D Urupina
email: darya.urupina@imt-lille-douai.fr

J Lasne

M N Romanias

V Thiery

P Dagsson-Waldhauserova

F Thevenet

Uptake and surface chemistry of SO2 on natural volcanic dusts

Keywords: v-dust, heterogeneous reactions, sulfur dioxide, sulfate formation, uptake coefficient, DRIFTS

V-dust (v-dust) is a highly variable source of natural particles in the atmosphere, and during the period of high volcanic activity it can provide a large surface for heterogeneous interactions with other atmospheric compounds. Icelandic v-dust was chosen as a case study due to frequency of volcanic eruptions and high aeolian activity in the area. In this study, we focus on the kinetics and mechanism of the reaction of sulfur dioxide (SO2) with natural v-dust samples under atmospheric conditions using coated wall flow tube reactor and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Steady state uptake coefficients determined are in the range of 10 -9 to 10 -8 depending on the considered v-dust. Concomitantly with SO2 uptake, both sulfites and sulfates are monitored on the surface of v-dust, with sulfates being the final oxidation product, attesting of SO2 surface reaction. Surface hydroxyl groups play a crucial role in the conversion of SO2 to sulfites as evidenced from both flow tube and DRIFTS experiments. Based on these experimental results, a mechanism for SO2 interaction with different surface sites of vdust is proposed and discussed. This study provides original insights in the kinetics of SO2 uptake under simulated atmospheric conditions and its mechanism and transformation on volcanic material. To that regards, it brings an accurate perspective on SO2 heterogeneous sinks in the atmosphere.

INTRODUCTION

Volcanic eruptions comprise one of the most significant natural hazards directly threatening people living in the proximity, and, in the extreme case, can affect livelihoods of the humankind on the global scale. Large volcanic eruptions of the past have significantly influenced Earth's climate by injecting vast amounts of volcanic gases and aerosols into the atmosphere [START_REF] Stevenson | Atmospheric impact of the 1783-1784 Laki eruption: Part I Chemistry modelling[END_REF]. Volcanic emissions can cause both regional and global changes to climate by affecting monsoon circulation, causing either excessive or limited rainfall thus leading to floods and droughts [START_REF] Stevenson | Atmospheric impact of the 1783-1784 Laki eruption: Part I Chemistry modelling[END_REF], [START_REF] Highwood | Atmospheric impact of the 1783-1784 Laki Eruption: Part II Climatic effect of sulphate aerosol[END_REF], ("Tambora and the 'Year Without a Summer ' of 1816," 2016). Volcanic particles have an ability to affect the climate by scattering solar radiation and thus changing the Earth radiation budget [START_REF] Langmann | Volcanic Ash versus Mineral Dust: Atmospheric Processing and Environmental and Climate Impacts[END_REF].

Besides, volcanic ash, having optical and thermal properties similar to those of the black carbon, can absorb solar radiation [START_REF] Arnalds | The Icelandic volcanic aeolian environment: Processes and impacts -A review[END_REF]. Vernier et al. highlighted the significance of the radiative impact of ash, in particular in the tropical latitudes due to the Brewer-Dobson circulation that sustains ash in the stratosphere for longer than was generally assumed (Vernier et al., n.d.).

Finally, volcanic ash particles can affect the climate through influencing cloud formation processes and acting as ice nucleation sites during plume rise [START_REF] Durant | Ice nucleation and overseeding of ice in volcanic clouds[END_REF]. The impact of v-dust on Earth's atmosphere is governed by the physical and chemical surface properties of the particles. However, the physico-chemical processes that govern the modifications of the particle surface in the plume and in the cloud when ash is in contact with volcanic gases remain poorly investigated.

Volcanic eruptions are a highly variable sources of solid particles ranging from 33 million tons (Mt) on an average year to over 100,000 Mt of ash after a major volcanic eruption, as was the case for the eruption of Mount Tambora in 1815 [START_REF] Andreae | Climate Effects of Changing Atmospheric Aerosol[END_REF]. With respect to atmospheric chemistry the most significant impact would be expected to come from the particles that are in the 0.002 to 10 µm range as they can be carried over thousands of kilometers before eventually being deposited onto land or into the ocean by gravitational settling and wet deposition [START_REF] Finlayson-Pitts | Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, and Applications[END_REF], [START_REF] Langmann | Atmospheric distribution and removal of volcanic ash after the eruption of Kasatochi volcano: A regional model study[END_REF], [START_REF] Dagsson-Waldhauserova | Long-term variability of dust events in Iceland (1949-2011)[END_REF].

When settled on land, v-dust (v-dust) can then be once again remobilized by the wind and entrained into the atmosphere. Some places on Earth are particularly prone to high aeolian activity. Such is, for example, the case for Iceland, that experiences on average about 135 dust days per year, partly due to frequent volcanic eruptions and re-suspension of volcanic materials [START_REF] Dagsson-Waldhauserova | Long-term variability of dust events in Iceland (1949-2011)[END_REF]. In fact, this volcanic island of North Atlantic with an area of 103,000 km 2 lying south of the Arctic Circle is one of the dustiest areas of the world as well as the largest desert in Europe and the Arctic [START_REF] Dagsson-Waldhauserova | Long-term variability of dust events in Iceland (1949-2011)[END_REF]. It is also one of the most volcanically active areas. There are about 30 active volcanic systems and volcanic eruptions occurring every 3-5 years on average [START_REF] Thordarson | Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history[END_REF], [START_REF] Schmidt | Assessing hazards to aviation from sulfur dioxide emitted by explosive Icelandic eruptions[END_REF]. Frequent dust events in Iceland transport dust over long distances, often exceeding 2,500 km, towards High Arctic (>80° N) and Europe [START_REF] Ovadnevaite | An experimental study of SO2 reactions with silicate glasses and supercooled melts in the system anorthite-diopside-albite at high temperature[END_REF], [START_REF] Groot Zwaaftink | Temporal and spatial variability of Icelandic dust emissions and atmospheric transport[END_REF], [START_REF] Moroni | Mineralogical and Chemical Records of Icelandic Dust Sources Upon Ny-Ålesund (Svalbard Islands)[END_REF], [START_REF] Dordevic | Can Volcanic Dust Suspended From Surface Soil and Deserts of Iceland Be Transferred to Central Balkan Similarly to African Dust (Sahara)?[END_REF]. Located only 1000 km from mainland Europe, it makes a particularly interesting case study due to its proximity to densely populated European countries.

In the stratosphere, emissions of volcanic ash were linked to ozone reduction recorded after major volcanic eruptions, such as El Chichón eruption in Mexico in 1982 and Mount Pinatubo eruption in Philippines in 1991 [START_REF] Brasseur | Future changes in stratospheric ozone and the role of heterogeneous chemistry[END_REF], ("Reactions on Mineral Dust -Chemical

Reviews (ACS Publications)," n.d.). Even if the effect of volcanic ash in the chemistry of the troposphere has not yet been evaluated, it is suggested that ash particles act as a long-range transporting carrier for various species adsorbed on their surface and as a solid support for their reactions with atmospheric trace gases [START_REF] Li | Kinetics and mechanism of heterogeneous oxidation of sulfur dioxide by ozone on surface of calcium carbonate[END_REF], [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF]. Some of these species are formed during eruption, when a variety of gases are released along with volcanic ash.

Sulfur dioxide (SO2) is typically the third most emitted volcanic gas after water and carbon dioxide [START_REF] Durant | Atmospheric and Environmental Impacts of Volcanic Particulates[END_REF]. SO2 is known to contribute to the formation of sulfuric acid aerosol, which, when injected in the stratosphere, stays there for up to 2 years effectively cooling the troposphere below [START_REF] Stevenson | Atmospheric impact of the 1783-1784 Laki eruption: Part I Chemistry modelling[END_REF], [START_REF] Highwood | Atmospheric impact of the 1783-1784 Laki Eruption: Part II Climatic effect of sulphate aerosol[END_REF].

Volcanic ash and SO2 are likely to participate in a variety of heterogeneous reactions that could potentially influence the balance of other atmospheric species. SO2 has already been shown

to exhibit heterogeneous reactivity towards many mineral oxides such as MgO, Al2O3, Fe2O3 and TiO2 [START_REF] Usher | A laboratory study of the heterogeneous uptake and oxidation of sulfur dioxide on mineral dust particles[END_REF]. Along SO2 uptake on these materials, sulfite and sulfate formation was detected on the surface of most mineral oxides and synthetic mineral dust and it was proposed that hydroxyl groups and surface-active oxygen are responsible for oxidation of sulfites to sulfates [START_REF] Zhang | Heterogeneous Reactions of Sulfur Dioxide on Typical Mineral Particles[END_REF]. Nevertheless, limited data exist on the mechanism of the interaction of SO2 gas with natural multi-component samples, such as volcanic ash as well as on the stability of the surface species formed during reaction.

It was recently evidenced by Maters et al. that volcanic ash is reactive to the uptake of weakly acidic SO2 gas with a reactivity that is proportional to the abundance of strongly basic sites, such as those affiliated with alkaline and alkaline earth metals [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF].

Although very informative of the processes occurring on the short scale, the initial uptake and the total uptake capacity measured by the authors have limited ability to explain the interactions on the longer time scale. Besides, the above-mentioned experiments were performed in a Knudsen cell under very low pressure and dry conditions that are not compliant with the atmospheric processes [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF]. It was therefore decided to undertake a comprehensive study of the interactions of SO2 with v-dust under atmospheric conditions. Iceland was chosen as a location of interest for sample collection required to study heterogeneous reactivity of volcanic ash and SO2 gas. The scope of this study is threefold. First, different natural volcanic and Icelandic samples were collected and characterized using various techniques, such as SEM (Scanning Electron Microscopy) and ICP-MS (Inductively Coupled Plasma-Mass Spectrometry). Second, the uptake of atmospheric SO2 by the surface of selected v-dusts was investigated using a flow tube reactor. Finally, Diffuse Reflectance Fourier Transformed Spectroscopy (DRIFTS) employed for the in-situ monitoring of adsorbed phase was used to identify the surface species formed and to clarify the surface reaction mechanisms. The in-situ spectroscopic approach has been complemented by HPLC (High-Performance Liquid Chromatography) technique employed for quantification of the surface products. Maelifellssandur (63°48'48.7"N 19°07'42.5"W) and Dyngjusandur (64°50'41,885"N 16°59'40,78"W) v-dusts are described by [START_REF] Arnalds | The Icelandic volcanic aeolian environment: Processes and impacts -A review[END_REF]. Overall, these areas are subjected to extremely large aeolian erosion due to frequent dust storms, and have extensive areas (10 to 140 km 2 ), hence providing a large supply (31-40 million tons annually) of v-dust to the atmosphere. Samples were collected from the top surface layer of each dust hot spot. The samples were dry and did not come in contact with soil or other organics. The fifth sample of Eyjafjallajökull (63°33'50.4"N 19°27'28.8"W) volcanic ash was collected and stored dry during the second phase of the explosive-effusive volcanic eruption on April 27. It is a typical volcanic ash sample as characterized by [START_REF] Gislason | Characterization of Eyjafjallajökull volcanic ash particles and a protocol for rapid risk assessment[END_REF].

EXPERIMENTAL SECTION

Characterization of the samples

Scanning Electron Microscopy / Morphology of the samples was analyzed by SEM carried out on a Hitachi S-4300SE/N SEM in high vacuum mode. The images of different v-dust particles are presented in Figure 1. These are small particles with rough edges. Dust from Eyjafjallajökull (Figure 1e) has larger particle size with many particles reaching 500 µm in diameter, while the other four dusts (Figure 1a-d) have a much finer structure and range from 10 to 50 µm in diameter due to glaciofluvial processes leading to their generation. Specific surface area measurements / To avoid any overestimation of the uptake coefficients by the use of the geometric surface of the sample bed, it was decided to use the specific surface area (SSA) in the uptake calculations as discussed in the latest IUPAC evaluation [START_REF] Crowley | Evaluated kinetic and photochemical data for atmospheric chemistry: Volume V -heterogeneous reactions on solid substrates[END_REF]. In brief, the determination of the uptake coefficient using the geometric surface -i.e. the geometric area of the tube covered with the volcanic dust as noted in equation 2 (see section 2.2.1) of the manuscript-overestimates the uptake since it does not consider all the surface of the dust as accessible to gaseous SO2. However, in a series of experiments it was evidenced that the entire surface area of the sample is accessible to the gas environment and thus the total surface area of the dust deposited in the tube should be used to determine the real uptake coefficient. The SSA is a physical property of solids, which represents the total surface area of a material per mass unit (m 2 g -1 ), and is used for the determination of the kinetic and sorption parameters (see section 2.2.1). To determine SSA, the Brunauer-Emmett-Teller (BET) method was employed. Nitrogen (N2) adsorption measurements were performed with a laboratory gas sorption analysis system [START_REF] Joshi | Investigating water adsorption onto natural mineral dust particles: Linking DRIFTS experiments and BET theory[END_REF], [START_REF] Ibrahim | Water Interaction with Mineral Dust Aerosol: Particle Size and Hygroscopic Properties of Dust[END_REF] within 0.05 to 0.3 relative pressure range (P/P0) of N2.

