C Bisch

X Mougeot

M.-M Bé

A.-M Nourreddine

Development of a system for measuring the shape of β spectra using a semiconductor Si detector

A system for the measurement of beta energy spectra has been developed. It is based on a silicon semi-conductor detector operating at liquid nitrogen temperatures, under ultra high-vacuum. Monte-Carlo simulations were made to optimize the detection chamber and the source holder. Descriptions of the electronic and mechanical systems are included, as well as the first measured spectra.

I.

INTRODUCTION

The study of the shape of β spectra is experiencing a resurgence of interest having been little studied since the late 1970s. Precise knowledge of the shape of energy spectra, coupled with well established uncertainties, are sought by users from the nuclear power industry (decay heat calculations), medical care sector (dose calculations) or ionizing radiation metrology (reduction of uncertainties in activity measurements using the liquid scintillation counting technique). As the primary laboratory for measurements of radioactivity in France, the Laboratoire National Henri Becquerel (LNHB) is developing a computer code, BetaShape [START_REF] Mougeot | Proceedings of the LSC2010 International Conference[END_REF], to meet the demands of users. BetaShape calculates analytically the beta spectra for allowed and forbidden unique transitions and takes into account the screening effect due to the electron cloud, the finite nuclear size effect with regard to the electron Compton wavelength, and the radiative corrections. This program must be tested and validated by experiments. Hence, an operational device using a semiconductor Si detector (easy to implement, linear response function, good energy resolution) has been developed in order to measure spectral shapes and to quantify uncertainties.

II. REQUIREMENTS OF THE SYSTEM

The main difficulties in beta spectrometry arise because the spectra may be distorted by the detection system. Specific requirements are necessary to limit the sources of deformation and to maximize the signal-tonoise ratio [START_REF] Grégoires | [END_REF]. Silicon detectors are operable at room temperature, however, thermally-generated leakage currents contribute heavily to the noise level. Operation at liquid nitrogen temperature offers marked improvements in resolution and to the signal-to-noise ratio, especially for low energies. Operation at low temperature, in turn, dictates a further design criterion: the detector must be kept under vacuum to prevent atmospheric moisture from condensing and freezing on the detector's surface [3]. However, the vacuum pump induces microphonics which have to be attenuated. Otherwise, physical phenomena like self-absorption in the source, detector dead zones, angle of incidence, scattering and backscattering or bremsstrahlung have also a great influence on the measured spectral shape. A high-vacuum provides another advantage, namely a low scattering environment. However, we must also carefully choose the materials and define the geometry of the detection chamber and source holder. Finally, for each radionuclide, a chemical study must be undertaken to determine the best method for preparing thin and homogeneous sources [4].

III. GEANT4 SIMULATIONS

We performed Monte Carlo simulations to study the impact of materials and geometry on the spectral shape. The model of the detector, the detection chamber and the source holder are relatively simple because here we are just interested in qualitative results. We have implemented in Geant4 [5] a new class based on our BetaShape [START_REF] Mougeot | Proceedings of the LSC2010 International Conference[END_REF] code.

We use it rather than the G4RadioactiveDecay class for simulating beta sources since it is more reliable. Our simulations were carried out using an isotropic 90 Y source.

A. Study of the detection chamber

The detection chamber is a cube milled on its six faces. Clearly the bigger the cube, the lower the scattering, but we must also avoid a cumbersome system. We tested various sizes of the detection chamber between 150 to 250 mm for two materials: stainless steel and aluminum. The source position from the detector was varied from 10 to 40 mm. Close to the detector the scattering effect is almost negligible but another source of deformation is induced due to the detector thickness. Fig. 1 shows that aluminum is to be preferred to stainless steel. Thus the dimensions of the cube could be reduced. The best trade-off found is a cube with 170 mm sides made from aluminum.

FIG. 1. Study of the spectrum deformation as a function of the material and dimensions of the detection chamber. The simulation was performed with a 5 mm thick detector. The curve in black is the theoretical spectrum calculated with Be-taShape [START_REF] Mougeot | Proceedings of the LSC2010 International Conference[END_REF].

B. Study of the source holder

We want to avoid, as far as possible, the detection of backscattered electrons coming from the source holder. Backscattering is more pronounced for electrons of low energy and absorbers with high atomic number [6]. Geant4 simulations were conducted from the Geant4 benchmark TestEm5 adapted to our application. We tested two materials for the source holder: aluminium and PEEK (polyether ether ketone) which is an organic polymer thermoplastic. PEEK is the best plastic to use for vacuum applications because it does not outgas. A 500 keV mono-energetic beam was simulated, impinging on a full block of material and then, conserving the same geometry, with a 1 cm recess milled into the block. Results are shown in Table I. Finally, the geometry adopted for the source holder is shown in Fig. 2. The notches on the side allow pressure equalization across the source.

IV. DESIGN OF SI ELECTRON SPECTROMETER A. Physical description

As we can see in Fig. 3 and Fig. 4 there are three gate valves, hence the detector chamber remains under constant vacuum. When the experimental setup is cooled we readily reach an ultra high vacuum of about 10 -7 mbar. During the change of the source, the gate valves of the cross are closed and the linear feed-through, below the cross, can be removed. On its top, the source holder (Fig. 2) is attached which can be easily removed in order to use another source. Two vacuum gauges, one behind the detection chamber and the other at the cross, permanently monitor the vacuum. The linear feed-through has a range of 152 mm, thus one can study spectral deformation as a function of the source-detector distance. A detector holder in copper, attached to the cold finger from the dewar, is used to uniformly cool the detector. Two basic electrical wires -one to carry information and to polarise the detector, the second one to ground the detector -are connected from the detector to an electrical feed-through located on top of the detection chamber. This electrical feed-through uses a microdot-BNC connector. On the other side, a preamplifier is directly plugged to this connector, which ensures a minimum distance between the detector and the preamplifier, hence minimizing the capacitance.

B. Electronics

The electronics system is a classic spectromerty chain composed of a preamplifier connected to a high voltage device, an amplifier and a multichannel analyzer. The detector is a PIPS (Passivated Implanted Planar Silicon) manufactured by Canberra. The main advantages of this technology for our application are a very thin entrance window of 50 nm Si equivalent and a low leakage current. The energy-loss of electrons in 50 nm of silicon is about 1.2 keV (see e.g. range tables in Penelope [START_REF] Salvat | PENELOPE-2008: A Code System for Monte Carlo Simulation of Electron and Photon Transport[END_REF]). Given the dead layer, the electronic noise and the resolution, the cutoff in energy of the PIPS detector is about 10 keV. The detector is 500 µm thick and has an active surface area of 450 mm 2 . Electrons having an energy of 320 keV have a range of 500 µm in silicon. Hence with the PIPS detector, we plan to measure spectra with an end-point lower than 350 keV. The first stage of the electronics system, as close as possible to the detector, is an Amptek "CoolFET" charge sensitive preamplifier cooled by the Peltier effect. The preamplifier must be kept stable, and we observed a significant decrease in the signalto-noise ratio when it was not positioned correctly. There are then two NIM ORTEC modules; a linear amplifier and an ADC for pulse processing and shaping. The settings (gain, shaping time, P/Z Cancellation, ADC LLD and zero ADC) of the NIM modules must be such as to find the best trade-off between the counting rate and the resolution. Finally, we use the Maestro spectrometry software for recording and analyzing our spectra.

