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Ozone Uptake by Clay Dusts Under Environmental Conditions Jerome Lasne, Manolis N. Romanias, and Frederic Thevenet ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00057 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

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ACS Earth and Space Chemistry

1

Ozone Uptake by Clay Dusts Under Environmental

2

Conditions

3

Jérôme Lasne,* Manolis N. Romanias and Frederic Thevenet

4

IMT Lille Douai, Univ. Lille, SAGE, 59000 Lille, France.

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* corresponding author

6 7

KEYWORDS: Ozone; Uptake by dust; Clay; Reaction mechanism; Surface interaction; Physical

8

chemistry.

9

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ABSTRACT: Clay is the most emitted type of dust in the atmosphere, and offers a large surface

11

for heterogeneous interactions with atmospheric compounds. In this study, we focus on the

12

uptake of ozone (O3) by montmorillonite and kaolinite dust samples at atmospheric pressure in a

13

coated-wall flow tube reactor. The influence of relevant environmental parameters, such as O3

14

concentration, relative humidity and temperature is determined. A mechanism for O3 interaction

15

with the surface sites of clays is described, and atmospheric implications are discussed. Although

16

the impact of O3 uptake by fresh clay dust seems limited in the atmosphere, this work highlights

17

the need to consider relevant and thoroughly defined uptake coefficients in models. Moreover,

18

the presence of a pressure-dependence in the mechanism of O3 uptake by dust suggested in this

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work helps reconciling measurements made at atmospheric pressure with those performed at low

20

pressure in Knudsen cells.

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1. Introduction

22

Mineral dust is a major aerosol species in the atmosphere of Earth. Atmospheric loadings of clay

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dusts are among the largest, accounting for more than half of total dust loadings.1 Emission

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fluxes of clay dusts are the largest, accounting for almost half of the total dust flux (i.e., ca. 109

25

tons per year) and providing a large and continuous supply of fresh clay dust to the atmosphere.

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During their journey in the atmosphere, aerosols encounter gas molecules, among which

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pollutants of biogenic and anthropogenic origin, such as nitrogen oxides (NOx), sulfur dioxide

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(SO2) and ozone (O3).

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Ozone is a strong oxidant, the presence of which in large amounts in the troposphere is

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associated with pollution events in urban and rural areas where the lower NOx budget represents

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a smaller O3 source.2 The toxicity of O3 makes it a major concern for health, of growing

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importance with climate change.3,4 To gain knowledge on processes regulating the abundance of

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ozone in the atmosphere is therefore of importance to predict pollution events and protect the

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health of the population.

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Although reactions between gas-phase molecules generally dominate atmospheric chemistry,

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heterogeneous reactions of gas molecules on surfaces have gained interest in the past decades

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and have been shown to be of major importance for crucial processes, such as the destruction of

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the stratospheric ozone layer.5-8 Regarding dust surfaces, a correlation between high dust particle

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contents and a drop in O3 concentration in the troposphere has been highlighted.9-15 This drop

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can be due to either direct, or indirect effects; the latter seem to prevail, as exemplified by Bauer


2

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et al.16 who show that removal of HNO3 by dust from the atmosphere largely lowers the budget

42

of this O3 precursor. Overall, it suggests that the tropospheric ozone budget is linked with the

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presence of dust, i.e. of a solid surface available for heterogeneous reactions to take place.

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Several laboratory studies have investigated the heterogeneous interaction of ozone with dust;

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see the work of Crowley et al.17 and Tang et al.18 for detailed reviews. Meanwhile, few studies

46

were conducted under atmospheric-relevant conditions of pressure, relative humidity and nature

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of the bath gas, either in a flow-tube reactor similar to the one used in the present study19,20, in an

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environmental chamber21 or more recently in a U-shape fixed bed reactor.22 Most studies were

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conducted in Knudsen cells,23-27 and took place under the experimental conditions prevailing in

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this type of setup. This means that the total pressure in the cell is kept typically below 10 mTorr

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(i.e., 10-5 atm), most of which is produced by the ozone partial pressure. These studies with a

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large amount of O3 and a small fraction of bath gas are designed to quantify the interaction of the

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compound of interest with the surface of dust in absence of water. However, pressure-dependent

54

reactions such as that evidenced in this study for O3 point to a better relevance to the natural

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atmospheric environment of atmospheric-pressure studies using air as bath gas and low O3

56

concentrations.

57

Moreover, most studies were designed to determine initial uptakes, in contrast with the steady-

58

state uptakes determined in flow-tube experiments. Initial uptakes are relevant to describe the

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early stages of the interaction of a gas with a surface, regardless of its nature, and are usually

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orders of magnitude larger than steady-state uptakes. These values being representative of the

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initial time of contact between the gas and the surface, they are therefore of limited relevance for

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the timescales necessary to reach equilibrium in the atmosphere. Following Nicolas et al.,19 we

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suggest that steady-state values are more appropriate to describe the equilibrium composition of


3


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the atmosphere. In this study, steady-state uptakes of O3 by montmorillonite and kaolinite dust

65

surfaces at atmospheric pressure are determined under atmospheric-relevant conditions of O3

66

concentration (20–200 ppb), temperature (250–353 K) and relative humidity (0–93%).