To determine the range of uncertainty, three adsorption measurements were conducted for each sample. The results of the BET specific surface area (SSABET) of the 5 samples are displayed in Table 1.

Chemical composition /

The bulk elemental composition of the samples of Icelandic v-dust used in this study was determined by ICP-MS using a Perkin Elmer NeXion 300x spectrometer, the results are presented in Table 2. Prior to the analysis, from 2 to 7 mg of each sample were treated in a mixture of acids (HF/HNO3/H2O2) in a microwave oven (Milestone Ultrawave) at 500 K and 35 bar for 15 min [START_REF] Alleman | PM10 metal concentrations and source identification using positive matrix factorization and wind sectoring in a French industrial zone[END_REF]. Six measurements per sample were performed to evaluate their chemical heterogeneity. Repeated measurements were carried out on acid blanks, quality control standard solutions and standard reference material (NIST SRM 1648a and SRM 2584) to evaluate detection limits, accuracy and to validate the whole procedure. As can be seen from Table 2 silicon is the most abundant element of the v-dust, followed by iron, calcium, and aluminum. Within the five selected v-dust samples the elemental composition seems to differ only slightly, except for Eyjafjallajökull v-dust which exhibits a higher amount of silicon than other samples and a lower amount of calcium, iron and magnesium. The results of ICP-MS analysis for Eyjafjallajökull v-dust agrees well with the literature [START_REF] Gislason | Characterization of Eyjafjallajökull volcanic ash particles and a protocol for rapid risk assessment[END_REF]. The difference in the composition of Eyjafjallajökull v-dust in comparison with other samples is most probably due to the difference in the composition of magma that produced it. Indeed, the volcanic systems that supply Dyngjusandur dust hotspot (Bárðarbunga and Kverkfjöll volcanic systems)

and Mýrdalssandur and Maelifellssandur dust hotspots (Katla volcanic system) have magmas of a predominantly basaltic composition, while magma of Eyjafjallajökull is andesitic, meaning that it is higher in silicon ("Icelandic Volcanoes," n.d.), [START_REF] Vogel | Reference data set of volcanic ash physicochemical and optical properties[END_REF]. The decrease of calcium, iron and magnesium with increasing silicon content is in accordance with the expected variation of the composition of magma [START_REF] Vogel | Reference data set of volcanic ash physicochemical and optical properties[END_REF]. Other processes that can influence the composition of magma during a particular eruption include crystallization [START_REF] Clague | Role of olivine cumulates in destabilizing the flanks of Hawaiian volcanoes[END_REF], melting of the crustal rocks [START_REF] Deegan | Processes of Magma-crust Interaction Insights from Geochemistry and Experimental Petrology[END_REF] and mixing of original magma with another magma that evolved separately ("Icelandic Volcanoes," n.d.). Since composition of magma can change within the same eruption, one might expect to see different compositions of v-dust both on the time scale and on the location scale. Transported dust can also have a different composition further away from the source. Globally, when compared to other types of mineral dusts, such as Saharan dusts or Gobi dust [START_REF] Joshi | Investigating water adsorption onto natural mineral dust particles: Linking DRIFTS experiments and BET theory[END_REF], the amount of silicon in all five v-dust samples is considerably lower while the amount of iron, magnesium, and titanium is higher [START_REF] Romanias | Heterogeneous Interaction of Isopropanol with Natural Gobi Dust[END_REF], [START_REF] Langmann | Volcanic Ash versus Mineral Dust: Atmospheric Processing and Environmental and Climate Impacts[END_REF]. Throughout the manuscript, gas concentrations are given in ppmv and ppbv (parts per million and parts per billion by volume, respectively). Under usual experimental conditions (T = 296 K, P = 1 atm), the conversion to a concentration is given by 1 ppbv (SO2) ≈ 2.5 × 10 10 molecules cm -3 . Note that this conversion factor takes a different value when the temperature or total pressure changes.

Experimental set ups 2.2.1 Coated wall flow tube reactor

The heterogeneous interaction of SO2 with v-dust was investigated in a horizontal double wall flow tube reactor represented schematically in Figure 2. The experiments were conducted under relevant atmospheric conditions, i.e. SO2 concentration in the ppb range, atmospheric pressure, room temperature, under dry conditions and 30% RH. The objective of this series of experiments was to determine the uptake coefficients of SO2 on the various Icelandic v-dusts.

The setup was described in detail in a previous paper [START_REF] Lasne | Ozone Uptake by Clay Dusts under Environmental Conditions[END_REF] it mainly consists of three parts; (i) the gas mixing line, (ii) the reactor and (iii) the analytical device. The gas preparation line is used for the mixing of zero dry/humid air with SO2 resulting in a flow with the desired proportion of RH and SO2 concentration. The reactor is made of a double wall Pyrex glass and is thermostated by circulating water in between the double wall surrounding the flow tube. Inside and along the axis of the reactor, a Pyrex tube covered on its inner wall with the vdust is introduced. To coat the inner surface of a Pyrex tube with v-dust (orange color in Figure 2), a defined mass of dust is first inserted in a tube. A small amount of water is added to form a slurry and the tube is shaken to deposit an even coating on the Pyrex surface. Then, the tube is heated slightly above 380 K for 10 minutes to evaporate excess water, placed in the reactor and flushed overnight with dry air. Potential modifications of the volcanic dust upon its dissolution in water in the process of slurry preparation and subsequent drying process include displacement of soluble salts that might be found on the surface of v-dust particles as well as leaching of alkali and alkaline earth metals from the aluminosilicate network at ash surfaces [START_REF] Witham | Volcanic ash-leachates: a review and recommendations for sampling methods[END_REF].

In the case of Hagavatn, Mýrdalssandur, Maelifellssandur and Dyngjusandur natural v-dust samples, that were likely previously exposed to water the above mentioned phenomena does not seem to effect the results because the soluble salts would be expected to leach out by the time of the experiment. In the case of fresh Eyjafjallajökull dust leaching of the soluble salts formed during ash-gas interaction in the plume presents a valid concern. In order to minimize removal of soluble salts the amount of water added to prepare a slurry was as small as necessary to spread the dust evenly, i.e. in the case of Eyjafjallajökull 1:3 ash (g) to water (mL) ratio was used, which is much smaller than the 1:25 ash-to-water ratio recommended for preparation of ash leachates [START_REF] Witham | Volcanic ash-leachates: a review and recommendations for sampling methods[END_REF]. In addition, contact time with water was minimized to around 10 seconds shaking followed by rapidly drying the slurry, which is also much faster than 90 min agitation procedure recommended by [START_REF] Witham | Volcanic ash-leachates: a review and recommendations for sampling methods[END_REF]. One could also expect that drying the slurry would lead to redisposition of the displaced salts, even though some of it could be redeposited on the surface of the glass tube and not the v-particles. Two Viton O-rings are placed around the Pyrex tube to fix its position inside of the reactor. The gas mixture is flowed through a movable injector (internal diameter of 0.3 cm) with a flow rate ranging between 250 and 500 sccm, ensuring laminar flow conditions with a Reynolds number, Re < 50. The role of the moveable injector is to either isolate (placed at the downstream end of the flow tube) or to expose the dust surface to the gas environment (upstream end of the reactor). The outgoing flow is then directed to the SO2 analyzer (Model 43C, Thermo Environmental Instruments Inc.) for the real time gas phase monitoring of SO2, with a time resolution of 10 seconds. We didn't noticed any loss of dust during the uptake experiments. Note that the tubes with the deposited dust are weighted before and after the experiments and the variations noticed were in the range of the mass scale uncertainty (<1%). In addition, considering that the flow rate during the experiments is relative slow, the experiments are carried out under atmospheric pressure and no pressure variation takes place between the reactor and the analyzer, it is highly improbable to experience any loss of dust during our measurements. Furthermore, we have not observed (at least visually)

any large particles to fail to adhere to the Pyrex tube. However, even if this was the case, in the horizontal flow tube, these particles would still be subjected to the flow of gas and thus remain accessible to the gas. To conclude, the total mass of the dust inside the reactor is maintained during the entire experiment and is accessible to the gas environment and hence the total surface area does not change and no correction is required to determine the uptake coefficients. The uptake of SO2 by v-dust is studied under dark conditions at T = 296 K, [SO2] ≈ 75 ppb (1.88 × 10 12 molecules cm -3 ) in air, and a relative humidity of either 30 % or under dry conditions (i.e.

RH < 0.1 %). Regarding RH, studying the uptake under dry conditions provides an evaluation of 12 the interaction of the probe gas with the direct surface of volcanic material, while increasing RH to 30% contributes to the understanding of the impact of the water coverage on the mineral aerosol [START_REF] Joshi | Investigating water adsorption onto natural mineral dust particles: Linking DRIFTS experiments and BET theory[END_REF]. Theoretical SO2 concentration profile is represented in Figure 3. During a typical flow tube experiment SO2 is flowed through the reactor, the dust being left unexposed initially. After a stable SO2 concentration, [SO2]0, is set, the injector is pulled out and the dust is exposed to SO2;

the change in SO2 concentration related to its uptake by dust is recorded by the SO2 analyzer.

After a steady-state is reached, the injector is pushed back in to control the [SO2]0. What is obtained as a result of the flow tube experiment is a time evolution of the trace gas concentration at the tube exit, the so-called breakthrough curve, that is used to calculate the steady state uptake coefficient [START_REF] Huthwelker | The Uptake of Acidic Gases on Ice[END_REF]. In the case of SO2 a long-lasting tailing of the breakthrough curve is observed, similar to the one described for the uptake of nitric acid (HNO3) on ice [START_REF] Huthwelker | The Uptake of Acidic Gases on Ice[END_REF]. This phenomena leads to the following question: at which point can we say that a steady state is reached? In the absence of the generally accepted understanding of the nature of tailing and for the practical reasons the following pragmatic approach was adopted.

Steady state is considered to be reached when the variation of the signal falls within 1.5% of its value for at least three hours. On average it takes about 12 hours to reach the steady state under 30% humidity, which is much longer than 1 hour needed to achieve a steady-state for the uptake of ozone by clay dust [START_REF] Lasne | Ozone Uptake by Clay Dusts under Environmental Conditions[END_REF]. Preliminary experiments were conducted to determine a possible contribution of the Pyrex surfaces to the uptake of SO2. They showed negligible contribution to the parameters determined in the current study.

The measurements of [SO2] at steady-state and at the initial level, [SO2]0, together with the knowledge of the parameters defining our setup are necessary to conduct the analysis leading to the determination of the uptake coefficients under equilibrium conditions, [START_REF] Lasne | Ozone Uptake by Clay Dusts under Environmental Conditions[END_REF]. Assuming first order kinetics for the uptake of SO2 by v-dust surfaces, the observed constant of reaction, kobs (in s -1 ), is determined by (1): v ln 1

where v is the flow in the reactor (in cm s -1 ) and L is the length of the dust coating (in cm). The value of kobs (in s -1 ) is corrected for diffusion of SO2 with a constant kdiff (in s -1 ) to give the diffusion-corrected constant kkin (in s -1 ). In this work, a diffusion coefficient of SO2 in the air derived from [START_REF] Massman | A review of the molecular diffusivities of H2O, CO2[END_REF] is used, D(296 K) = 95.28 Torr cm 2 s -1 . This value is in excellent agreement with D = 94 ± 13 Torr cm 2 s -1 suggested by Tang et al [START_REF] Tang | Compilation and evaluation of gas phase diffusion coefficients of reactive trace gases in the atmosphere: volume 1. Inorganic compounds[END_REF]. The diffusion-corrected constant, kkin, is then used to determine γss, (2): 4 2

where V and Sgeom are the volume (in cm 3 ) and geometric surface (in cm 2 ) of the region where the reaction takes place, respectively, and c is the average molecular speed (in cm s -1 ). The diffusion-correction accounts for ca. 5 % of the measured uptake coefficients.

