domain: sde chim

text:
Badr R'mili

Rafal S Strekowski
email: rafal.strekowski@univ-amu.fr

Brice Temime-Roussel

Henri Wortham

Anne Monod
email: anne.monod@univ-amu.fr

Important effects of relative humidity on the formation processes of iodine oxide particles from CH 3 I photo-oxidation

Keywords: Iodine oxides, Iodides, Aerosols, Particles, Particle formation, Methyl iodide, Mass Spectrometry, SMPS, Granulometry, Microscopy, Electron microscopy

In this work, laboratory chamber experiments of gas-phase methyl iodide photolysis in the presence of ozone at three relative humidity conditions were performed to study the formation and physico-chemical properties of iodine oxide particles. The obtained results revealed significant morphological changes of iodine oxide particles that were observed to depend on relative humidity. The formed iodine oxide particles under dry condition were supposed to be agglomerates of fine hygroscopic crystals. On the other hand, a humid atmosphere was observed to favor the formation of isomeric, tetragonal and orthorhombic hygroscopic crystals potentially composed of HIO 3 likely formed from progressive hydration of iodine oxide clusters. This process leads to a release of molecular iodine, I 2 , which may indicate a potential role of I 2 O 4 in the particles' evolution processes. The obtained results on the iodine oxides' behavior are important to the nuclear power plant safety industry since many of the organic iodides that may be released during a major nuclear power-plant accident contain radioactive isotopes of iodine that are known to have lethal or toxic impacts on human health.

Introduction

Organic iodides are of great interest for both the atmospheric science community and the nuclear power plant safety industry. They are involved in the chemical processes driving the oxidizing capacity of the atmosphere [1] and atmospheric new particle formation involved in marine aerosol and cloud formation [START_REF] O'dowd | Marine aerosol formation from biogenic iodine emissions[END_REF]. This is important since atmospheric aerosols and clouds are known to scatter incoming solar radiation, thus, regulating the Earth's radiation budget [START_REF] O'dowd | Marine aerosol formation from biogenic iodine emissions[END_REF][START_REF] Slingo | Sensitivity of the Earth's radiation budget to changes in low clouds[END_REF]. Further, organic iodides are known to play a significant role in the tropospheric [START_REF] Davis | Potential impact of iodine on tropospheric levels of ozone[END_REF][START_REF] Mcfiggans | A modeling study of iodine chemistry in the marine boundary layer[END_REF] and stratospheric [START_REF] Solomon | On the role of iodine in ozone depletion[END_REF] ozone depletion cycles.

Organic iodides have also gained a considerable interest in the nuclear power plant safety industry following three major power plant accidents that include Three Mile Island (U.S.A.) [START_REF] Cline | Measurements of 129 I and radioactive particulate concentrations in the TMI-2 containment atmosphere during and after the venting[END_REF], Chernobyl (Ukraine) [START_REF] Noguchi | Physicochemical speciation of airborne 131 I in Japan from Chernobyl[END_REF] and Fukushima (Japan) [START_REF] Kinoshita | Assessment of individual radionuclide distributions from the Fukushima nuclear accident covering central-east Japan[END_REF] when the nuclear safety engineering community has come to understand that many different organic iodides are formed within the nuclear power plant containment building during such an accident [START_REF] Hala | Radioactivity, Ionizing Radiation, and Nuclear Energy[END_REF][START_REF] Ball | Behaviour of iodine project: Final report on organic iodide studies[END_REF]. These newly formed organic iodides may then be released into the environment if the reactor building containment is breached. Further, following the Fukushima nuclear power plant accident, atmospheric transport models have clearly shown the importance to understand the atmospheric emissions and behavior of iodine species released from point sources [START_REF]Modélisation du comportement de l'iode dans l'atmosphère[END_REF][START_REF] Fortin | Études par simulations numériques et moléculaires de la réactivité atmosphérique de l'iode[END_REF]. This is important, since many of these newly released organic iodides contain radioactive isotopes of iodine, namely 131 I and 129 I, that are known to have a lethal or toxic impact of human health. Primary compounds released during a major nuclear power plant accident include molecular iodine and methyl iodide (CH 3 I).

The CH 3 I is thought to be formed from the interactions of gas-phase iodine with the organic content present within the containment paint ingredients under gamma irradiation [START_REF] Jubin | A literature Survey of Methods to Remove Iodine from Off-gas Streams Using Solid Sorbents[END_REF][START_REF] Wren | The interaction of iodine with organic material in containment[END_REF][START_REF] Tietze | Formation of organic iodides from containment paint ingredients caused by gamma irradiation[END_REF].

The formed CH 3 I is subsequently photolyzed by gamma or solar radiation in the UV region of the solar spectrum to produce •CH 3 and •I radicals. The latter radicals are known to react with ozone (reaction R1), an oxidizing molecule readily available in ample quantities in the containment area of a nuclear power plant following a major accident, to produce •IO radicals, a known precursor of iodine oxide aerosols [START_REF] Hoffmann | Iodine oxide homogeneous nucleation: An explanation for coastal new particle production[END_REF][START_REF] Jimenez | New particle formation from photooxidation of diiodomethane (CH 2 I 2 )[END_REF].

•I + O 3 → IO • +O 2 (R1)
The gas-phase •IO radical's self-reaction has four potential thermodynamically accessible reaction channels that include the formation of OIO• and I• radicals (reaction R2a), the formation of I 2 O 2 (reaction R2b), the formation of I• radical and molecular oxygen (reaction R2c) and the formation of molecular iodine and molecular oxygen (reaction R2d) [START_REF] Misra | Computational investigations of iodine oxides[END_REF][START_REF] Bloss | Kinetics and products of the io selfreaction[END_REF][START_REF] Atkinson | Evaluated kinetic and photochemical data for atmospheric chemistry: Volume II -gas phase reactions of organic species[END_REF][START_REF] Cox | OIO and the atmospheric cycle of iodine[END_REF]:

IO • +IO• → OIO • +I• (R2a) M -→ I 2 O 2 (R2b) → I • +O 2 (R2c) → I 2 + O 2 (R2d)
Only the reactive channel R2a that leads to the formation of OIO• and I• radicals is considered to be important and the reactive channels R2b to R2d are considered to be negligible or recycled further to regenerate IO • radicals [START_REF] Gómez Martín | Kinetic and mechanistic studies of the I 2 /O 3 photochemistry[END_REF][START_REF] Bloss | Kinetics and products of the io selfreaction[END_REF][START_REF] Martín | Determination of the O-IO bond dissociation energy by photofragment excitation spectroscopy[END_REF]. It is understood that the gas-phase OIO• and IO• radicals react further to form I 2 O 4 or I 2 O 3 (reactions R3 and R4) [START_REF] Gómez Martín | Kinetic and mechanistic studies of the I 2 /O 3 photochemistry[END_REF].

OIO • +OIO• M -→ I 2 O 4 (R3) OI • +OIO• M -→ I 2 O 3 (R4)
While studying the gas-phase CH 2 I 2 /hv/O 3 reactivity in an atmospheric simulation chamber, [START_REF] Hoffmann | Iodine oxide homogeneous nucleation: An explanation for coastal new particle production[END_REF] [START_REF] Hoffmann | Iodine oxide homogeneous nucleation: An explanation for coastal new particle production[END_REF] suggested that the self-reaction of •OIO (reaction R3) that leads to I 2 O 4 formation is the key nucleation precursor step responsible for aerosol formation. These authors' deduction is based on direct identification of I 2 O 4 molecules present within the formed aerosols and propose the following polymerization mechanism (reactions R3 and R5) to explain the nucleation process:

I 2 O 4 + (OIO•) n → [-I -O -IO 2 -O-] (1+ n 2 ) (R5)
This observation is supported by theoretical and chamber work of Jimenez et al. (2003) who studied iodine oxide particle formation using the same reactive system as Hoffmann et al.

(2001), namely CH 2 I 2 /hv/O 3 , carried out under two different relative humidity experimental conditions, namely, dry and ≈65%RH [START_REF] Jimenez | New particle formation from photooxidation of diiodomethane (CH 2 I 2 )[END_REF]. Jimenez et al. (2003) [START_REF] Jimenez | New particle formation from photooxidation of diiodomethane (CH 2 I 2 )[END_REF] argued that the self-reaction of •OIO radicals leads to the formation of I 2 O 4 , a conclusion that was based on the observed hygroscopic behavior of aerosols. These investigators argued that low hygroscopicity aerosols are best explained by the nucleation process involving I 2 O 4 and not highly hygroscopic I 2 O 5 .

To the contrary, in their chamber work on the reactivity of the I 2 /hv/O 3 system, Saunders and Plane (2006) [START_REF] Saunders | Formation pathways and composition of iodine oxide ultrafine particles[END_REF] proposed that polymerization involves I 2 O 5 , formed by successive oxidation of I 2 O 2 , I 2 O 3 and I 2 O 4 in the presence of ozone. These authors base their conclusion on an EDX analysis and the calculated average atomic ratio O/I of 2.45 obtained on the sampled iodine oxide particles [START_REF] Saunders | Formation pathways and composition of iodine oxide ultrafine particles[END_REF].

