Heterogeneous Photo-oxidation of SO2 in the Presence of Two

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Heterogeneous Photooxidation of SO2 in the Presence of Two Different Mineral Dust Particles: Gobi and Arizona Dust Jiyeon Park, Myoseon Jang, and Zechen Yu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00588 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 30, 2017

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Environmental Science & Technology

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Heterogeneous Photooxidation of SO2 in the Presence of Two Different Mineral Dust Particles: Gobi and Arizona Dust Jiyeon Park, Myoseon Jang,* and Zechen Yu Department of Environmental Engineering Sciences, P.O. Box 116450, University of Florida, Gainesville, FL, 32611, USA

Abstract

8

The impact of authentic mineral dust particles sourced from the Gobi Desert (GDD)

9

on the kinetic uptake coefficient of SO2 was studied under varying environments (humidity,

10

O3, and NOx) using both an indoor chamber and an outdoor chamber. There was a significant

11

increase in the kinetic uptake coefficient of SO2 ( , ) for GDD particles under UV

12

light compared to the value ( , ) under dark conditions at various relative humidities

13

(RH) ranging from 20% to 80%. In both the presence and absence of O3 and NOx, ,

14

and , greatly increased with increasing RH. The resulting , of GDD

15

particles was also compared to that of Arizona Test Dust (ATD) particles.

16

values of GDD were 2 to 2.5 times greater than those of ATD for all RH levels. To

17

understand the photocatalytic act of dust particles, both GDD and ATD were characterized for

18

the metal element composition of fresh particles, the aerosol acidity of aged particles, and the

19

hygroscopic properties of both fresh and aged particles. We conclude that the difference in

20

the formation of sulfate between GDD and ATD particles is regulated mainly by the quantity

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of the semi-conductive metals in dust particles and partially by hygroscopic properties.

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ACS Paragon Plus Environment

The ,


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TOC/Abstract art

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1. Introduction

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Mineral dust particles are one of the largest contributors to particle mass loading in

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the ambient atmosphere, with an estimated annual emission of 1000−3000 Tg yr-1.1,

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Mineral dust particles larger than 100 µm in diameter quickly settle near the desert regions,

34

whereas dust particles smaller than 20 µm are globally transported over thousands of

35

kilometers due to their long lifetimes.3-5 During long-range transport, they provide significant

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surfaces for the heterogeneous reaction with atmospheric trace gases such as SO2,6 O3,7-9

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NO2,10,

38

significantly impact radiation balance,17-19 cloud formation by serving as cloud condensation

39

nuclei and ice nuclei,20 marine productivity due to dust-bonded iron,21-23 visibility

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impairment,24, 25 and human health.26, 27

11

N2O5,12,

13

HNO3,10,

14, 15

2

and H2O2.16 In addition, mineral dust particles can

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SO2 is a major air pollutant and can be heterogeneously transformed into sulfate on

42

the surface of dust particles. The kinetics of heterogeneous oxidation of SO2 have been

43

studied on both pure metal oxides (e.g., TiO2, Fe2O3, and Al2O3)28-30 and authentic mineral

44

dust (i.e., Saharan dust).6, 31, 32 The uptake coefficient of SO2 onto pure metal oxides (~10−4)6,

45

32

46

and dry China loess (3 (±1) × 10-5),28 due to the significant fraction of SiO2. Huang et al.

47

have found that the uptake coefficient of SO2 on Asian dust and Arizona Test Dust (ATD)

48

particles is known to be significantly influenced by relative humidity (RH).

49

uptake coefficient has been focused on autoxidation in open air under dark conditions.

50

date, it is not fully understood how the uptake coefficient of SO2 is influenced by

51

meteorological variables (humidity, temperature, and light intensity) and by atmospheric

52

conditions due to other air pollutants (NOx and O3), particularly in the presence of air-

53

suspended mineral dust particles.

54

is generally larger than authentic dust particles, such as Saharan dust (an order of 10−5)28, 32 33

Typically, the To

Semi-conductive metal oxides were known to be responsible for heterogeneous



55

photooxidation of tracer gases such as NO2, H2O2, O3, and SO2.16, 34 Ndour et al.11, 35 have

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studied the uptake coefficient of NO2 on Saharan dust particles, and the values were higher

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under UV light by two orders of magnitude compared to the values under dark conditions.

58

Romanias et al. reported that the initial uptake coefficients of H2O2 on Al2O3 and Fe2O3 were

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similar under dark and UV conditions.36

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intensity on the heterogeneous interaction of H2O2 with ATD.37 Chen et al. reported that UV

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light significantly enhances the uptake of O3 on both Fe2O3 and TiO2 surfaces, whereas Al2O3

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exhibits no uptake capacity for O3 under UV light.38 Our recent study39 showed that UV light

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significantly increased the kinetic uptake coefficient of SO2 in the presence of air-suspended

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ATD particles. The mechanism of mineral dust photooxidation is electron-hole pair theory.

