Gas-Particle Partitioning of Carbonyl Compounds in the Ambient

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Gas-Particle Partitioning of Carbonyl Compounds in the Ambient Atmosphere Hengqing Shen, Zhongming Chen, Huan Li, Xi Qian, Xuan Qin, and Wenxiao Shi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01882 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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Environmental Science & Technology

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Gas-particle Partitioning of Carbonyl Compounds in the Ambient

2

Atmosphere

3

Hengqing Shen, Zhongming Chen*, Huan Li, Xi Qian, Xuan Qin, Wenxiao Shi

4

State Key Laboratory of Environmental Simulation and Pollution Control, College of

5

Environmental Sciences and Engineering, Peking University, Beijing 100871, China

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ABSTRACT

7

Despite their crucial roles in health and climate concerns, the gas-particle

8

partitioning of carbonyl compounds is poorly characterized in the ambient atmosphere.

9

In this study, we investigate their partitioning by simultaneously measuring six

10

carbonyl compounds (formaldehyde, acetaldehyde, acetone, propionaldehyde, glyoxal,

11

and methylglyoxal) in gas and particle phase at an urban site in Beijing. The

12

field-derived partitioning coefficients ( ) are in the range of 10−5−10−3 m3 µg−1, and

13

corresponding effective Henry’s law coefficients ( ) should be 107–109 M atm−1.

14

The Pankow’s absorptive partitioning theory and the Henry’s law both significantly

15

underestimate concentrations of particle-phase carbonyl compounds (105–106 times

16

and >103 times, respectively). The observed “salting in” effects only partially explain

17

the enhanced partitioning to particles, approximately one order of magnitude. The

18

measured values are higher at low relative humidity and the overall effective

19

vapor pressure of these carbonyl species are lower than their hydrates, indicating that

20

carbonyl oligomers potentially formed in highly concentrated particle phase. The

21

reaction kinetics of oligomer formation should be included if applying the Henry’s

22

law to low-to-moderate RH and the high partitioning coefficients observed need

23

further field and laboratory studies. These findings provide deeper insights into the

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formation of carbonyl secondary organic aerosols in the ambient atmosphere.

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ACS Paragon Plus Environment


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1 INTRODUCTION

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Secondary organic aerosols (SOA) form a significant fraction of atmospheric

28

fine particulate matter, and they play important roles in regional air quality and global

29

climate change.1 Traditional two-product models relying on absorptive partitioning

30

theory or volatility basis set (VBS) models usually significantly underestimate mass

31

concentrations and oxidation state of SOA in the ambient atmosphere, particularly in

32

polluted urban areas.1-3 The large discrepancy between field measurements and model

33

simulations could partly be ascribable to an incomplete understanding of SOA

34

formation pathways, including gas-particle partitioning of the oxidation products of

35

volatile organic compounds (VOCs), one of the critical processes determining the

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formation and growth of SOA in the atmosphere.

37

Carbonyl compounds are important oxidation intermediates of VOCs and crucial

38

composition of SOA.4 However, small carbonyls such as formaldehyde and glyoxal

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have been considered as an insignificant contributor to aerosol mass until recently due

40

to their high vapor pressures. Yet, increasing numbers of laboratory and field studies

41

confirm that these highly soluble and reactive small carbonyls actually contribute to

42

SOA formation much more than expected.5-8 In laboratory, Jang et al.7 observed that

43

the hydration of small carbonyls in the particle phase could contribute to particle

44

growth through further heterogeneous reactions and oligomerization, and Kalberer et

45

al.9 found that polymers formed from small carbonyls were major components of

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organic aerosols. Field evidence for the importance of small carbonyls in SOA

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formation was first proposed by Volkamer et al.,8 who found that a missing glyoxal

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sink account for 15% of the SOA mass in Mexico City. These small carbonyls

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entering particle phase significantly change the physicochemical properties of

50

aerosols, e.g., decreasing the surface tension of aerosols, and forming extremely

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light-absorbing compounds known as brown carbon.10-12 These studies highlight the

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importance of small carbonyls in the formation, growth, and evolution of urban SOA.