Contrary to the work of [START_REF] Saunders | Formation pathways and composition of iodine oxide ultrafine particles[END_REF] [START_REF] Saunders | Formation pathways and composition of iodine oxide ultrafine particles[END_REF] but in-line with the results presented by [START_REF] Hoffmann | Iodine oxide homogeneous nucleation: An explanation for coastal new particle production[END_REF] [START_REF] Hoffmann | Iodine oxide homogeneous nucleation: An explanation for coastal new particle production[END_REF] and Jimenez et al. (2003) [START_REF] Jimenez | New particle formation from photooxidation of diiodomethane (CH 2 I 2 )[END_REF], modeling and theoretical work carried out by Gomez Martin et al. (2007) [START_REF] Gómez Martín | Kinetic and mechanistic studies of the I 2 /O 3 photochemistry[END_REF] suggested that the formation of I 2 O 3 and I 2 O 4 that leads to aerosol formation is best explained by •OI and •OIO radical reactivity. In accord with these studies, Saunders et al. (2010) [START_REF] Saunders | Studies of the formation and growth of aerosol from molecular iodine precursor[END_REF] argue that iodine oxide aerosols are formed mainly by polymerization of I 2 O 4 with itself and with I 2 O 3 . In their experimental flow-tube study of the I 2 /O 3 system under "dark" experimental conditions, that is, in the absence of photolyzing and its dimer, once formed, will undergo polymerization, thus, initiating the aerosol nucleation process [START_REF] Martń | On the mechanism of iodine oxide particle formation[END_REF]. However, a more recent study by [START_REF] Sipilä | Molecular-scale evidence of aerosol particle formation via sequential addition of HIO 3[END_REF] suggested that aerosols form via sequential addition of HIO 3 , followed by intracluster restructuring to I 2 O 5 and recycling of water in the atmosphere or on dehydration [START_REF] Sipilä | Molecular-scale evidence of aerosol particle formation via sequential addition of HIO 3[END_REF]. The additional observations of HIO 3 and the singular role of H 2 O raises new questions about different reaction pathways leading to new iodine oxide particle formation. Under humid conditions, the gas-phase OIO • + • OH reaction or iodine oxides hydration may lead to form HIO 3 . However, the OIO • + • OH reaction is an unlikely source of HIO 3 because it is limited by the atmospheric concentrations of OH radicals.

Further, the atmospheric OH radicals may be scavenged by the I • + • OH reaction in the early stages of the atmospheric oxidation processes. Also, the HIO 3 atmospheric formation from the iodine oxide hydration reaction is known to be endothermic with a high gas-phase reaction energy barrier, therefore, thermodynamically unlikely to be important. [START_REF] Martín | A gas-to-particle conversion mechanism helps to explain atmospheric particle formation through clustering of iodine oxides[END_REF].

The exact nature and details of the reactive processes and the exact composition of the iodine oxide that polymerizes remain elusive and uncertain [START_REF] Kumar | Single-molecule catalysis revealed: elucidating the mechanistic framework for the formation and growth of atmospheric iodine oxide aerosols in gas-phase and aqueous surface environments[END_REF][START_REF] Martín | A gas-to-particle conversion mechanism helps to explain atmospheric particle formation through clustering of iodine oxides[END_REF]. That is, all the intermediate reactions that lead to aerosol formation and the final aerosol composition remain poorly understood and the chemical composition of the iodine oxide aerosol is a subject of much discussion and controversy [START_REF] Martín | A gas-to-particle conversion mechanism helps to explain atmospheric particle formation through clustering of iodine oxides[END_REF]. This is worrisome for the Nuclear Safety Industry since tests carried out by the Institute of Radio-protection and Nuclear Safety (IRSN, France) show that the bulk of aerosols released within the confinement zone of the nuclear power plant following a major power-plant accident are in the form of iodine oxides. As a result, detailed chemical and physical characterization of iodine oxide aerosols are needed to better understand the chemical and physical processes responsible for the atmospheric formation, transport and fate of different fission products to better apply and execute appropriate mitigation strategies to protect the public.

In this work, we have performed laboratory chamber investigations of CH 3 I photolysis in the presence of ozone under controlled temperature, reactant gas-phase concentration, humidity and UV-radiation. The impact of relative humidity on the physico-chemical properties of the formed iodine oxide particles is investigated. A new analytical approach based on on-line gasphase and particle formation and growth analysis and off-line transmission electron microscopy (TEM) characterization is used. Significant differences in the way the particles form and grow were observed to depend on the given relative humidity experimental conditions, best explained by particles' morphological changes. The obtained results give a better insight into the singular hygroscopic behavior of the particles and the chemical processes that lead to the iodine oxide particle formation, growth and their final chemical composition.

Experimental approach

The experimental approach described in this work is similar to the one employed in other emission studies of environmental hazard interest [START_REF] Zhang | Études cinétiques de l'oxydation radicalaire en phase gazeuse d'iodures organiques et de la formation de particules d'oxydes d'iode sous conditions simulées de l'enceinte d'un réacteur nucléaire en situation d'accident grave[END_REF]. The experiments involved a series of offline and on-line and continuous analyses of both the gas and aerosol phases using state-of-the-art analytical methods that include Selective Reagent Ionization Time-of-Flight Mass Spectrometry All experiments were performed within the double-wall VCE1000 (Vötsch Industrietechnik) electro-polished stainless-steel emission test chamber. The internal chamber volume was 916 L. The temperature within the chamber was maintained constant by allowing a temperaturecontrolled air to circulate through the external air-jacket housing located between the inner chamber volume and the outer wall. All experiments were performed using purified compressed nitrogen carrier gas. The nitrogen flow rate was 2 standard liters per minute (SLPM) and the corresponding exponential carrier gas exchange rate was 0.13 chamber volume per hour. The nitrogen gas was produced using the N2 LCMS-1 nitrogen generator (Claind Srl, Italy) based on air separation by mechanical means by taking filtered atmospheric air, pressurizing it in an air compressor and then separating it using the pressure swing adsorption technique to deliver a continuous stream of dry nitrogen. However, a presence of oxygen could not be excluded. The carrier gas throughput dynamics through the chamber were such that there was always a slight over-pressure within the chamber with respect to the atmospheric pressure to prevent outside air from leaking in. The chamber temperature was measured using the platinum resistance temperature detector (Pt100, Class B) and the chamber humidity was measured using a capacitance hygrometer. The experimental chamber temperature was T( • C) = 25±0.4 and the experimental relative humidity was preconditioned from dry to 57%RH. The listed uncertainty in temperature was an estimated experimental value. In this work, the humidity sensor that was used had a lower limit humidity value of 2%RH and the humidity accuracy was within ±2%RH. It was considered that the chamber volume was under dry experimental conditions if the relative humidity sensor reading was < 2%RH. The relative humidity witin the chamber was measured using the Vötsch Industrietechnik chamber relative humidity probe. The probe's humidity range was % 5

to 95 and the humidity deviation in time was % ±1 to ±3.

The UVP Pen-Ray low pressure mercury lamp (Fisher Scientific, France) was used as a photolytic light source. The pen-ray lamp was fitted into a jacketed quartz cylinder to eliminate the λ = 184.9 nm Hg atomic line. Water was allowed to circulate throughout the outer jacket of the cylinder to filter out infrared radiation (heat) while a dry nitrogen gas was allowed to flow through the tube fitted with the Hg-lamp to remove oxygen, thus, eliminating its photolysis leading to unwanted ozone formation within the lamp housing. Even though a UV lamp was used as a source of photolyzing radiation, Hughey et al. (2021) [START_REF] Hughey | Preliminary studies of uv photolysis of gas-phase ch3i in air: Time-resolved infrared identification of methanol and formaldehyde products[END_REF] reported the CH 3 I UV photolysis rate constant to be 5.92 × 10 -4 s -1 , a value that is in agreement with the gammairradiation rate, albeit the radiolysis induced CH 3 I decomposition kinetics were observed to depend on the initial methyl iodide concentration and the dose rate [START_REF] Tang | Kinetics of. gamma.-induced decomposition of methyl iodide in air[END_REF][START_REF] Hughey | Preliminary studies of uv photolysis of gas-phase ch3i in air: Time-resolved infrared identification of methanol and formaldehyde products[END_REF].

As stated above, a state-of-the-art instrumentation park was employed to monitor gas-phase reactant and product temporal profiles and aerosol formation/behavior within the chamber before and after the inception of photolysis. Reactant introduction and the concurrent gas-phase reactant/product concentrations and aerosol measurement experimental details are described in more detail below.

Reactants

The CH 3 I (Linde France, Saint-Priest, France) stated minimum purity was 99%. To limit any photo-catalytic or thermal decomposition, CH 3 I liquid was stored under dark conditions at T = 279K. Deionized water used to humidify the chamber had resistivity greater than 18 MΩ, TOC< 2 ppb, and was prepared by allowing tap water to pass first through a reverse osmosis demineralization filter (ATS Groupe Osmose) and then through a commercial deionizer (Milli-Q, Molsheim, France). The ozone was generated and injected by allowing molecular oxygen (O 2 , purity > 99.999%, Linde) to flow through a corona discharge generator (CERTIZON C25 Sanders). The oxygen flow rate was 0.3 L•min -1 and the resulting ozone flux was 25 mg O 3 h -1 .