A series of experiments were carried out as a function of Mýrdalssandur v-dust mass deposited that showed a linear increase in the 0 -250 mg range. The linear increase of the uptake with mass and the lack of saturation reflect the fact that the entire surface of the dust is accessible to SO2 molecules from the gas and thus the specific surface area SSABET is used for the calculation of the uptake coefficient, γ ss,BET , (3):

,"#$ % 3 where As, is the effective surface area of the dust, obtained from multiplying SSABET (m 2 g -1 ) by the mass of the sample (g).

The error on the uptake coefficients is the root-mean-square deviation of the values measured. It was calculated with the precision of the signal (0.5% of the measured concentration) and its propagation to kobs measurement, the SSA determination (~ 25%) and all relevant uncertainties, i.e. on the gas flow measurement, temperature, mass weighting, and length of the exposed dust coating (~8%). The total error calculated for the γ values was estimated to be ca 35% in all experiments and a safe limit of 40% is given. Although the quoted uncertainty is significant, it reflects the real error accounted for the measurement of slow uptake processes (uptakes on the order of 10 -9 ) in the flow tube.

Besides uptake coefficients, the transient initial number of SO2 molecules taken up per surface area of v-dusts Ns (molecules cm -2 ) is determined at 30% of RH; by integrating the area of the initial uptake process in a typical uptake experiment (Figure 3) divided by the effective surface area of the dust, As, according to (4):

' ( ) * % +,* +, -. 4
where Ft is the flow rate (molecules min -1 ) of SO2 molecules through the reactor. The total error in Ns determination is estimated to be ca. 30% and includes all systematic uncertainties and the error in SSA determination.

It should be noted that since the SO2 concentration recorded at the steady-state was lower than its pre-exposure concentration, for the determination of Ns solely the transient initial uptake removal of SO2 is considered (dashed area in Figure 3). The long tailing of the breakthrough curve observed points to additional physico-chemical processes besides the absorption/desorption. Therefore for the determination of integration parameter a similar to twothird criterion approach was adopted as recommended by Huthwelker et al., where "the surface uptake is considered finished once the breakthrough curve rises to two-thirds of its initial value" [START_REF] Huthwelker | The Uptake of Acidic Gases on Ice[END_REF]. The integration limits for the determination of the number of molecules Ns were chosen from the time of the exposure of SO2 to the dust to 300 min after the exposure to make sure that the change in the breakthrough curve towards steady state has occurred for all the dusts. This criterion was adopted in order to be able to compare the uptake capacity of the vdusts at the initial stage of the interaction with SO2. On longer scale, samples can be compared based on the γss,ΒΕΤ values.

DRIFTS experiments

The heterogeneous interaction between SO2 gas and v-dusts was studied in-situ inside of the optical DRIFTS cell operated at atmospheric pressure and room temperature under dry and 30% RH conditions. The objective of this series of experiments was to monitor the adsorbed species formed on the surface of v-dusts, aiming to elucidate the mechanism of the interaction under both dry and humid conditions.

The DRIFT experimental setup consists of three parts: (i) the gas supply line, (ii) the optical reactor, and (iii) the analytical device as described previously by Romanias et al. [START_REF] Romanías | Investigating the Heterogeneous Interaction of VOCs with Natural Atmospheric Particles: Adsorption of Limonene and Toluene on Saharan Mineral Dusts[END_REF]. A representation of the system is given in At the beginning of the experiment the crucible sample holder inside the DRIFTS cell is filled with about 80-110 mg of v-dust. The cell is then tightly closed and the infrared beam is focused on the surface of the dust. The gas flowed through the DRIFTS cell is made up of different proportions of dry air, humid air and SO2 gas. Prior to the introduction of the SO2 gas volcanic samples are heated to 423 K for 1.5 hours to remove any pre-adsorbed species. Samples are then allowed to cool down to room temperature and are purged overnight with either dry or humid air depending on the experimental conditions in order to equilibrate the system. A background spectrum is recorded right before introduction of SO2 gas. DRIFT spectra of the v-dust in the presence of SO2 gas are recorded from 650 to 4000 cm -1 using Omnic software with 100 scans per spectrum, a spectral resolution of 4 cm -1 , and a time resolution ranging from 3 minutes to 1 hour depending on the stage of the experiment. The formation and loss of surface species are observed as positive and negative absorption bands respectively. Thus, a typical experiment lasts about 4 days and consists of thermal pre-treatment (1.5 hr), system equilibration (16 hr), SO2 adsorption phase (72 hr) and desorption upon flushing phase (6 hr). The SO2 concentration in the mixed gas is 175 ppm. Note that this is over three orders of magnitude higher than the atmospherically relevant SO2 concentration of ~75 ppb used in the flow-tube reactor experiments.

Large difference in SO2 concentrations used for flow tube versus DRIFTS experiments comes from different technical constrains. For instance, DRIFT spectroscopy is used for the in situ characterization of surface adsorbed species but is characterized by a relatively low sensitivity, depending on the gas-surface interactions. Likewise, the observation of the steady state in flow tubes, when the flowing gas is in ppm level, is not possible. In this work two different reactors serve two different purposes: flow tube is used to study kinetics by providing uptake coefficients under atmospherically relevant conditions while DRIFTS is used to study surface reaction mechanism. It is important to keep in mind that while the kinetics is changed by the concentration because the surface coverage is changed by the concentration, the reaction mechanism is not likely to be changed as a function of gas phase concentration. Thus, both techniques bring complimentary and valuable information.

Ion analysis

A reverse-phase HPLC method with indirect photometric detection for the simultaneous determination of sulfates and sulfites was used for quantification of sulfates and sulfites formed following experiments with DRIFTS. To prevent oxidation of sulfites to sulfates, 1.0 mL of 1% formalin in pure water is used as an extracting solvent as recommended by Michigami [START_REF] Michigami | Sulphite stabilizer in ion chromatography[END_REF] acetonitrile solution to produce a charged surface as recommended by Zuo et al. [START_REF] Zuo | Simultaneous determination of sulfite, sulfate, and hydroxymethanesulfonate in atmospheric waters by ion-pair HPLC technique[END_REF] The HPLC instrument is operated isocratically at ambient temperature using Methanol-Potassium Hydrogen Phthalate Buffer 1.0 mM, adjusted to pH 6.5 with dilute potassium hydroxide (1:99, v/v) mobile phase and run at a flow rate of 1 mL/min for 15 min. The injection volume is 10 µL. Detector is set at 255 nm. All chemicals and solvents used for HPLC analysis are of analytical grade.

RESULTS AND DISCUSSION

3.1 Exploring SO2 uptake on v-dust from the gas phase: flow tube study 3.1.1 First insight on SO2 uptake on various natural v-dusts under ambient conditions

The uptake of atmospheric relevant concentration of SO2 (i.e. ≈ 75 ppb) by the surface of five selected v-dusts under typical atmospheric conditions of ambient temperature, pressure and 30%

RH was investigated using the flow tube reactor [START_REF] Lasne | Ozone Uptake by Clay Dusts under Environmental Conditions[END_REF]. A typical uptake profile depicted in Figure 5 shows the [SO2] uptake of Mýrdalssandur dust in humid (RH = 30%) conditions. It is clearly observed from the profile that after the initial uptake of the gas upon exposure of the v-dust the system reaches a steady-state [SO2] that is distinct from the initial state

[SO2]0, evidencing continuous consumption of the title molecule at least for the duration of the experiment. All the samples under investigation demonstrated both an initial uptake and a steady state uptake (similar to Figure 5). The latter is worth investigating as it suggests the ability of the v-dusts to exhibit a long-term effect on the equilibrium composition of the atmosphere. The steady state coefficients are in the order of 10 -9 to 10 -8 (Figure 6, Table 3). These values are much lower than the values obtained by Maters et al., who reported the initial uptakes of SO2 on volcanic ash and glass powders at 10 -3 to 10 -2 range [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF]. Large difference in values is not surprising though for a number of reasons. First, the initial uptake describes an uptake on the fresh surface at the first instances of its interaction with gas, while the steady state uptake is an ongoing phenomenon reflecting the ability of the surface to adsorb gas continuously. Second, while determining initial uptakes geometric surface area was used by Maters et al. in order to calculate uptake coefficient [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF], while specific surface area was used in this study in order to account for all the surface accessible to the SO2 molecules on longer time scales.

Using geometric vs specific surface area gives an upper value for the uptake coefficient [START_REF] Crowley | Evaluated kinetic and photochemical data for atmospheric chemistry: Volume V -heterogeneous reactions on solid substrates[END_REF]. Finally, in the study of Maters et al. experiments were performed under dry conditions contrary to 30% RH used in this study. Moreover, in the adsorption/desorption experiments on the volcanic glasses performed by Schmauss and Keppler it was observed that the first layer of SO2 molecules was adsorbed on the surface of volcanic glass irreversibly and could not be removed [START_REF] Schmauss | Adsorption of sulfur dioxide on volcanic ashes[END_REF].

Interestingly, the obtained results show somewhat contrasted behaviors from one sample to another. In the literature the differences in the uptake of SO2 on solid material are commonly linked to the differences in elemental composition or minerology [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF], [START_REF] Maters | Controls on the surface chemical reactivity of volcanic ash investigated with probe gases[END_REF], [START_REF] Usher | A laboratory study of the heterogeneous uptake and oxidation of sulfur dioxide on mineral dust particles[END_REF], [START_REF] Zhang | Heterogeneous Reactions of Sulfur Dioxide on Typical Mineral Particles[END_REF], [START_REF] Harris | Sulfur isotope fractionation during heterogeneous oxidation of SO<sub>2</sub> on mineral dust[END_REF]. Using synthetic and natural silicate glasses as proxies for fresh unweathered volcanic ash is commonly done due to the fact that glass often represents a major component of v-dust. Besides, a thin layer of glass covering the crystalline components of ash surfaces was observed [START_REF] Delmelle | High Temperature Reactions Between Gases and Ash Particles in Volcanic Eruption Plumes[END_REF]. Several studies indicated that sulfates are more likely to get adsorbed on the glass fraction of volcanic dust [START_REF] Schmauss | Adsorption of sulfur dioxide on volcanic ashes[END_REF], [START_REF] Farges | Sulfur K-edge XANES study of S sorbed onto volcanic ashes[END_REF]. While investigating trends in adsorption of SO2 on glass, elemental composition is commonly investigated [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF]. This approach though ignores the crystalline phase of the volcanic dust. Maters et al. emphasized that the influence of crystallinity on the adsorption of SO2 gas is not yet understood [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF]. On the other hand author working with natural mineral dust samples, such as desert dusts often approximate composition of dusts using simple mineral oxides and look for trends in adsorption in relationship with the mineral phase composition. In our study v-dusts differ significantly in their crystalline fraction. Volcanic samples used in this study contain from 20%

(Hagavatn) to 80-90% (Mýrdalssandur, Maelifellssandur and Dyngjusandur) of amorphous material [START_REF] Baratoux | Volcanic Sand in Iceland: Diverse origins of aeolian sand deposits revealed at Dyngjusandur and Lambahraun, Iceland[END_REF], [START_REF] Moroni | Mineralogical and Chemical Records of Icelandic Dust Sources Upon Ny-Ålesund (Svalbard Islands)[END_REF]. The remaining part is crystalline.