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A better understanding of gas-particle partitioning of small carbonyls is critical 2


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for estimating their contribution to the property and concentration of SOA. Gas-phase

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carbonyls could distribute into the aerosol organic or aqueous phase, and these two

56

progresses are expected to follow the Pankow’s absorptive partitioning theory

57

(Pankow’s partitioning coefficient, ) and the Henry’s law (Henry’s law coefficient,

58

), respectively. However, laboratory-determined particle-phase carbonyls have been

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found several orders of magnitude higher than that being predicted by their

60

corresponding ,7,13 which is contrast to other types of compounds such as

61

polycyclic aromatic hydrocarbons and n-alkanes that are well predicted.14 The

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measurements for small carbonyls also have uncertainties of several orders of

63

magnitude,15,16 hence the applicability of in highly concentrated aerosol phase are

64

still in doubt. Furthermore, the field-derived gas-particle partitioning coefficients ( )

65

or the effective Henry’s law coefficients ( ) of small carbonyls especially for

66

glyoxal and methylglyoxal are very limited and highly uncertain,8,17 and the influence

67

of various environmental factors (such as relative humidity and salts) on their

68

partitioning are not carefully assessed.18,19 Additionally, the distribution of their

69

different forms (monomer, hydrates or oligomers) has not been determined in ambient

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particles. Overall, despite excess (more than predicted) concentrations of carbonyls

71

have been detected in ambient particles,17,20 their detailed partitioning mechanism and

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behavior are poorly characterized in the atmosphere.17,19,21

73

In this study, we simultaneously measured gas- and particle-phase concentrations

74

of six carbonyls (formaldehyde, acetaldehyde, acetone, propionaldehyde, glyoxal, and

75

methylglyoxal) at an urban site in Beijing, China, and estimated their and in

76

the field. Then, observed and are compared with their theoretical

77

partitioning coefficients. We further explore the salts effects and relative humidity

78

dependence in their partitioning in the ambient atmosphere.

3



79

2 MATERIAL AND METHODS

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2.1 Field Sampling

81

Ambient gas and particle measurements were obtained on the roof of a six-story

82

academic building (about 20 m above the ground level) on the campus of Peking

83

University (PKU, 39.99°N, 116.30°E), from 6 November to 17 November 2014. This

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site is located in the northwest of Beijing and is regarded a representative urban site.

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Except for two main traffic roads, there are no significant stationary pollution sources

86

in the area.

87

The gas-phase and particle-phase samples were collected using two independent

88

samplers. The gas-phase samples were collected using Sep-Pak DNPH-Silica gel

89

cartridges (Waters) with an ozone scrubber cartridge (Waters) filled with potassium

90

iodide (KI) being connected to its front to eliminate ozone interference. Ultrapure

91

nitrogen gas was added and mixed with the sampled ambient air to reduce the relative

92

humidity (RH), as high RH would lead to KI deliquescing and ozone scrubber

93

blocking. The sampling duration was 3 hours between 07:20–22:20, and 9 hours

94

between 22:20–07:20. The field blank DNPH cartridge samples were collected every

95

three days by placing it near the gas inlet for the same duration.

96

A four-channel sampler (TH-16A, Wuhan Tianhong, China) was used to collect

97

PM2.5 samples at a flow rate of 16.7 L min–1. Two sample sets, one daytime sample

98

(08:00–19:30) and one nighttime sample (20:00–07:30), were obtained daily. Each set

99

included two Teflon filters and two preheated (550 °C for 6 hours) quartz filters

100

(Whatman, 47 mm diameter). The Teflon filters were weighted by a semi-micro

101

balance (Sartorius, Germany) to obtain the mass concentration of PM2.5 and extracted

102

to measure water-soluble inorganic compounds (Na+, NH4+, K+, Mg2+, Ca2+, Cl–, NO3–,

103

and SO42–) by DIONEX ICS-2500 and ICS-2000 ion-chromatograph, and the quartz

104

filters were used for carbonyls analysis. Filters were stored at –18 °C in darkness

105

immediately after the sampling and analyzed within two weeks. Blank measurements

106

were made every three days by placing a filter on the PM2.5 inlet without sucking air. 4


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Backup quartz filters were used to evaluate the sampling artifacts (i.e., gaseous

108

carbonyls adsorption on the filters) through an independent experimental study.