67 68

2. Experimental Section

69

2.1. Experimental reactor

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The uptake experiments presented in this paper are conducted in a horizontal double-wall flow-

71

tube reactor operated at atmospheric pressure using zero air as bath gas (Figure 1). The dust

72

sample of interest is deposited on the inner surface of a Pyrex tube, which is introduced in the

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reactor along its axis. Two viton o-rings are placed around the Pyrex tube to keep it in position

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inside the reactor. Air and O3 are flowed through a movable injector with an internal diameter of

75

0.3 cm. Zero air is used as bath gas, with a total flow rate ranging between 1100 and 1300 sccm,

76

ensuring laminar flow conditions (Reynolds number Re < 100). Further, we have checked that the

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roughness of the coating did not create turbulence in the flow, as detailed in SI. A

78

thermoregulation unit (Huber, Ministat 230) regulates the temperature by circulating a fluid in

79

between the double-wall of the flow-tube. The temperature profile along the reactor was

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recorded by inserting a K-type thermocouple in the movable injector. The temperature profile of

81

the gas is usually constant along the flow-tube and consistent with the value displayed by the

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thermoregulation unit (± 1%). However, under extreme temperature conditions (i.e. T < 278 K

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and T > 340 K), small variations of the temperature of the gas flow were observed when the

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injector was placed at the upstream end of the flow tube. To overcome this issue, the injector was


4

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pre-cooled or preheated at the desired temperature, ensuring temperature homogeneity of the gas

86

flow. Table 1 summarizes the experimental conditions prevailing in this setup.

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Coated-wall flow-tube

Injector

88 89

Figure 1. Schematic representation of the Coated-Wall Flow-Tube (CWFT) reactor used in this

90

study. The space filled with the coolent in between the two walls is shaded in blue. The dust

91

sample coating the inner surface of the Pyrex tube is shown in grey.

92 Parameter

Value

Pressure (P)

1 atm

Temperature (T)

(250–353) ± 1 K

Reactor inner diameter

1.8 cm

Reactor length

43 cm

Flow rate

(1100–1300) ± 7 sccm

Linear velocity

(24.6–29.0) ± 1.5 cm s-1

Reynolds number (Re)

78 ± 5


5


Molecular mean free path at

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94.4 ± 0.4 nm

296 K Dust-coated tube length (L)

(14–16) ± 0.1 cm

Dust-coated tube diameter

1.00 ± 0.001 cm

Residence time

(0.48–0.65) ± 0.04 s

93 94

Table 1. Parameters describing the experimental conditions in the CWFT reactor used in this

95

study.

96

The gas flowed through the reactor is pre-mixed in gas lines. Zero air used as bath gas is

97

supplied by a Claind ZeroAir 2020 generator. In experiments requiring humid air, a second flow

98

of zero air going through a bubbler filled with ultrapure water (milli-Q, resistivity 18.2 MΩ cm)

99

is mixed with the dry air flow in proportions necessary to reach the relative humidity (RH)

100

targeted. Ozone is synthesized by a generator (Model 165, Thermo Environmental Instruments

101

Ltd.) supplied with zero air. Dry air, humid air and ozone flows are controlled with MKS mass-

102

flo® controllers (100 to 1000 sccm) connected to a 4-channel MKS type 247 readout unit; they

103

are mixed at the entrance of the injector. Downstream of the reactor, the outgoing flow is

104

connected to the ozone analyzer (Model O342M, Environnement SA).

105

Throughout the manuscript, O3 gas concentrations are given in ppbv (parts per billion by

106

volume). Under standard experimental conditions (T = 296 K, P = 1 atm), the conversion to a

107

concentration is given by 1 ppbv (O3) ≈ 2.5 × 1010 molecules cm-3. Note that this conversion

108

factor changes with temperature or total pressure.

109


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110

2.2. Preparation of the dust samples

111

Our investigations of clay dust surfaces are directed on some of the most abundant types of clays

112

in the atmosphere: montmorillonite and kaolinite.1 Montmorillonite and kaolinite clay powders

113

are used as supplied by Sigma-Aldrich (montmorillonite K 10; natural kaolinite). Both minerals

114

are

115

montmorillonite ((Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·nH2O) lacks water and has a variable layer

116

spacing. The elemental composition of the samples was determined using an ICAP7400 ICP-

117

AES instrument (Inductively Coupled Plasma - Atomic Emission Spectroscopy; Thermo

118

Scientific) for major elements listed in Table 2. To prepare the samples for ICP-AES analysis,

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montmorillonite and kaolinite dust samples (2 to 7 mg of dust) were treated with a mixture of

120

acids (HF/HNO3/H2O2) in a microwave oven (Milestone Ultrawave) at 500 K and 35 bar for 15

121

min.28 Triplicate measurements were performed with both types of dust to evaluate their

122

chemical heterogeneity. Measurements were carried out on acid blanks, quality control standard

123

solutions and standard reference material (NIST SRM 1648a and SRM 2584) to evaluate the

124

detection limits, the accuracy and to validate the whole procedure. Si dominates in both minerals,

125

but we note the much larger Al content of kaolinite. Montmorillonite is composed of 4.6 % Fe,

126

almost 6 times the Fe content of kaolinite. Other metals (K, Mg, Ca, Ti and Na) account for less

127

than 4 % of the mass budget of these minerals.

aluminosilicates;

kaolinite

(Al2Si2O5(OH)4)

has

a

hydrated

structure,

whereas

128 Element

Si

Mass composition of

Mass composition of

Montmorillonite (%)

Kaolinite (%)

65.9 ± 8.6

53.6 ± 3.1


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Page 8 of 42

Al

16.0 ± 0.2

39.7 ± 0.4

Fe

4.6 ± 0.04

0.8 ± 0.008

K

2.9 ± 0.07

3.8 ± 0.14

Mg

2.1 ± 0.03

0.2 ± 0.003

Ca

1.4 ± 0.01

0.7 ± 0.2

Ti

0.6 ± 0.008

0.5 ± 0.005

Na

0.4 ± 0.02

0.08 ± 0.07

129 130

Table 2. Mass composition of montmorillonite and kaolinite dust samples used in this study, as

131

determined by ICP-AES experiments.