V. MEASUREMENTS A. 109 Cd source calibration

As an initial test we have measured the spectrum of a 109 Cd source, which emits mono-energetic conversion electrons (Table II) that appear as sharp peaks in the spectrum (Fig. 5). To check the influence of the temperature of the detector, two spectra were acquired with and without liquid nitrogen cooling. As expected, the energy resolution increases with temperature. At 300 K, the full width at half maximum (FWHM) is about 8 keV, while at 77 K it is about 5 keV. We believe the resolution may yet improve, compared to these preliminary spectra, if we can further optimize our electronics system settings. The response function for this energy range seems to be approximately linear. Monte Carlo calculations are still in progress in order to complete the analysis. A preliminary measurement of a 36 Cl source was carried out. The radioactive material was deposited on an ion-exchange resin. As expected, the electrons of high energy that go through of our 500 µm thickness detector induce a folding of the spectrum, i.e. only a fraction of the energy of the electrons is deposited in the active volume of the detector resulting in an overcounting at low energy and an undercounting at high energy. The realistic geometry of the experimental II). FIG. 6. Comparison between the 36 Cl measured and simulated beta spectra. The data were adjusted using an approximate linear energy calibration.

setup was implemented in our simulations. We used the experimental shape factor extracted from the measurement of Rotzinger et al. [START_REF] Rotzinger | [END_REF] for the primary emitted beta spectrum. The data were adjusted using an approximate linear energy calibration. As shown in Fig. 6, our simulation reproduces well the main distortion of the measured spectrum. A complete and careful study of the detector response function will provide a precise energy calibration and should improve the agreement with our simulation.

VI. CONCLUSION

Our experimental setup, dedicated to measuring the shape of beta spectra, is finalized and initial measurements have been carried out with success. Some distortion, due to the detection system, persists in the measured spectra but owing to the important development and simulation work, we should be able to characterize and quantify this with our Monte Carlo simulations. The next step will be to precisely study the response function of the detector in order to determine an experimental distortion function. Then we will carry out a series of measurements of well-known spectra to validate our benchmark. Finally, we will focus on unique and non-unique forbidden beta spectra to validate our code BetaShape. This particular setup is well suited for measuring beta spectra with a maximum energy of about 350 keV. It is also easily adjustable to a 5 mm thick Si(Li) detector, for measuring beta spectra up to about 2.5 MeV.

FIG. 3 .

 3 FIG. 3. Technical drawing of the beta spectrometer. Gate valves are indicated by the red circles and the detection chamber is indicated by the green circle.

FIG. 5 .

 5 FIG.5. Measured conversion electron spectrum following the EC decay of 109 Cd. Spectra were measured with the PIPS detector at room temperature (red curve) and liquid nitrogen temperature (blue curve). The 85 keV peak is a mixture of the L and M conversion electrons (see TableII).

TABLE I .

 I Backscattered electrons depending on the material and geometry of the source holder.

		Fraction reflected in	Fraction	reflected
		full source holder	in recessed source
			holder
	Aluminium 11.4 %	5.1 %
	PEEK	2.7 %	1.3 %

FIG. 2. View of the source holder.

TABLE II .

 II Evaluated data for the emitted electrons that follow the electronic capture (EC) in the 109 Cd decay[START_REF] Bé | Table of Radionuclides -Monographie BIPM-5[END_REF].

		109 48 Cd	T 1/2	:	461,4(12)d -→	109 47 Ag
	Electrons
	Energy (keV) Intensity (%) Type	Origin
	21.7	20.6			Auger K	Ag-109
	62.52	40.8			ec K	Ag-109
	84.46	44.8			ec L	Ag-109
	87.67	9.28			ec M	Ag-109
				B. 36 Cl

ACKNOWLEDGEMENTS

The authors wish to thank Mr G. Gaudiot, Mr C. Schawb and the technicians of the mechanical workshop of the Institut Pluridisciplinaire Hubert Curien in Strasbourg for their helpful suggestions and their work on this experimental setup.

The research within this EURAMET joint research project receives funding from the European Community's Seventh Framework Programme, ERA-NET Plus, under Grant Agreement No. 217257.


Bastien Pellegrin
email: bastien.pellegrin@cea.fr

Philippe Berne

Hervé Giraud

Arthur Roussey

Exploring the potential of electrostatic precipitation as an alternative particulate matter filtration system in aircraft cabins

Keywords: Aircraft air quality management, Electrostatic precipitation, Spectral collection efficiency, Ozone generation

On modern airliners, cabin air pressurization, heating and renewal is achieved using air supplied from the gas turbine engines during flight. This air intake impairs the motors yield and needs to be conditioned, leading to energy overconsumption. Recent advances in thermal management enable aircraft manufacturers to reduce further the intake airflow needed to maintain cabin temperature at high altitude. Nevertheless, for lower air renewal rates, an appropriate air filtration system will be needed to maintain acceptable air quality in the cabin. In this context, Clean Sky 2 Joint Undertaking (CS2JU) project EC2S (Environment Control Secondary System) aims at developing an integrated filtration system to be implemented in existing cabin air management systems (so called Environmental Control System -ECS). The EC2S unit will include three filtration units addressing separately Volatile Organic Compounds (VOCs), CO2 and Particulate Matter (PM). Circulated air in the ECS is conventionally filtered on pleated HEPA filters that generate substantial pressure drop. Since the EC2S VOCs and CO2 filtration units would generate additional pressure drop in the ECS system, electrostatic precipitation is foreseen as a low flow resistance alternative for PM removal. This paper reports the development and performance assessment of a two-stage electrostatic precipitator (ESP) designed for aircraft recirculated air filtration. The ESP prototype presents single pass high particle collection rates (i.e. over 90% for airborne particles with an aerodynamic diameter of 0.5 μm or larger), low pressure drop (i.e. 4 Pa at nominal flowrate) and a limited ozone generation rate (i.e. below 8 mg.h -1 ).

INTRODUCTION

Cabin air management system

Over the last few decades, significant efforts have been made by aircraft manufacturers to reduce the overall energetic consumption of aircraft engines. Besides providing thrust and electric power, an engine must also provide pressurized fresh air to the cabin. A substantial part of the outside air compressed by the engine is "bled" off the engine and passed through air conditioning packs to be cooled and supplied to the cabin. This "bleed-air" pressurizes and heats the cabin to overcome extreme external conditions (low temperature and pressure at high altitudes). As passengers and crew are confined in the relatively low volume of the aircraft cabin, a high flow of fresh air must be provided to the cabin. To reduce demand of bleed air on the engine for cabin thermalization and pressurization, aircraft manufacturers implemented an air recirculation system, which mixes bleed-air with air recirculated from the cabin with a recirculation fan (Figure 1). Nowadays, on most commercial airliner models, the air flowing in the cabin comprises between 30% and 50% of recirculated air [START_REF] Michaelis | Aircraft Cabin Air Filtration and Related Technologies: Requirements, Present Practice and Prospects[END_REF]. Modern aircraft recirculation systems include a filtration unit which comprises a High Efficiency Particulate Air (HEPA) filter with stated single pass PM removal efficiencies up to above 99% [START_REF] Pecho | Design of air circuit disinfection against COVID-19 in the conditions of airliners[END_REF]. Optionally the HEPA filters may be coupled with Activated Carbon (AC) layers to remove VOCs from recirculated air.