Eyjafjallajokull is also dominated by glass [START_REF] Gislason | Characterization of Eyjafjallajökull volcanic ash particles and a protocol for rapid risk assessment[END_REF]. Plagioclase, pyroxene, and olivine are the mineral phases reported in all five samples, while magnetite was only found in Myrdalssandur and Maelifellsandur [START_REF] Moroni | Mineralogical and Chemical Records of Icelandic Dust Sources Upon Ny-Ålesund (Svalbard Islands)[END_REF], [START_REF] Baratoux | Volcanic Sand in Iceland: Diverse origins of aeolian sand deposits revealed at Dyngjusandur and Lambahraun, Iceland[END_REF]) [START_REF] Gislason | Characterization of Eyjafjallajökull volcanic ash particles and a protocol for rapid risk assessment[END_REF]. A very small amount of crystalline silica was found in Eyjafjallajokull and none was reported for other samples [START_REF] Gislason | Characterization of Eyjafjallajökull volcanic ash particles and a protocol for rapid risk assessment[END_REF].

Since amorphous fraction dominates composition of v-dusts, an attempt to find trends in the uptake vs elemental composition was undertaken. Among the authors linking the differences in uptake coefficient to the elemental composition Maters et al. observed increase of the initial uptake of SO2 on the surface of synthetic volcanic glasses with decrease of their silica (Si) content and increase of the sum of their sodium (Na), potassium (K), magnesium (Mg) and calcium (Ca) content [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF]. As for the natural volcanic samples the relationship was not as straightforward, even though a dependence of the initial uptake on the total surface content of the sum of Na, K, Mg and Ca was suggested [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF]. In our studies no clear correlation between the bulk elemental composition of a v-dust and its uptake of SO2 was established. It can be due to the fact that the elemental concentration in the bulk sample may not represent the availability of the elements on the surface. Therefore, one might expect the information derived from elemental surface composition to be more accurate, yet care must be taken in interpreting such results. One should keep in mind that a given element when incorporated in glass versus crystalline mineral might behave differently. Farges et al. observed that crystalline quartz does not adsorb any sulfur species while amorphous silica does [START_REF] Farges | Sulfur K-edge XANES study of S sorbed onto volcanic ashes[END_REF]. The heterogeneous nature of the sample might further complicate finding the trends as elements forming different mineral phases will behave differently depending on the nature of mineral phase. For example, Fe is incorporated in both ilmenite (FeTiO3) and fayalite (Fe2SiO4), yet the SO2 uptake is significantly higher for the former mineral [START_REF] Harris | Sulfur isotope fractionation during heterogeneous oxidation of SO<sub>2</sub> on mineral dust[END_REF].

Earlier studies can be used to understand potential contribution of crystalline fraction of the volcanic ash to the uptake of SO2. Both quartz (SiO2) and magnetite (Fe3O4) can be found in volcanic ash. Comparison of the uptakes of SO2 on individual mineral oxides, such as CaO, Al2O3, CaCO3, Fe3O4 and Fe2O3 showed that the uptake of SO2 depends on the nature of the mineral oxide with higher uptakes observed when SO2 interacted with iron-containing compounds [START_REF] Usher | A laboratory study of the heterogeneous uptake and oxidation of sulfur dioxide on mineral dust particles[END_REF]. These observations were further tested by Zhang et al. who compared reactivity of different oxides taking into consideration their specific area and placed them in the following order: Fe2O3>MgO>TiO2>FeOOH>mixture>Al2O3>SiO2 [START_REF] Zhang | Heterogeneous Reactions of Sulfur Dioxide on Typical Mineral Particles[END_REF]. Both studies indicate that conversion of SO2 per unit surface of Fe2O3 is the highest, suggesting that particles with the highest amount of iron might be the most reactive in reactions with SO2. Furthermore, the reactivity of the mixture, that was obtained by mixing the different oxides based on their abundance in the continental crust, was measured to be twice its theoretical value, which demonstrates its synergistic effect [START_REF] Zhang | Heterogeneous Reactions of Sulfur Dioxide on Typical Mineral Particles[END_REF]. Even though most studies use simple mineral oxides and synthetic dust as a substitution for natural samples, such approach is problematic, as it undermines the importance of more complex mineralogy in the uptake of SO2.

While studying the uptake of SO2 on Saharan dust Harris et al. distinguished ilmenite (FeTiO3), rutile (TiO2) and iron oxides (Fe2O3, Fe3O4, FeOOH) (γ BET(mixture of ilmenite and rutile)=3×10 -5 ) as major phases of dust responsible for uptake and oxidation of SO2 [START_REF] Harris | Sulfur isotope fractionation during heterogeneous oxidation of SO<sub>2</sub> on mineral dust[END_REF]. Uptake on feldspar minerals such as KAlSi3O8, NaAlSi3O8 or CaAl2Si2O8 and quartz (SiO2) was found to be slow (γ BET(feldspar)=9×10 -7 , γ (quartz with basic components)=4×10 -8 ) [START_REF] Harris | Sulfur isotope fractionation during heterogeneous oxidation of SO<sub>2</sub> on mineral dust[END_REF]. To identify the elements associated with adsorption of SO2 by crystalline material Harris et al. analyzed individual Saharan dust grains that were relatively rich in S content after their exposure to SO2 and studied their elemental profile using single-particle SEM-EDX analysis [START_REF] Harris | Sulfur isotope fractionation during heterogeneous oxidation of SO<sub>2</sub> on mineral dust[END_REF].

Ti, Fe and Ca were identified as the most important reactive elements, while Na, Mg, Al, Si showed no relationship to oxidizing capacity of dust [START_REF] Harris | Sulfur isotope fractionation during heterogeneous oxidation of SO<sub>2</sub> on mineral dust[END_REF]. While studying the uptake of SO2 on v-dusts it would be informative to investigate the uptake of SO2 on other complex mineral phases present in v-dust such as pyroxene, plagioclase, amphibole, biotite, and olivine even though higher uptakes of SO2 are linked to the presence of minerals lacking silicates, such as ilmenite (FeTiO3) and magnetite (Fe3O4) [START_REF] Harris | Sulfur isotope fractionation during heterogeneous oxidation of SO<sub>2</sub> on mineral dust[END_REF]. The number of SO2 (Ns) molecules removed during the transient initial uptake process per surface area was also investigated and was calculated to be in the order of 10 13 molecules cm -2 (Table 3). These values are comparable to the values obtained by Maters et al., who reported the total uptake capacity of SO2 on volcanic ash and glass powders under dry conditions at 10 11 to 10 13 range [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF]. As with initial uptakes, total uptake capacity of SO2 on the surface of synthetic volcanic glasses increased with decrease of their Si content and increase of their total Na, K, Mg, Ca content [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF]. As for the natural volcanic ash samples the relationship was less clear and the dependence on total surface Na, K, Mg, Ca content was suggested [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF]. Furthermore, as displayed in Figure 7, the transient number of molecules adsorbed Ns at the initial stage of the exposure, and the steady state uptake coefficients do not follow the same trend. On one hand, Mýrdalssandur seems to adsorb the highest amount of molecules and it has the highest uptake coefficient. On the other hand, Hagavatn that has the smallest uptake coefficient adsorbs almost the same amount of molecules as Eyjafjallajökull and more than Maelifellssandur even though the latter two have an uptake coefficient that is twice as high as uptake coefficient of Hagavatn. The discrepancies between Ns and γ ss,BET suggest that the surface is modified upon its initial exposure to SO2. Indeed, one should keep in mind that adsorption behavior of SO2 on each volcanic dust was characterized by measuring the steady state uptake coefficients after 12 hours of exposure, i.e. after long processing, while the number of molecules taken up was measured after the first 5 hours of the interaction where a transient uptake was observed at the initial stage of the experiment. Differences in the observed trends could indicate that at the initial stage of the experiment the surface of the dusts is modified due to SO2 uptake and the corresponding formation of sulfites and sulfates, and these modifications affect the surface reactivity at longer time scale when the γ ss,BET values are determined. The modifications could explain why no correlation was noticed between the γ ss,BET and Ns with the elemental composition of the v-dusts. Finally, the discrepancies could be due to the fact that the nature of the governing processes behind the initial fast uptake and thus the high amount of molecules adsorbed in the initial stage and those dominating the steady state uptake are fundamentally different [START_REF] Huthwelker | The Uptake of Acidic Gases on Ice[END_REF].

The role of relative humidity on the uptake of SO2 on v-dusts

Several studies have highlighted the role of RH on SO2 uptake [START_REF] Li | Kinetics and mechanism of heterogeneous oxidation of sulfur dioxide by ozone on surface of calcium carbonate[END_REF], [START_REF] Zhou | Knudsen cell and smog chamber study of the heterogeneous uptake of sulfur dioxide on Chinese mineral dust[END_REF], [START_REF] Huang | Kinetics of Heterogeneous Reaction of Sulfur Dioxide on Authentic Mineral Dust: Effects of Relative Humidity and Hydrogen Peroxide[END_REF]. In order to investigate the role that RH plays in the steady state uptake of SO2 on v-dust samples the relationship was investigated for two contrasted samples based on their steady-state uptakes -Mýrdalssandur and Hagavatn (Figure 8). For Mýrdalssandur we observed that the steady state uptake coefficient of SO2 at 30% of RH was significantly higher than under dry conditions (i.e. γss, BET (30% RH) = 6.2 × 10 -8 and γss, BET (dry) = 2.0 × 10 -8 ). For the Hagavatn v-dust a similar trend was observed (i.e. γss, BET (30% RH) = 6.6 × 10 -9 and γss, BET (dry) = 1.0 × 10 -9 ). The enhanced uptake trends observed in the presence of water indicate that water plays an important role promoting the partitioning of SO2 to the adsorbed phase. There might be numerous reasons for the observed increased uptake of SO2 in the presence of water vapor. Adsorbed water molecules can (i) participate in the reactions as reactants, (ii) serve as a medium for the reaction to take place or (iii) change the physical properties of the particle shifting the equilibrium towards higher SO2 uptake. Water molecules can also form water layers that can help the product of the reaction to diffuse along the surface and thus promote the renewal, i.e. the turnover, of surface sites [START_REF] Shang | Heterogeneous reaction of SO2 on TiO2 particles[END_REF]. The intrinsic ability of metal oxides to adsorb water molecules and form surface hydroxyls was linked to higher reactivity in the heterogeneous reaction with SO2 [START_REF] Zhang | Heterogeneous Reactions of Sulfur Dioxide on Typical Mineral Particles[END_REF].

In the literature the analysis of the influence of RH on the uptake of SO2 on natural mineral dusts shows complex trends. An influence of water vapor on the interaction of SO2 with Saharah dust at 258 K showed no effect on initial uptake within statistical error (i.e. γ BET (27% RH) = (6.0 ± 1.0) × 10 -5 and γ BET (dry) = (6.4 ± 0.7) × 10 -5 ) . Likewise, no dependency of the initial uptake coefficient on humidity was found for Adobe clay soil taken from Los Angeles area [START_REF] Judeikis | Laboratory measurement of SO2 deposition velocities on selected building materials and soils[END_REF]. Alternatively, when Huang et al. subjected three authentic dusts to SO2 at RH ranging from 0 to 90% an increase in uptake coefficient was observed for Tengger Desert dust (i.e. γ BET (dry) ≈ 3.8 × 10 -5 and γ BET (90% RH) ≈ 5.5 × 10 -5 ) and for Arizona test dust (i.e. γ BET (dry) ≈ 1.3 × 10 -5 and γ BET (90% RH) ≈ 2.76 × 10 -5 ), but a decrease was observed for Asian mineral dust (i.e. γ BET (dry) ≈ 3.2 × 10 -5 and γ BET (90% RH) ≈ 1.9 × 10 -5 ) [START_REF] Huang | Kinetics of Heterogeneous Reaction of Sulfur Dioxide on Authentic Mineral Dust: Effects of Relative Humidity and Hydrogen Peroxide[END_REF].