132 133

To coat the inner surface of Pyrex tubes with clay dust, a defined amount of dust is inserted in a

134

tube. A small amount of water is then added to form a slurry. Kaolinite has proved difficult to

135

stick to Pyrex when suspended in water, and ethanol was therefore used preferentially. Hanisch

136

and Crowley25 claim that deposition in ethanol has an impact on their Ca-montmorillonite films,

137

in contrast with Saharan dust for which they observed no impact. However, in our experiments,

138

the linear mass-dependence of the geometric uptake coefficient presented in Figure 3 is

139

unchanged by the change of solvent. Hence, the solvent used during deposition (i) is completely

140

evaporated and (ii) does not affect the dust coating in such an extent that its uptake of ozone is

141

modified. Once the solvent is added to the dust, the tube is shaken, not stirred, until a

142

homogeneous solution is obtained. The remaining slurry is evacuated, and the tube heated to 450

143

K for 10 minutes to evaporate the excess solvent. It is then left in an oven at 393 K overnight.


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The dust masses used in our experiments correspond to coatings with densities of 0.2–2.7 mg

145

cm-2 for montmorillonite and 0.2–3.3 mg cm-2 for kaolinite. The dust-coated area is similar in all

146

experiments, and therefore these ranges of values reflect the variation in total masses employed,

147

since the dust is evenly spread over the whole surface. These values are at the low end of those

148

used with kaolinite by Fuentes et al.20 in a laminar flow reactor (2.8±0.5 mg cm-2) and by

149

Hanisch and Crowley25 on Saharan dust in a Knudsen cell (2.3–14.2 mg cm-2), but lower than

150

those used by Karagulian and Rossi27 in a Knudsen cell (10–205 mg cm-2). This parameter is

151

directly related to the thickness of the dust coating, and therefore controls whether the whole

152

sample can be accessed by O3 molecules or not. Hence, it is crucial that this value remains in a

153

range corresponding to the linear regime of the mass-dependence of the uptake coefficient,

154

which is achieved in this study and described in section 3.1.

155 156

2.3. Experimental protocol and determination of the uptake coefficients

157

In many studies dealing with O3 interaction with mineral dust, initial uptakes are determined,

158

whereas steady-state uptakes rarely are. Initial uptakes have values very different from steady-

159

state uptakes, as these values reflect different regimes of the uptake process, and therefore

160

different processes might be at play. Moreover, atmospheric pressure CWFT reactors are

161

intrinsically bound to measure steady-state uptakes, as the time resolution of these setups is

162

usually not low enough to make an accurate analysis of the initial part of the uptake process.19

163

For these reasons, our study focuses on the determination of steady-state uptakes.

164

During a typical flow-tube experiment shown in Figure 2, O3 is formed by the generator and

165

flowed through the reactor, the dust being initially left unexposed. After a stable O3


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concentration is set, the injector is moved to expose the dust surface to O3; the change in O3

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concentration, [O3], related to its uptake by dust is recorded by an O3 analyzer located

168

downstream of the reactor. The system is then left to equilibrate until a steady [O3] is reached,

169

allowing the calculation of steady-state uptake coefficients; under our experimental conditions,

170

this is achieved within about 80 minutes. After the steady-state is reached, the injector is pushed

171

back all the way in the reactor to control the ozone level, [O3] 0. In all the experiments presented

172

in this paper, the steady-state [O3] is clearly distinct from [O3] 0.

173

Preliminary experiments were conducted to determine the possible contribution of the Pyrex

174

surfaces to the uptake. They show a strong contribution of the Pyrex to the initial uptake, and a

175

non-negligible contribution on the steady-state. As a consequence, the contribution of the Pyrex

176

glass to the steady-state uptake of O3 is subtracted from the data presented in this paper, where it

177

can account for as much as 10 % of the total ozone consumption observed at room temperature

178

under dry conditions. It was observed that O3 uptake by the Pyrex surfaces decreases

179

exponentially with increasing O3 concentration, decreases linearly with temperature and also

180

with relative humidity.

181


10

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182 183

Figure 2. Evolution of the O3 concentration with time recorded during a typical experiment. A

184

tube coated with 50.5 mg of montmorillonite is exposed to 42 ppb of O3 at atmospheric pressure,

185

T = 296 K and a relative humidity (RH) of 30 % under dark conditions. Once a stable O3 flow is

186

set, the dust surface is exposed to O3; after a steady-state is reached, the injector is pushed back

187

to the “control” position.