Cabin air pollution

Cabin Air Quality (CAQ) is impacted by multiple pollution sources which may be internal or external depending on aircraft position and activity on ground (maintenance, taxiing and idling periods) or during flight phases.

On the ground, in the vicinity of airport aprons and runways, the aircraft main engines combustion appears to be a predominant emission source for PM (and gaseous air pollutants such as NOx, CO, VOCs or SO2) [START_REF] Masiol | Aircraft engine exhaust emissions and other airport-related contributions to ambient air pollution: A review[END_REF]. Based on onsite concentration measurements, several studies estimated the PM emissions directly attributed to aircrafts in different international airports: depending on the airport capacity, results ranged from 3.5 up to 71.5 tons per year [START_REF] Unal | Airport related emissions and impacts on air quality: application to the Atlanta International Airport[END_REF][START_REF] Song | Emissions of greenhouse gases and air pollutants from commercial aircraft at international airports in Korea[END_REF][START_REF] Yang | Characterization of aircraft emissions and air quality impacts of an international airport[END_REF]. Yang et al. estimated that the aircraft engines emissions account for almost 50% of fine particles (PM2.5) concentration recorded at Beijing airport, the rest being mostly attributed to other diesel or gasoline vehicles, fuel related activities and aircraft landing or braking [START_REF] Yang | Characterization of aircraft emissions and air quality impacts of an international airport[END_REF]. Some recent studies [START_REF] Marie-Desvergne | Assessment of nanoparticles and metal exposure of airport workers using exhaled breath condensate[END_REF][START_REF] Lobo | Measurement of Aircraft Engine Non-Volatile PM Emissions: Results of the Aviation-Particle Regulatory Instrumentation Demonstration Experiment (A-PRIDE) 4 Campaign[END_REF][START_REF] Stacey | Evaluation of ultrafine particle concentrations and size distributions at London Heathrow Airport[END_REF][START_REF] Xiang | Distinct Ultrafine Particle Profiles Associated with Aircraft and Roadway Traffic[END_REF] specifically focused on the nano-sized fraction of PM, i.e. ultrafine particles (UFPs, with diameter below 100 nm). They reported the emission by the aircrafts engines of a characteristic non-volatile nano-aerosol composed of soot and other components including zinc, aluminum, and titanium. Those non-volatile UFPs were found to present a particle size distribution (PSD) with a mode at 20 nm, notably smaller than typical PSD that can be monitored in urban locations [START_REF] Xiang | Distinct Ultrafine Particle Profiles Associated with Aircraft and Roadway Traffic[END_REF]. Additionally, the formation of a secondary, volatile nano-aerosol, with a mode below 10 nm (nucleation mode), originating from conversion processes of sulfur and various organic gases, was also reported [START_REF] Masiol | Aircraft engine exhaust emissions and other airport-related contributions to ambient air pollution: A review[END_REF]. Since the intake air is usually not filtered before entering the cabin, the presence of high levels of airborne particles on airport area (reaching up to 10 7 particles.cm -3 within runways vicinity [START_REF] Zhu | Aircraft emissions and local air quality impacts from takeoff activities at a large International Airport[END_REF]) may drastically impair the CAQ when airplanes are operated at ground level.

After take-off, air from higher altitude is provided to the aircraft cabin, and is considered to be clean, with a very low PM and UFPs concentrations (~50 particles.cm -3 ) [START_REF] Reus | Particle production in the lowermost stratosphere by convective lifting of the tropopause[END_REF]. Similarly, at high altitudes, VOCs are present at even lower concentrations than in typical outdoor air (<10 ppb), as they are oxidized by highly reactive radicals [START_REF] Hofzumahaus | Amplified Trace Gas Removal in the Troposphere[END_REF]. Ozone is also present in the atmosphere, and its concentration depends on cruising altitudes. Typical concentrations are ~0.03 ppm at sea level, ~0.1 ppm at 40 000 ft (12.2 km) and ~5 ppm at 50 000 ft (15.5 km), but ozone concentrations depend also on aircraft routes and encountered weather [START_REF] Bhangar | Atmospheric ozone levels encountered by commercial aircraft on transatlantic routes[END_REF]. For example, ozone-rich transients up to 0.5 ppm have been measured in the upper troposphere between 10 and 12 km [START_REF] Marenco | Measurement of ozone and water vapor by Airbus in-service aircraft: The MOZAIC airborne program[END_REF]. Therefore, after take-off, outdoor air can be considered as clean, only containing potentially high ozone concentration.

In addition to outside air quality, internal pollution sources such as engine fluid leaks (fume events) or passengers and their activities may have an impact on CAQ. Malfunction of the air compressing system on the aircraft engine may lead to the dispersion of potentially harmful engine fluids (oil, anti-icing fluid…) in the cabin. This incidental situation, commonly referred to as fume event, is well known and documented [START_REF]HVP.23: Characterisation of the toxicity of aviation turbine engine oils after pyrolysis[END_REF][START_REF] Michaelis | Oil bearing seals and aircraft cabin air contamination[END_REF]. It may lead to gas phase or mist pollution of the cabin with engine oil which comprises hazardous chemicals such as organophosphates (OPs) [START_REF] Schuchardt | Cabin air quality -Quantitative comparison of volatile air contaminants at different flight phases during 177 commercial flights[END_REF]. Besides fume events, passengers themselves (and their activities) represent the major pollution source during flight. Wang et al. estimated that nearly 30% of VOC concentrations in aircraft cabins were due to services and humans [START_REF]) Committee on Air Quality in Passenger Cabins of Commercial Aircraft, The Airliner Cabin Environment and the Health of Passengers and Crew[END_REF]. Cao et al. recently showed a higher particle concentration in the cabin during boarding and deplaning of a retired aircraft. They even proposed an emission rate of PM10 per passenger of ~15 µg.m - 3 .pax -1 during boarding/deplaning, and ~5-6 µg.m -3 .pax -1 while sitting [START_REF] Cao | Prediction of particle deposition around the cabin air supply nozzles of commercial airplanes using measured in-cabin particle emission rates[END_REF]. Finally, passengers are also a source of bioaerosol. As reviewed in reference [START_REF]) Committee on Air Quality in Passenger Cabins of Commercial Aircraft, The Airliner Cabin Environment and the Health of Passengers and Crew[END_REF], concentrations of airborne bacteria and fungi were however found to be generally lower than those common in indoor air.