The negative dependence of humidity on SO2 uptake in the case of Asian mineral dust was explained by the presence of water-soluble inorganic coating, which made surface more acidic and inhibited uptake of SO2 [START_REF] Huang | Kinetics of Heterogeneous Reaction of Sulfur Dioxide on Authentic Mineral Dust: Effects of Relative Humidity and Hydrogen Peroxide[END_REF]. When the Asian mineral dust was washed and coating was removed, the uptake trend got reversed and showed positive correlation (i.e. γ BET (dry) ≈ 0.85 × 10 -5 and γ BET (90% RH) ≈ 1.32 × 10 -5 ) [START_REF] Huang | Kinetics of Heterogeneous Reaction of Sulfur Dioxide on Authentic Mineral Dust: Effects of Relative Humidity and Hydrogen Peroxide[END_REF]. Then again, while the samples from Inner Mongolian desert, characterized by high carbonate contents, demonstrated considerably increased uptakes at higher humidity (i.e. γss, BET (40% RH) = 1.0 × 10 -6 and γss, BET (dry) = 1.7 × 10 -7 ), Xinjiang sierozem natural mineral dust showed only a modest increase in steady-state uptake (i.e. γss, BET (40% RH) ≈ 2.4 × 10 -7 and γss, BET (dry) ≈ 2.2 × 10 -7 ) [START_REF] Adams | The uptake of SO2 on Saharan dust: a flow tube study[END_REF]. The authors suggest that difference in the minerology could be responsible for the difference in the observed trends [START_REF] Zhou | Knudsen cell and smog chamber study of the heterogeneous uptake of sulfur dioxide on Chinese mineral dust[END_REF]. Higher carbonate component of the Inner

Mongolian desert dust can promote the SO2 uptake. Indeed, while investigating carbonate particles Zhang et al. observed that the value of the steady state uptake γss, BET drastically increases from γss, BET (1%) = 0.32× 10 -8 to γss, BET (85% RH) = 13.9 × 10 -8 ), at 85% RH reaching 43 times its value at 1% [START_REF] Zhang | The influence of relative humidity on the heterogeneous oxidation of sulfur dioxide by ozone on calcium carbonate particles[END_REF]. In our study volcanic dusts, void of carbonates, cannot be directly compared to carbonate-rich samples, but the observed trend follows the one of Tengger Desert dust and Arizona test dust discussed earlier. More information about influence of humidity on the steady state uptake of SO2 on the surface of volcanic glass and ash is necessary to obtain a more comprehensive picture.

While the increase in the SO2 uptake with the increase of relative humidity is observed for a number of samples, it is not at all the case for the uptakes of other species, such as nonpolar volatile organic compounds [START_REF] Romanias | Heterogeneous Interaction of Isopropanol with Natural Gobi Dust[END_REF], hydrogen peroxide (H2O2) [START_REF] Romanias | Heterogeneous Interaction of H2O2 with TiO2 Surface under Dark and UV Light Irradiation Conditions[END_REF] or ozone (O3) [START_REF] Lasne | Ozone Uptake by Clay Dusts under Environmental Conditions[END_REF]. In fact, the uptake coefficient can decrease with increased humidity, such as the case for the steady state uptake of O3 on montmorillonite clay dust reflecting the tendency for the molecules in question and molecules of water to compete with each other for the sorptive sites on the clay surface [START_REF] Lasne | Ozone Uptake by Clay Dusts under Environmental Conditions[END_REF].

The observation of the steady state nature of the uptakes points out that there are important and maybe cyclic processes occurring on the surface of the v-dust. The gas phase monitoring limits us to the determination of kinetic parameters; in order to investigate the mechanism of the SO2 uptake surface monitoring is necessary. Increased SO2 uptake at higher RH suggests that humidity plays a particularly important role in the adsorption of SO2 to the surface. To monitor the adsorbed phase, determine the functional groups that are involved in the adsorption of SO2 and to further evaluate the role of water to the reaction system, a series of experiments were carried out employing DRIFT spectroscopy. 3.2 Exploring SO2 uptake on v-dust from the adsorbed phase: DRIFTS study While flow tube reactor monitors changes in the gas-phase concentration, DRIFTS focuses on changes on the solid phase. It is commonly used to study fine particles and powders, and can be used to investigate adsorption of molecules on solid surfaces. In this case spectra are collected as difference spectra with the unexposed solid as background. As the reaction proceeds, growing or decreasing peaks can be attributed to the species formed or consumed on the surface.

Infrared spectroscopy based on light diffuse reflectance offers information about the vibrational modes associated with stretching, bending, and rocking motions of the adsorbed molecules.

However, in order to monitor these vibrations the sample has to have high enough reflectance to insure an adequate signal in response to changes in the adsorbed phase, and the surface concentration needs to be high enough due to the low sensitivity of the technique. If the sample is highly absorbent, which is the case of v-dust samples, it is usually diluted in a non-absorbent matrix, such as KBr. In our case, however, this type of matrix could not be used because of the interactions of KBr with SO2 and thus the use of pure v-dust was required. The background obtained for each v-dust under dry conditions showed that all of them absorb light strongly and, as a result, a weak IR signal was recorded in response to changes in the adsorbed phase. To study the adsorption of SO2 on v-dust, backgrounds of five samples were taken to choose the least absorbent varieties. In comparison with other v-dusts the background for Hagavatn and Eyjafjallajökull shows higher reflectance in the area from 800 to 1350 cm -1 where sulfite and sulfate species usually absorb. Because of its higher reflectance and higher specific surface area, Hagavatn v-dust was selected as the most appropriate sample for DRIFTS experiments. Besides, results obtained from the flow tube studies showed similar trends in SO2 adsorption by the vdusts based on gas-phase monitoring of SO2, i.e. rapid consumption of SO2 gas at the initial stage of surface exposure followed by a steady state removal of the probe molecule. Therefore, it appeared justified to choose one dust as a representative sample to study the mechanism of product formation on the surface of aerosols. Aiming to further investigate the observations from the flow tube study and to further assess the role of humidity on the reaction mechanism, the DRIFTS experiments were performed under dry conditions and 30% RH using Hagavatn as a sample material.

Typical DRIFT spectra of SO2 interaction on v-dust

Figure 9 and Figure 10 show DRIFT spectra of Hagavatn v-dust at increasing SO2 exposure time intervals at room temperature under 0% and 30% RH respectively. Table 4 introduces the assignment of peaks of different adsorbed sulfur-containing species as well as water and hydroxyl groups on the surface of v-dust investigated and different metal oxides found in the literature.

In Particular, the negative and positive absorption peaks in the area from 4000 to 3500 cm - 1 correspond to the free OH groups that are naturally present on the surface of v-dust [START_REF] Hair | Hydroxyl groups on silica surface[END_REF].

These groups are attached to the surface atoms, such as Si for example, and can be either completely isolated or hydrogen-bonded to each other [START_REF] Hair | Hydroxyl groups on silica surface[END_REF]. Adsorption of molecules containing a lone-pair of electrons is often associated with isolated surface OH groups, while neighboring hydrogen-bonded surface OH groups are involved in the adsorption of water [START_REF] Hair | Hydroxyl groups on silica surface[END_REF], [START_REF] Nanayakkara | Sulfur dioxide adsorption and photooxidation on isotopically-labeled titanium dioxide nanoparticle surfaces: roles of surface hydroxyl groups and adsorbed water in the formation and stability of adsorbed sulfite and sulfate[END_REF]. The exact assignment of each individual peak is not possible due to the complex response reflecting slightly different vibrational energies of OH groups bonded to different minerals composing the v-dust. A peak from 3500 to 2600 cm -1 and another peak from 1800 to 1400 cm -1 centered at 1620 cm -1 (not displayed) are attributed to vibration modes in molecular water [START_REF] Hair | Hydroxyl groups on silica surface[END_REF].

A weak broad peak from 2600 to 2400 cm -1 , visible under humid conditions, could be assigned to the S-H stretch vibration of the HSO3 -bisulfite species that is only found in the literature as a theoretical value calculated by Zhang et al. for aqueous solution and reported to be in the range of 2620 to 2450 cm -1 [START_REF] Zhang | Infrared spectroscopy of SO2 aqueous solutions[END_REF]. This peak is not visible under dry condition. Sulfur-containing products appear in the area from 1385 to 700 cm -1 under humid conditions and from 1367 to 716 cm -1 under dry conditions. Spectra under humid conditions are better resolved and several peaks can be distinguished: 3040, 2500, 1364, 1220, 1135, 930 and 872 cm -1 . Peaks under dry conditions are much noisier, nevertheless the following peaks are observed: 3242, 1340, 1230 and 1150 cm -1 . A double peak centered at 1364 cm -1 under humid conditions and at 1340 cm -1 under dry conditions appears rapidly upon exposure of the surface to SO2 gas and disappears right away from the spectra after ceasing the SO2 flow. It was therefore assigned to the weakly, reversibly physisorbed SO2 species. The peak corresponds well to the literature values for SO2 adsorption under dry conditions on the surfaces of aluminum, magnesium or titanium oxides identified in the previous studies at 1330 cm -1 and its gas-phase value of 1362 cm -1 [START_REF] Nanayakkara | Sulfur dioxide adsorption and photooxidation on isotopically-labeled titanium dioxide nanoparticle surfaces: roles of surface hydroxyl groups and adsorbed water in the formation and stability of adsorbed sulfite and sulfate[END_REF], [START_REF] Goodman | Heterogeneous Uptake of Sulfur Dioxide On Aluminum and Magnesium Oxide Particles[END_REF]. Another band appears rapidly upon exposure of SO2 to the surface between 1053 and 700 cm -1 under humid conditions.

A corresponding band between 975 and 716 cm -1 under dry conditions takes more time to develop and is hard to interpret since its intensity is almost at the limit of detection of the DRIFTS instrument. The assignment of this band is based on the previous assignments of SO2 adsorption on the surface of different metal oxides found in the literature and corresponds to the stretching motion of sulfite and bisulfite species that are bonded to the surface in a monodentate or bidentate mode. Different coordination environments as well as heterogeneous nature of v-dust contribute to the broadening of the band [START_REF] Usher | A laboratory study of the heterogeneous uptake and oxidation of sulfur dioxide on mineral dust particles[END_REF]. In particular, Goodman et al [START_REF] Goodman | Heterogeneous Uptake of Sulfur Dioxide On Aluminum and Magnesium Oxide Particles[END_REF]., [START_REF] Usher | A laboratory study of the heterogeneous uptake and oxidation of sulfur dioxide on mineral dust particles[END_REF] and Zhang et al. [START_REF] Zhang | Heterogeneous Reactions of Sulfur Dioxide on Typical Mineral Particles[END_REF] assigned the band between 1100 to 850 cm -1 to adsorbed sulfite and bisulfite on α-Al2O3 and MgO. Li et al. [START_REF] Li | Kinetics and mechanism of heterogeneous oxidation of sulfur dioxide by ozone on surface of calcium carbonate[END_REF] and [START_REF] Wu | Effects of temperature on the heterogeneous oxidation of sulfur dioxide by ozone on calcium carbonate[END_REF] assigned a 1000-900 cm -1 band on CaCO3 to a stretching vibration of sulfite. A peak from 1200 to 800 cm -1 on TiO2 was assigned to sulfite and bisulfite species by Nanayakkara et al. [START_REF] Nanayakkara | Sulfur dioxide adsorption and photooxidation on isotopically-labeled titanium dioxide nanoparticle surfaces: roles of surface hydroxyl groups and adsorbed water in the formation and stability of adsorbed sulfite and sulfate[END_REF]. On hematite particles Wang et al.

assigned to sulfites a peak between 950 and 800 cm -1 under dry conditions and a series of peaks from 1050 to 900 cm -1 under humid conditions (Wang et al., 2018b). Finally, a band from 1320 to 975 cm -1 with features at 1230 and 1150 cm -1 in dry conditions and from 1302 to 1055 cm -1 with features at 1220 and 1135 cm -1 under humid conditions was attributed to sulfate and bisulfate species. The assignment agrees well with the results published previously, where sulfates/bisulfates were found around 1300-1100 cm -1 on α-Al2O3 and MgO [START_REF] Usher | A laboratory study of the heterogeneous uptake and oxidation of sulfur dioxide on mineral dust particles[END_REF], [START_REF] Zhang | Heterogeneous Reactions of Sulfur Dioxide on Typical Mineral Particles[END_REF]; between 1240 and 1012 cm -1 on CaCO3 with features at 1198, 1127, and 1090 cm -1 [START_REF] Wu | Effects of temperature on the heterogeneous oxidation of sulfur dioxide by ozone on calcium carbonate[END_REF], [START_REF] Li | Kinetics and mechanism of heterogeneous oxidation of sulfur dioxide by ozone on surface of calcium carbonate[END_REF]. Sulfates were reported between 1300 and 1100 cm - OH groups (4000-3500 cm -1 ), adsorbed water (3500-2600 cm -1 ), physisorbed SO2 (1385-1320 cm -1 ), sulfates/bisulfates (1320-975 cm -1 ), sulfites/bisulfites (975-716 cm -1 ) as a function of time: black line -before introduction of SO2, red line-after 1.5 h of exposure to SO2, blue line-after 2.5 h of exposure to SO2, green line-after 22 h of exposure to SO2, yellow line-after 71 h of exposure to SO2.