109

Several major trace gases (CO, O3, SO2, NO, and NO2) and meteorological

110

parameters (temperature, RH, wind direction, and wind speed) were monitored at the

111

same site. The PM2.5 mass concentration was also measured online using TEOM

112

1400a, and the half-day averaged data were very consistent with those obtained by

113

weighing (Figure S1). The meteorological parameters as well as measured

114

concentrations of trace gases and PM2.5 are provided in Table S1.

115

2.2 Sample Extraction and Analysis

116

The DNPH cartridges were eluted with 5 mL acetonitrile (LC grade, Merck), and

117

analyzed by high performance liquid chromatography (HPLC) with an ultraviolet (UV)

118

detector. The detailed analysis method is described in our earlier report.22 The quartz

119

filters were extracted with 5 mL acetonitrile, 1 mL DNPH acetonitrile solution (~10–2

120

M), and 30 µL H2SO4 solution using a shaker (Shanghai Zhicheng ZWY 103D, China)

121

at 180 rpm and 4 °C for 180 minutes. The solution was kept in darkness for 12–24

122

hours derivatization and analyzed using the same method as with the gas phase

123

detection. Note that the measured particle-phase carbonyls may include their original

124

monomer forms and the reversibly formed hydrates and oligomers. The carbonyls are

125

identified by a mixed liquid standard solution of ten carbonyls (formaldehyde,

126

acetaldehyde, acetone, propionaldehyde, methylacrolein, butaldehyde, methyl vinyl

127

ketone, benzaldehyde, glyoxal, and methylglyoxal). The limits of detection are

128

approximately 1 ng m–3 for particle-phase carbonyls and below 50 pptv for gas-phase

129

carbonyls. We also quantify particle-phase formaldehyde using the deprotonated ion

130

of m/z = 209 [formaldehyde-DNPH-H]– measured by LC-MS and compared with that

131

of HPLC-UV detection in an independent experiment, and confirm that formaldehyde

132

is not a positive bias from other compounds reacting with DNPH and co-eluting. A

133

total of 72 gas-phase samples and 23 particle-phase samples were obtained, and all

134

reported data in this paper are blank-corrected. 5



135

2.3 Sampling Artifacts

136

Quartz filters have been widely used for measuring gas-particle partitioning of

137

VOCs, and similar method as described in this study were used in the sampling of

138

glyoxal and methylglyoxal in many previous studies.23-25 However, the lack of using

139

denuders may lead to positive artifacts by adsorption of gas-phase carbonyls on the

140

filters, which have been evaluated by some previous studies.26-29 In field study,

141

Odabasi et al.29 reported that about 36% of the measured particle-phase formaldehyde

142

was caused by adsorbed formaldehyde on quartz filters.

143

In this study, independent experiments were carried out in field to evaluate the

144

possible adsorption artifacts. The atmospheric conditions (RH, temperature and PM2.5)

145

during the artifact experiments are provided in the Supporting Information, which

146

actually cover the atmospheric conditions of samples that are used for the partitioning

147

observation. We placed a backup quartz filter after the particle sampling quartz filter

148

using an independent filter holder, then the first filter would collect the particles, and

149

the gas flow carrying gas-phase carbonyls would pass through the second filter. If the

150

adsorption by the filter dominates the measured particle-phase carbonyls, the ratio of

151

the measured carbonyls in the second filter to that in the first would be large (close to