In a typical experiment, the desired ozone mixing ratio (1.5±0.1 ppmV) within the chamber was established within ten minutes. This ozone mixing ratio has been chosen so that its concentration was in excess of that of methyl iodide under the experimental conditions employed. The ozone mixing ratio was monitored using a commercial ozone analyser (O3 41M; Environment S.A., Poissy, France). To correct the flow dynamics requirements of the ozone analyzer (2 L•min -1 ), an additional and independent flow of dry nitrogen to the analyzer was used in a T-configuration

(2:1 ratio).

The gas phase CH 3 I was introduced into the chamber using the gas saturation method first developed by Regnault in 1845 [START_REF] Regnault | Études sur l'hygrométrie[END_REF]; one of the oldest and most versatile ways of studying gasliquid equilibria involving low vapor pressure compounds. Further, the gas saturation method used in this work is similar to the one employed in a number of other studies of atmospheric interest [START_REF] Zhang | Kinetic study of the reaction of OH with CH 3 I revisited[END_REF][START_REF] Zhang | Kinetic study of the reaction of OH with CH 2 I 2[END_REF][START_REF] Zhang | Temperature dependence kinetic studies of the reaction of O( 3 P) with CHI 3 and C 2 H 5 I and the 298K reaction of OH(X 2 Π) with CHI 3[END_REF][START_REF] Knispel | Adduct formation of OH radicals with benzene, toluene, and phenol and consecutive reactions of the adducts with NOx and O 2[END_REF] where precise reactant concentrations of volatile or semi-volatile compounds needed to be known. Its operation principle and the calculation of the concentration of CH 3 I leaving the saturation system have been presented in detail elsewhere [START_REF] R'mili | Quantification of the gas phase methyl iodide using O + 2 as the reagent ion in the PTR-ToF-MS technique[END_REF]. Briefly, the nitrogen carrier gas was allowed to enter the volume containing the CH 3 I liquid that itself was mixed with glass beads to increase its total surface area and supported on a fritted glass surface.

The saturator volume itself was immersed in a temperature-controlled fluid (water) and kept at constant temperature (T (K) = 279.0 ± 0.1) and pressure (p = 962 Torr) using a thermostat with an accuracy of ±0.1K. The temperature inside the saturator volume was measured using a Type-J thermocouple (Omega, Manchester, United Kingdom) with an accuracy of ±0.1K. The carrier gas was allowed to enter the saturator volume, come to a thermal equilibrium with the sample, and was then allowed to exit through a glass tube. The geometry of the exiting glass tube was such that the diameter of the glass tube increased with increasing length. This was done to avoid any sample condensation as the sample and the carrier gas were allowed to leave the saturator system. The concentration of CH 3 I leaving the saturation system was calculated from the given vapor pressures [START_REF] Stull | Vapor pressure of pure substances. organic and inorganic compounds[END_REF], mass flow rates, and total pressure within the saturator [START_REF] R'mili | Quantification of the gas phase methyl iodide using O + 2 as the reagent ion in the PTR-ToF-MS technique[END_REF]. Under typical experimental conditions, the vapor pressure of CH 3 I within the saturator was calculated to be 183 ± 4 Torr and the gas phase CH 3 I mixing ratio within the saturation system was about 19%.

This vapor pressure was further diluted with a dilution ratio of 1500:1 using dry nitrogen carrier gas that was allowed to flow through a system of mass flow controllers before its introduction into the chamber. Under typical experimental conditions, the reactant introduction lasted about 22 minutes to achieve a homogeneous chamber mixing ratio of [CH 3 I]= 102 ± 5 ppbV. This methyl iodide mixing ratio is on the lower side of what has been modeled to be emitted during a major nuclear power plant accident [START_REF] Tang | Kinetics of. gamma.-induced decomposition of methyl iodide in air[END_REF].

2.2. On-line physical and chemical analyses

Gas phase chemical analysis

A commercial Selective Reagent Ionisation Time-of-Flight Mass Spectrometer (SRI-ToF-MS 8000 Ionicon Analytik GmbH, Innsbruck, Austria) was used to measure the gas-phase CH 3 I and other gas-phase products that include selected iodide oxides. Similar to our previous work on detection of gas-phase methyl iodide, the chemical ionization (CI) reagent was the molecular oxygen ion (O 2 + ) [START_REF] R'mili | Quantification of the gas phase methyl iodide using O + 2 as the reagent ion in the PTR-ToF-MS technique[END_REF]. The drift-tube pressure (p drift ) was 2.10 mbar, drift tube voltage was 544

V and the drift tube temperature was held constant at T=333.15K. The corresponding E/N ratio was 127 Td. (1Td=10 -17 V • cm 2 ) where E is the electric field strength applied to the drift tube and N is the buffer gas density. Data were analyzed using the Tofware software V2.5.10.

Granulometric assessment

Particle size distribution was measured using the Scanning Mobility Particle Sizer (SMPS).

The SMPS was equipped with a TSI model 3077A 85Kr neutralizer (TSI Inc.), a long col-

umn

Aerosol physio-chemical analysis

The non-refractory aerosol fraction (evaporates rapidly at T=600 • C under vacuum conditions) was analyzed in-situ and continuously using the HR-ToF-AMS. The AMS analysis allowed to chemically characterize both the non refractory organic and inorganic fraction of aerosols in the aerodynamic diameter size ranging from ∼70 nm to 1000 nm. The instrument was used under manufacturer's standard operating conditions (vaporizer temperature was 600 • C and the electron impact (IE) detector voltage was 70 eV). The AMS mass spectra analyses of the aerosols allowed to distinguish the iodinated species from the organic fraction. Data were preprocessed using the standard ToF-AMS analysis toolkit SQUIRREL V.1.20O. High-resolution peak fitting and integration procedures were performed using the PIKA software (v1.600). Typically, AMS mass spectra are measured using the nitrate equivalent mass temporal mass evolution or nitrate equivalent mass [START_REF] Jayne | Development of an Aerosol Mass Spectrometer for Size and Composition Analysis of Submicron Particles[END_REF]. That is, ammonium nitrate particles are used to calibrate the aerosol mass spectrometer because their physical properties such as density and shape are well established, and they are known to fully vaporize within the ionization zone. Consequently, the AMS mass response to ammonium nitrate signal is used to calculate the "ionization efficiency", that is, number of ions produced per molecule. Therefore, when the ionization efficiency of specific compound is unknown, it is a common practice to express their quantity in "equivalent nitrate" concentrations.

Off-line analysis

To evaluate the physio-chemical and morphological nature of the formed aerosols, off-line point samples were collected by filtration using the Mini-Particle Sampler (MPS) [START_REF] R'mili | Particle sampling by TEM grid filtration[END_REF]. Here, sampled air was allowed to flow through the Transmission Electron Microscope (TEM) TEMporous grids (Quantifoil, AGAR Scientific, 1.2/1.3 400 mesh). This procedure allowed to sample the aerosols directly on a given support, thus, minimizing any additional sample preparation or sampling artifacts [START_REF] R'mili | Particle sampling by TEM grid filtration[END_REF]. The filtration flow rate was 0.3 L min -1 and the sampling efficiency was known. The filtered aerosols were then characterized using the TEM (JEOL JEM2010 microscope, CINaM Electron Microscopy Service, Marseille, France) fitted with a LaB 6 electron gun under a 200 kV accelerating voltage coupled with energy-dispersive X-ray and EDX spectrometer (BRUKER, Quantax) equipped with a XFlash R silicon drift detector.

Results

The performed experiments are summarized in Table 1. Experiments D0 and D1 were carried out under dark conditions, that is, the UV-lamp was turned off, in the absence and presence of CH 3 I at a given O 3 mixing-ratio value. The Li (i = 0 to 4) and Li* (i = 2,3,4) experiments were carried out in the presence of the photolyzing UV-radiation (UV-lamp was tuned on) at

given gas-phase CH 3 I and O 3 mixing-ratio values and given relative humidity conditions. A and L4, respectively, when the O 3 and CH 3 I mixing ratios have been observed to reach the corresponding instrument lower detection limit values. That is, the L2*, L3* and L4* experiments were carried out without prior chamber cleaning procedure and following the re-injection of O 3 . ‡ Below the limit of quantification, LOQ < 0.23 ± 0.10 ppbv [START_REF] R'mili | Quantification of the gas phase methyl iodide using O + 2 as the reagent ion in the PTR-ToF-MS technique[END_REF] cleaning procedure was used prior to every experiment with the exception of Li* (i = 2,3,4) experiments. Briefly, the internal walls of the chamber were first wiped clean with deionized water using kimwipes. Then, a flow of purified nitrogen gas was allowed to flow through the chamber volume at a rate of 4 liters per minute and was maintained for more than 72 hours.

The cleanless of the chamber was then verified for particle formation using the SMPS and AMS and an organic contamination using the SRI-ToF-MS under given experimental conditions that included a presence and absence of UV-radiation and different relative humidity values. As listed in Table 1, the L0 experiments were carried out in the absence of CH 3 I at a given O 3 mixingratio value and L1 experiments were carried out in the absence of O 3 at given CH 3 I mixing-ratio values. The influence of relative humidity in the presence of both gas-phase reagents, namely, CH 3 I and O 3 , were tested in the experiments L2, L3 and L4. As shown in Table 1, experimental conditons that lead to particle formation require the presence of gas-phase methyl iodide, ozone and UV-radiation.