32

Figure 10: DRIFTS spectra of SO2 uptake on Hagavatn v-dust at 30% RH and 296 K depicting evolution of bands for free surface OH groups (4000-3500 cm -1 ), adsorbed water (3500-2600 cm -1 ), SH groups (2600-2400 cm -1 ), physisorbed SO2 (1367-1320), sulfates/bisulfates (1302-1050 cm -1 ), sulfites/bisulfites (1053-700 cm -1 ) as a function of time: black line -before introduction of SO2, red line-after 1.5 h of exposure to SO2, blue line-after 2.5 h of exposure to SO2, green line-after 22 h of exposure to SO2, yellow line-after 71 h of exposure to SO2.

Surface reaction mechanism.

Changes in the DRIFT spectra of the surface of Hagavatn v-dust are clearly seen upon introduction of SO2 (Figure 9 and 10). In comparison with the studies of the interaction of SO2 with pure metal oxides reported earlier by different researchers, it is evident that the peaks for sulfur-containing species appear much slower, most probably due to low sensitivity when dealing with dark v-dusts where the reflectance of the infrared light is limited [START_REF] Zhang | Heterogeneous Reactions of Sulfur Dioxide on Typical Mineral Particles[END_REF], [START_REF] Usher | A laboratory study of the heterogeneous uptake and oxidation of sulfur dioxide on mineral dust particles[END_REF], [START_REF] Goodman | Heterogeneous Uptake of Sulfur Dioxide On Aluminum and Magnesium Oxide Particles[END_REF], [START_REF] Li | Kinetics and mechanism of heterogeneous oxidation of sulfur dioxide by ozone on surface of calcium carbonate[END_REF], [START_REF] Wu | Effects of temperature on the heterogeneous oxidation of sulfur dioxide by ozone on calcium carbonate[END_REF], [START_REF] Nanayakkara | Sulfur dioxide adsorption and photooxidation on isotopically-labeled titanium dioxide nanoparticle surfaces: roles of surface hydroxyl groups and adsorbed water in the formation and stability of adsorbed sulfite and sulfate[END_REF], [START_REF] Shang | Heterogeneous reaction of SO2 on TiO2 particles[END_REF], (Wang et al., 2018b). The time evolution of five peaks corresponding to different species, such as free OH groups, molecular water, physisorbed SO2, sulfites/bisulfites and sulfates/bisulfates is followed.

Dry conditions

From our flow tube experiments presented earlier it is apparent that SO2 gas is massively lost from the gas phase upon its exposure to the v-dust surface. Interestingly, the first peaks that appear in the DRIFT spectra after introduction of SO2 gas are positive peaks for physisorbed SO2 gas and negative peaks for free OH groups (Figure 9). Note that although the sample has been thermally pretreated and the experiment is performed under dry conditions, surface OH groups are present on the surface of the sample. The complete dehydroxylation of the surface would have required stronger thermal pretreatment. The loss of surface free OH groups reaches its local minimum at around 1.5 hr exposure to SO2 before returning to its initial pre-exposed level where it starts fluctuating around zero. The peak for SO2 gas is stable, but hard to integrate due to its small size and overlap with a much more pronounced peak for bulk water. Bulk water peak is highly unstable. The disappearance of free OH groups and appearance of physisorbed SO2 suggest that in the first instants of the reaction, SO2 gas gets weakly bonded to hydroxyl groups of the for γ-alumina [START_REF] Datta | Claus catalysis. 1. Adsorption of SO/sub 2/ on the alumina catalyst studed by FTIR and EPR spectroscopy[END_REF], [START_REF] Lo | SO2 Adsorption and Transformations on γ-Al2O3 Surfaces: A Density Functional Theory Study[END_REF]. Maters et al. suggested that due to the weakly acidic nature of SO2 gas its uptake is likely to happen on strongly basic sites affiliated with alkaline and alkaline earth metals on the surface of the volcanic glass (-K-OH, -Mg-OH, -Ca-OH, Mg-OH) [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF]. At the same time it is also possible for the SO2 gas to get adsorbed on metallic Lewis acid sites of mineral phase as proposed by Wang et al. (Wang et al., 2018a).

Hence, the first interaction of SO2 gas with the surface under dry conditions is its distribution on different sites of the v-dust leading to a reversible physisorption of SO2, as indicated by subscript (ads) (Reaction 1). Throughout the manuscript the chemisorbed species or chemical groups that form a chemical bond with the surface are indicated as (surface).

Reaction 1

SO2(g) ↔ SO2(ads)

Physisorbed SO2 can then participate in reactions leading to chemisorbed species. Sulfites and sulfates, under dry conditions first appear after several hours of exposure probably due to the low sensitivity of our DRIFT setup, as abovementioned. Sulfites can be formed on the lattice oxygen sites that can be viewed as Lewis basic sites (Wang et al., 2018a) as in Reaction 2.

Reaction 2

SO2(ads) + O 2-(lattice) →SO3 2-(surface)
Alternatively, sulfites can be formed in the presence of hydroxyl groups as SO2 can react with either one hydroxyl group to form bisulfite (Reaction 3) or two neighboring hydroxyl groups to form sulfite and water (Reaction 4) (Wang et al., 2018a). Surface hydroxyl groups themselves can be bonded to a single metal atom or bridge two, creating different environments for interactions with SO2 [START_REF] Ullerstam | The kinetics and mechanism of SO2 oxidation by O-3 on mineral dust[END_REF]. Chemisorption of SO2 gas on oxide anions (O 2-) and hydroxyl groups of volcanic ash and glass leading to the formation of sulfites and bisulfites was suggested earlier by Maters et al. and detection of sulfites in the leachates confirmed the conversion [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF].

From Figure 11 that shows time evolutions of sulfites and sulfates, it can be observed that, under dry conditions, sulfites (SO3 2-) get completely stabilized after 22 hours of exposure, but sulfates (SO4 2-) keep growing linearly, suggesting that sulfites are the intermediate species in the oxidation of SO2 to sulfates. Oxidation of sulfites to sulfates on the surface of volcanic glass was observed by Farges et al. [START_REF] Farges | Sulfur K-edge XANES study of S sorbed onto volcanic ashes[END_REF]. Sulfites can be oxidized to sulfates via several pathways. Active oxygen derived from molecular oxygen adsorbed on the active sites (Reaction 7) can oxidize sulfites to sulfates and bisulfites to bisulfates as in Reaction 5 and Reaction 6 (Wang et al., 2018a). Contrary to our observations and to results reported by [START_REF] Shang | Heterogeneous reaction of SO2 on TiO2 particles[END_REF], who also noticed a stable rate of sulfate production on the surface of titanium dioxide particles in the presence of oxygen, Usher et al. [START_REF] Usher | A laboratory study of the heterogeneous uptake and oxidation of sulfur dioxide on mineral dust particles[END_REF] noted complete saturation of mineral oxide surfaces with SO2 gas and observed no sulfate formation even upon exposure of SO2-treated surface to air and oxygen. It could be due to the fact that the minerals under investigation were not able to convert molecular oxygen into active oxygen under the specific experimental conditions of these measurements. where e -is a conductive electron trapped in the vacant oxygen site on the surface of some oxides that are prone to having defect sites, such as on the surface of Fe2O3, for example (Wang et al., 2018b), [START_REF] Baltrusaitis | Adsorption of sulfur dioxide on hematite and goethite particle surfaces[END_REF]. Noticeably, reaction proceeds in the absence of light [START_REF] Baltrusaitis | Adsorption of sulfur dioxide on hematite and goethite particle surfaces[END_REF]. Reaction 7 was reported to enable the conversion of sulfites to sulfates on the surface of mineral phase, but a similar process can be expected to happen on the amorphous fraction of the volcanic ash due to the high defect population on silicate glasses (A [START_REF] Leed | Computer Modeling of Water Adsorption on Silica and Silicate Glass Fracture Surfaces[END_REF], [START_REF] Farges | Sulfur K-edge XANES study of S sorbed onto volcanic ashes[END_REF] .

Additionally, certain metal centers that can be readily reduced, such as Fe 3+ to Fe 2+ , contribute to the formation of sulfates from bisulfites as per Reaction 8 (Wang et al., 2018a). Finally, bisulfates can be oxidized to sulfates by hydroxyl groups as per Reaction 9 (Wang et al., 2018a). On ceasing the gas flow and flushing the system for 6 hours, the peak for sulfites decreases due to its oxidation as per Reaction 5, Reaction 6 and Reaction 8, while the peak for sulfates increases for a couple of hours before getting stable, at the same time slightly changing its shape.

The peak centered at 1230 cm -1 gets smaller in comparison with the peak at 1135 cm -1 suggesting indeed further conversion of bisulfates to sulfates. A decrease in sulfite peak with simultaneous increase in the sulfate peak upon cutting off SO2 supply and addition of O3 was observed by Li et al. [START_REF] Li | Kinetics and mechanism of heterogeneous oxidation of sulfur dioxide by ozone on surface of calcium carbonate[END_REF]. Thus, sulfates appear to be the final product of oxidation of SO2 gas and they keep being formed on the surface as long as there are sulfites available for oxidation. Sulfates were also reported as a final irreversible product of the interaction of SO2 gas with volcanic glass by Farges et al. and by Maters et al. (Farges et al., 2009), [START_REF] Maters | Reactive Uptake of Sulfur Dioxide and Ozone on Volcanic Glass and Ash at Ambient Temperature[END_REF].

Humid conditions

After exposure of v-dust to SO2 gas under humid conditions the following changes are observed in the analysis of DRIFT spectra as seen in Figure 10 Sulfates are first observed in the DRIFT spectra at 2.5 hours exposure to SO2. Sulfates can be formed by three different pathways shown in Reaction 5, Reaction 6, and Reaction 8 (Wang et al., 2018a). Unlike sulfites, the peak for sulfates keeps growing linearly with time once again suggesting that sulfate is the final product of SO2 oxidation and that the surface does not get saturated within the duration of the experiment. A peak for molecular water is hard to evaluate at 30% RH as water is being constantly supplied in large excess. Initially, the peak for water seems to decrease, which could be due to the competition with SO2 gas molecules for active sites on the surface (Figure 10). At 14 hours the water peak stabilizes at a positive value, indicating that the surface is now storing more H2O molecules than before introduction of SO2 gas (Figure 10).

Formation of sulfite and sulfate species could have changed equilibrium for adsorption of water on the dust surface (e.g. due to chemical changes of the mineral structure that increase the hygroscopicity of the samples). Particularly, it was argued that adsorption of bidentate sulfate on the surface of Fe-Mn/TiO2 catalyst increases the Lewis acid character of the metal ion to which it is attached, thus potentially favoring the adsorption of water on the metal center (Jiang et al., 2010). Finally, when the flow of SO2 is stopped, the peak for sulfite gradually decreases while the peak for sulfate increases for a couple of hours, stabilizes and stays in the spectra for at least six hours. Like under dry conditions the peak for bisulfates/sulfates changes its shape by decreasing the peak associated with bisulfates (at 1220 cm -1 ) suggesting its conversion to sulfates.

Comparison between dry and humid conditions

Although DRIFTS is a semi-quantitative technique, it is feasible to compare the results obtained under dry and humid conditions. Thus, in comparison with the reaction under dry conditions the reaction under humid conditions is much more pronounced, resulting in both faster reaction rates and higher integration values obtained for all sulfur containing species involved.