152

one). We provide the measured results in Figure S4. The fraction is usually lower than

153

20% for measured formaldehyde, glyoxal and methylglyoxal, and within 30% for

154

acetaldehyde and acetone. The higher fraction of adsorbed acetaldehyde and acetone

155

is reasonable considering their high gas-phase concentrations and relatively low

156

particle-phase concentrations (Section 3.1). Compared with the results of Odabasi et

157

al.,29 the adsorbed formaldehyde is relatively small in our study (13% vs. 36%). This

158

may be caused by glass fiber filters used in their study, which are more alkaline and

159

therefore likely to adsorb more slightly acidic aldehyde. In addition, different

160

temperature dependence of particle partitioning and filter adsorption may also

161

contribute to this discrepancy, as the sampling temperature is different (8.5 °C vs.

162

18 °C).

6


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We also approximate the artifacts using the equation derived by Mader and

163 164

Pankow26, which has been evaluated by Liggio28:

,

, = (1 + ) TSP

(1)

165

Where , is the measured carbonyl particle-phase concentration and is the

166

actually concentration in the particles, , is the gas-filter partitioning coefficient

167

adopted from Liggio,28 is the filter area, is the gas-particle partitioning

168

coefficient, TSP is particle concentrations and is the sample volume. We also

169

measured the gas-filter partitioning coefficient for formaldehyde in laboratory (see

170

Supporting Information), which are similar with that adopted from Liggio28 (9.2 × 10–

171

4

172

estimation, the artifacts caused by filter adsorption are about 24% for formaldehyde, 4%

173

for glyoxal and 9% for methylglyoxal. For acetaldehyde and acetone, the artifacts

174

would be about 60%, larger than the measured fractions in the field. However, even in

175

extreme cases that the actual concentrations of acetaldehyde and acetone in the

176

particles only account for 40% of the measured concentrations, the derived

177

gas-particle partitioning coefficients would only decrease about two, and according to

178

the evaluation of field measurements, the artifacts would be smaller. Therefore,

179

though gas-phase carbonyls could be in part adsorbed on the quartz filter, the

180

uncertainty caused by the adsorption would usually be no more than 30% based on the

181

field measurements, especially for the three most concerned carbonyls, formaldehyde,

182

glyoxal and methylglyoxal, and it thus will not affect the validity of our conclusions.

183

2.4 Aerosol Water Content

vs. 1.0 × 10–3 m3 cm–2) and have a slight increase with increasing RH. According the

184

In order to derive the effective Henry’s law coefficients for the measured

185

carbonyls, aerosol water content (AWC) is needed. The thermodynamic model

186

ISORROPIA-II is used here to calculate the equilibrium inorganic ion composition

187

and AWC based on the measured aerosol composition (Na+, NH4+, K+, Mg2+, Ca2+, Cl–, 7



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NO3–, and SO42–) and meteorological parameters (temperature and RH).30 Both

189

forward and reverse modes are used assuming a metastable system, and both modes

190

output similar results. Gas-phase ammonia estimated from the observed NOx

191

concentration with an empirical equation derived from Meng et al.31 is included, but it

192

does not substantially affect calculated AWC. The AWC prediction has been evaluated

193

and shown previously to provide reasonable performance compared with particle

194

water measurements.32,33 The water contributed by organic fraction is neglected in this

195

study due to the lack of data on the particulate organic matter, however, previous

196

studies have shown that the mass fraction of organic matter induced particle water

197

accounts for less than 10% of total AWC.34

198

2.5 Estimation of Partitioning Coefficient

199 200

Here, for each carbonyl compound i, the field gas-particle partitioning coefficient ( ) (m3 µg–1) is determined according to equation (2):

, =

, , TSP

(2)

201

where C, (ng m–3) and C, (ng m–3) are the particle- and gas-phase concentrations

202

of compound i, respectively; and TSP (µg m–3) is the mass concentration of suspended

203

particles (mass concentrations of PM2.5 are used here). The value could represent

204

the capability of compound i partitioning into the particles under specific conditions

205

in the ambient atmosphere.

