, m-tolualdehyde, 2-oxopentanal	2017
	PFBHA	Methanol in Impinger	Cleaning	Terpenes (limonene, terpineol,	GC/MS	GLY, MGLY	Forester et al.,
				geraniol)		4-oxopentanal	2007a; 2009
	PFBHA	Methanol in Impinger	Freshener's	b-Ionone (+OH/O3)	GC/MS	GLY, MGLY	Forester et al.,
			emissions				2007b

Table 8 .

 8 Instrumentation used in the intercomparison of GLY, MGLY and NO 2 measurements carried out in the atmospheric simulation chambers EUPHORE (Valencia, Spain) and NCAR (Boulder, Colorado, USA). Based on Thalman et al. (2015). a CU = University of Colorado, Boulder, USA; NCAR = National Center for Atmospheric Research, Boulder, USA; Leic = University of Leicester, UK; CEAM = CEAM, Spain. b G = GLY, M = MGLY, N = NO 2 .

	Instrument

Commercial LOPAP/Custom Build LOPAP/2x IBBCEAS/ToF-CIMS High correlation but significant differences between the instruments, reasons were not clarified, potential spacial variability, different inlet position[START_REF] Crilley | Intercomparison of nitrous acid (HONO) measurement techniques in a megacity ( Beijing )[END_REF]