Based on the above considerations, CAQ will highly depend on the aircraft mission and will be particularly impacted by its ground time: as fresh air is not directly filtered and enters the cabin before a first filtration (Figure 1), contaminated air will enter the cabin if the pollution sources are located outside the aircraft. Several studies were conducted on UFPs and PM concentrations in aircraft cabin and confirmed this trend [START_REF] Spengler | In-flight/onboard monitoring: ACER[END_REF][START_REF]EASA_REP_RESEA_2014_4 CAQ Preliminary cabin air quality measurement campaign[END_REF][START_REF] Guan | Temporal variations of ultrafine particle concentrations in aircraft cabin: A field study[END_REF][START_REF] Nagda | Measurement of cabin air quality aboard commercial airliners[END_REF][START_REF] Cheng | Air Quality in transport cabin-Part I[END_REF][START_REF] Poon | Indoor air quality investigation on commercial aircraft[END_REF]. Recorded UFPs concentrations were found to be low at high altitude (50-100 p.cm -3 ) [START_REF] Spengler | In-flight/onboard monitoring: ACER[END_REF][START_REF] Guan | Temporal variations of ultrafine particle concentrations in aircraft cabin: A field study[END_REF] and higher during on-ground operations such as taxiing (10 3 -10 4 p.cm -3 ) [START_REF] Guan | Temporal variations of ultrafine particle concentrations in aircraft cabin: A field study[END_REF].

Cabin air filtration

According to the Federal Aviation Administration (FAA Title 14) and the American National Standards Institute (ANSI/ASHRAE standard 161), air filters are mandatory to be mounted on all commercial aircrafts. Nowadays, modern airliners ventilation systems are equipped with HEPA filters as standard [START_REF] Xu | Investigation on the performance of airliner cabin air filter throughout the lifetime usage[END_REF]. The HEPA cabin air filters definition in aerospace industry is based on a minimum removal efficiency of 99.97 % when tested using a DOP challenge (mean size 0.3 µm, according to ASTM publication D 2986-95 -Boeing criteria) or 99.99 % when tested with a sodium chloride challenge (mean size 0.58 µm, according to British Standard BS.3928 -Airbus criteria) [START_REF] Michaelis | Aircraft Cabin Air Filtration and Related Technologies: Requirements, Present Practice and Prospects[END_REF][START_REF] Michaelis | Health and Flight Safety Implications from Exposure to Contaminated Air in Aircraft[END_REF]. Typical ECS HEPA filters dimensions are roughly about 60 cm by 60 cm, with a 15-20 cm thickness and weight approximately 3-6 kg. This thickness accommodates the pleated structure of the filter medium, which substantially increases the available filter area within the 60 by 60 cm frame, reducing the pressure required to force the air through the filter. No regulation currently enforces the replacement frequency of used filters but various airliners' maintenance manuals mention a service life ranging from 3 000 to 6 000 flight hours [START_REF] Xu | Investigation on the performance of airliner cabin air filter throughout the lifetime usage[END_REF][START_REF] Michaelis | Health and Flight Safety Implications from Exposure to Contaminated Air in Aircraft[END_REF].

Very limited scientific information is available in the literature on performance assessment of HEPA filters dedicated to filtration of airplane cabin air. Xu et al. [START_REF] Xu | Investigation on the performance of airliner cabin air filter throughout the lifetime usage[END_REF] reported a comparative study of two commercially available HEPA filters from Pall (Model CD00973F2) and Donaldson (Model KK2972941) made of glass fibrous media. Pristine filters, when challenged at their respective nominal flowrates (1970 m 3 .h -1 and 1600 m 3 .h -1 ), exhibited pressure drop (ΔP) of approximately 280 Pa for Pall filter and 100 Pa for Donaldson filter. The efficiency of Donaldson filter (assessed according to European norm EN 1822 protocol) was shown to be 86 %, for a Most Penetrating Particle Size (MPPS) of 150 nm. For the Pall filter, a 99 % efficiency was observed for a MPPS of 55 nm. The MPPS value of the Pall filter is lower than values commonly reported in the literature for HEPA filters, which are usually in the range of 120 nm to 300 nm [START_REF] Podgórski | Application of nanofibers to improve the filtration efficiency of the most penetrating aerosol particles in fibrous filters[END_REF], most probably due to the presence of electrostatic charges on the media. Xu et al. [START_REF] Xu | Investigation on the performance of airliner cabin air filter throughout the lifetime usage[END_REF] additionally investigated the changes of filters characteristics along with filter usage. Due to fouling, for an equivalent of 6 000 flight hours, Donaldson filter efficiency increased by 10 % (reaching 97%) and ΔP increased by 800 % (reaching 950 Pa). A recent study from Cao et al. [START_REF] Cao | Prediction of particle deposition around the cabin air supply nozzles of commercial airplanes using measured in-cabin particle emission rates[END_REF] monitored the particle deposition in ECS of commercial airliners. Based on PM concentration monitored in various airports and the relationship established by Xu et al. [27] between dust loading and pressure drop, they calculated an estimated HEPA lifespan as low as 800 flight hours for aircraft operated in highly polluted airports in China (considering that the filter needs to be replaced when ΔP reaches a level that is twice the initial resistance).

ESP as an alternative to HEPA

Electrostatic precipitation (ESP) is a process to remove particulate matter in a gas using electrostatic force. ESP acts by imparting charge on airborne particles, which, in the presence of an electric field, are then directed to and deposited on a metallic collection plate [START_REF] Kettleson | Soft-X-Ray-Enhanced Electrostatic Precipitation for Protection against Inhalable Allergens, Ultrafine Particles, and Microbial Infections[END_REF]. It has been widely used in industry since the beginning of the 20th century for fume filtration. ESP plays a prevailing role in high flowrate atmospheric waste treatment for major sectors such as metalworking and chemical industries, cement production plants and coal-fired power plants [START_REF] Jaworek | Modern electrostatic devices and methods for exhaust gas cleaning: A brief review[END_REF], applications which operate at temperatures too high for fiber-based filters.

In the field of aerosol filtration in indoor air, the prevailing technology is the mechanical filtration on fibrous media. Nevertheless, the energy consumption of fiber based air filtration systems mainly depends on the density of the fibrous filter: the denser the filter mesh, the higher the pressure drop is. To overcome higher pressure drop, fans need to operate at higher speeds, therefore consuming more energy [START_REF] Wen | The key energy performance of novel electrostatic precipitators[END_REF]. Additionally, throughout its life, the fibrous filter collects particles that accumulate on fibers' surface, gradually blocking the pores. This particle accumulation leads to higher collection efficiency but simultaneously generates higher pressure drop over the lifespan of the filter. This pressure drop increase requires the filter to be replaced periodically to avoid energy overconsumption or damages on the air circuit. This is why selection of a fibrous filter needs to be a trade-off where the filter is dense enough to capture the required percentage of particles while being open enough to be economically viable.