188 189

The measurement of [O3] at steady-state and of the initial level, [O3] 0, together with the

190

knowledge of the parameters defining our setup listed in Table 1, is necessary to conduct the

191

analysis leading to the determination of the uptake coefficient. Assuming pseudo first-order

192

kinetics with the active surface sites in excess for the uptake of O3 by clay dust surfaces, the

193

observed constant of reaction, kobs (in s-1), is determined (1):


11


𝑘𝑜𝑏𝑠 =

v [𝑂 ] × ln( 3 0⁄[𝑂 ]) 3 𝐿

Page 12 of 42

(1)

194

where v is the flow in the reactor (in cm s-1) and L is the length of the dust coating (in cm). The

195

hypothesis of a pseudo first-order kinetics for the reaction has been confirmed experimentally by

196

the linear increase of ln([O3] 0/[O3]) with t, the residence time of ozone in contact with dust (see

197

Figure S1 of the Supplementary Information). Moreover, integration of the signal in Figure 2

198

provides the number of O3 molecules taken up by the surface in a typical experiment: assuming

199

that 1 O3 molecule reacts with only 1 surface site, we calculate a density of reactive surface sites

200

of 1.6 × 1011 cm-2. This value certainly overestimates the density of reactive surface sites because

201

Figure 2 shows that O3 decomposition is catalytic, therefore some surface sites are regenerated

202

and decompose several O3 molecules. The calculated density of a monolayer of packed O3

203

molecules is 7.4 × 1012 cm-2, i.e. a factor of roughly 50 larger than the calculated density of

204

reactive sites. This suggests that roughly 2 % of surface reactive sites have actually reacted, and

205

therefore that they are in large excess with respect to O3. O3 uptake by clay surfaces can

206

therefore be considered a pseudo first-order process with the surface sites in excess. kobs is

207

corrected for O3 diffusion with a constant kdiff (in s-1), and gives the diffusion-corrected kinetic

208

reaction constant kkin (in s-1) following (2): 1 𝑘𝑜𝑏𝑠

209

=

1 𝑘𝑘𝑖𝑛

+

1 𝑘𝑑𝑖𝑓𝑓

(2)

kdiff can further be expressed as (3):

1 𝑘𝑑𝑖𝑓𝑓

𝑅2 =( 1.75 ) × 𝑃 𝑇 3.66 × 𝐷(296𝐾) × ( ⁄296)

(3)


12

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210

where R is the inner radius of the Pyrex tube (in cm), P and T are the pressure (in Torr) and the

211

temperature (in K) of the gas in the reactor. D is the diffusion coefficient (in Torr cm-2 s-1) of O3

212

in air, and to our knowledge it has not been measured.29 In this work, we use a diffusion

213

coefficient of O3 in air derived from the calculation of Massman30 at 1 Torr and 273 K: D(296 K)

214

= 127 Torr cm2 s-1. It should be noted that this value is different from that calculated more

215

recently by Ivanov et al.31 for O3 diffusion in air, D(296 K) = 96.3 Torr cm2 s-1. Nevertheless,

216

changing the value of the diffusion coefficient for Ivanov’s has negligible impact on the results

217

presented in this article (< 3 %).

218

The diffusion-corrected constant, kkin, is then used to determine the uptake coefficient, γ, (4):

𝛾=

4𝑘𝑘𝑖𝑛 𝑉 𝑐𝑆𝑔𝑒𝑜𝑚

(4)

219

where V and Sgeom are respectively the volume (in cm3) and geometric surface (in cm2) of the

220

flow tube, and c is the average molecular speed (in cm s-1). The diffusion-correction accounts for

221

2 to 10 % of the uptake coefficients calculated. The lowest corrections are of about 2 % for the

222

highest O3 concentrations (200 ppb) and increase with decreasing [O3]. The diffusion-correction

223

accounts for about 5 % of γ at 250 K, and to about 10 % of γ at 353 K.

224 225

The error on the uptake coefficients is the root-mean-square deviation of the measured values. It

226

is calculated with the precision of the signal and the random error. To evaluate the random error,

227

we use the errors given by the manufacturers of the flow controllers, relative humidity and

228

temperature sensors; errors on dust mass, specific surface area and length of the dust coating are

229

measured. The total error calculated for the γ values is lower than 15 % in all experiments.


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Page 14 of 42

230

These formulae pave the way between the measurement of O3 concentrations during exposure of

231

a dust surface and the determination of the steady-state uptake coefficients of O3 on clay dust

232

under various experimental conditions. In the following, the results obtained under different

233

conditions of dust mass, ozone concentration, relative humidity and temperature are presented.

234 235

3. Results and Discussion

236

3.1. Mass-dependence of O3 uptake by clay dust

237

The geometric uptake at steady-state of ozone by montmorillonite and kaolinite dust surfaces is

238

determined at T = 296 K under dark and dry conditions (RH < 0.1 %), under a flow of air

239

containing 40 ppb O3 (1 × 1012 molecules cm-3), and displayed in Figure 3. A linear increase of

240

the uptake from 10 to 130 / 160 mg respectively by montmorillonite / kaolinite is observed, and

241

saturation is not reached at the highest masses investigated. The linear increase of the uptake

242

with mass and the absence of saturation reflect the fact that the entire dust surface is accessible to

243

O3 molecules of the gas.

244


14

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245 246

Figure 3. Geometric steady-state uptake of 40 ppb O3 by montmorillonite (black circles) and

247

kaolinite (red squares) dust surfaces at 296 K under dry and dark conditions. Dashed lines show

248

linear fits of the data. The error on mass is estimated as 5 % from measurements of the dust mass

249

coating the tube before and after the experiments. The error shown on the uptake coefficients is

250

the root-mean-square deviation calculated as described in section 2.3.