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When a photon is absorbed by metal oxides in dust particles, an electron-hole pair can be

66

produced via the activation of an electron from the valence band to the conduction band of

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metal oxides, leading to the formation of OH, HO2 and other radicals.39-42

However, Zein et al. showed the impact of UV

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Here, we extensively investigated the kinetic uptake coefficient of the photocatalytic

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oxidation of SO2 in the presence of two different mineral dust particles: Gobi Desert Dust

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(GDD) particles and ATD particles using a 2-m3 indoor photo-irradiation chamber.

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Heterogeneous photooxidation of SO2 on the surface of GDD particles was studied in the

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presence and absence of O3 and NOx over a wide range of RH conditions under UV light.

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To better understand how the chemical characteristics of two different dust particles influence

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their photocatalytic oxidation, studies were taken for the hygroscopic property of fresh and

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aged particles, the aerosol acidity of aged particles, and the metal composition of fresh dust

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particles using the outdoor chamber.

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2. Experimental section

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2.1 Characterization of dust particles

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Two types of mineral dust particles were used: GDD particles collected from the


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Tsogt-Ovoo soum in the Umnugovi province, Mongolia between March and May, 2015 and

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sieved to ~20 µm diameter and ATD (size ranges: 0–3 µm, Powder Technology Inc.,

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Minnesota, USA). The GDD sample was collected from the dust source regions where the

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dust was deposited. The specific surface area was measured using the BET method (NOVA

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2200). The BET surface areas of GDD and ATD particles were 39.6 and 47.4 m2 g-1,

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respectively. Elements of GDD and ATD particles were analyzed using energy dispersive

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spectroscopy (EDS, model 6505, Oxford Inc., England) (Figure S1 (a) in Section S1). The

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elements of the two types of mineral dust particles are similar, but fractions of Ti and Fe in

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GDD particles were higher than those in ATD particles. The similar results were observed for

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elements of each GDD and ATD particle samples obtained from the previous studies.34, 43, 44

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The size distribution of GDD and ATD particles was measured using both the Scanning

91

Mobility Particle Sizer (SMPS; TSI 3080, USA) and the Optical Particle Counter (OPC; TSI

92

3330, USA) (Figure S1 (b)). The size distribution of GDD particles was much broader at

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larger sizes. The concentrations of water soluble ions on the surface of GDD and ATD were

94

measured with a Particle Into-Liquid Sampler (Applikon, ADISO 2081) coupled with Ion

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Chromatography (Metrohm, 761 Compact IC) (PILS-IC). Figure S1 (C) shows that the

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amount of water soluble ions on GDD was higher than that on ATD.

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2.2 Indoor Chamber Experiments

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Heterogeneous photooxidation of SO2 on the surface of GDD and ATD particles was

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performed at varying RH levels in the presence and absence of O3 and NOx using a 2 m3

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Teflon indoor chamber under UV-visible light conditions. Spectral irradiance of the light

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sources used in the current study was measured by a fibro-optical portable spectrometer

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(EPP2000, Stellar Net Inc., USA) (Figure S2 in Section S2). The detailed procedures for the

103

indoor chamber experiments have been reported in the SI (Section S3).39, 45 The experimental

104

conditions for SO2 photooxidation in the presence of GDD and ATD are summarized in Table



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1. Although the chamber air was initially flushed with clean air, photochemical reactions

106

inside the chamber can produce OH radicals via photolysis of the HONO off-gassing from

107

the wall and photolysis of the small amount of formaldehyde. To confirm the reproducibility

108

of the OH radicals in the gas phase, the initial concentration of formaldehyde in the chamber

109

was

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hydroxylamine hydrochloride (PFBHA) (Fluka, USA) and analyzed using a Gas

111

Chromatograph–Ion Trap Mass Spectrometer (CP-3800 GC, Saturn 2200 MS, Varian Inc.). In

112

addition to the presence of photolytic species, the chamber is not perfectly air tight and is

113

diluted by a 1st-order rate, which also allows intrusion of air from outside of the chamber.

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The initial concentration of HCHO was 3.66 ± 2.11 ppb, which is much lower than ambient

115

concentrations (∼20 ppb).46

116

2.3 Measurements of the hygroscopic property and the aerosol acidity of dust particles

measured

by

derivatizing

formaldehyde

by

O-(2,3,4,5,6-pentafluorobenzyl)

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Table S1 summarizes outdoor chamber experimental conditions for SO2

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photooxidation in the presence of GDD and ATD. The detailed experimental procedures are

119

described in the SI (Section S4).47, 48

120

ATD particles was determined using a Fourier Transform Infrared spectroscope (FTIR)

121

combined with an optical flow chamber (refer to Section S5).

122

([H+]C-RUV, µmol L-1 by aerosol volume) of aged GDD and ATD was measured using a

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colorimetry integrated with a reflectance UV-Visible spectrometer (C-RUV).