This observation is in line with the results obtained from the flow tube experiments, where significantly higher steady state uptakes were determined for SO2 under humid compared to dry conditions. Faster sulfite formation is most likely related to the dramatic increase in the amount of hydroxyl groups covering the surface under humid conditions, driving the sulfite and bisulfite formation through Reaction 3 and Reaction 4. Indeed, under humid conditions the amount of hydroxyl groups lost in the first minutes is much larger than under dry conditions, thus adsorbed SO2 molecules convert rapidly to form sulfites (Figure 9 and Figure 10). Huang et al. reported that the rate of sulfate formation on the surface of Arizona test dust increased with RH and reached its maximum at 70% RH [START_REF] Huang | Kinetics of Heterogeneous Reaction of Sulfur Dioxide on Authentic Mineral Dust: Effects of Relative Humidity and Hydrogen Peroxide[END_REF]. Importance of surface OH groups in the heterogeneous oxidation of SO2 gas was discussed by Zhang et al. who compared basic, neutral and acidic Al2O3 in their ability to oxidize SO2 and noted that basic Al2O3 was much more efficient due to its higher concentration of hydroxyl groups on the surface [START_REF] Zhang | Heterogeneous Reactions of Sulfur Dioxide on Typical Mineral Particles[END_REF]. It was also suggested that reactivity of different oxides to SO2 uptake could be predicted by evaluating their intrinsic ability to form surface hydroxyl groups. Thus, metal oxides that have empty or half-empty d atomic orbitals, such as Al2O3, easily adsorb molecular oxygen and gaseous water and show excellent performance in the reactions with SO2 while metal with full d orbitals, such as MnO2, show weak reactivity [START_REF] Zhang | Heterogeneous Reactions of Sulfur Dioxide on Typical Mineral Particles[END_REF]. Reaction of SO2 gas with the lattice oxygen as in Reaction 2 seems to proceed much slower. The observation is in line with the results of experiments on dehydroxylated surfaces that show very little reactivity towards product formation, suggesting low reactivity of lattice oxygen groups [START_REF] Nanayakkara | Sulfur dioxide adsorption and photooxidation on isotopically-labeled titanium dioxide nanoparticle surfaces: roles of surface hydroxyl groups and adsorbed water in the formation and stability of adsorbed sulfite and sulfate[END_REF]. Absence of S-H stretch from the spectra of the v-dust under dry conditions could be due to the fact that bisulfites and bisulfates are formed in much lower quantities than in humid conditions.

Surface coverage of v-dust with SO4 2-ions

The uptake of SO2 gas on the surface of the v-dust proved to be very long and the process of conversion to H2SO4 was not declining for as long as the SO2 gas was supplied, at least for the duration of the experiments. At the same time no product leaving the surface was identified. This is puzzling since there seems to be no reason for continuous uptake in the absence of some kind of catalytic process leading to regeneration of active sites. In this case it only makes sense that sooner or later the process should come to an end. In order to evaluate a potential for further uptake and conversion of SO2 to H2SO4 it was decided to evaluate the surface coverage of SO4 2- at the end of the experiment. The amount of the product formed on the surface was quantified using HPLC. A 109 mg sample of Hagavatn dust was exposed to 175 ppm of SO2 gas for 24 hours at 23°C and 30% RH and then flushed for 6 hours to allow complete oxidation of sulfites into sulfates. The dust was then extracted in 1 mL of 1% formalin. A calibration curve was constructed using Na2SO4 as a standard solution. The amount of SO4 2-recovered was calculated at 62.79 µg corresponding to 0.57 mg of SO4 2-ion per 1g of Hagavatn dust. The number of molecules formed after 24 hours of continuous exposure was calculated as 7.95 × 10 13 molecules cm -2 . Considering the linear trend that we observed in DRIFTS experiments, the coverage of SO4 2-ions after 72 hours of exposure was estimated at ca. 2.3 × 10 14 molecules cm -2 . To approximate whether the monolayer coverage was reached it was decided to compare the amount of molecules formed on the surface to its theoretical monolayer coverage.

The calculated value for the amount of sulfuric acid molecules necessary to form a monolayer was calculated to be 4.22 × 10 14 molecule cm -2 . This value is consistent with the usually reported surface density of sorptive sites on mineral oxides, ca. 3 x 10 13 -5 x 10 14 sites cm -2 [START_REF] Kulkarni | Isopropanol oxidation by pure metal oxide catalysts: number of active surface sites and turnover frequencies[END_REF]. The coverage of SO4 2-ions after 72 hours of exposure was estimated to be almost by a factor of 2 lower than the theoretical monolayer coverage of the sample by H2SO4 molecules. At this point it should be noted that experimentally, the molecules are not expected to pack together perfectly since the shape of the molecules and hindering interactions would render some of the active sites on the surface inaccessible. Thus, the experimental monolayer coverage would be assumed to be lower than the theoretical one [START_REF] Hudson | The Interaction of Methanol, Acetone, and Acetaldehyde with Ice and Nitric Acid-Doped Ice: Implications for Cirrus Clouds[END_REF], [START_REF] Diaz | Investigation of factors affecting the adsorption of functional molecules onto gel silicas. 1. Flow microcalorimetry and infrared spectroscopy[END_REF].

The rough approximation points to the fact that the surface coverage θ after 72 hours of exposure is approximately θ = 0.5. Therefore, it can be concluded that there are still available sites on the surface of v-dust for the conversion of SO2 to occur and the subsequent sorption of sulfates.

Moreover, the process does not seem to slow down. Longer exposure times are necessary to determine how the sulfate formation will behave at θ = 1 when a monolayer of sulfate ions forms.

Further on, what happens after the formation of monolayer? Will the process stop, and if it continues what are the driving forces behind it?

CONCLUSIONS AND ATMOSPHERIC IMPLICATIONS

In this study the uptake of SO2 gas by Icelandic v-dust was investigated. Uptake of SO2 by v-dusts was observed even though all of them have already been exposed to SO2 in the volcanic conduit and plume owing to their production/emission mode. Upon exposure of the v-dust to SO2

the system reaches a steady state clearly distinct from the initial state indicating continuous consumption of SO2 molecules. The steady state uptake coefficients for v-dusts at 30% RH range from (6.6 ± 2.6) × 10 -9 for Hagavatn to (6.2 ± 2.5) × 10 -8 for Mýrdalssandur. Moreover, increased uptake of the SO2 gas in humid conditions suggests that water plays an important role in the above-mentioned processes. The DRIFTS data indicate the presence of sulfite and sulfate ions on the surface. Presence of sulfates was further detected by HPLC analysis and it was also noted that after 3 days of exposure of v-dust to 175 ppm SO2 the surface coverage by SO4 2-ions is ca. θ = 0.5, i.e. lower than the monolayer coverage suggesting that the surface of the v-dust is still active in uptake processes that are occurring on the surface. It is most likely that SO2 gas reacts at the surface of the v-dust and forms sulfites or bisulfites, which are then converted to sulfates.

Hydroxyl groups play a major role in the conversion of SO2 gas to sulfites, while oxidizing agents in the form of active oxygen or metal centers are necessary to their conversion to sulfates.

Furthermore, sulfates formed on the surface of v-dust are a stable chemisorbed species not easily desorbed even after flushing for hours in the absence of SO2 gas.

During an episode of volcanic eruption volcanic dust will react with SO2 gas both in subvolcanic and atmospheric environments [START_REF] Ovadnevaite | An experimental study of SO2 reactions with silicate glasses and supercooled melts in the system anorthite-diopside-albite at high temperature[END_REF], [START_REF] Schmauss | Adsorption of sulfur dioxide on volcanic ashes[END_REF].

Initially, at near-magmatic high temperatures of over 800 °C there is a compelling evidence that the adsorption of SO2 gas is driven by diffusion of Ca 2+ cations from the interior of the glass to its surface followed by precipitation of CaSO4 [START_REF] Delmelle | High Temperature Reactions Between Gases and Ash Particles in Volcanic Eruption Plumes[END_REF]. This initial stage is characterized by high concentration of gases and high density of ash particles, but is very short in duration lasting only a couple of minutes even in the case of Earth's largest explosive eruptions [START_REF] Delmelle | High Temperature Reactions Between Gases and Ash Particles in Volcanic Eruption Plumes[END_REF]. The pathway is of limited importance in small and medium size eruptions and/or eruptions of volcanoes lacking a deep-seated magma chamber [START_REF] Ayris | SO2 sequestration in large volcanic eruptions: High-temperature scavenging by tephra[END_REF].

Subsequent cooling rapidly brings the system to the subzero temperatures (Textor et al., n.d.). At ambient and low temperatures where effective diffusion of cations to the ash surface is no longer possible the driving force of SO2 adsorption is its physisorption on the surface of ash followed by chemical reaction with active sites that are available on the surface. Previous research under these conditions points out that the rate of sulfate formation is higher at low temperatures compared to room or elevated temperatures of up to 150 °C [START_REF] Wu | Effects of temperature on the heterogeneous oxidation of sulfur dioxide by ozone on calcium carbonate[END_REF] , [START_REF] Schmauss | Adsorption of sulfur dioxide on volcanic ashes[END_REF].

Interestingly, adsorption of SO2 remains very strong at very low partial pressures, which confirms that adsorption is likely to happen in diluted cold parts of the plume [START_REF] Schmauss | Adsorption of sulfur dioxide on volcanic ashes[END_REF]. Furthermore, an increase in sulfur load on ash surfaces was observed to increase with increase of distance from the vent during 2010 Eyjafjallajökull eruption [START_REF] Bagnato | Scavenging of sulphur, halogens and trace metals by volcanic ash: The 2010 Eyjafjallajökull eruption[END_REF].

Thus, one can expect that most of the adsorption of SO2 gas during a medium-size volcanic eruption would happen in the horizontal umbrella part of the volcanic ash cloud. In this case it is important to use the steady state uptake coefficient to evaluate the uptake as it is better fit to describe the phenomena of the continuous process. In this research it was demonstrated that even after 3 days of exposure of natural v-dust to the SO2 gas there are still active sites available for the reaction to take place. Therefore one can expect a v-dust particle travelling through the air to remain active to the adsorption of SO2 gas.

In order to evaluate potential impact of SO2 adsorption on v-dust on atmosphere its atmospheric lifetime was calculated using equation 5.

. 8 * 4 9 5

where γ is the uptake coefficient, c is the mean molecular velocity (m s -1 ), and D is the volcanic dust surface area density (m 2 m -3 ). Considering (i) concentrations of volcanic ash in a vertical column of the volcanic cloud passing over Faroe Islands located about 650 km from the Eyjafjallajökull volcano measured during its eruption on April 15 2010 ranging from 200 to 6000 µg m -3 [START_REF] Gudmundsson | Ash generation and distribution from the April-May 2010 eruption of Eyjafjallajökull, Iceland[END_REF] and (ii) the SSA of Eyjafjallajökull dust sample of 0.75 m 2 g -1 , D can be calculated by direct multiplication of the dust concentration and the SSA of the vdust and was found to be in the range from 1.5×10 -4 m 2 m -3 to 4.5×10 -3 m 2 m -3 for the lower and higher ash load respectively. This leads to the calculation of lifetime of SO2 molecule as a result of its heterogeneous loss on Eyjafjallajökull v-dust particle to 81 to 2.7 years. If we consider a higher 7.5 m 2 g -1 SSA values for Eyjafjallajökull dust obtained by Maters et al. the atmospheric

Figure 1 :

 1 Figure 1: SEM images of five v-dusts: (a) Mýrdalssandur, (b) Dyngjusandur, (c) Hagavatn, (d) Maelifellsandur, (e) Eyjafjallajökull.

Figure 2 :

 2 Figure 2: Schematic representation of the Coated wall Flow Tube (CWFT) reactor used in this study. The space filled with the coolant in between the two walls is shaded in blue. The dust sample coating the inner surface of the Pyrex tube is shown in orange.

Figure 3 :

 3 Figure 3: Theoretical SO2 concentration profile during a typical flow tube experiment with v-dust exposed to SO2. At the beginningof the experiment where surface is isolated from the gas mixture the initial SO2 concentration is [SO2]0, then the dust is exposed to the gas and the concentration of SO2 falls to its minimum, after which it recovers till a steady-state is reached. Note that the steady-state concentration [SO2] is different from the initial state concentration [SO2]0. Initial concentration is controlled when the injector is pushed in and the SO2 gas in no longer in contact with the dust. Shaded area corresponds to the integrated area that is used to obtain the number of molecules during the transient initial uptake of SO2 on v-dusts Ns (molecules cm -2 ). 300 min was chosen as integration limits for the determination of Ns.