251 252

The experiments presented in this paper are performed with dust masses in the linear regimes

253

determined here for montmorillonite and kaolinite, respectively, to ensure that the whole dust

254

surface could be accessed by O3 and that the BET uptake coefficients could be calculated. To

255

compensate for the overestimation of the uptake coefficients by the use of the geometric surface,

256

we consider in the following the specific surface area SBET.32 SBET values have been measured for

257

montmorillonite and kaolinite dust by gas sorption analysis:33 SBET = 225 ± 68 m2 g-1 for


15


Page 16 of 42

258

montmorillonite and 9.2 ± 2.8 m2 g-1 for kaolinite. This allows the determination of BET uptake

259

coefficients, γBET, (5):

𝛾𝐵𝐸𝑇 = 𝛾 ×

𝑆𝑔𝑒𝑜𝑚 𝑆𝐵𝐸𝑇

(5)

260 261

3.2. Initial O3 concentration-dependence

262

The initial O3 concentration flowed through the reactor is varied in two series of experiments

263

performed at 296 K, under dry and dark conditions with montmorillonite and kaolinite dust

264

masses in the linear regime. Figure 4 shows that for [O3] 0 ranging from 20 to 200 ppb (5 × 1011

265

to 5 × 1012 molecules cm-3), the steady-state BET uptake of O3 on montmorillonite and kaolinite

266

follows an exponential decay. The inverse variation of γ with [O3] 0 is generally observed for the

267

heterogeneous interaction of ozone with a surface,18-22,25,26,34 and reflects the increasing

268

competition between O3 molecules for surface sites as the gas-phase O3 concentration increases.

269


16

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270 271

Figure 4. BET steady-state uptake of O3 by montmorillonite (black circles) and kaolinite (red

272

squares) dust surfaces as a function of initial O3 concentration at T = 296 K, under dry (RH
4. In kaolinite, the Si/Al mass ratio is closer to unity

289

(1.35). One can therefore expect montmorillonite to be mostly composed of Si-oxides, in a

290

higher proportion than kaolinite. Moreover, montmorillonite has a 2:1 structure; the basal

291

surfaces are therefore only siloxanes. Kaolinite however has a 1:1 structure, with basal surfaces

292

being composed of siloxanes for half, and Al-OH or Mg-OH groups for the other half. Michel et

293

al.23,24 have shown that O3 uptake is larger on Al2O3 than on SiO2, and the reactivity of -OH

294

groups is known to be higher than that of siloxanes. On this basis, one expects the uptake of O3

295

by montmorillonite to be lower than by kaolinite. This is in agreement with what is observed at

296

all the concentrations investigated, giving support to the interpretation of the uptake in terms of

297

dust composition and surface group functionality.

298 299

3.3. Influence of temperature


18

Page 19 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300

Ozone uptake by clay dust was investigated over the 250 to 353 K temperature range, at RH =

301

0.1 %, [O3] 0 = 40 ppb and with dust masses in the linear regime. γSS,BET varies between roughly

302

10-9 at 251 K and 10-8 at 353 K for montmorillonite, and from 10-8 at 250 K to 10-7 at 353 K for

303

kaolinite. Figure 5 shows the Arrhenius plot of the BET steady-state coefficient as a function of

304

temperature. An increase of γSS,BET with T is observed on montmorillonite and on kaolinite, the

305

uptake being higher on kaolinite than on montmorillonite by a factor of 10. The uptake of a

306

molecule from the gas by a surface is expected to be favored as temperature decreases, and not

307

by an increase of T. Therefore, it seems that the rate-determining step of O3 removal by the

308

surface is the decomposition of O3 or the regeneration of the surface, pointing to a complex

309

mechanism not limited to the adsorption process. Linear fits of the data give similar activation

310

energies for O3 interaction with montmorillonite (16 ± 3 kJ mol-1) and kaolinite (16 ± 2 kJ mol-

311

1

312

that these activation energies relate to the reactional process globally, not to one specific step of

313

the mechanism. To our knowledge, this is the first study showing the temperature dependence of

314

O3 uptake by clay dust. Therefore, no other data on the activation energy of this reaction on clay

315

is available, and these results can only be compared with measurements made on surfaces of

316

single metal oxides, such as alumina. Hanning-Lee et al.32 also observed an increase of O3

317

uptake on alumina with increasing temperature. Michel et al.24 studied the temperature

318

dependence of O3 uptake on α-Al2O3 in a Knudsen cell and observed a weak temperature

319

dependence; the activation energy was determined as 7 ± 4 kJ mol-1, in agreement with previous

320

studies on metal oxides.35,36 This is less than half of the value found on clay dusts, and likely

321

reflects the higher reactivity of metal oxide surface sites.

); errors are given as the first standard deviation of the linear fit to the data. It is worth noting

322


19


Page 20 of 42

323 324

Figure 5. Arrhenius plot of the BET steady-state uptake coefficient of 40 ppb O3 by

325

montmorillonite (black circles) and kaolinite (red squares) dust surfaces as a function of inverse

326

temperature under dry (RH < 0.1 %) and dark conditions. Dashed lines show linear fits of the

327

data. The error on temperature (1 %) is a conservative upper limit of the error on the temperature

328

of the circulating fluid. Errors on ln(γss,BET) and 1000/T are determined by error propagation.

329 330

3.4. Influence of relative humidity

331

To investigate the influence of gas-phase water on the uptake of O3 by clay dust, experiments

332

were conducted under different relative humidity (RH) conditions. The experiments are run at

333

296 K, [O3] 0 = 40 ppb (1 × 1012 molecules cm-3) and with dust masses in the linear regime under

334

dark conditions. Figure 6 shows the variation of γSS,BET on montmorillonite and kaolinite dust for

335

RH ranging from 0 to 93%.


20

Page 21 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


336

337 338

Figure 6. BET steady-state uptake of 40 ppb O3 by montmorillonite (black circles) and kaolinite

339

(red squares) dust surfaces as a function of RH at T = 296 K under dark conditions. Dashed lines

340

show linear fits of the data. The error on RH is given by the error on the hygrometer used to

341

measure it.