124

capacity of the surface of both GDD and ATD (discussed in Section 3.3.2) was estimated by

125

comparing the actual aerosol acidity, as measured by C-RUV, to the aerosol acidity predicted

126

using the inorganic thermodynamic model (E-AIM II)49 using an inorganic composition from

127

PILS-IC.47 Measurements of both the hygroscopic properties and the aerosol acidity were

128

performed using the University of Florida Atmospheric Photochemical Outdoor Reactor (UF-

129

APHOR) due to its large chamber volume (104 m3).

The hygroscopic property of fresh and aged GDD and


The actual aerosol acidity

The reaction

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3. Results and discussion

3.1 The kinetic uptake coefficient of SO2 via autoxidation: ,

A first-order rate coefficient for autoxidation ( , s-1) of SO2 in the presence of

mineral dust particles was calculated from the decay rate of SO2 under dark conditions:39

134 135 136

137 138

[ ] = − [ ] − % [ ] "

where [ ] is the concentration of gaseous SO2 in the chamber (mol m-3) and % is the rate constant of SO2 wall loss (s-1). is defined by the kinetic uptake coefficient of SO2 ( , ) on dust particles under dark conditions as given below:

=

& ,

140

(

eq. 2

4 where & is the mean molecular velocity of gaseous SO2 (m s-1) and ( is the geometric surface area of dust particles (cm2 m-3). & is defined using eq. 3:

& = ) 139

eq. 1

8+, -.

eq. 3

where R is the gas constant (J mol K−1), T is the absolute temperature (K), and . is the

molecular weight of SO2 (kg mol−1). ( is determined using eq. 4:

( = / exp(−4 ")

eq. 4

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where S0 is the total concentration of the initial geometric surface area of dust particles (cm2

142

m-3) and kp is the first-order rate constant for the wall loss of dust particles (s-1). By

143

substituting eqs. 2–4 with eq. 1, the analytical solution is described as

ln

& , / [ ] = −(1 − exp(−4 ")) − % " [ ]/ 4 4

eq. 5

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The adsorbed SO2 can produce sulfate via heterogeneous reactions on dust surfaces. The

145

present study assumes that SO2 decay after correction of SO2 loss to the wall equals

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formation of sulfate. Thus, observed [ ] is determined by subtracting the sulfate

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concentration ([9 : ] ) from the wall loss-corrected gaseous [ ]/, as shown in eq. 6 ACS Paragon Plus Environment


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below:

[ ] = [ ]/ exp(−% ") − [9 : ]

149

eq. 6

where [ ]/ is the initial concentration of gaseous SO2 (mol m-3), [ ] is the

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concentration of SO2 at time t (min) (mol m-3), and [9 : ] is the amount of sulfate

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produced via the heterogeneous reaction of SO2 on dust particles at time t (min) (mol m-3).

152

Thus, this made it possible to kinetically characterize the formation of sulfate on dust

153

surfaces.

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3.2 The kinetic uptake coefficient of SO2 via photooxidation: ,;?

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To estimate the kinetic uptake coefficient for the heterogeneous photocatalytic

156

oxidation of SO2 ( , ), the sulfate concentrations observed in the chamber data need

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several corrections for the following: preexisting indigenous sulfate, the wall procedure of

158

gaseous species (SO2, NOx and O3) and particles, autoxidation, and SO2 oxidation in the gas

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phase.

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(1)

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GDD mass ranged from 0.9%–1.8%.

162

experiment under dark and UV light conditions were corrected for the quantity of indigenous

163

sulfate.

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(2)

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loss to the wall were obtained from separate experiments under dark conditions and applied

166

to both , and , . The 4 for GDD and ATD were 0.31 × 10-1 min-1 and

Preexisting indigenous sulfate: The measured fraction of indigenous sulfate to total All sulfate data obtained from each chamber

Chamber wall procedure: The rate constants (1st-order process) of particle and SO2

167

0.66 × 10-2 min-1, respectively. In the GDD/SO2 system % at 20%, 55%, and 80% RH

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were 7.79 × 10-4 min-1, 1.42 × 10-3 min-1, and 2.47 × 10-3 min-1, respectively.

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(3)

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reaction of adsorbed SO2 with an oxygen molecule in the absence of O3 under the dark

171

conditions. To estimate , solely by heterogeneous photocatalytic oxidation, the

Impact of autoxidation: The autoxidation of SO2 is an oxidation process via the


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sulfate data obtained under UV light is also corrected for sulfate via autoxidation.

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, is determined at three different RH levels in the presence and absence of O3 and

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NOx (D1-9 in Table 1).

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(4)

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SO2 is decoupled into sulfate from oxidation in the gas phase (non-dust origin sulfate) and the

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sulfate formation via dust-driven photooxidation.