In contrast, with airflow occurring tangentially to the collection surfaces, ESP is characterized by low-pressure drop and, subsequently, has lower power requirement than its fibrous counterparts. Generally speaking, ESP may be considered less efficient in removing particles from air stream than HEPA filters when very high collection is intended (>99.9%) [START_REF] Kim | A novel electrostatic precipitator-type small air purifier with a carbon fiber ionizer and an activated carbon fiber filter[END_REF]. However, the usefulness of ESP technology in mitigating both biological and non-living aerosols has been demonstrated, even for ultrafine particles. Regarding biological matter, it is well documented that mechanical filtration may present a risk of contamination and proliferation because the surface of fibrous filters, where microorganisms and nutriments accumulate, can represent a suitable ecosystem for microorganisms growth [START_REF] Liu | A review of air filtration technologies for sustainable and healthy building ventilation[END_REF][START_REF] Mittal | Survival of Microorganisms on HEPA Filters[END_REF]. For ESP, research has shown that electrical charging could impair the survival rates of bacterial cells and spores, thus limiting their proliferation [START_REF] Mainelis | Effect of electrical charges and fields on injury and viability of airborne bacteria[END_REF][START_REF] Yao | Inactivation of Microorganisms Using Electrostatic Fields[END_REF]. Consequently, over the last decade, ESP have gained increasing attention for the removal of particulate matter (mainly dust particles and airborne microorganisms) in confined indoor spaces [START_REF] Anses | Identification et Analyse des Différentes Techniques d'Epuration d'Air Intérieur Emergentes[END_REF][START_REF] Afshari | Electrostatic Precipitators as an Indoor Air Cleaner -A Literature Review[END_REF][START_REF] Chen | Electrostatic precipitator for fine and ultrafine particle removal from indoor air environments[END_REF].

This study reports the sizing, design and performances assessment (single-pass collection efficiency, pressure drop and ozone emission rate) of an ESP prototype dedicated to aircraft cabin air filtration. The potential benefit of substituting HEPA filters for ESP is discussed.

METHODS

ESP Prototype

Figure 2 presents a simplified schematic of the developed ESP prototype. The device comprises two distinct sections: the charging stage and the collection stage.

The charging stage (where airborne particles acquire electric charges) operates using unipolar ions generated by corona discharge. Corona discharge is an operational condition in which a dense cloud of free electrons and unipolar ions is established around a discharge electrode. This complex phenomenon originates from a strong, non-uniform electric field, generated by high potential gradients on curved electrodes (such as wire, tip or sharp blade geometries). A simple geometry to enable corona discharge is the wire-plate geometry: a potential difference is applied between a low-radius conductive wire, which acts as a high-voltage induced corona electrode and two metal plate counter-electrodes. Since it is widely known that positive corona discharge emits much lower ozone concentrations than negative polarity corona [START_REF] Mizuno | Electrostatic Precipitation[END_REF], positive corona discharge configuration will be preferentially investigated.

The collection stage relies on the migration of the charged particles under the influence of an electric field and their deposition at the surface of collection electrodes. For this purpose, an electric field is created by applying a potential difference between the collection electrode and a counter-electrode. The selected geometry for the collection stage is a parallel plates configuration. Final assembly comprises the charging stage and two consecutive collection stages, with an outer section of 355x355 mm 2 and a total length of 640 mm. For the charging stage, the corona discharge is performed using thirteen Ø 80 μm stainless steel wires located at a distance of 10 mm from their plate counter-electrodes. The wires are positively charged, while the counterelectrodes are grounded. The collection stages comprise two sections of 200 mm long coppercoated glass fiber plates, which are alternatively grounded and positively charged. The main specifications of the ESP prototype are summarized in Figure 3 below.

Experimental apparatus

Assessment of the single pass particle collection efficiency, pressure drop and ozone emission rate was conducted on a dedicated test rig. This experimental setup was inspired from test equipment for measuring spectral efficiency and air flow resistance of air filters for general ventilation in buildings (as described in EN ISO 16890-2:2016) and of high efficiency air filters (as described in EN 1822-1:2019). The detailed dimensions and constituting elements of the test unit are schematically presented in Figure 4. It comprises a square section (305x305 mm 2 ) duct in which the ESP prototype is integrated. The air flow in the duct is controlled by a fandriven power ventilator equipped with a frequency convertor to adjust speed (airflow rate is monitored with a KIMO AMI 300 propeller anemometer). Unless otherwise specified, the air flowrate in the duct (Q) is fixed at 540 m 3 .h -1 (corresponding to an airflow velocity of 1.61 m.s -1 ) to match the nominal feed flow of the ESP prototype. This air flowrate was calculated to match the air renewal rate needed for half a cabin of a regional airliner (100 passengers capacity) with a 70% recirculated air ECS design.

The air blower forces the air through an H14 HEPA filter and a concentrated challenge aerosol (generation means are described hereinafter) is mixed with the clean airstream. The airstream charged with the challenge aerosol then passes through the ESP prototype and is collected through the laboratory exhaust system. The experimental test rig is equipped to monitor in real time:

-The aerosol concentration and size distribution upstream and downstream of the ESP prototype (aerosol characterization means are described hereinafter), -The ozone concentration downstream of the ESP prototype, using an ENVEA o342e ozone analyzer, -The ESP prototype pressure drop, using a KIMO AMI 300 manometer.

All the experiments were conducted at 20 ± 2 °C and 45 ± 10% relative humidity.

Challenge aerosols

To produce challenge aerosols with different particle size distributions (PSD), two types of aerosol generators were implemented. After generation, the challenge aerosols were systematically brought to a neutral charge distribution state using an in-house developed neutralizer that produces positive and negative ions by soft X-ray irradiation. Neutralization step effectiveness was confirmed by measuring the neutralized aerosols net charge with a highcapacity Faraday cup aerosol electrometer (PALAS Charme).

Generation of sub-micron and micron sized aerosol was performed using a rotating brush RBG 1000 aerosol generator (PALAS). This equipment produces an aerosol by mechanical solicitation of a powder. The powder to be aerosolized is placed in a container and inserted into the dispersion head. The powder is then conveyed onto a rotating brush at a precisely controlled feed rate. A compressed air stream tears the particles out of the brush, generating an aerosol at adjustable flowrate. The powder selected for this study is a calibrated calcium carbonate. According to manufacturer specifications, ESKAL 500 (KSL staubtechnik gmbh) is an inorganic mineral powder obtained from natural calcium carbonate (CaCO3) sieved to achieve a narrow particle size distribution (fine and coarse particles are almost eliminated), with a mass based median particle size at approximately 5 µm. When used in the RBG 1000 aerosol generator and after dilution in the clean air stream of the duct, this powder produces a polydisperse aerosol with a number-based PSD (presented in Figure 5-a) ranging from below 500 nm, up to 4 µm (with a mode at approximately 1 µm).