CH 3 I temporal profile

To evaluate the gas-phase loss of CH 3 I within the chamber, its temporal evolution was studied using the SRI-ToF-MS. Some typical normalized CH 3 I temporal profiles observed following the introduction of CH 3 I/H 2 O/N 2 and CH 3 I/O 3 /H 2 O/N 2 mixtures into the chamber in the presence and absence of photolysis are shown in Figure 1. As shown in Figure 1, the CH 3 I temporal profiles were observed to be exponential. As a result, pseudo-first-order conditions were assumed. The kinetic data shown in Figure 1 provide information on the CH 3 I gas-phase loss rate within the chamber due to photolysis, wall loss or secondary reactions. From the kinetic data using CH 3 I/O 3 /H 2 O/N 2 mixtures in the absence of a photolytic light source (Experiments D1), the obtained decay rate constant was k obs = (3.3 ± 0.1) × 10 -5 s -1 , 1σ. This value corresponds well with the chamber N 2 renewal flow rate of 2 L min -1 or exponential rate of 3.6 × 10 -5 s -1 . As a result, under the experimental conditions employed in this work, it was assumed that the gas-phase CH 3 I does not react with ozone in the absence of photolytic radiation and the wall losses may be considered to be negligible.

From the kinetic data using CH 3 I/H 2 O/N 2 mixtures in the presence of a photolytic light source (Experiments L1) we obtain k obs = (6.3 ± 0.1) × 10 -5 s -1 , 1σ. On the other hand, from the kinetic data using CH 3 I/O 3 /H 2 O/N 2 mixtures in the presence of a photolytic light source (experiments L2, L3 and L4) we obtain k obs = (7.2 ± 0.4) × 10 -5 s -1 , 1σ. From the calculated k obs values we conclude that the gas-phase CH 3 I is mainly consumed by photolysis. The kinetics of CH 3 I loss in the presence of a photolytic light source appears to be independent of humidity but dependent on the ozone mixing ratio within the chamber.

Particle formation and growth

A typical SMPS temporal evolution of the particle number size distribution as well as the corresponding average electric mobility diameter values (d m , solid line) are shown in Figure 2.

The d m was obtained using the aerosol total mass distribution data shown in Figure 3. The UVP lamp was switched on at time t 0 (t 0 = 0 s), initiating the photooxidation of CH 3 I inducing particle formation as shown in Figure 2 and Figure 3. Particle number size distributions were measured between 5.94 and 224.7 nm every 135 seconds. The d m were observed to evolve exponentially to reach their maximum size after 7 to 8 hours of reaction time, a range that represents a typical temporal evolution of particle formation observed in the simulation chamber or in the real atmosphere [START_REF] Wang | Design of a new multi-phase experimental simulation chamber for atmospheric photosmog, aerosol and cloud chemistry research[END_REF]. As shown in Figures 2a and2b, under dry experimental conditions (L2: < 2%RH) and at a low relative humidity value (L3: 22%RH), the nucleation mode was observed to persist for up to 3 hours with an average aerosol electric mobility diameter of ≈8 nm and 14 nm, respectively. However, as may be seen in A typical temporal evolution of the total particle mass size distribution and the corresponding d m is shown in Figure 3. Here, the shape of the banana-plot illustrates the evolution of the aerosol accumulation mode. Under dry experimental conditions (L2: < 2%RH), the aerosol formation evolve toward a broad particle size distribution regime with an observed maximum average electric mobility diameter of 173 ± 6 nm (Figure 3a). At 22%RH, the aerosols have been observed to evolve toward a narrower particle size distribution with a smaller maximum average mobility diameter of 103±8 nm (Figure 3b). At 57%RH, two accumulation modes were observed to form, evolving toward two distinct maximum average electric mobility diameters of ≈ 138 ± 8 nm and 205 ± 8 nm with the average electric mobility diameter value of (192 ± 8) nm (Figure 3c). The two aerosol accumulation modes were observed to distinguish themselves at t = 80 to 100 minutes (4.8 × 10 3 s to 6 × 10 3 s, Figure 3c) when the average electric mobility aerosol diameter exceeded 70 nm.

The nitrate equivalent mass measured using the AMS is shown in Figure 4. As discussed before, ammonium nitrate particles are used to calibrate the aerosol mass spectrometer because their physical properties such as density and shape are well established, and they are known to fully vaporize within the ionization zone [START_REF] Jayne | Development of an Aerosol Mass Spectrometer for Size and Composition Analysis of Submicron Particles[END_REF].

The aerosol fraction is observed to be constituted of iodides, organics and, to a lesser extent, sulfates, nitrates and ammonium ions that are believed to represent residual impurities present within the chamber. The aerosol chemical composition total mass time evolution is observed to remain relatively stable as soon as the AMS aerosol detection limit is reached (vacuum aerodynamic mobility diameter, d va > 70 nm). This means that after the aerosols' formation, their composition remains homogeneous during growth. As shown in Figure 4, an increase in humidity results in the total mass decrease. However, between 22%RH and 57%RH an increase in the total mass is observed. This phenomenon is thought to be due to the impact of humidity on particle size and morphology. This important result is explained in section 3.5.

On the average, the aerosol iodine component fraction represents 73 ± 4% of the total mass concentration followed by the organic fraction (19 ± 4%) and traces of ammonium (1 to 6%), sulfates and nitrates (1 to 2% The organic fraction of aerosols may be attributed mostly to the CO 2 + fragment (m/z 43.98, org44) (and by default to the CO + fragment (m/z 27.99, org28, according to the standard fragmentation approach: org28 = org44 x 1)) which represents the most oxidized fraction of organic compounds. These two fragments account for 96±1% of the total mass attributed to the aerosols' organic fraction, independent of the experimental relative humidity conditions. The remaining organic fraction mass spectra fragments include CH Similar to the situation described by Pieber and coworkers (2016), it is assumed that the reactive behavior of the iodine oxides is equivalent to the NH 4 NO 3 and (NH 4 ) 2 SO 4 salt reactivity within the AMS ionization source [START_REF] Pieber | Inorganic salt interference on CO + 2 in aerodyne AMS and ACSM organic aerosol composition studies[END_REF]. As a result, it is assumed that the contribution of the highly oxidized CO 2 + and CO + fragments to the total aerosol content may be ignored and that the aerosols formed under given experimental conditions are principally composed of iodine oxides. Further, possible origins of other organic fragments include residual organic compounds present within the AMS ionization source that include CH 3 I and its organic radicals formed following the photooxidation of CH 3 I and to a much lesser extent the photooxidation of other trace organic impurities present in our experimental chamber.

Particle morphology

Off-line samples were collected to better interpret and understand the aerosols' morphology and physico-chemical information that includes particle number and size distribution and par-ticle mass concentration, acquired using the SMPS and AMS techniques. Here, samples were collected by filtration on porous TEM grids at a specific time under given experimental conditions. A typical TEM image of a sample taken over a 6 minute time interval after 3 hours of reaction time (starting at t ≈ 3 hours) is shown in Figure 5. Additional off-line samples were collected over a period of 6 minutes after 7 hours of reaction time (starting at t ≈ 7 hours). At this moment, the aerosols have been observed to approach their maximum electric mobility diameter.

Typical TEM images observed at three different relative humidity values are shown in Figure 5.

Compared to t ≈ 3 hours, the morphology of the collected particle remained unchanged.

As shown in Figure 5, the morphology of the collected particles was observed to be explicitly distinct at different relative humidity values. That is, the morphology of the particles formed at < 2%RH and at 22%RH are observed to be mostly compact or near-spherical in nature. At 57% RH the formed particles are observed to be mostly like-crystal in nature. Particles collected at 22%RH (Figure 5) were observed to be clearly smaller in size compared to the particles collected at < 2%RH and at 57%RH (Figure 5). This result is in accord with the SMPS analysis shown in Figure 3.

As shown in Figure 6 the collected particles may be classified into 4 series. Also, the corresponding particle number size distributions obtained using the SMPS averaged over the TEM grid sampling period (three scan from t ≈ 7 hours) is shown in Figure 6 .

The Series 1 taken under dry conditions may be attributed to the accumulation mode 1. In a similar manner, the Series 2 observed at 22±1%RH may be attributed to the accumulation mode 2 and the Series 3 observed at 57 ± 3%RH may be attributed to the accumulation mode 3 (Figure 6). Since the SMPS measurements operate in the transition regime [START_REF] Decarlo | Particle morphology and density characterization by combined mobility and aerodynamic diameter measurements[END_REF], the elongated particles observed under dry condition are influenced by their shape and orientation within the Dimensional Mobility Analyzer (DMA) electric flow fields which in turn contribute to the broadening of the corresponding particle size distributions [START_REF] Zelenyuk | From agglomerates of spheres to irregularly shaped particles: Determination of dynamic shape factors from measurements of mobility and vacuum aerodynamic diameters[END_REF]. Likewise, depending on aerosols' orientation within the DMA electric field flow, a selection of the series 4 aerosols, may be divided between the accumulation mode 3 and the accumulation mode 4.