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autoxidation (correction 3), the sulfate data produced under UV light should also be corrected

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by the sulfate formed from SO2 oxidation in the gas phase to estimate , . To confirm

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reproducibility of the chamber experiments, SO2 oxidation was performed in triplicate (error

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range: 2% of the total sulfate).39

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Sulfate formation in the gas phase: The sulfate formation via photooxidation of

In addition to the correction of

The SO2 oxidation in the presence of mineral dust particles under UV light can be written as,

[ ] B B [ ] − C [D][ ] − E [ ] = −(@A "

eq. 7

− [ ] − % [ ]

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where (@A is a 1st order rate constant for photocatalytic SO2 uptake on the surface of dust B

particles (s-1), C is a rate constant for the gas-phase reaction of SO2 with OH radicals (m3

186

molecules-1s-1), and E is a rate constant for SO2 uptake on H2SO4 particles (s−1). To

187

estimate the reaction rate of SO2 for given dust particles, the gas-phase photooxidation of SO2

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B (terms C [D][ ] and E [ ]) is subtracted from eq. 7.

189

B

[ ] B = −(@A [ ] − [ ] − % [ ] "

(@A is related to , as follows: B

(@A = B

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& , 4

(

eq. 8

eq. 9

By substituting eqs. 3, 4, 6, and 9 with eq.8, the analytical solution is derived as



ln

& , / [ ] = −F1 − expF−4 "GG [ ]/ 4 4 − (1 − exp(−4 "))

& , / 4

4

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eq. 10

− % "

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To use eq. 10, the sulfate concentration measured in the chamber experiments was corrected

192

for indigenous sulfate and the sulfate formation in the gas phase.

193

The concentrations of sulfate originating from the gas-phase photooxidation of SO2

194

were determined at three different RH levels in the presence and absence of O3 and NOx (G1-

195

9 in Table 1).

196

was inevitable because ammonia off-gasses from the chamber wall during the daytime due to

197

the decomposition of ammonium sulfate carried over from previous chamber experiments.

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Our PILS-IC data suggest that most aerosol systems comprising NH4+and SO42- are

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effloresced (NH4+ mol/SO42- mol: 1.0 ∼ 2.0), except the simulations at 80% RH. Among the

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terms in eq. 7, the loss of SO2 to the wall (3rd term, % ") is the largest over the course of

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the chamber experiment.

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80% RH and 88% loss at 20% RH.

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dust particles, the sulfate formation in the presence of GDD can be slightly underestimated.

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The estimated uncertainty in , due to the subtraction method ranges from 1%

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(SO2/GDD experiment) to 17% (SO2/NO2/GDD at low RH) after a 2 hour reaction time and

206

is negligible for SO2/GDD at a high RH (see the footnote in Table 1).

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insignificant compared to the difference between the , values produced under

208

different environmental variables such as humidity, NOx and O3 (see section 3.2).

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, produced in this study (Table 1) are valuable to study the effect of environmental

210

variables and dust compositions sourced from different types of dust.

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kinetic uptake coefficients reported in Table 1, the BET surface area was used for ( in eqs.

Partial neutralization of gas-phase H2SO4 (non-dust phase) with ammonia

For example, 99% of SO2 consumption is due to the wall loss at By subtracting the gas phase H2SO4 formed without


Such uncertainty is

Thus, the

To estimate the

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2 and 9. Both , and , values were semiempirically fit to the

213

experimental data using eqs. 5 and 10 (Figure S3 in Section S6).

214

3.3 The effects of light, NOx, O3, and humidity

215

In Figure 1 (a)-(c), , and , at three different RH levels are

216

illustrated for the GDD/SO2 system, GDD/SO2/O3 system, and GDD/SO2/NOx system.

217

Overall, both , and , increased with increasing RH for all systems as

218

shown in Figure 2 (a)-(d). The amount of SO2 on dust surfaces becomes greater by increasing

219

the water content in particles at the higher RH and thus, forms more sulfate via heterogeneous

220

reactions.

221

produces the more OH radicals via the reaction of the water molecule with electrons or holes

222

(Section S7 and Figure S4).

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in the previous study (section S8).39

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will be discussed in Section 3.4.1.

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In addition to the partitioning process, the higher water content on dust particles

Similar experiments were performed for the ATD/SO2 system More details about the water content on dust surfaces

In the GDD/SO2/O3 system, , was one order of magnitude higher than

226

, , suggesting that photochemistry plays an important role in heterogeneous SO2

227

oxidation. Photocatalytic conductive metal oxides can promote the production of dust-surface

228

OH radicals,39,

229

, in the GDD/SO2/O3 system (Exp. D4–6 and L4–6) were also greater than those in

230

the GDD/SO2 systems (Exp. D1–3 and L1–3). O3 directly reacts with a photochemically

231

generated electron on metal oxides (Figure S4), which can produce the ozonide radical (O3-

232

).38 O3- reacts with water to generate the OH radical.38 Even under dark conditions, SO2

233

irreversibly adsorbs as sulfite (SO32-) on dust surfaces and rapidly oxidized to sulfate in the

234

presence of O3.28, 30

42

which can oxidize adsorbed SO2 (Section S7). Both , and

235

At high levels of NOx, the production of sulfate is known to be suppressed in the

236

absence of the dust particle due to the competition of the OH radical between NO2 and SO2.32



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237

Through dust-induced heterogeneous photochemical processes (Section S7), NO2 can form

238

HONO, which can be a source of OH radicals via photolysis and enhance sulfate formation.40,

239

50

240

chamber data showed that , of GDD/SO2/NOx (Figure 1 (c)) is slightly lower than

241

that of GDD/SO2 (Figure (a)) suggesting that sulfate formation is somewhat suppressed by

242

NOx.