206 207

The field-derived effective Henry’s law coefficient ( ) is calculated by equation (3):

, = 10 208 209 210

, , AWC/ρ# $%

(3)

where c, (µg m–3) and c, (atm–1) are particle- and gas-phase concentrations, respectively; AWC (µg m–3) is the aerosol water content modeled by ISORROPIA-II; (g mol–1) is relative molecular weight; ρ# $% (g cm–3) is the density of water. In 8


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calculation, we do not consider aerosol phase state assuming that equilibrium is

212

reached between gas phase and aerosol liquid water. It should be noted that the

213

modeled lowest AWC is about 0.2 µg m–3 during the observation, with an everage

214

value of 4.16 µg m–3. The calculated here represents the apparent effective

215

Henry’s law coefficient of compound i in ambient particles.

216

The detailed description about the calculaiton of theoretical equilibrium

217

absorptive partitioning coefficient ( $ ) (m3 µg–1) based on the method developed by

218

Pankow35 is provided in the Supporting Information. The vapor pressure ('() ), which

219

is the primary determinant in $ estimation, is calculated using the extended aerosol

220

inorganic model (E-AIM).

221

3 RESULTS AND DISCUSSION

222

3.1 Carbonyls in the Gas and Particle Phase

223

Four monocarbonyls (formaldehyde, acetaldehyde, acetone, and propionaldehyde)

224

and two dicarbonyls (glyoxal and methylglyoxal) are quantified in both the gas and

225

particle phase. Figure 1 shows their temporal variations and Table S2 summaries their

226

average concentrations and range. Formaldehyde, acetaldehyde, and acetone are the

227

most abundant carbonyls in the gas phase, with average concentrations of 3.85 ppbv,

228

2.22 ppbv, and 3.52 ppbv, respectively. The average gas-phase concentrations of two

229

dicarbonyls, glyoxal and methylglyoxal, are 0.10 ppbv and 0.31 ppbv, respectively.

230

Formaldehyde is also the most abundant carbonyl in the particle phase, with an

231

average concentration of 23.20 ng m–3. This value is consistent with previous field

232

measurements, where the presence of dozens ng m–3 formaldehyde in the particle

233

phase in both rural and urban areas are confirmed.20,29,36 The average particle-phase

234

concentrations of glyoxal and methylglyoxal are 12.01 ng m–3 and 8.27 ng m–3,

235

respectively, which are in the range of previously reported values in Beijing.23,37,38

236

Note that the particle-phase propionaldehyde is only detected in five samples. The

237

temporal variations of four monocarbonyls (methylacrolein, butaldehyde, methyl 9



238

vinyl ketone, and benzaldehyde) only detected in the gas phase are shown in Figure

239

S5. The result that benzaldehyde are not observed in the particle phase suggests that

240

the volatility of carbonyls may not be the factor dominating their particle-phase

241

abundant, as the volatility of benzaldehyde are much lower than formaldehyde (over

242

103 times). The lower Henry’s law coefficients and gas-phase concentrations as well

243

as the larger molecular structures, which are detrimental to the polymerization due to

244

steric-hindrance effect, may account for the undetectable particle-phase concentrations

245

of these four monocarbonyls. In the following section, we analyze the gas-particle

246

partitioning of the six carbonyls detected in both the gas and particle phase.

247 248

Figure 1. Temporal variations of the six carbonyls detected in both the gas (left axis,

249

ppbv) and particle phase (right axis, ng m–3).

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3.2 Measured versus Predicted Partitioning

251

3.2.1 Pankow’s absorptive partitioning

252

The field-derived gas-particle partitioning coefficients ( ) for the six carbonyls

253

are provided in Table 1. The average values of for these six carbonyls are in the

254

f range 10−5−10−3 m3 µg−1, following the order of Kp,

255

f Kp,

256

the first report of simultaneously measured for these six carbonyls in the ambient

257

atmosphere.