Based on the TEM analyses, the dry-formed Series 1 aerosols induce diffraction fringes and, therefore, appear to be agglomerates of fine crystals of compact and elongated nature. Unfortunately, we were not able to better study the fine crystals' diffraction fringes under higher magnification because the the particles' instabilities under the electron beam of the TEM. According to the SMPS measurements (Figure 2a), these aerosols have likely been formed by nucleation for d m ≤ 20 nm and then evolved to form larger agglomerates. Series 2 and 3 particles formed at 22%RH and partially at 57%RH have been observed

to have more compact structures than those shown in Series 1. These structures appear to be intermediate between those of Series 1 and those of the crystals in Series 4. Based on SMPS measurements (Figure 3c), the Series 4 particles were observed to form when d m > 70 nm.

Sequential samples taken on TEM grids showed that crystals in series 4 progressively evolved from a isomeric or tetragonal shape to orthorhombic structures. It is likely that the formation of these crystals is probably due to a sequential addition in a preferential direction of the molecules that make up the crystal lattice.

To better identify the elemental composition of the aerosols, the Energy Dispersive X-Ray (EDX) analysis was performed. The EDX analysis revealed the presence of oxygen and iodine, a result that is in agreement with the AMS aerosol measurements. However, the caculated O/I ratio is not observed to yield any usefeul information from one particle to another, resulting from the particles' instability under the electron beam.

Particle density

To obtain a mass concentration from SMPS measurements, the density ρ p of the measured particles must be defined. This information may be obtained from the ratio of the vacuum average aerodynamic diameter measured using the AMS (d va ) and the average mass-weighted electric mobility diameter measured using the SMPS (d m ), equation 1 [START_REF] Jimenez | New particle formation from photooxidation of diiodomethane (CH 2 I 2 )[END_REF][START_REF] Decarlo | Particle morphology and density characterization by combined mobility and aerodynamic diameter measurements[END_REF],

ρ p = ρ 0 d va d m (1)
where ρ 0 is the unit density in g/cm 3 (ρ 0 = 1 g/cm 3 ). The equation 1 is valid only if the particles are spherical or close to spherical. For non-spherical particles, this relation depends on several considerations related to the geometrical shape of these particles. In this case, the density measured with equation 1 is a so-called effective density ρ eff , relative to a spherical particle that would have the same electric mobility diameter as well as its aerodynamic equivalent in vacuum [START_REF] Jimenez | New particle formation from photooxidation of diiodomethane (CH 2 I 2 )[END_REF][START_REF] Decarlo | Particle morphology and density characterization by combined mobility and aerodynamic diameter measurements[END_REF].

The plot of the average aerodynamic mobility diameter obtained from the dominant fragment I + (m/z 126.90) measured using the AMS as a function of the average mass-weighted electric mobility diameter, d m , measured using the SMPS at three relative humidity values is shown in Figure 7. Based on microscope measurements, particle formed at < 2%RH and at 22%RH are observed to be mostly compact or near-spherical in nature (Figures 5). In these cases, it can be considered that in dry conditions ρ eff ≈ ρ p = 2.04 g•cm -3 and at 22%RH ρ eff ≈ ρ p = 2.82 g•cm -3 .

As shown in Figure 5, at 57% RH the formed particles are observed to be mostly of crystal structure in nature and the overall effective density ρ eff = 2.61 g•cm -3 .

The effective density values calculated in this work are observed to be lower than the known iodine oxides potentially formed in our system (Table 2) [START_REF] Lide | CRC handbook of chemistry and physics[END_REF][START_REF] Daehlie | Iodine oxides. part i. on I 2 O 3 • SO 3 , I 2 O 3 • 4 SO 3 • H 2 O, I 2 O 3 • SeO 3 and I 2 O 4[END_REF][START_REF] Fjellvég | The crystal structure of I 2 O 4 and its relations to other iodineoxygen-containing compounds[END_REF]. Otherwise, in their chamber work using the CH 2 I 2 /O 3 /hv reactive system, Jimenez et al. ( 2003) obtained particle densities of 0.86 g cm -3 and 1.22 g cm -3 under dry conditions and values ranging from 1.6 g cm -3 to 2.2 g cm -3 at 65%RH. In the absence of morphological data, these investigators hypothesized that the obtained low density values were a result of the formed particles' aspherical or fractal nature [START_REF] Jimenez | New particle formation from photooxidation of diiodomethane (CH 2 I 2 )[END_REF]. However, based on the morphological observations reported in our work, we argue that the In addition to the morphological considerations, other criteria include particle porosity and particle matrix structure that may contain other materials of lower density. However, the measured particle density was observed to be lower than the density of the materials present within the matrix structure. That is, the AMS analyses concluded that iodine oxides represent by far the majority in the chemical composition of the particles. As a result, we conclude that the particle matrix does not play a significant role in the obtained effective particle density. However, particle porosity may be important, in particular for dry-formed particles because the TEM analysis shows that they appear to be agglomerates of small crystals, favoring a porous structure, resulting in a lower effective density value.

Total mass concentration

The particle total mass concentration was calculated using the SMPS data that includes the particle density ρ p and the volume equivalent diameter, d ve [START_REF] Decarlo | Particle morphology and density characterization by combined mobility and aerodynamic diameter measurements[END_REF].

m p = π 6 ρ p d 3 ve (2)
Since the particles formed at < 2%RH and 22%RH were globally observed to be nearspherical in nature, the volume equivalent diameter ( d ve ) was approximately equivalent to the electric mobility diameter (d m ) measured using the SMPS. As a result, it was assumed that the temporal evolution of the total particle mass concentration depended solely on the temporal evolution of the particle number size distribution. At 57%RH, the determination of the actual total particle mass concentration evolution was more challenging due to the crystal shape that directly influence the density and the volume equivalent diameter calculations.

The non-spherical morphology of the collected particles help to explain poor reproducibility of the aerosol total mass measurements from one test to another under given experimental relative humidity conditions (Figure 8). At 57%RH, d ve is related to d m by the dynamic shape factor χ t,Θ which is a function of the flow regime (transition in this case) and the orientation of the particles within the electric flow field of the SMPS [START_REF] Zelenyuk | From agglomerates of spheres to irregularly shaped particles: Determination of dynamic shape factors from measurements of mobility and vacuum aerodynamic diameters[END_REF].

The direct measurement of total mass concentration using the SMPS based on the effective density, ρ eff = 2.61 g/cm 3 , is biased. The SMPS total aerosol mass measurement data shown in Figure 8 indicate that the greater the second accumulation mode (Figure 8a) relative to the crystal particle (series 4 crystals presented in Figure 6), the higher the total particle mass (Figure 8b). This phenomenon is known to induce, in some cases, a higher mass reading than the one observed under dry (< 2%RH) conditions (Figure 9a). On the other hand, the corresponding AMS analyses show that the total mass measurement data is always greater under dry conditions than under elevated relative humidity conditions (Figure 9b). Actually, the transition from a dry to humid experimental environment appears to induce a decrease in the particle total mass.

Furthermore, the increase in total mass observed between 22%RH and 57%RH is due to the formation of isomeric tetragonal and orthorombic crystals.

The masses measured using the SMPS at 57%RH appear to be overestimated due to the effect of particles' shape on the measured electric mobility diameter within the DMA of the SMPS. This effect is especially important for the more elongated crystal shapes as they travel across the DMA.

On the other hand, the total mass measurements using the AMS are likely to be underestimated. For example under dry conditions, a significant difference between the SMPS and AMS total mass concentration data is observed as shown in Figure 9. AMS data are given in terms of nitrate equivalent mass. A calibration specific to the iodine oxides formed therefore needs to be carried out. To date, it is difficult to establish with certainty the true mass of iodine oxide particles formed at 57% relative humidity. Therefore, we propose that relative humidity inhibits the formation of iodine oxides. This proposition is in-line with the results on the inhibition of iodine oxide formation by relative humidity reported by Saunders et al. (2010) [START_REF] Saunders | Studies of the formation and growth of aerosol from molecular iodine precursor[END_REF] and Galvez et al.

(2013) [START_REF] Gálvez | A theoretical study on the formation of iodine oxide aggregates and monohydrates[END_REF]. Further, our results show that at a given value of relative humidity, a well-defined geometric crystal shape formation is favored.

Particle hygroscopic properties

In their chamber work, Jiménez et al. 2003 [START_REF] Jimenez | New particle formation from photooxidation of diiodomethane (CH 2 I 2 )[END_REF] investigated the hygroscopic properties of iodine oxide aerosol formation using the CH 2 I 2 /hv/O 3 reactive system. A reduction in the diameter of particles previously formed in a dry atmosphere was observed when they are exposed to moisture. In the case of pre-formed particles at 65%RH, exposure to moisture did not show a significant influence [START_REF] Jimenez | New particle formation from photooxidation of diiodomethane (CH 2 I 2 )[END_REF]. The authors deduce the formation of I 2 O 4 , judged to be nonhygroscopic, and explain the shrinkage of the dry-formed particles by the fractal nature of the particles, which collapse by capillary forces induced by water vapor.