Generation of nano-sized aerosol was performed using a constant output atomizer, model 3076 (TSI). Water vapor is systematically removed from the atomized aerosols using a diffusion dryer (model 6062, TSI). The aerosol precursor for this study is an aqueous sodium chloride (NaCl) solution at a concentration of 1 g.L -1 . The PSD (recorded by Scanning Mobility Particle Size spectrometer -SMPS) of the aerosol generated from this solution is presented in Figure 5-b. NaCl aerosol number based PSD ranges from below 20 nm, up to 200 nm (with a mode between 40 nm and 45 nm).

Aerosol characterization and collection efficiency calculation

ESKAL 500 aerosols were monitored using an Aerodynamic Particle Sizer (APS model 3321, TSI). APS relies on particles time-of-flight measurement (using light scattering intensity measurement of two partially overlapping laser beams), providing aerosol concentration and PSD for particle aerodynamic size ranging from 0.5 µm to 20 µm, with a concentration scale from 0.001 p.cm -3 up to 1 000 p.cm -3 . APS operates at 1 Hz sampling rate.

For nano-sized (NaCl) aerosol monitoring, Scanning Mobility Particle Sizer spectrometer (TSI SMPS 3080) was implemented. The SMPS is an electrostatic classifier that comprises a Differential Mobility Analyzer (DMA, TSI 3081) and a Condensation Particle Counter (CPC, TSI 3785). The DMA separates airborne particles based on their electrical mobility and the CPC optically counts the individual particles (after enlarging them by nucleation growth of droplets in a supersaturated atmosphere). The SMPS provides concentration values and number based PSD for particle size ranging from 5 nm up to 1 000 nm, for concentration values as high as 10 7 p.cm -3 [START_REF] Asbach | Measurement Methods for Nanoparticles in Indoor and Outdoor Air, Indoor and Outdoor Nanoparticles[END_REF].

The single-pass particle collection efficiency is obtained by alternatively measuring the particle concentration and PSD upstream and downstream (hereunder respectively referred to as US and DS) of the ESP prototype. More specifically, for ESKAL 500 aerosol challenge, measurement is performed and averaged for 30 s US of the ESP prototype, followed by 30 s DS and another 30 s US to compensate for any concentration change in the test rig. The singlepass particle collection efficiency η(Dp) for particles with diameter Dp corresponding to each channel of the spectrometer (SMPS or APS) is then calculated as:

𝜂(𝑑𝐷 𝑝 ) = (1 - 𝑁 𝐷𝑆 (𝐷 𝑝 ) 𝑁 𝑈𝑆1 (𝐷 𝑝 )+𝑁 𝑈𝑆2 (𝐷 𝑝 ) 2 ) . 100 (1)
With: 𝑁 𝑈𝑆 (𝐷 𝑝 ) and 𝑁 𝐷𝑆 (𝐷 𝑝 ) the average particle number concentrations for particles with diameter Dp, respectively measured upstream and downstream of the ESP prototype.

Ozone emission rate calculation

Ozone (O3) concentration change was monitored in the air duct, downstream to the ESP prototype using an ozone analyzer from ENVEA, model o342e. For each set of operational parameters investigated, sampling was performed at 0.1 Hz frequency until reaching steady O3 concentration for 10 min ([O3]s). The ozone emission rate (E, in mg.h -1 ) was then calculated as:

𝐸 = ([𝑂 3 ] 𝑠 -[𝑂 3 ] 0 ).𝑄.𝑀.𝑃 𝑅.𝑇 . 10 -3 (2)
With: [O3]0 the initial ozone O3 concentration in the air duct (in ppbv), M the molar mass of O3 (47.998 g.mol -1 ), P the atmospheric pressure (in Pa), T the temperature in the air duct (in K) and R the universal gas constant (8.314 462 618 J.K -1 .mol -1 ).

RESULTS AND DISCUSSION

Key prototype operating variables were investigated, such as the corona current intensity (Iw) at the charging stage, the plate/plate voltage (Up) at the collection stage and the filtration air flowrate (Q). The following sections report the results of this parametric study on ESP prototype performances.

To remain in operating conditions that ensure the absence of spark discharge, upper limits were respectively set to 500 µA and 8 kV for corona current intensity (Iw) and plate/plate voltage (Up). Maximal value of 500 µA for the charging stage Iw was experimentally obtained for a wire/plate voltage (Uw) of 4.9 kV and the positive corona onset voltage was measured at a Uw value of approximately 4.0 kV. The current-voltage and power consumption-voltage characteristic curves are reported in supplementary materials (Figure S1).

Single-pass collection efficiency assessment

Effect of the applied voltage Up at collection stage

To investigate the influence of Up on collection efficiency, a series of experiments was conducted. The corona current intensity Iw was set to its maximum value of 500 µA, so that the aerosol challenge particles acquire the highest electrical charge achievable. The airflow Q was maintained at 540 m 3 .h -1 , corresponding to a filtration velocity of 1.61 m.s -1 .

The ESP prototype was challenged with ESKAL 500 aerosol and the influence of Up on collection efficiency was assessed for values ranging from 0 kV to 8kV. The spectral collection efficiencies as a function of particle size, obtained in the particle size range of ESKAL 500 aerosol are reported in Figure 6a. The overall ESKAL 500 collection efficiencies (calculated on the overall number based concentrations) are also shown in Figure 6b. The applied Up has a major influence on collection efficiency, which increased from 14% up to 94% when Up increased from 0 kV to 8 kV. Within particle size range of 500 nm to 4 µm, regardless of the Up value applied, the spectral efficiencies decrease when the particle size diminishes. This observation is in good agreement with Deutsch-Anderson equation [START_REF] Lin | A Modified Deutsch-Anderson Equation for Predicting the Nanoparticle Collection Efficiency of Electrostatic Precipitators[END_REF] that stipulates that collection efficiency in ESP is driven by particles electrical mobility, which, for micron-sized particles with fixed charging state, decreases with particle size [START_REF] Lin | A Modified Deutsch-Anderson Equation for Predicting the Nanoparticle Collection Efficiency of Electrostatic Precipitators[END_REF]. The spectral efficiencies demonstrate that PM2.5 (coarse particles, with an aerodynamic diameter of 2.5 µm and higher) are efficiently filtered (above 90%) for Up values as low as 4 kV whereas finer particles require Up to be set above 7 kV to be as efficiently collected.

Effect of corona current Iw at charging stage

To study the effect of corona intensity on particle collection efficiency, Up was fixed to its maximum value of 8 kV, so that the electric field intensity is maximized at the collection stage, and Q was maintained at 540 m 3 .h -1 .

The ESP prototype was here challenged with both ESKAL 500 and NaCl aerosols, to assess its efficiency on a broad range of particle size (micron/sub-micron sized and UFPs). The spectral collection efficiencies as a function of particle size corresponding to ESKAL 500 aerosol are reported in Figure 7a for positive Iw values (spectral collection efficiencies obtained with negative Iw values are available in supplementary materials, Figure S2). The overall ESKAL 500 collection efficiencies (calculated on the overall number based concentrations) are also shown in Figure 7b. Similarly, spectral and overall collection efficiencies corresponding to NaCl aerosol challenge are respectively reported in Figure 8a and Figure 8b.