  Figure 4. The heterogeneous reactions between SO2 and different v-dust samples are monitored in situ inside of the optical DRIFTS cell (Praying Mantis Kit, Harrick Scientific Corp.) fitted with zinc selenium (ZnSe) windows. DRIFT spectra are recorded by a Nicolet 6700 FTIR spectrometer equipped with a mercury cadmium telluride (MCT) detector cooled with liquid nitrogen. The temperature of the sample is measured using a thermocouple placed right below of the sample holder and is monitored/controlled with a Harrick temperature controller (Pleasantville, USA).

Figure 4 :

 4 Figure 4: Scheme of the DRIFT experimental setup. MFC: Mass Flow Controllers are used to supply the optical cell with a defined mixing ratio of dry air, humid air, and SO2.

Figure 5 :

 5 Figure 5: SO2 concentration recorded by the analyzer during exposure of Mýrdalssandur v-dust at T = 296 K, RH = 30 %.

Figure 7 :

 7 Figure 7: Number of SO2 molecules taken up on the surface of different v-dusts during the transient initial uptake process when exposed to [SO2]0 ≈ 75 ppb, T=296K, RH=30%, dark conditions. The errors quoted reflect the total error for the Ns determinations.

Figure 8 :

 8 Figure 8: Steady-state uptake coefficients of the Mýrdalssandur and Hagavatn v-dust samples at RH = 30 % (blue bars) and RH=0% (red bars), [SO2]0 ≈ 75 ppb, T = 296 K, under dark conditions. The errors quoted reflects the total error for the steady state uptake coefficient determination.

Figure 9 :

 9 Figure 9: DRIFTS spectra of SO2 uptake by Hagavatn v-dust at 0% RH and 296 K depicting evolution of bands for free surface

  surface as first proposed by Datta et al. and later proved by theoretical calculations by Lo et al.

Figure 11 :

 11 Figure 11: Integrated absorbance for (a) sulfites/bisulfites, (b) sulfates/bisulfates as a function of time under 0% RH and 296K. Upper graph: sulfites/bisulfites are stabilized after 22 hours of exposure. The solid line is an exponential fitting of experimental results to better display the observed trends. Lower graph: A linear growth of sulfates/bisulfates with time is noticed; note that

  and Figure 12. At the very beginning of the experiment, the free OH groups get consumed and reach a minimum after 1.5 hours exposure to SO2, at the same time sulfites form rapidly and at an almost linear rate. The peak for physisorbed SO2 increases much slower and the peak remains small for the duration of the experiment. The initial time period corresponds well with the reaction mechanism proposed by Wang et al. in Reaction 3 and Reaction 4 both of which require consumption of free OH groups to produce sulfites and bisulfites. From 1.5 to 2.5 hours consumption of OH groups slows down and finally reaches a stable level. It is important to stress out that, when working under humid conditions, surface OH groups are continuously regenerated. The mechanism for metal oxide surface hydroxylation proposed by Tamura et al. assumes that an exposed oxygen center of the metal oxide lattice acts as Lewis base with water to form a surface terminal hydroxyl group and a hydroxide ion as shown in Reaction 10 (Tamura et al., 2001). In other words, a surface oxide ion gets neutralized by water to become OH -(surface), while water itself loses a proton to become hydroxide ion OH -(ads). Reaction 10 O 2-(lattice) + H2O → OH -(surface) + OH -(ads)

Figure 12 :

 12 Figure 12: Integrated absorbance for (a) free hydroxyl groups, (b) physisorbed SO2, (c) sulfites/bisulfites, (d) sulfates/bisulfates as a function of time under 30% RH and 296 K. The free OH groups get consumed very fast at the beginning of the experiment and reach a minimum at 1.5 hours, then the consumption of OH groups decreases and reaches a stable level at 2.5 hr. The solid line is an empirical fitting of experimental results to better display the observed trends. The peak for physisorbed SO2 is quite nosiy but increases slowly and remains weak for the duration of the experiment. The solid line is an empirical fitting of experimental data points. Sulfites/bisulfites grow rapidly and almost linearly for the first 1.5 hr and then stabilize. The solid line is an exponential fitting of experimental results to better display the observed trends. The peak for sulfates/bisulfates is stable at low values for the first 2.5 hr and then increases in the linear fashion. The solid line is the linear fit of experimental results.

Table 1 :

 1 BET Specific Surface Area of the v-dust samples used in this study.

	Origin of the Icelandic v-dust sample	BET Specific Surface Area
		(m 2 g -1 )
	Mýrdalssandur	1.5 ± 0.38
	Dyngjusandur	7.0 ± 1.8
	Hagavatn	4.5 ± 1.1
	Maelifellssandur	8.2 ± 2.0
	Eyjafjallajökull	0.75 ± 0.19

  . After 20 min of mechanical shaking the extracted solution is passed

	through a 0.2 µm PTFE membrane and a filtered leaching solution is analyzed by Waters HPLC
	system. Chromatography equipment consists of Waters 2695 Series HPLC System equipped with
	2487 UV/VIS Dual wavelength absorbance detector, (Waters Corporation, Milford, MA).
	Empower 2 Data Acquisition System for LC (Copyright 2005, Waters Corporation, Milford, MA)

is used to analyze the data. Analysis is performed using Restek Ultra Column C18, 5 µm, Length 250 mm, I.D. 4.60 mm column dynamically coated with 1.0 mM cetylpyridinium chloride in 7%

Table 3 :

 3

SO2 steady-state uptake coefficients (γSS,BET) and transient initial number of molecules adsorbed (Ns) on the surface of v-dusts. Experimental conditions: [SO2]0 ≈ 75 ppb, RH = 30 %, T = 296 K, under dark conditions.

Origin of the Icelandic v-dust γSS,BET Ns (molecules cm -2 )

	Mýrdalssandur	(6.2 ± 2.5) × 10 -8	(2.5 ± 0.8)×10 13
	Dyngjusandur	(1.6 ± 0.6) × 10 -8	(2.1 ± 0.6)×10 13
	Hagavatn	(6.6 ± 2.6) × 10 -9	(1.2 ± 0.4)×10 13
	Maelifellssandur	(3.2 ± 1.3) × 10 -8	(9.6 ± 2.9)

×10 12 Eyjafjallajökull (3.3 ± 1.3) × 10 -8 (1.2 ± 0.4)×10 13

Zhang et al., 2006) 1361, 1297, 1172, 1116, 1050, 1000 (Nanayakkara et al., 2012) 1300-1100 (Shang et al., 2010) 1130 (Li et al., 2006) Dry: 1339-1014 Humid: 1208-1222(Wang et al., 2018b) free OH groups 4000-3500 4000-3500 (Hair, 1975) 3748, 3707 (Goodman et al., 2001) 3800-3600 (Nanayakkara et al., 2012) 1666, 3628 (Shang et al., 2010) 3704 (Wang et al., 2018b) OH groups of acids 3447, 3548 (Wang et al

	Molecular species	V-dust (this	Si2O3	Al2O3	MgO	TiO2	CaCO3	Fe2O3	Theoretical
		work)							calculations
	(wavenumbers are
	expressed in cm -1 )
	SH stretch of HSO3 -	2600-2400	2600-2400 (Hair,						2620 to 2450 (Zhang
			1975)						and Ewing, 2002)
	SO2	1364, 1340		1330, 1149	1330, 1149	1139, 1325 (Nanayakkara et		1400 (Nanayakkara et al.,
				(Goodman et al.,	(Goodman et al.,	al., 2012)		2012)
				2001)	2001)
	HSO3 -/SO3 2-	1053-700		1200-900 (Goodman		1200-800 (Nanayakkara et		Dry:
		975-716		et al., 2001)		al., 2012)		950-800
				1100-850 (Zhang et		1080 (Shang et al., 2010)		Humid:
				al., 2006)				1050-900 (Wang et al.,
								2018b)
	SO3 2-				1125-800	Monodentate:	1000-900 (Li	Dry:891 (Wang et al.,
					(Goodman et al.,	1033, 971, 923	et al., 2006)	2018b)
					2001)	(Nanayakkara et al., 2012)	1000-885	Humid: 981 (Wang et al.,
						Bidentate:	(Wu et al.,	2018b)
						1006, 886(Nanayakkara et	2011)
						al., 2012)
	HSO3 -					1077 (Nanayakkara et al.,			1054, 1154, 1219
						2012)			(Zhang and Ewing,
									2002)
	HSO4 -/SO4 2-	1320-975		1245, 1170 (Usher et	1150, 1050		1240-1012
		1302-1055		al., 2002)	(Usher et al.,		(Wu et al.,
					2002)		2011)
	HSO4 -	1220, 1230						1219 (Wang et al., 2018a)
	SO4 -			1300-1100(Zhang et	1300-1100
				al., 2006)	(.,	3622 (Zhang and
								2018b)	Ewing, 2002)
	molecular water	3500-2600	3450, 1630	3524, 3296		3160, 1654 (Shang et al.,		3208, 1642 (Wang et al.,
		1400-1800	(Hair, 1975)	(Goodman et al.,		2010)		2018b)
	739			2001)

Table 4 :

 4 Vibrational wavenumbers (cm -1 ) of sulfur-containing species adsorbed on v-dust and metal oxides.

on TiO2(Nanayakkara et al.,

2012),[START_REF] Shang | Heterogeneous reaction of SO2 on TiO2 particles[END_REF]; and at 1220 cm -1 on Fe2O3(Wang et al., 2018b). Just as sulfites, sulfates can be bonded to the surface in bidentate or monodentate modes(Jiang et al., 2010). A peak centered at 1220 cm -1 under humid conditions and at 1230 cm - 1 under dry conditions could be attributed to the formation of bisulfate species(Wang et al., 2018a). The process of sulfate/sulfite formation is explained later in the reaction mechanism section.

lifetime falls to 8.1 years to 99 days [START_REF] Maters | Controls on the surface chemical reactivity of volcanic ash investigated with probe gases[END_REF]. For comparison, the lifetime of SO2 in the atmosphere as measured over the Eastern United States in the absence of volcanic activity ranges from 15 h in summer to 65 hours in winter [START_REF] Lee | SO2 emissions and lifetimes: Estimates from inverse modeling using in situ and global, space-based (SCIAMACHY and OMI) observations[END_REF]. On the base of these calculations it appears that the heterogeneous uptake of SO2 molecule on the surface of v-dust particle is a negligible process in comparison with the gas phase oxidation by aqueous (i.e., H2O2) and gas-phase (i.e., OH) processes. However, in case of higher volcanic ash concentrations in the plume, for example 2.0×10 6 µg m -3 estimated during 1991 Pinatubo eruption (SSA 1.5 m 2 g -1 [START_REF] Maters | Controls on the surface chemical reactivity of volcanic ash investigated with probe gases[END_REF]) at the direct proximity to the vent [START_REF] Witham | Modelling concentrations of volcanic ash encountered by aircraft in past eruptions. Atmospheric Environment, Volcanic ash over Europe during the eruption of Eyjafjallajöekull on Iceland[END_REF], and considering the same uptake coefficient measured in this study for Eyjafjallajökull, calculations lead to a radically shorter lifetime of 36 hr and make heterogeneous uptake of SO2 on volcanic dust particles equally important. Furthermore, the lifetime of SO2 could be further decreased for volcanic particles with higher specific surface area. These calculations point out that adsorption of SO2 on the surface of volcanic ash is most likely to happen in the part of volcanic cloud closest to the vent where ash concentration is the highest. The process of SO2 loss due to heterogeneous reactions with v-dust particles could be further enhanced in the presence of oxidizing species (i.e., O3, OH) and sunlight, and needs to be quantitatively evaluated.

ACKNOWLEDGMENTS

The authors acknowledge Mr Vincent Gaudion and Dr Mohamad Zeineddine (SAGE, IMT Lille Douai) for their assistance in the lab. We are grateful to Mr Bruno Malet and Dr Laurent Alleman (SAGE, IMT Lille Douai) for conducting the ICP-MS experiments. This work was achieved in the frame of Labex CaPPA, funded by ANR through the PIA under contract ANR-11-LABX-0005-01, and CPER CLIMIBIO project, both funded by the Hauts-de-France Regional Council and the European Regional Development Fund (ERDF). J. Lasne acknowledges support from the Labex CaPPA and CPER CLIMIBIO projects and the Hauts-de-France Regional Council for his post-doctoral fellowship.