243

3.4 ,;? in different dust particles: GDD vs. ATD

244

However, NO2 can compete for dust-originated OH radicals with SO2.

Our indoor

Figure 1 (d) compares , of GDD to that of ATD at three different RH levels.

245

For comparison, , of ADT, recently reported by Park and Jang (2016),39 were

246

reconstructed using the BET surface area and the scale factor for light intensity (1.64) as

247

shown in Table S2.

248

1) was greater by 2 to 2.5 times for all RH levels (Exp. L11-13). The , values of

249

GDD were also higher than the values of ADT (by approximately 3 times) (Figure S5 in

250

Section S8).

251

from SO2 photooxidation at presence of GDD and ATD using UF-APHOR under natural

252

sunlight and demonstrate GDD’s strong photocatalytic ability compared to ATD. To

253

comprehensively understand why GDD particles produce more sulfate than ATD, we focused

254

on the following chemical characteristics of dust particles: (1) the hygroscopic properties of

255

dust particles, (2) the amount of alkaline carbonate and metal oxides, which can react with

256

sulfate and nitrate formed on dust surfaces, and (3) photoactivation of conductive metal

257

oxides of mineral dust particles (more detailed explanation in sections 3.4.1–3.4.3).

258

3.4.1 Hygroscopic properties of fresh and aged GDD and ATD particles

Compared to ATD particles, , with GDD (Exp. L1-3 in Table

The sulfate mass concentrations shown in Figure S6 (Section S9) were sourced

259

To understand the influence of the hygroscopic properties of GDD and ATD on

260

, , the water content of both fresh and aged mineral dust particles was measured


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261

using the FTIR spectrometer over a wide range of RH levels (10%–80%). Most particles

262

impacted on the FTIR window originate from dust particles due to the high collection

263

efficiency of larger particles (i.e., little influence by H2SO4 produced via gas-phase reaction).

264

The water content in Figure 2 is defined as the water mass normalized by the dry particle

265

mass. The water contents of both fresh GDD and fresh ATD were small with values below 40%

266

RH and increased rapidly above 50% RH (Figures 2(a) and 3(b)).

267

hygroscopic property of fresh dust particles is consistent with the trends in both ,

The trend of the

268

and , values measured at three different RH levels (Figure 1).

269

suggests that the water content of dust particles can significantly affect the heterogeneous

270

oxidation of SO2 under different RH conditions via a pseudo-multiphase mechanism.

271

Although the mass fraction of water soluble indigenous inorganic ions (e.g., sulfate) of GDD

272

was significantly greater than that of ATD (Figure S1 (C)), the water content sourced from

273

water soluble ions was negligible (Figure 2).

274

carbonates and metal oxides, might determine the humidity-dependent uptake coefficients of

275

SO2.

This accordance

Thus, other dust constituents, such as alkaline

276

The water content of fresh GDD was approximately 2 times higher than that of the

277

fresh ATD between 40% and 80%RH, showing a substantially greater affinity for water with

278

GDD. Overall, we found , to be higher for GDD compared to ATD under all of the

279

RH conditions.

280

even at 20% RH, where water content for both GDD and ATD particles were very small.

281

This suggests that the water content of the dust surface is not enough to explain the difference

282

between two particles in , .

283

The difference between GDD and ATD in , was still substantial

Figure 2 (c) and (d) shows the water contents of photochemically aged GDD and ATD

284

in the presence of SO2 as a function of RH.

285

(b)), the water contents of aged GDD and ATD particles increased exponentially from 60%

Unlike the fresh dust particles (Figure 2 (a) and



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286

RH. This occurred due to the production of sulfate by SO2 photooxidation on the surface of

287

the dust particles. To quantify the contribution of water soluble inorganic species on the

288

hygroscopic property of aged dust particles, the water content by inorganic species was

289

determined by integration of PILS-IC data, which mainly consist of sulfate and ammonium,

290

into the inorganic thermodynamic model.

291

photooxidation of SO2 influences the hygroscopic property of aged particles as shown in

292

Figure 2 (c) and (d).

293

because of the involvement of various reactions on the dust surface.