For both challenge aerosols, increasing Iw drastically improves the collection efficiency. When Iw was increased from 0 µA to approximately 500 µA, ESKAL 500 and NaCl overall collection efficiencies respectively increased from 29% to 94% and from 45% to 88%. This observation is in agreement with theory since Iw increase generates higher unipolar ion concentrations (Ni), leading to higher charging efficiencies, which is reflected by higher particle collection at fixed Up. Additionally, one can notice that the ESKAL 500 collection efficiency is substantially higher for positive Iw as compared to equivalent negative Iw.

Analysis of spectral efficiencies obtained for NaCl aerosol challenge shows that, regardless of the Iw applied, the collection efficiency reaches a maximum for particle electrical-mobility diameter between 40 and 80 nm. For smaller particles (20 nm < Dme < 40 nm) and for bigger particles (80 nm < Dme < 200 nm), the efficiency slightly drops. A most penetrating particle size (MPPS) in the range of 120 nm to 200 nm classically results from the Deutsch-Anderson equation, whereas the efficiency decrease observed for particles below a few tens of nanometers may be attributed to partial charging phenomenon. Indeed Lin et al. [START_REF] Lin | A Modified Deutsch-Anderson Equation for Predicting the Nanoparticle Collection Efficiency of Electrostatic Precipitators[END_REF] suggested a modified Deutsch-Anderson equation accounting for the probability of nanoparticles to hold one elementary charge. This partial charging phenomenon implies that, below 100 nm, an increasing particles fraction remains at neutral state making its collection by electrostatic effect impossible.

Effect of the filtration air flowrate Q

To study the effect of the air velocity on single pass particle collection efficiency, Up and Iw were fixed at their maximum values (respectively 8 kV and 500 µA).

The ESP prototype was challenged with ESKAL 500 aerosol and the influence of Q on collection efficiency was assessed for values ranging from 120 m 3 .h -1 to 540 m 3 .h -1 . Spectral efficiencies are presented in Figure 9a and overall efficiencies are reported in Figure 9b. As one can expect, lowering the air flowrate improves the overall system efficiency. When Q decreased from 540 m 3 .h -1 down to 120 m 3 .h -1 , the single-pass collection efficiency of ESKAL 500 increased from 94% up to over 99.8%. Two distinct phenomena may explain this efficiency improvement: lowering the air velocity in the prototype firstly increases the particles residence time in the charging stage and secondly decreases the tangential component of particles velocity, enabling for more efficient displacement of the particles towards the collection plates.

Pressure drop assessment

One of the main benefits of ESP over mechanical filtration using non-woven fibrous media filters is its very low theoretical pressure drop, which is beneficial in terms of power consumption. Experimental measurement of the ESP prototype pressure drop was conducted for various air flowrates, as presented in Figure 10. The system pressure drop increases linearly with the air flow and is approximately equal to 4 Pa for the nominal flow rate (540 m 3 .h -1 ). ESP pressure drop is therefore 25 to 75 times lower than typical fibrous media filters, which usually exhibit initial pressure drops in the range of 100-300 Pa. Additionally, the ESP system, unlike the fibrous media, does not suffer from pressure drop increase due to fouling and clogging during operation. Indeed, this clogging phenomenon may be considered as the main drawback of fibrous media and can cause pressure drop to be multiplied by up to 10 over the filter lifespan: as reported by Xu et al. [START_REF] Xu | Investigation on the performance of airliner cabin air filter throughout the lifetime usage[END_REF], the pressure drop of a Donaldson KK2972941 airplane cabin filter was measured at 100 Pa before use and reached 950 Pa after dust loading representative of 6000 flight hours. Nevertheless, additional study will be needed to evaluate the long-term ESP prototype behavior. Indeed, as collected particles accumulate on plates surface, the pressure drop may not significantly change but the efficiency could potentially be impaired due to particles re-entrainment phenomenon [START_REF] Mizuno | Electrostatic Precipitation[END_REF] (liberation of a fraction of collected particles in the inter-electrode spacing).

Ozone emission assessment

The ozone generation by corona discharge is a well-recognized phenomenon. Since tropospheric O3 is considered as an air pollutant [START_REF]Air Quality Criteria for Ozone and Related Photochemical Oxidants (Final Report)[END_REF][START_REF]Health Risks of Ozone from Long-range Transboundary Air Pollution[END_REF], any device using corona discharge and being used for indoor applications should be designed to ensure that the ozone level does not exceed acceptable limits. For this matter, the ozone emission characteristics of the ESP prototype have been studied for various corona current intensities Iw at charging stage.

Results, expressed in O3 mass emission rate (E, in mg.h -1 ) are reported in Figure 11, together with the associated ESKAL 500 collection efficiency. E was found to be directly proportional to Iw ranging from -500 µA to +500 µA. As previously documented [START_REF] Mizuno | Electrostatic Precipitation[END_REF], E appears to be higher (about 6.6 times) when negative Iw is applied, as compared to positive Iw. The linear relationship between O3 mass emission rate and positive corona current (in µA) is given by the following relationship: E = 0.0155*Iw. For optimum collection efficiency, the ESP would thus approximately generate 8 mg.h -1 of ozone. Considering this value, and using a simplified mass balance model (presented in supplementary materials), calculation can be performed to estimate the O3 concentration change in an airplane cabin over time. Parameters and results of this calculation are presented in Figure 12. It is worth mentioning that this calculated O3 concentration increase can be considered as the worst case scenario since it does not take into consideration the influence of natural O3 decay. Calculation results estimate that the implementation and full-time operation of two ESP prototypes would lead to less than 20 ppbv O3 increase in the overall aircraft cabin air (in approximately 60 min) of a regional airliner.

CONCLUSION

An ESP prototype sized for aircraft cabin air PM filtration has been successfully designed and its performances in-depth experimentally assessed. The ESP prototype design was made for a half-system, treating half of the total recirculated air flow of a standard regional aircraft cabin (i.e. 540 m 3 .h -1 ). The particulate matter collection efficiency as a function of operation parameters (corona current Iw, plate/plate voltage Up and air flowrate Q) was assessed for nano-, submicron-and micron-sized airborne particles. For nominal operation parameters, a collection efficiency over 90% was demonstrated for airborne particles with an aerodynamic diameter of 0.5 µm or larger. For ultrafine particles (UFP), collection efficiency was assessed at 88% with a NaCl aerosol challenge. The particle charging technology based on corona discharge inherently produces ozone but the optimized design of the ESP prototype was found to limit this unwanted production down to 8 mg.h -1 , which would induce, even in the worst case scenario, very restrained concentration increase (<20 ppbv) in the aircraft cabin. Lastly, for nominal operation flowrate, the ESP prototype exhibited a pressure drop below 4 Pa, which is 25 to 75 times lower than conventional pristine media filters.