342 343

A linear decrease of O3 uptake with increasing RH is observed on montmorillonite; on kaolinite

344

however, the datapoints are more scattered, and the best fit gives a constant, suggesting that the

345

uptake is RH independent. The equations given by the linear fits to the data for montmorillonite

346

(8) and kaolinite (9) are reported: 𝛾𝑆𝑆,𝐵𝐸𝑇 (𝑅𝐻) = −3.25 × 10−11 × 𝑅𝐻(%) + 4.64 × 10−9

(8)


21


Page 22 of 42

𝛾𝑆𝑆,𝐵𝐸𝑇 (𝑅𝐻) ≈ 4.6 × 10−8

(9)

347

An interpretation of these results with the help of the structure and composition of the clay

348

minerals is discussed here. Kaolinite has a fully hydrated structure. Addition of water to the

349

environment (i.e., an increase in RH) is thus not expected to modify its structure.

350

Montmorillonite however, has a structure lacking water; adding water through an increase of RH

351

would gradually change its structure, the mineral expanding while the interlayer spaces are filled

352

with water molecules. The sites in the interlayer spaces would then not be available for ozone

353

uptake any longer, which could explain the observed decrease of the uptake with increasing RH.

354

The chemical formula of kaolinite (Al2Si2O5(OH)4) suggests that the surface of kaolinite dust

355

exhibits

356

((Na,Ca)0,3(Al,Mg)2Si4O10(OH)2·nH2O). An increase of RH leads to a higher water coverage at

357

the surface of dust, and therefore to a higher density of dangling OH groups for an adsorbate to

358

interact. The formation of a water monolayer on natural samples of kaolinite and

359

montmorillonite has been observed at RH = 20-28 %.20,37 Above these values, the surface of dust

360

is expected to be covered with water, although the water layer would not form a uniform

361

coverage but rather forms islands, leaving the surface of dust partly accessible. At the surface of

362

kaolinite dust, increasing RH would show little difference from the perspective of the adsorbate,

363

because the density of OH groups would remain constant, hence the constant uptake of O3 by

364

kaolinite with respect to RH. On montmorillonite, we observe a decrease of O3 uptake when RH

365

increases. The increase in surface OH groups’ density seems, in that case, to be outdone by the

366

structural change hypothesized above and by the competition with H2O for access to reactive

367

sites. For montmorillonite, the structural interpretation seems more consistent with the

368

experimental results.

a

higher

density

of

OH

groups

than

the

surface

of

montmorillonite


22

Page 23 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


369 370

Our measurements can be compared to the steady-state uptake of O3 on SiO2 dust doped with a

371

few percent TiO2 determined in a CWFT reactor at atmospheric pressure:19 under dark

372

conditions, at RH = 24% and 298 K, γSS,BET ≈ 5 × 10-9 at 50 ppb O3. Extrapolation of our results

373

obtained under dry conditions for montmorillonite to RH = 24% leads to γSS,BET = 4.4 × 10-9, a

374

value consistent with Nicolas et al.19 A decrease of O3 uptake with increasing RH is also

375

observed on alumina21,38 and on α-Fe2O3 surfaces.21 It was interpreted as a result of competitive

376

adsorption between water and ozone molecules, H2O blocking the sites where O3 would

377

decompose. This interpretation is not in contradiction with the “structural” interpretation given

378

above, and offers a complementary point of view. In contrast, no RH-dependence of O3 uptake

379

on alumina was observed between RH = 0 and 75 %;26,34 although their values of the uptake are

380

scattered within at least a factor of 2, the authors claim that this spread is not significant. In our

381

experiments, the uptake coefficient of O3 by montmorillonite between RH = 0 and 93 % varies

382

by a factor of about 2. Roscoe and Abbatt38 have shown that after exposure of an alumina surface

383

to O3, exposure to H2O restores its reactivity towards O3. Therefore, the global behavior

384

observed results from the competition between the two effects described above: hindering of O3

385

access to the surface by H2O molecules, and regeneration of the reactive surface sites by H2O.

386 387

3.5. Reaction mechanism for O3 uptake by dust surfaces

388

A thorough discussion requires a description of the nature of the surface sites available for O 3 to

389

interact. On a partially hydrated TiO2 structure, Bulanin et al.39 show with infrared spectroscopy

390

that at least 3 types of reactive sites are present: (I) hydroxyl groups of the surface can form


23


Page 24 of 42

391

hydrogen bonds with O3, with little modification of its structure; (II) weak Lewis acid sites

392

where O3 forms a complex with a distortion of its structure; (III) strong Lewis acid sites where

393

O3 dissociates into O2 and O, the O atom remaining adsorbed on the surface. Physisorption of O3

394

on the surface would also be more significant if the temperature was low enough,39 which is not

395

the case in the present study. It is also suggested that O3 can react with surface OH groups by

396

exchange of an O atom with the hydroxyl group, and subsequent recombination forming

397

H2O.25,42 This appears to be in contradiction with the existence of type I surface sites, OH groups

398

forming an H-bond with O3 and leaving it undissociated; Hanisch and Crowley25 do not see

399

desorption of O3 after its uptake, and conclude that type I surface sites play a minor role in the

400

uptake process at room temperature. Our non-detection of O3 left unreacted at the surface of clay

401

minerals at the end of our experiments, as described below, is in agreement with the observation

402

of Hanisch and Crowley.25

403

Based on the literature available, we propose a reaction mechanism for O3 uptake by dust

404

surfaces. When interacting with initially unreacted surface sites (SS), successive reactions (R1a +

405

R1b) and (R2) with O3 form SS-O and SS-O2 species.17,21,41,42 The adsorption of molecular O3 on

406

surface sites is also considered (R1a).