The sulfate produced by the heterogeneous

However, the hygroscopic property of aged dust particles is complex

CaCO3 + H2SO4 → CaSO4 + H2O + CO2

R1

Al2O3 + 3H2SO4 (aq) → Al2(SO4)3 (aq) + 3H2O

R2

Fe2O3 + 3H2SO4 (aq) → Fe2(SO4)3 (aq) + 3H2O

R3

294

Acidic sulfate reacts with alkaline carbonates and depletes carbonates by evaporating CO2

295

(R1). The formation of CaSO4 generally decreases the hygroscopic property of dust particles

296

while the formation of Al2(SO4)3 and Fe2(SO4)3 (R2 and R3) via the reaction of sulfuric acid

297

with metal oxides can increase water affinity.

298

3.4.2 The aerosol acidity of aged dust particles

299

When sulfuric acid on dust particles reacts with alkaline carbonate or metal oxides,

300

actual aerosol acidity drops. The modification of aerosol acidity varies with the chemical

301

composition of dust particles. To investigate the chemical reactions that occur between dust

302

particles and sulfates on dust surfaces, aerosol acidity ([H+], mol L-1) of photochemically

303

aged GDD and ATD particles was measured using C-RUV ([H+]C-RUV) and compared to [H+]

304

predicted from the PILS-IC data and the E-AIM Model II ([H+]PILS-IC). Figure 3 (a) shows the

305

concentration of water soluble inorganic compositions of photochemically aged GDD and

306

ATD (e.g., NH4+ and SO42-). During the course of the chamber experiments, the neutralization

307

of acidic sulfate was unavoidable due to the ammonia off-gassing from the chamber wall


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during the daytime. [H+]C-RUV was determined directly based on the filter sample without

309

water extraction using an in situ optical technique. The [H+]C-RUV in the aerosol collected on

310

the filter will thermodynamically interact with an indicator and show color.

311

influenced by both the neutralization of sulfuric acid with ammonia and any reaction of acidic

312

sulfate with dust constituents. Figure 3 (b) compares [H+]C-RUV and [H+] PILS-IC of the aged

313

GDD and ATD. The results show that [H+]C-RUV was much lower than [H+] PILS-IC, suggesting

314

there is more to the reaction than the neutralization of sulfuric acid with ammonia (R1-R3).

[H+]C-RUV is

315

The reaction capacity of the surface of both the aged GDD and ATD was also

316

estimated by comparing the actual aerosol acidity, as measured by C-RUV, to the aerosol

317

acidity predicted by the inorganic thermodynamic model (E-AIM II) using an inorganic

318

composition from PILS-IC.47, 49 It was assumed that CaCO3 and CaSO4 (reaction R1) cannot

319

affect the aerosol acidity due to their low solubility in the aqueous phase. The quantity of

320

sulfate which reacted with the alkaline carbonates and metal oxides of dust particles is limited

321

to the reaction capacity of the mineral dust surface.

322

reacted with the dust particle at an excess acidity ([H+]C-RUV > 0) was normalized by the dust

323

mass.

324

technique.

325

, cannot be explained by the different reaction capacities.

326

3.4.3 The metal components of fresh GDD and ATD particles

The concentration of the sulfate that

The values for the aged GDD and ATD were 6.9% and 7.0%, respectively, using our This suggests that the difference between the values of GDD and ATD in

327

As shown in Sections 3.4.1 and 3.4.2, both the hygroscopic property and the reaction

328

capacity of GDD and ATD can partially influence the heterogeneous production of sulfate,

329

but they may not be major contributors to the variation in the photocatalytic activity of

330

different dust particles.

331

catalytic reactions due to conductive metal oxides, leading to the formation of highly reactive

332

oxidants, such as OH, HO2, and other radicals.42, 51 As shown in Figure S1 (a), the metal

In the presence of UV light, both GDD and ATD can undergo



Page 16 of 26

333

oxides of Fe, Al, and Ti, which can act as a photocatalyzer, are higher in GDD particles. The

334

same observation has been reported in the laboratory study by Ndour et al. for measuring the

335

uptake coefficient of NO2 on the surface of Saharan dust particles collected from different

336

locations.11

337

higher fraction of Ti in the total dust. Previous chamber study reported that Fe2O3, Al2O3 and

338

TiO2 particles play an important role in heterogeneous photooxidation of SO2.40, 52

339

4. Uncertainties and Atmospheric Implication

In their study, higher reactivity occurred in the Saharan dust that contained a

340

In estimating , , the amount of the sulfate via dust phase photooxidation of

341

SO2 is estimated by subtracting the sulfate produced by gas phase oxidation from the total

342

sulfate (eqs. 7 and 8). By this approach, the uncertainty in , occurs as seen in

343

Figure 1 (notice error bars) and the footnote in Table 1.

344

relatively small in our experimental conditions, further studies are needed in the future

345

through the simulation using a kinetic solver integrated with light intensity.

346

properties of the dust particles could possibly change due to the chemical reaction, such as

347

the reaction of sulfuric acid with metal oxides (i.e., Fe and Ti), and could dynamically

348

modulate , .