258

Table 1 Field-derived for the six carbonyls compared with theoretically predicted

259

values ( $ ) at 283 K (units: m3 µg–1).

formaldehyde ,

Compounds

Chemical

N

Formula

propionaldehyde

f > Kp,

>

f > Kp, acetone . To our knowledge, this is

Average

Range

$

a

methylglyoxal

/ $

Formaldehyde

HCHO

23

1.72×10–4

(0.5–4.8)×10–4

6.18×10–11

2.78×106

Acetaldehyde

CH3CHO

23

1.61×10–4

(0.2–6.7)×10–4

2.95×10–10

5.47×105

CH3COCH3

23

0.71×10–4

(0.06–3.4)×10–4

1.23×10–9

0.58×105

CH3CH3CHO

5

2.05×10–4

(0.9–3.8)×10–4

9.13×10–10

2.25×105

CHOCHO

23

1.44×10–3

(4.6–35.0)×10–4

1.27×10–9

1.13×106

CH3COCHO

23

4.19×10–4

(0.8–18.6)×10–4

2.73×10–9

1.53×105

Acetone Propionaldehyde Glyoxal Methylglyoxal 260

f f Kp, acetaldehyde , Kp,

glyoxal

a

N indicates the number of samples.

261

Table 1 also lists the theoretical Pankow’s absorptive partitioning coefficients

262

( $ ). The $ values are about 105–106 times smaller than their corresponding .

263

We explore the possible influencing factors (activity coefficient, molecular weight,

264

absorbing fraction and temperature) on $ estimation in the Supporting Information.

265

However, the positive deviation from is too large to be explained by these factors. 11



266

Therefore, it is most likely that the vapor pressure '() in theoretical equation changed.

267

This is possible, as the measured particle-phase carbonyls include their reversible

268

products (hydrates or oligomers) formed through particle-phase reactions. These

269

products have much lower vapor pressures than their precursors, and return to their

270

original carbonyl monomers during analysis.

271

Actually, unexpectedly high carbonyl partitioning coefficients are well known in

272

laboratory studies,7,13,39 and are also reported in several field measurements.17,19,21 The

273

reported laboratory values ( * ) for glyoxal and methylglyoxal by Healy et al.,13 are

274

(4–7) × 10–5 and (0.7–5.9) × 10–5 m3 µg−1 respectively, which are still more than one

275

magnitude lower than the values in our study as well as other field

276

measurements.17,19 Similarly, laboratory studies by Kroll et al.40 and Healy et al.13

277

indicated that the reactive uptake of small monocarbonyls by aerosols is negligible,

278

whereas abundant monocarbonyls, especially formaldehyde, are detected in our study,

279

and even larger ratios of particle-phase formaldehyde (as high as 0.3) are observed in

280

other field studies.20 These results suggest that the ambient atmosphere may be more

281

favorable for the condensation of carbonyls, and as a result, small carbonyls could

282

remarkably condense into ambient particles.

283

3.2.2 Henry’s law-type partitioning

284

Based on measured concentrations of gas-phase carbonyls, modeled AWC and

285

their respective Henry’s law coefficient, we estimate the theoretical particle-phase

286

concentrations of these six carbonyls (Table 2). The predicted particle-phase

287

concentrations are more than two orders of magnitude lower than the field measured

288

values, indicating that more carbonyls partition into the aerosol phase than expected

289

by the Henry’s law. In a recent study, Sareen et al.34 modeled particle-phase glyoxal

290

and methylglyoxal in the continental U.S. using a regional transport model based on

291

their Henry’s law coefficients. The modeled highest particle-phase concentrations of

292

glyoxal and methylglyoxal are all within 10–3–10–1 ng m–3 and this may significantly

293

underestimate their particle-phase concentrations particularly for methylglyoxal.34 12


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Previous modeling studies also found that just considering the Henry’s law

295

coefficients would lead to several orders of magnitude underestimation on the

296

modeled carbonyl-SOA than that using the surface-limited uptake method.41

297

Therefore, it may be impossible for a model just relying on the existing Henry’s law

298

coefficients to simulate the particle-phase carbonyls accurately. A more suitable

299

method or a modified Henry’s law that considers more parameters to increase its

300

applicability to a wider area are needed to accurately simulate carbonyl concentrations

301

in ambient particles and the abundance of carbonyl-SOA.