To evaluate the hygroscopic behavior of particles formed under given experimental conditions, we performed additional tests on the collected iodine oxide aerosols. Here, the TEM grid samples were collected under given three experimental relative humidity conditions. More, two successive samples were collected under each experimental condition. Then, the first grids were immediately placed in a sealed pocket under dry nitrogen. The others were exposed to a flow of Knowing the TEM grid collection efficiency [START_REF] R'mili | Particle sampling by TEM grid filtration[END_REF] and the total volume sampled (6 minutes at 300 cm 3 min), the particle size distribution may be obtained from TEM images using "Im-ageJ" image processing and analysis software. Figure 11 shows the particles size distribution obtained from images of the unexposed grid (figure 10a) and distribution form that of exposed to 83% RH (figure 10b) as well as the corresponding SMPS particle size distributions averaged over the sampling time. The diameter used to define the distributions from TEM images is an area equivalent diameter (d ae ) calculated from the air occupied by a particle. Distributions are obtained between dae = 46 nm and dae = 368 nm. The particles of smaller diameters obtained by SMPS were perceptible under TEM on the unexposed grids, however they are very unstable As presented in figure 11a, the particle size distribution obtained from the unexposed TEM grid is in agreement with that obtained using the SMPS. On the other hand, as shown in figure 11b, the particle size distribution obtained from the TEM grid exposed to 83%RH differs. By comparing the two distributions (Figure 11c), it appears that exposing the particles collected on TEM grids to 83%RH causes a decrease in the number of particles with a diameter between d ae = 68 nm and d ae = 154 nm and an increase in particles of higher d ae . As can be seen visually in figure 10, the particle morphology is observed to evolve as the TEM grid is exposed to humidity.

By exposure to 83 ± 1%RH, particles of initially elongated morphology probably collapse and tend towards a more compact form. It is also probable that rearrangements of particles at the surface of the TEM grid take place by deliquescence, coagulation and further efflorescence which is the origin of the new particle size distribution and morphology observed after exposure to 83% RH. Obviously, these observations require further study. However, our observations of collected particles on TEM grids reflect the hygroscopic nature of the iodine oxides particles formed under dry conditions and particles' non-hygroscopic nature formed under humid conditions.

Formation of gas-phase I 2

Elucidation of a reaction mechanism that leads to the formation of gas-phase molecular iodine is important since such mechanism will give crucial information on reactant precursors, namely IO 11a, the gas-phase SRI-ToF-MS analyses carried out during the L4 experiments at 57%RH revealed an important molecular iodine signal. The formed I 2 probably partitions between the gas and particle phases and is deposited on the simulation chamber walls. During L3* and L4* tests, the re-injection of O 3 (arrow 1 in Figure 12b) under UV-visible irradiation induced a release of I 2 that has been adsoprbed on chamber walls (arrow 1 in Figure 12). This process is accompanied by the formation and evolution of a small number of particles.

Consequently, particles formed during the L3* and L4* tests under given experimental conditions of relative humidity are likely the result of a photo-oxidation of desorbed or volatilized secondary I 2 that has been formed during the L3 and L4 tests and adsorbed by the walls via dry and humid deposition on the chamber walls.

Discussion

The results obtained in this chamber work using the CH 3 I/hv/O 3 /H 2 O gas-phase system show a formation of new particles. That is, when a known quantity of gas-phase CH 3 I and O 3 was allowed to enter the simulation chamber at a given relative humidity value, aerosols were observed to form as soon as the UV lamp was turned on, inducing a nucleation process. This nucleation process was assumed to be initiated by a direct photolysis of gas-phase CH 3 I and the resulting major fraction of the formed aerosols is identified to contain iodine oxides (H x O y I z ).

The identified H x O y I z aerosol fraction was observed to form in the aerosols' nucleation mode initiated by the UV radiation and evolve towards the accumulation mode, when the total aerosol mass and number were observed to increase.

The formation of iodine oxide aerosols was studied at three experimental conditions of relative humidity, namely, < 2%RH or "dry" conditions, (22 ± 1)%RH and (57 ± 3) %RH, and at atmospheric pressure and ambient temperature. Relative humidity was observed to play a major role in the way particles initially form and evolve in time. An increase in the experimental relative humidity was observed to influence the size and morphology of the formed aerosols and induce a decrease in the total aerosol mass. The aerosols formed in "dry" conditions and at 22%RH were observed to be compact in nature. On the other hand, the aerosols formation at the experimental relative humidity value of 57%RH was observed to favor additional crystal formation.

The aerosol chemical composition total mass time evolution was observed to remain relatively stable as soon as the AMS aerosol detection limit was reached (d va > 70 nm). That is, once formed, the aerosol composition remained homogeneous during growth.

The values of the average mass-weighted electric mobility diameter (d m ) and the free-molecular regime aerodynamic diameter (d va ) were used to calculate the effective particle density, a value that is equivalent to the density of spherical particles. This is the case for particles that were observed to form in this work in "dry" conditions (< 2%RH) and at 22%RH. The calculated particle densities were found to be lower than the particle densities of other known iodine oxides that may have potentially formed in our experimental system [START_REF] Lide | CRC handbook of chemistry and physics[END_REF][START_REF] Daehlie | Iodine oxides. part i. on I 2 O 3 • SO 3 , I 2 O 3 • 4 SO 3 • H 2 O, I 2 O 3 • SeO 3 and I 2 O 4[END_REF]. At this point we cannot explain this discrepancy. However, it is well known that the particle density is lower than the density of matter if the volume occupied by the particle is greater than the volume occupied by the matter. This is particularly the case for porous agglomerates and particles containing vacuum or gases [START_REF] Decarlo | Particle morphology and density characterization by combined mobility and aerodynamic diameter measurements[END_REF].

In this work, the off-line TEM analyses of the collected particles indicated a presence of diffraction fringes. Unfortunaly, we were not able to further explore the observed diffraction fringes beacause of the instabilities of the collected iodide oxide particles under the electron beam. Nonetheless, the observed diffraction fringes are indicative of a crystalline and/or semicrystalline character of the collected particles. That is, in dry conditions (< 2%RH), initial aerosol nucleation likely results in a production of fine crystals. Then, these fine crystals grow to reach an electric mobility diameter of d m ≈ 20 nm and finally agglomerate to reach the d m ≈ 173 nm. Under higher relative humidity experimental conditions, namely, 22%RH, the aerosols appear to be in an intermediate arrangement state between the near-spherical geometric shape and the crystals geometric shape. The collected crystals at 57%RH have been clearly observed to be of isomeric, tetragonal and orthorhombic geometric shape. These geometric shape suggests that the aerosols were formed by sequential addition in a preferential direction of the constituent compound. The iodic acid, HIO 3 , is the likely candidate for the particle's observed morphology. That is, it is known that HIO 3 crystallizes in an orthorhombic space group [START_REF] Rogers | The crystal structure of iodic acid[END_REF][START_REF] Wu | Crystal structure of a new polymorph of iodic acid, δ-HIO 3 , from powder diffraction[END_REF].

Other iodine oxides (I 2 O 4 and I 2 O 5 ) are unlikely candidates for the observed morphology since they are known to crystallize in a monoclinic space group that likely favorize needle shape single crystals [START_REF] Selte | Iodine oxides. part ii. on the systel H 2 O-I 2 O 5[END_REF][START_REF] Fjellvég | The crystal structure of I 2 O 4 and its relations to other iodineoxygen-containing compounds[END_REF].

The obtained result in this work is in accord with field studies that reported iodate ions (IO 3 -)

to constitute a major fraction of iodides found within coastal and marine aerosols [START_REF] Pechtl | Modelling iodide -iodate speciation in atmospheric aerosol: Contributions of inorganic and organic iodine chemistry[END_REF]. [START_REF] Pechtl | Modelling iodide -iodate speciation in atmospheric aerosol: Contributions of inorganic and organic iodine chemistry[END_REF] reported that the main source of IO 3 -ions in marine aerosols is gas-phase HIO 3 and HI [START_REF] Pechtl | Modelling iodide -iodate speciation in atmospheric aerosol: Contributions of inorganic and organic iodine chemistry[END_REF]. This observation is important since a more recent study by [START_REF] Sipilä | Molecular-scale evidence of aerosol particle formation via sequential addition of HIO 3[END_REF] suggested that aerosols form via sequential addition of HIO 3 , followed by intracluster restructuring to I 2 O 5 and recycling of water through the atmosphere or dehydration [START_REF] Sipilä | Molecular-scale evidence of aerosol particle formation via sequential addition of HIO 3[END_REF].

It is known that atmospheric gas-phase HIO 3 may be formed in the reaction of OH radicals with OIO radicals and this reaction may occur by several reactive pathways.

OIO • + • OH (+M) → HIO 3 (R6a) → IO • + HO 2 • (R6b) → HOI + O 2 (R6c)
However, only the reaction pathway R6a that leads to form HIO 3 is important since the energy barriers of reactions R6b and R6c are considered to be too high to be of atmospheric significance [START_REF] Glasow | Modeling halogen chemistry in the marine boundary layer 1. cloud-free MBL[END_REF][START_REF] Plane | An experimental and theoretical study of the reactions OIO + NO and OIO + OH[END_REF].

While the reaction R6a represents a possible gas-phase source of the observed aerosol HIO 3 inhibits the particle formation and growth, a result that is consistent with our observations. A more recent theoretical study by Martin et al. 2020 [START_REF] Martín | A gas-to-particle conversion mechanism helps to explain atmospheric particle formation through clustering of iodine oxides[END_REF] suggested that the formation of HIO 3 particles does not proceed via the slow gas-phase reaction of water with iodine oxides but rather the particle formation is initiated by progressive hydration clustering of iodine oxides IxOy.