Although this uncertainty is

The chemical

The chemical compositions of dust particles differ from authentic

349

dust sourced in different locations and yields different , values. The indoor

350

chamber lighting conditions also differ from natural sunlight in intensity as well as the shape

351

of the corresponding light spectrum. With regard to applying , to ambient

352

conditions, the , reported in the present study should be revisited using simulations

353

in an outdoor chamber.

354

Asian dust storms frequently passed through the polluted urban area53 where the

355

concentrations of O3 and NOx are significant. As seen in Figure 1, NOx can somewhat

356

suppress the formation of sulfate, while ozone can significantly enhance sulfate formation, in


Page 17 of 26


357

particular at high RH levels. In ambient conditions, the intensity of sunlight and humidity are

358

dynamic and the formation of sulfate will have a diurnal pattern. During daytime, humidity

359

becomes lower as the intensity of sunlight increases and , at low RH regions

360

becomes important.

361

, via autoxidation becomes significant. With ozone in the ambient air, ,

At nighttime, humidity reaches the levels higher than 80% and

362

at a high RH (80%) are comparable to , values at low RH regions (Figure 1),

363

suggesting the importance of both nighttime and daytime chemistry for heterogeneous

364

oxidation of SO2.

365

underestimated at regional scales and the evaluation of the effect of meteorological

366

parameters and tracer gases on sulfate formation can be inaccurate.

367

ASSOCIATED CONTENT

368

Supporting Information

369

Supplementary experiment information and data related to this article can be found in

370

Supporting Information:

371

Fractions of elements, size distribution, and the mass of water soluble ions normalized by the

372

dust mass (Figure S1 in Section S1). The chamber experiment details of heterogeneous

373

oxidation SO2 at presence of mineral dust particles using an indoor chamber (Figure S2 in

374

Section S2 and Section S3) or using a large smog chamber (Section S4 Section S5 and Table

375

S1). The estimation of kinetic uptake coefficient of SO2 (Figure S3 in Section S6). The

376

mechanisms of heterogeneous oxidation of SO2 (Figure S4 in Section S7). , of

377

SO2 on the surface of GDD and ATD (Figure S5 and Table S2 in Section S8). The sulfate

378

mass concentration sourced from SO2 photooxidation at presence of GDD and ATD using

379

UF-APHOR under natural sunlight (Figure S6 in Section S9).

380

Corresponding Author

381

*Email: [email protected]. Phone: +1 352-846-1744, Fax:+1 352-392-3076.

Without considering of , , the prediction of sulfate would be



382

The authors declare no competing financial interest.

383

Acknowledgments

384

This work was supported by grants from the National Institute of Metrological

385

Sciences (NIMS-2016-3100), the Ministry of Science, ICT, and Future Planning at South

386

Korea (2014M3C8A5032316), and the Fulbright Scholar (from USA to Mongolia).

387

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388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425

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548 549



Page 22 of 26

Table 1. Experimental conditions and kinetic uptake coefficients ( , and ) for SO2 oxidation on the surfaces of GDD and ATD particles in the absence and presence of O3 and NOx under different RHs

Exp. No.a D1 D2 D3

Type of dust

Dust massb (µg m-3)

RHC (%)

Temp. C (°C)

Off

GDD

320

20.5

24.8

N.A..g

108.90

1.87/1.70

2.20

(2.25 ± 0.09) × 10-7

Off

GDD

206

54.9

25.4

N.A.

101.32

1.67/1.05

0.64

(2.94 ± 0.13) × 10-7

Off

GDD

252

80.3

27.6

N.A.

83.15

2.47/1.80

0.83

(8.97 ± 0.27) × 10-7

On

N.A.f

N.A.f

20.5

23.5

17

77.08

2.33/2.42

2.31

N.A.f

On

N.A.f

N.A.f

55.4

26.7

34

89.67

2.25/2.01

0.88

N.A.

On

N.A.

f

N.A.

f

80.3

27.6

47

77.83

2.34/2.33

1.05

N.A.

On

GDD

207

20.5

24.8

3.4

80.59

1.87/1.70

2.20

(1.37 ± 0.03) × 10-6

On

GDD

269

54.9

25.4

20

72.54

1.67/1.05

0.64

(2.14 ± 0.34) × 10-6

On

GDD

126

80.3

27.6

26

60.54

2.47/1.80

0.83

(4.04 ± 0.54) × 10-6

RH effect under light with ATD

On

ATD

297

20.1

23.1

15

63.74

0.87/0.65

1.23

(5.01 ± 0.02) × 10-7

On

ATD

298

55.3

24.5

5.9

79.40

1.57/1.58

0.89

(7.74 ± 0.21) × 10-7

On

ATD

279

80.2

24.1

10

57.52

0.23/1.52

2.34

(1.96 ± 0.40) × 10-6

O3 effect at dark condition with GDD

Off

GDD

172

20.5

27.3

N.A.