Details of the simplified mass balance model to simulate [O3] and PM in an airplane cabin

The model considers a medium size regional airliner cabin volume V of 136 m 3 , accommodating for 100 passengers. Fresh air is supplied to the cabin with a flowrate Qf, while the air inside the cabin is recirculated and filtered with a total flowrate Qr according to the Figure S3 below. The cabin is assumed to be pressurized at the atmospheric pressure and thermalized at 20 °C. The air in the cabin is assumed to be well mixed, so that the pollutant concentration (PM or O3) is essentially uniform and equal to a single value c. This assumption is quite common (see for instance the model for PM in [START_REF] Cao | Prediction of particle deposition around the cabin air supply nozzles of commercial airplanes using measured in-cabin particle emission rates[END_REF]) and allows to ignore the complex air flow pattern in the cabin. With this assumption, the pollutant mass balance in the cabin reads: -𝑐 𝑒𝑥𝑡 is the outside pollutant concentration -𝑚̇𝑖 is the internal pollutant source -𝑐 𝑟 is the concentration at the outlet of the filters.

For consistency, in this equation flowrates are expressed in m 3 .s -1 , concentrations in kg.m -3 and sources in kg.s -1 . All are evaluated under cabin temperature and pressure conditions.

Simulation of ESP operation on [O3] in the airplane cabin

O3 emission by the ESP prototype operation is assimilated here to the O3 internal pollution source 𝑚̇𝑖. For this simulation, to be able to discriminate the impact of ESP operation on [O3] in the cabin, the outside O3 concentration ( 𝑐 𝑒𝑥𝑡 ) is set as a constant, and the higher O3 concentrations that may be found at high altitudes are considered to be regulated by the presence of an airplane ozone scrubber located on the bleed airflow path. Other potential contamination sources and the O3 natural decay inside the cabin are neglected. Since the O3 produced by the ESP is accounted for as internal pollution source (in other words: cr = c), the mass balance equation can be simplified as follows:

Q f Q f Q r c ext c r c c 𝑚 𝑖 ̇
Selected parameters and resulting influence of ESP prototype operation on [O3] in an airplane cabin (calculated from equation S.3) are reported on Figure 12.

Simulation of ESP operation on PM in the airplane cabin

The same model can be used to evaluate the impact of filter efficiency on PM concentration in the cabin over time. Since PM concentration on the tarmac may reach very high concentrations, the worst situations for PM pollution are the ground operations such as taxiing. Here the internal PM pollution source 𝑚̇𝑖 can be neglected and cr is defined through the PM filter singlepass efficiency. The mass balance equation may then be expressed as: Using (S.5), the PM concentration change as a function of time can be calculated for various filter efficiencies. Figure S4 presents the PM concentration change in an airplane cabin during a phase (such as landing and taxiing) where the initial concentration in the plane (fixed at 100 p.cm -3 ) is substantially lower than the outside concentration (fixed at 100 000 p.cm -3 ). In this situation, the cabin PM concentration rapidly increases. Without filtering system, PM concentration in the cabin would tend to increase until equalizing the outside concentration.

With the implementation of a filter, the increase would be contained at a level that depends on the filter efficiency. A 99.9% efficiency HEPA filter would limit the concentration in the cabin at 30 081 p.cm -3 , while the implementation of the ESP prototype developed in this study (94% PM10 efficiency and 88% UFP efficiency) would maintain concentration in the range of 31 182 to 32 615 p.cm -3 . Those steady state concentrations are reached within approximately 30 min. In this situation, replacing an HEPA filter by the ESP prototype would lead to a concentration increase in the airplane cabin by at most 3.7% (PM10) to 8.4% (UFP).

Figure 1 .

 1 Figure 1. Simplified flow chart of a typical air supply on commercial aircraft, including a High Efficiency Particulate Air filter (HEPA) sometimes combined with an Activated Carbon (AC) filter in the recirculation loop of the ECS.

Figure 2 .

 2 Figure 2. Schematic of the ESP prototype developed in this study.

Figure 3 .

 3 Figure 3. ESP prototype main specifications and illustration.

Figure 4 .

 4 Figure 4. Experimental setup for ESP prototype performance assessment.

Figure 5 .

 5 Figure 5. Number-based PSD of (a) ESKAL 500 aerosol (obtained by APS) and (b) NaCl aerosol (obtained by SMPS).

Figure 6 .

 6 Figure 6. ESP prototype spectral (a) and overall (b) collection efficiencies, for various Up values (Q = 540 m 3 .h -1 , Iw = 500 µA, ESKAL 500 challenge).

Figure 7 .Figure 8 .

 78 Figure 7. ESP prototype spectral (a) and overall (b) collection efficiencies, for various Iw values (Q = 540 m 3 .h -1 , Up = 8 kV, ESKAL 500 challenge).

Figure 9 .

 9 Figure 9. ESP prototype spectral (a) and overall (b) collection efficiencies, for various Q values (Up = 8 kV, Iw = 500 µA, ESKAL 500 challenge).

Figure 10 .

 10 Figure 10. ESP prototype pressure drop as a function of air flowrate Q.

Figure 11 .*

 11 Figure 11. ESP prototype O3 mass emission rate E and collection efficiency η (with ESKAL 500 challenge), as a function of corona current Iw.

Figure 12 .

 12 Figure 12. Simulated O3 concentration change in an aircraft cabin due to ESP operation during flight.

Figure S3 .

 S3 Figure S3. Simplified diagram of the air supply system.

  𝑐 𝑒𝑥𝑡 + 𝑚̇𝑖 + 𝑄 𝑟 𝑐 𝑟 -(𝑄 𝑓 + 𝑄 𝑟 )c (S.1) Where:

  (𝑐 𝑒𝑥𝑡 -𝑐) + 𝑚̇𝑖 (S.2)The solution of this simplified equation is then calculated as:𝑐 = 𝑐 𝑒𝑥𝑡 + 𝑚̇𝑖 𝑄 𝑓 (1 -𝑒𝑥𝑝 (-𝑄 𝑓 𝑡 𝑉 )) (S.3)

  𝑐 𝑒𝑥𝑡 -c(𝑄 𝑟 η + 𝑄 𝑓 ) (S.4) Considering an initial PM concentration in the airplane cabin ci, the solution of this simplified equation can then be calculated as: c(t) = 𝑐 𝑖 . exp (-(𝑄 𝑟 η+𝑄 𝑓 )𝑡 𝑉 ) + 𝑄 𝑓 𝑐 𝑒𝑥𝑡 (𝑄 𝑟 η+𝑄 𝑓 ) (1 -exp (-(𝑄 𝑟 η+𝑄 𝑓 )𝑡 𝑉 )) (S.5)

ACKNOWLEDGMENTS

This project has received funding from the Clean Sky 2 Joint Undertaking (CS2JU REG-IAD) under grant agreement No. 831963. The JU receives support from the European Union's Horizon 2020 research and innovation program and the Clean Sky 2 JU members other than the Union.

SUPPLEMENTARY MATERIALS

It is worth mentioning that the described scenario points out the limitations of the ECS design implemented on nowadays airliner, with the absence of bleed air filtration before being mixed with the recirculated air.