These investigators proposed a mechanism that leads to the formation of HIO 3 by hydration of I 2 O 5 units present in OIO(I 2 O 5 ) n clusters according to the reaction R7. However, the results obtained in this work that are based on the SRI-ToF-MS analysis suggests a formation of gasphase I 2 in humid conditions whereas I 2 is not supposed to form during the experiments carried out in dry conditions. As a result, we argue in this work that HIO 3 is likely accompanied by a release of molecular iodine I 2 . Therefore, we suspect an additional role of I 2 O 4 units according to the reaction R8 or another iodine oxide cluster that may react with H 2 O to release I 2 .

Also, the observed hygroscopic behavior of the particles formed under dry and humid conditions is important. Only dry-formed particles show a noticeable growth and change in size and morphology under the effect of relative humidity. This finding is consistent with the hygroscopic behavior observations of Jimenez et al. (2003) [START_REF] Jimenez | New particle formation from photooxidation of diiodomethane (CH 2 I 2 )[END_REF]. However, these authors argue that I 2 O 4 is central to particle formation under dry and humid conditions. In our work, we argue that HIO 3 is a likely candidate involved in iodine oxide particle formation under humid conditions.

According to [START_REF] Selte | Iodine oxides. part ii. on the systel H 2 O-I 2 O 5[END_REF] [START_REF] Selte | Iodine oxides. part ii. on the systel H 2 O-I 2 O 5[END_REF], the formation of HIO 3 is irreversible and its stable form α-HIO 3 can be stored at room temperature and humidity. According to [START_REF] Murray | Glass formation and unusual hygroscopic growth of iodic acid solution droplets with relevance for iodine mediated particle formation in the marine boundary layer[END_REF], HIO 3 has a very low hygroscopicity and its deliquescence point is high at %RH = 82.5 [START_REF] Murray | Glass formation and unusual hygroscopic growth of iodic acid solution droplets with relevance for iodine mediated particle formation in the marine boundary layer[END_REF]. Thus, particles formed under a dry atmosphere (< 2%RH ) are thought to be composed of porous agglomerates of hygroscopic fine crystals which, on prolonged exposure at 83%RH, evolve to form more compact particles of HIO 3 . The same hydration proccess may occur in the aerosol phase. As proposed by Martin and co-workers (2020) [START_REF] Martín | A gas-to-particle conversion mechanism helps to explain atmospheric particle formation through clustering of iodine oxides[END_REF] the nucleation process is thought to be driven by I x O y cluster reactions. Under dry experimental conditions, the nuleating particles evolve progressively as agglomerates of small or fine crystals. Under humid atmosphere conditions, water will progressivly hydrate I x O y clusters and promote a well-defined HIO 3 geometric shape crystals of low hygroscopicity.

Further, in our chamber experiments we observe that humidity tends to inhibit particle formation and growth. This observation is consistent with the results of Saunders et al. (2010) [START_REF] Saunders | Studies of the formation and growth of aerosol from molecular iodine precursor[END_REF] and Galvez et al. (2013) [START_REF] Gálvez | A theoretical study on the formation of iodine oxide aggregates and monohydrates[END_REF] who argue that iodine oxides form relatively stable complexes with water and may block nucleation processes [START_REF] Saunders | Studies of the formation and growth of aerosol from molecular iodine precursor[END_REF][START_REF] Gálvez | A theoretical study on the formation of iodine oxide aggregates and monohydrates[END_REF]. Our observational results complement the results obtained by these investigators. That is, the lower total mass that was observed under humid conditions may also originate from the I 2 loss (reaction R8).

Conclusions

Iodine oxide aerosol formation and growth from the photo-oxidation of gas-phase CH 3 I was studied under controlled laboratory conditions of temperature, relative humidity, reactant gasphase concentration and UV radiation. The work aimed to characterize the impact of relative humidity in the physico-chemical properties of the formed particles and to identify the chemical processes that lead to aerosol initiation and growth. The CH 3 I degradation kinetics were observed to increase with increasing ozone concentration and did not appear to be significantly influenced by humidity. The chemical analyses of the formed aerosols showed that the particles were mainly composed of iodine oxides. The temporal evolution of the formed iodine oxide particle size distribution was observed to change depending on the experimental relative humidity within the chamber. An increase in the experimental relative humidity was observed to influence the size, morphology and hygroscopicity of the formed particles and induce a decrease in the total particle mass. Off-line transmission electron microscopy analysis showed that humidity favored the formation of isomeric, tetragonal and orthorhombic crystals whose chemical composition was likely iodic acid, HIO 3 , known to crystallize in an orthorhombic space group.

The formation of new iodine oxide particles is initiated by the gas-phase recombination or The experimental conditions of temperature and relative humidity may be somewhat different from the actual nuclear power plant accident scenario. In view of the results obtained in this work, it is probable that under given conditions, the iodine oxide particles formed from iodomethane are of the HIO 3 crystal type, stable, but likely to deliquesce relative humidity values above 83%RH. This will likely be accompanied by a secondary formation of gaseous molecular iodine, itself strongly involved in the formation of new iodine oxide particles. We gratefully acknowledge this support. TEM images were taken at the Interdisciplinary Center of Nanoscience of Marseille (CiNAM), CNRS, Aix-Marseille University, France.

Acknowledgements

(

  SRI-ToF-MS), High Resolution Time-of-Flight Aerosol Mass Spectrometry (HR-ToF-AMS) and Scanning Mobility Particle Sizer (SMPS) among others. The experimental details relevant to this work are given below.

  cylindrical Differential Mobility Analyzer (DMA, TSI 3081) and a Condensation Particle Counter (CPC, TSI 3776). The DMA sheath air flow was 15 liters per minute (LPM) and the aerosol flow was 1.5 LPM. The aerosol mobility time resolution within the DMA was 135 s over 5.94 < d m (nm) < 224.7, where d m (nm) is the aerosol electric mobility diameter in units of nanometers, nm (1 nm = 10 -9 m), and was obtained by changing the voltage exponentially from -30 VDC to -7000 VDC. Similar to the situation listed above for the ozone analyzer, to correct the flow dynamics requirements of the SMPS sampling flow, an additional and independent flow of filtered room air (2:1 ratio) was used in a T-configuration.

Figure 1 :

 1 Figure 1: Typical temporal evolution of the concentration of CH 3 I observed using SRI-ToF-MS, with the UVlamp off and on with and without O 3 . Experimental conditions: (a) Experiments D1, UV-lamp off, [CH 3 I]=102±5 ppb, [O 3 ]=1.5±0.1 ppm, (b) Experiments L1, UV-lamp on, [CH 3 I]=102±5 ppb, (c) Experiments L2, L3 and L4, UV-lamp on, [CH 3 I]=102±5 ppb, [O 3 ]=1.5±0.1 ppmV

Figure 2 :

 2 Figure 2: A typical temporal evolution of the total particle number size distribution and the corresponding average electric mobility diameter (d m , solid line) obtained using the SMPS. The d m was calculated from the mass distribution measurements. The pattern of the matrix illustrates the initiation of particles followed by the particle growth evolution from the nucleation mode to the accumulation mode. Experimental conditions: (a) L2: < 2%RH, (b) L3: RH = 22%RH, (c) L4: 57%RH.

Figure 3 :

 3 Figure 3: A typical temporal evolution of the total particle mass size distribution and the corresponding average electric mobility diameter (d m , solid line) calculated from the mass distribution. The data has been corrected using the density values presented in section 3.4. The shape of the matrix shows the evolution of the accumulation mode. Experimental conditions: (a) L2: < 2%RH, (b) L3: 22%RH, (c) L4: 57%RH.

Figure

  Figure 2c, at higher relative humidity value (L4: 57%RH), the nucleation contribution to aerosol number quickly evolved to become negligible and gave rise to the accumulation mode of the aerosol growth.

Figure 4 :

 4 Figure 4: Example of the temporal evolution of the total mass (nitrate mass equivalent) measured by the AMS as well as the proportional evolution ratio to the total mass of each family of compounds within the aerosol. Experimental conditions: (a) L2: < 2%RH, (b) L3: 22%RH, (c) L4: 57%RH.

Figure 5 :

 5 Figure 5: Selected TEM images of particles observed under three relative humidity conditions. Top images: TEM images of samples taken over a period of 6 minutes at t ≥ 3 hours reaction time. Bottom images: samples taken over a period of 6 minutes after 7 hours of reaction (from t ≈ 7h).

Figure 6 :

 6 Figure 6: Classification of the observed particles at three different relative humidity values and the SMPS particle number size distributions averaged over the TEM grid sampling period shown in Figure 5. Experimental conditions: (•) open circles, L2: <2%RH; (---) Dashed line, L3: 22%RH; ( ) Solid line, L4: 57%RH. The presented particle TEM images do not respect the original scale and are shown only for their morphological structure and shape.