102.64

0.63/1.09

65.82

(2.93 ± 0.14) × 10-7

Off

GDD

208

55.8

27.8

N.A.

99.51

1.01/1.05

64.78

(4.71 ± 0.21) × 10-7

Off

GDD

243

80.3

28.3

N.A.

99.27

1.11/2.59

69.88

(1.89 ± 0.39) × 10-6

On

N.A.f

N.A.f

20.7

25.7

15

103.67

0.48/1.78

62.19

N.A.

On

N.A.

f

N.A.

f

55.1

25.1

41

112.65

1.48/2.08

63.95

N.A.

On

N.A.

f

N.A.

f

80.3

25.7

55

84.53

1.20/1.73

69.39

N.A.

On

GDD

117

20.5

27.3

7.4

82.58

0.63/1.09

68.67

(1.57 ± 0.02) × 10-6

On

GDD

120

55.8

27.8

13

99.84

1.01/1.05

67.05

(2.14 ± 0.32) × 10-6

RH effect at dark condition with GDD

G1 G2 G3 L1 L2

RH effect under light with and without GDD

L3 L10 L11 L12 D4 D5 D6 G4 G5 G6 L4 L5

or

UV

Purpose

O3 effect under light with and without GDD

SMPS volume Initial SO2 conc. Initial NO/NO2 Initial O3 conc. (ppb) conc. (ppb) (ppb) conc. (nL m-3)d


, e

Page 23 of 26


L6 D7 D8 D9

NO2 effect at dark condition with GDD

G7 G8 G9 L7 L8 L9

NO2 effect under light with and without GDD

On

GDD

182

80.3

28.3

23

61.01

1.11/2.59

63.92

(4.04 ± 0.35) × 10-6

Off

GDD

362

20.5

25.2

N.A.

95.62

80.71/97.43

1.73

(2.36 ± 0.01) × 10-7

Off

GDD

231

55.2

27.6

N.A.

96.34

62.94/112.27

1.36

(4.10 ± 0.26) × 10-7

Off

GDD

280

80.4

29.1

N.A.

85.32

74.89/82.94

1.02

(1.16 ± 0.40) × 10-6

On

N.A.f

N.A.f

20..4

23.3

7.2

80.2

48.71/103.53

0.84

N.A.

On

N.A.

f

N.A.

f

55.2

24.1

23

83.2

52.85/97.28

1.58

N.A.

On

N.A.

f

N.A.

f

80.4

24.6

18.4

102.8

78.1/56.3

1.10

N.A.

On

GDD

242

20.5

25.2

1.8

79.09

45.83/95.61

1.73

(1.53 ± 0.02) × 10-6

On

GDD

109

55.2

27.6

17

70.20

55.19/125.27

1.36

(2.58 ± 0.41) × 10-6

On

GDD

157

79.9

26.1

19.5

64.53

78.1/56.3

0.10

(3.41 ± 0.89) × 10-6

a

“D” denotes “Dark condition” experiments. “G” denotes “Gas-phase” experiments. “L” denotes “Light condition” experiments.

b

Mass concentration of dust particles were calculated from the SMPS and OPC data. The density of 2.65 g cm-3 for dust particles was used.

c

Accuracy of RH: ±5%; accuracy of temperature: ±0.5 °C.

d

The total volume concentration of the particles between 20 nm to 148 nm in diameter was measured using SMPS data. This data was applied to the estimation of the quantity of new particles formed via the gas phase oxidation of SO2, followed by neutralization with ammonia (NH4+–SO42aerosol). The errors associated with , include the uncertianty from experimental measurements and that cuased by subtraction of gas-phase H2SO4 from the total sulfate.

e

f

N.A.: not applicable (no mineral dust particles).

g

N.A.: not applicable (no light source).



Figure 1.

Page 24 of 26

(a)

(b)

(c)

(d)

, and , values of SO2 on GDD ((a)-(d)) and ATD ((d) only)

at three different RH levels in the presence and absence of O3 and NOx. The errors associated with , include the uncertianty from experimental measurements caused by the subtraction of gas-phase H2SO4 from the total sulfate.


Page 25 of 26


(a)

(b)

(c)

(d)

Figure 2. Water contents of (a) fresh GDD and (b) ATD particles and photochemically aged (c) GDD and (d) ATD particles in the presence of SO2 as a function of RH. Error bars were estimated from uncertainties in the FTIR absorbance at the O-H band and balanced.



(a)

(b) Figure 3. (a) The inorganic compositions of photochemically aged GDD and ATD particles in the presence of SO2. Error bars were estimated from the uncertainty of internal standard (LiBr), which was used for PILS-IC. (b) Comparison between [H+]C-RUV and [H+]PILS-IC. Error bars represent the standard deviation from the mean.


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Heterogeneous Photo-oxidation of SO2 in the Presence of Two

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