407 408

O3 + SS → SS-O3

(R1a)

409

SS-O3 → SS-O + O2

(R1b)

410

O3 + SS-O → SS-O2 + O2

(R2)

411


24

Page 25 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


412

Lampimäki et al.40 detail (R1) in a photoelectron spectroscopy (XPS) study. They show that the

413

initial step (R1a) consists in the reversible adsorption of O3 on Lewis acid sites, followed by its

414

dissociation (R1b) leading to the formation of O2 and an O atom that remains on the surface, in

415

agreement with a previous experimental and theoretical work.42 Co-adsorption of O3 with

416

pyridine, a Lewis base, completely suppresses O3 decomposition on alumina surfaces at 77 K.42

417

Hydrochloric acid (HCl) has also been shown to deactivate alumina surfaces for ozone

418

decomposition.32 Pre-treatment of alumina with HNO3 strongly decreases the uptake of O3,

419

whereas prior exposure to SO2 increases it.43 This confirms that availability of strong Lewis acid

420

sites at the surface is required to dissociate O3. The formation of peroxide species, in the form of

421

a reversibly-adsorbed precursor SS-O2, was proposed by Lampimäki et al.40.

422

Reaction (R1a) was further investigated under our experimental conditions. After exposure of

423

montmorillonite dust to O3, a nitrogen monoxide (NO) flow was set through the reactor. NO is

424

expected to react with O3 molecularly adsorbed on the surface, forming NO2. Consumption of

425

NO was not observed at 258 K nor at 338 K; production of NO2 and NOx species was not

426

detected at either temperature. This suggests that O3 does not remain on the surface of

427

montmorillonite dust in detectable amounts on the timescale of a few minutes, but rather

428

decomposes into fragments that are non-reactive with NO. This confirms the observation of

429

Ullerstam et al.44, who did not find evidence for O3 adsorption on Saharan dust at room

430

temperature. From the ±1 ppb uncertainty on the concentrations measured by our NOx analyzer,

431

the knowledge of the amount of NO molecules injected in the setup and the specific surface of

432

montmorillonite, we can derive upper limits of the concentration of SS-O3 sites on

433

montmorillonite at 258 and 338 K: N(SS-O3) < 1.4 × 108 cm-2 at 258 K and < 1.1 × 108 cm-2 at

434

338 K. These numbers are orders of magnitude lower than the number of O3 molecules in a


25


Page 26 of 42

435

monolayer, calculated as 7.4 × 1012 cm-2. (R1a) therefore does not leave stable O3 species at the

436

surface in detectable amounts under our experimental conditions. The surface oxidation reactions

437

that occur in the present study (R1-R2) are shown in blue (full symbols) on the reaction diagram

438

proposed on Figure 7.

439

The reaction of O3 with surface sites leaves oxygen species (O and O2) bound to the surface. We

440

suggest that SS sites and O3 can be regenerated by the reaction of SS-O sites with O2 from the air

441

bath (R3). Reaction (R3) is similar to the gas-phase termolecular reaction of oxygen atoms with

442

O2 and a third body, leading to the formation of O3.45 However, it is proposed here to occur at the

443

surface of dust for the first time. We suggest that (R3) should not be neglected in the reaction

444

mechanism of O3 with mineral dust surfaces, and more information about its contribution is

445

given in the next section. The oxygenated surface sites SS-O have been proposed to self-react to

446

release O2 and make fresh SS sites available (R4);21 Li et al.41 however ruled out this reaction

447

with isotopic substitution experiments and Raman spectroscopy. SS-O2 sites can self-decompose

448

and release O2,17,41 regenerating SS sites (R5) that are available for O3 uptake; this reaction is

449

favored by higher temperatures. The regeneration reactions (R3 and R5) are shown in green

450

(open symbols) on Figure 7.

451 452

SS-O + O2 → SS + O3

(R3)

453

SS-O + SS-O → 2SS + O2

(R4)

454

SS-O2 → SS + O2

(R5)

455


26

Page 27 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


456

Over the temperature range investigated in this study, the O3 concentration at steady-state is

457

clearly distinct from the “control” concentration recorded when the O3 flow does not interact

458

with the dust, as seen on Figure 2. This means that O3 molecules impinging the surface keep

459

finding sites where they can react and that the regeneration mechanism (reactions R3 and R5) is

460

efficient over the whole temperature range investigated. Part of the reaction process on clays is

461

hence catalytic, similarly to what is observed on metal oxides observed by others.21,23,42 The

462

regeneration reactions (R3 and R5) can account for the increased O3 uptake by clay dust as

463

temperature is raised.

464 465

Figure 7. Schematic representation of O3 reaction pathways with surface sites (SS) of clay dust.

466

Oxidation pathways of SS are shown in blue (full symbols); regeneration pathways of SS sites

467

are shown in green (open symbols).

468


27


Page 28 of 42

469

4. Atmospheric Implications

470

4.1. Comparison with literature

471

Although this study focuses on the determination of steady-state values of the uptake, it is worth

472

reminding the values of initial uptakes measured for ozone interaction with mineral dust to

473

broaden the scope of the discussion. Literature values of O3 uptake by mineral dust are compiled

474

in Figure 8A, where the atmospheric lifetime of O3, τ, determined with equations (10) and (11)

475

is plotted as a function of the uptake coefficient. (10) is an alternative expression of equation (4),

476

where A represents the surface/volume ratio of the aerosol, here taken as A = 1.5 × 10-6 cm2 cm-3,

477

a value measured by de Reus et al.10 during a dust event over the Canary Islands; (11) expresses

478

the atmospheric lifetime τ.