Figure 7 :

 7 Figure 7: Plot of the average aerodynamic mobility diameter under vacuum obtained from the main fragment I + (AMS measurement at m/z 126.90) as a function of the average mass-weighted electric mobility diameter (SMPS measurement). Experimental conditions: (a) L2: < 2%RH, ρ eff ≈ 2.04 ± 0.07 g cm -3 , (b) L3: 22%RH, ρ eff ≈ 2.82 ± 0.02 g cm -3 , (c) L4: 57%RH, ρ eff ≈ 2.61 ± 0.03 g cm -3 , 1σ. Solid line is the linear correlation plotted from target

Figure 8 :

 8 Figure 8: SMPS measurements. Experimental conditions: L4, 57%RH. (a) the particle size distributions in number of particles measured at the maximum of the total particles mass obtained (t max ) for 3 tests and (b) The corresponding temporal evolution of the total particle mass concentration.

Figure 9 :

 9 Figure 9: Temporal evolution of the particle total mass concentration under given experimental conditions of realtive humidity (a) measured by the SMPS and (b) by the AMS. (•) open circles, L2: dry (<2%RH) ; (---) Dashed line, L3: 22%RH; ( ) Solid line, L4: 57%RH. Separate data curves for each given experimental coditions of relative humidity shown in the two figures are used to better illustrate variability in temporal evolution.

Figure 10 :

 10 Figure 10: Comparative TEM images of dry-formed and dry-conditioned particles (RH < 2%) with those exposed to 83±1%RH for more 20 minutes. The collected particles shown on the left have not been exposed to humidity while the particles shown on the right have been exposed to 83%RH. TEM sampling time: 6 minutes

Figure 11 :

 11 Figure 11: (a) Typical particles size distribution obtained from images of the unexposed grid. (b) Particles size distribution obtained from the TEM grid exposed to 83%RH. The figures (a) and (b) also show the corresponding SMPS particle size distributions averaged over the sampling time. (c) Comparison of the two particle size distributions obtained from TEM images.

Figure 12 : 2 +

 122 Figure 12: (a) Typical temporal evolution profile of the particle size distribution in number of particles formed according to the L4 tests and evolution of the emissions of molecular iodine I 2 observable by SRI-ToF-MS (%RH = 57 ± 3). The I 2 signal (solide line) is normalized to 1 × 10 6 cps with respect to the O 2 + chemical ionization reagent signal (SRI-ToF-MS). (b) Arrow 1 denotes the initiation of the L4* test where (O 3 /hv) is re-introduced into the chamber, and arrow 2 denotes an increase in the carrier gas renewal rate (N 2 /O 2 ) within the chamber for rapid evacuation.

  cross-reactions of OIO • or IO • radicals to form I 2 O 3 , I 2 O 4 or I 2 O 5 . These gas-phase recombination or cross-reactions lead to form larger iodine oxide OIO(I 2 O 5 ) n clusers. In this work it was observed that the particles formed under humid conditions are accompagned by the release of I 2 which may indicate a potential role of I 2 O 4 in the particle evolution processes. Undergiven experimental conditions, a dry environnement was observed to favor the formation of agglomerates of fines crystals. On the other hand, humid conditions were observed to promote the formation of stable non-hygroscopic crystals of HIO 3 and the release of I 2 .

  This research was supported by the Mitigation of Releases to the Environment (MiRE) project launched by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) and funded by the French National Research Agency (ANR) under the convention number 11-RSNR-0013.

[ 1 ]

 1 Figgans, H. Coe, M. Flynn, others, Impact of halogen monoxide chemistry upon boundary

  radiation, Gomez Martin et al. (2013) argue that I 2 O 4 is responsible for the nucleation by polymerization processes, with a minor contribution of I 2 O 5 in the early stages of aerosol formation[START_REF] Martń | On the mechanism of iodine oxide particle formation[END_REF]. A hypothesis that is further supported by theoretical studies of (I 2 O 4 ) 2 dimers carried out by Glavez et al. (2013)[START_REF] Gálvez | A theoretical study on the formation of iodine oxide aggregates and monohydrates[END_REF].

There appears to be a convergence of experimental and theoretical results that indicate that the formation of new iodine oxide aerosols results from gas-phase reactions of •OIO or •OI radicals to form I 2 O 3 , I 2 O 4 or I 2 O 5 . It has been presumed that the most stable molecule, I 2 O 4

Table 1 :

 1 Summary of the experiments performed under "D" dark conditions and "L" in the presence of UV-radiation or "light". T( • C) = 25 ± 0.4. Listed uncertenties are experimental estimates.

	Experiments †	[CH 3 I] ppbV [O 3 ] ppmV	%RH	Lamp Max. Number cm -3
	D0	0	1.5±0.1	dry, 22±1, 57±3	Off	No particle formation
	D1	102±5	1.5±0.1	dry, 22±1, 57±3	Off	No particle formation
	L0	0	1.5±0.1	dry, 22±1, 57±3	On	No particle formation
	L1	102±5	0	22±1, 57±3	On	Not investigated
	L2	102±5	1.5±0.1	dry	On	1.2 × 10 6
	L2*	not re-injected ‡	1.5±0.1	dry	On	No particle formation
	L3	102±5	1.5±0.1	22±1	On	2.2 × 10 6
	L3*	not re-injected ‡	1.5±0.1	22±1	On	1.7 × 10 4
	L4	102±5	1.5±0.1	57±3	On	1.8 × 10 6
	L4*	not injected ‡	1.5±0.1	57±3	On	3.6 × 10 4
	† The L2*, L3* and L4* experiments were performed in series after the experiments L2, L3

  ). The signal of iodides is dominated by the ion fragment I + (m/z 126.90) followed by I 2 + (m/z 253.80). In addition to I + and I 2 + , the fragments attributed to the family of iodine compounds are the following, in order of importance, IO + (m/z 142.89), IH +

	(m/z 127.91), IO 2	+ (m/z 158.89), HIO + (m/z 143.90), I 2 O + (m/z 269.80), I 2 O 3	+ (m/z 301.79),
	HIO 3	+ (m/z 175.89), HIO 2	+ (m/z 159.90) and I 2 O 2	+ (m/z 285.79).

Table 2 :

 2 Physical properties of HIO 3 , I 2 O 4 and I 2 O 5 . Based on our result, the hypothesis of Jimenez et al (2003) only concerns the density values of the elongated particles and crystals but cannot explain the overall observed low density values.

	Name	Formula Molar mass	Color Crystal structure	Density
			(g mol -1 )		(g cm -3 )
	Iodic acid	HIO 3	175.910	incolor orthorhombic	4.63	[48]
				[51, 52]
	Diiodine tetroxide	I 2 O 4	317.807	yellow monoclinic [53]	4.2	[48]
					4.97	[49]
					2.57	[50]
	Diiodine pentoxide	I 2 O 5	333.806	white monoclinic [50]	4.98	[48]
	formed particles are globally spherical in nature at < 2%RH, 22%RH and partially at 57%RH.

  • ,OIO • ,I 2 O 4 and I 2 O 5 , that are involved in aerosol formation. As shown in Table1, L0 and Li* (i= 2,3,4) tests were performed without the CH 3 I reactant under the same experimental conditions of ozone mixing ratio, relative humidity, and the presence of photolyzing radiation. The difference is that the Li* tests were carried out in series after the Li tests with-

	out prior chamber cleaning procedure and following the re-injection of O 3 . The L0 and L2*
	experiments did not result in any particle formation.
	On the other hand, particle formation was observed in the L3* and L4* experiments that
	were carried out at relative humidity values of 22%RH and 57%RH, respectively (Table 1). The
	observed particle formation in the L3* and L4* experiments is best explained by the secondary
	formation of gas-phase I 2 which takes place in a humid environment. Indeed, during the L3 and
	L4 tests, the observed formation and evolution of particles at 22%RH and 57%RH, respectively,
	was observed to be accompanied by a release of molecular iodine, I 2 . As shown in Figure

  fraction, this reaction is of importance only under higher OH radical concentrations. On the other hand, the hydrolysis of I 2 O 4 and I 2 O 5 is also known to form HIO 3 . The reaction of I 2 O 5 with H 2 O results in the formation of HIO 3 (reaction R7)[START_REF] Selte | Iodine oxides. part ii. on the systel H 2 O-I 2 O 5[END_REF][START_REF] Khanniche | Computational study of the I 2 O 5 + H 2 O= 2HOIO 2 gas-phase reaction[END_REF] and the hydrolysis of I 2 O 4 may

	forms molecular iodine, I 2 , and HIO 3 (reaction R8) [49], .
	3I 2 O 5 + 3H 2 O → 6HIO 3	(R7)
	5I 2 O 4 + 4H 2 O → I 2 + 8HIO 3	(R8)
	In their work on photochemically generated nanoparticles from gas-phase I 2 in the presence
	of O 3 , Saunders and Plane (2006) [25] argue that I 2 O 5 aerosols are formed following a series
	of exothermic reactions of I 2 O 2 , I 2 O 3 and I 2 O 4 with ozone. However, based on theoretical
	calculations carried out by Kumar et al. (2018) and Khanniche et al. (2016), the I 2 O 5 hydration
	reaction (R7) is unlikely to occur in the gas phase [31, 57].
	Experimental and theoretical study carried out by Saunders et al. (2010) [26] and theoretical
	study carried out by Galvez et al. (2013) [28] support a hypothesis that I 2 O 4 is a key compound
	involved in the nucleation and formation process of iodine oxides through polymerization of
	I 2 O 4 with itself or with I 2 Oy (y= 3 to 5). It has also been shown that the presence of H 2 O