302

Table 2 Theoretical carbonyl particle-phase concentrations predicted by their Henry’s

303

law coefficients, and field-derived assuming that all carbonyls enter the particle

304

phase following the Henry’s law. Estimated

c

Henry’s law

Predicted

coefficient a

Cp b

M atm–1

ng m–3

Formaldehyde

1.14×104

5.50×10–3

2.04×108

1.04×107–2.04×108

Acetaldehyde

3.76×101

1.53×10–5

1.98×108

4.60×106–8.48×108

Acetone

7.02×101

5.96×10–5

9.52×107

2.39×106–5.18×108

Propionaldehyde

2.15×101

1.45×10–6

2.81×108

1.62×107–5.65×108

Glyoxal

1.57×106

3.80×10–2

1.66×109

1.80×108–6.21×109

Methylglyoxal

1.31×104

1.21×10–3

5.16×108

1.42×107–2.17×109

Compounds

Average

Range M atm-1

305

a

306

by ISORROPIA-II. c Estimated to match the measured particle-phase carbonyls.

307 308

Calculated from Sander et al.42 at 283 K. b Calculated using average AWC estimated

We provide field-derived effective Henry’s law coefficients ( ) needed to

match the measured particle-phase carbonyls in Table 2. The average values of

309

are in the range 107−109 M atm−1; glyoxal has the largest value, 1.66 × 109 M atm−1,

310

and acetone has the smallest value, 9.52 × 107 M atm−1. It should be noted is that the

311

for glyoxal is approximately three magnitudes larger than the recommended value

312

in pure water,42 while it is comparable to the value suggested by Volkamer et al.8 (4 × 13



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109 M atm–1) to interpret a missing sink of gas-phase glyoxal in field observation and

314

the value recommended by Ip et al.15 (>109 M atm–1) measured in sulfate solution.

315

The for the other carbonyls are much higher than existing laboratory measured

316

results.42

317

3.3 Salting Effects

318

In terms of a Henry’s law-type partitioning process (solubility is the dominator),

319

the salts concentration is an important factor. An exponentially increased solubility

320

with the salt concentration is known as a “salting in” effect, while the reverse

321

(exponentially decreased) is a “salting out” effect. The salting effects can be descried

322

by equation (4):16

log . 323 324 325 326

,# $% / = *$ , *$

(4)

where ,# $% and K 1,2345 are the Henry’s law coefficients of the organic

compound in pure water and in salt solution, respectively, and *$ is the salt concentration in molality. is the salting constant or Setschenow constant. The negative value of indicates a “salting in” effect.

327

Previous studies have shown that sulfate is probably the major factor for the

328

salting effect of dicarbonyls.15,16,43 Figure 2 shows the Setschenow plot log6,# $% /

329

7 of glyoxal and methylglyoxal versus aqueous sulfate concentration ( 8* $ , mol

330

kg−1 AWC) modeled by ISORROPIA-II assuming a metastable system. The slope of

331

the linear regression gives the salting constant. The negative salting constants indicate

332

the “salting in” effects for both glyoxal and methylglyoxal in ambient particles.