𝑘𝑘𝑖𝑛 =

𝛾𝑐𝐴⁄ 4

𝜏 = 1⁄𝑘

𝑘𝑖𝑛

(10)

(11)


28

Page 29 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


479

480 481

Figure 8. A) The atmospheric lifetime of O3, τ, is plotted as a function of the uptake coefficient

482

of O3 by mineral dust compiled from the literature. Initial uptakes are shown in black, steady-

483

state uptakes in red. The area shaded in grey corresponds to uptake coefficients larger than 10-5


29


Page 30 of 42

484

that are considered of significance in numerical simulations of the atmosphere.45 Numbers relate

485

to reference numbers; TW = this work. B) The uptake coefficients and O3 atmospheric lifetimes

486

have been corrected to account for the pressure in the experimental setups. Values of the pressure

487

were taken as those given in the references or as 0.1 mbar (7.5 mTorr), whichever value is the

488

lowest.

489 490

The geometric uptake overestimates the value of the uptake by a factor of 50 to 100. Hanisch and

491

Crowley25 measured the initial geometric uptake of O3 on kaolinite and illite in a Knudsen cell as

492

γ0,geom ≈ 10-4, at their limit of detection. The initial uptake observed on montmorillonite was

493

below their detection limit, i.e. γ0,geom < 10-4.25 Taking into account the BET specific surface of

494

the dust samples leads to lower values: Michel et al.24 measured γ0,BET = 3 × 10-5 for kaolinite.

495

This value is similar to that found by the same authors for Saharan dust23, γ0,BET = 6 × 10-5; the

496

stronger O3 uptake by Saharan dust with respect to kaolinite is explained by the presence of Fe

497

oxides in Saharan dust.24 Indeed, it has been shown23,24 that O3 uptake increases from SiO2 to

498

Al2O3 and from Al2O3 to Fe2O3,21 likely due to the presence of Lewis acid sites of increasing

499

strength. On Al2O3 dust, O3 uptake is lower than on Saharan dust, with γ0,BET = 1.5 × 10-5.26

500

Hanisch and Crowley25 tentatively determined the steady-state uptake of O3 interacting with

501

Saharan dust, and found γSS = 8 × 10-4 that they extrapolate to 7 × 10-6 under atmospheric

502

conditions of O3 concentration; in their experiments, Hanisch and Crowley25 use N2 a bath gas.

503

Michel et al.24 measured γSS,BET = 6 × 10-6 for O3 uptake on Saharan dust in a Knudsen cell, in

504

good agreement with Hanisch and Crowley25. γSS,BET = 2 × 10-6 for O3 uptake by kaolinite was

505

measured by Karagulian and Rossi27 also in a Knudsen cell.


30

Page 31 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


506

These values, including those determined at steady-state with the BET surface area, are much

507

higher than those determined in this study, which are typically γSS,BET < 10-7 on kaolinite and
10-5 used in models are not representative of O3 uptake by fresh clay dust.

594

Meanwhile, these values could describe the interaction of O3 with clay dust aged with organic

595

compounds in the atmosphere. Efforts are needed to quantify O3 interaction with aged dust under


34

Page 35 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


596

atmospheric conditions in order to make a proper assessment of the uptake coefficients used in

597

models;

598

• Last, the strong difference between experiments performed at atmospheric pressure (e.g., flow-

599

tube and environmental chamber experiments) and at low pressure (e.g., Knudsen cell

600

experiments) was highlighted. Experiments conducted at atmospheric pressure give much lower

601

values of the uptake of O3 by dust; we hypothesize that this may be explained by the pressure-

602

dependence of reactions (R3) and (R5). Studies conducted at atmospheric pressure are more

603

relevant to the atmospheric environment, and uptakes determined in these conditions should be

604

preferred in models for a more realistic description of such heterogeneous processes in the

605

atmosphere. This last point stresses the need for a more detailed study to validate the existence of

606

a pressure-dependence in the reaction mechanism.

607 608

AUTHOR INFORMATION

609

Corresponding Author

610

Jérôme Lasne: Phone: +33 (0)3 27 71 22 68; E-mail: [email protected]

611

Author Contributions

612

The manuscript was written through contributions of all authors. All authors have given approval

613

to the final version of the manuscript.

614

Funding Sources


35


Page 36 of 42

615

This work was achieved in the frame of Labex CaPPA, funded by ANR through the PIA under

616

contract ANR-11-LABX-0005-01, and CPER CLIMIBIO project, both funded by the Hauts-de-

617

France Regional Council and the European Regional Development Fund (ERDF).

618

Acknowledgements

619

The authors acknowledge Mr Vincent Gaudion and Mr Mohamad Zeineddine (IMT Lille Douai,

620

SAGE) for their assistance in the lab. Mr Bruno Malet and Dr Laurent Alleman (IMT Lille

621

Douai, SAGE) are gratefully acknowledged for conducting the ICP-AES experiments. JL

622

acknowledges support from the Labex CaPPA and CPER CLIMIBIO projects and the Hauts-de-

623

France Regional Council for a post-doctoral fellowship. The authors gratefully acknowledge

624

Prof. M.J. Rossi (Paul Scherrer Institute) for insightful discussions, and two anonymous referees

625

for constructive comments that improved the manuscript.

626 627

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