333

However, in laboratory studies, it has been found that the formation of

334

sulfate-glyoxal-hydrate complexes leads to a “salting in” effect for glyoxal,

335

while—presumably because of its increasing steric hindrance to fit in the ion

336

hydration shell—methylglyoxal presents a “salting out” effect.16,44 This discrepancy

337

implies that the salting effects on the solubility of carbonyls is distinguished between

338

the ambient atmosphere and laboratory conditions, which may be due to the much 14


Page 15 of 30


339

more complex composition of the ambient atmosphere and particles. On the one hand,

340

the laboratory studies about salting effects are carried out using single compound,

341

while the interactions between different components may play important roles in

342

ambient particles. On the other hand, the ionic strength in ambient particles is usually

343

higher than the bulk salt solutions used in the laboratory, e.g. the aqueous sulfate

344

concentration may reach metastable saturation or even supersaturated concentrations

345

in ambient particles. In Figure S7, we show that all the six carbonyls have strong

346

positive exponential relationships with 8* $, and the other four carbonyls show

347

similar “salting in” effects, which are also inconsistent with laboratory result.45

348 349

Figure 2. Setschenow plot for glyoxal and methylglyoxal using 1.57 × 106 mol kg–1

350

atm−1 and 1.31 × 104 mol kg–1 atm−1 as their Henry’s law coefficients in pure water at

351

283 K assuming unit density for pure water.42

352 353

It is worth noting that the difference in for different carbonyls is smaller

than expected. For example, the for glyoxal is only 5 ± 3 times higher than that

354

of methylglyoxal and for formaldehyde and acetaldehyde are very close.

355

However, the Henry’s law coefficients for methylglyoxal and acetaldehyde are more

356

than two orders of magnitude lower than glyoxal and formaldehyde respectively in

357

pure water.42 This indicates that other factors not just their solubility in water

358

dominate their partitioning. In this study, the “salting in” effects could partly improve 15



359

the theoretical estimated concentrations of particle-phase carbonyls; however, the

360

salting effects alone could not close the gap between measurements and predictions.

361

The derived salting constant of glyoxal, ,*9:; *, is –0.04 kg mol–1 in this study,

362

indicating much smaller “salting in” than the laboratory values obtained by Kampf et

363

al.16 (–0.24 kg mol–1) and Waxman et al.43 (–0.16 kg mol–1). Moreover, the nonzero

364

intercept of linear regressions showing in Figure 2 indicate that even eliminating the

365

effects of salts, the field-derived effective Henry’s law coefficients remain much

366

larger than in bulk pure water (102–105 times). Therefore, more mechanical and

367

quantitative studies into the roles of salts on the partitioning of carbonyls in both

368

laboratory and field are needed, and more importantly, there may be other factors

369

dominating their partitioning in the ambient atmosphere.

370

3.4 Relative Humidity Dependence

371

We choose glyoxal as a reference compound and analyze correlations of for

372

these carbonyls. The significant positive correlations of for different carbonyls

373

with glyoxal (Figure S8 a–d) suggest that the factors dominating their partitioning

374

may be the same or similar. The relatively small difference of values for different

375

carbonyls, and the large variations in different samples imply that their partitioning

376

processes are significantly influenced by environmental conditions. Two important

377

environmental factors that potentially influencing are the RH and the temperature.

378

We investigate the relationship between and temperature, however, no significant

379

correlation is observed, which may be due to the relatively small temperature range

380

(smaller than 10 °C) during the observation.

381

We calculate for the five carbonyls at different RH bins, and find that the

382

field-derived has strong dependence on RH (Figure 3). It decreases significantly

383

when RH increases from 0.9). However, it should be noted that the highest RH observed in this study is

431

∼60% (Figure S2); thus, the partitioning behaviors for carbonyls at higher RH need

432

more laboratory and field studies, and the fitting formulas given here also need to be

433

tested at higher RH.

434

3.5 Volatility in Ambient Particles

435

The distribution and volatility of carbonyl species (hydrates or oligomers) in

436

ambient particles are rarely directly determined, as these products easily return to their

437

original forms during analysis. To verify the potential forms of the reversible carbonyl

438

products in ambient particles, we calculate the effective vapor pressures of these

439

carbonyl species based on their measured values using the same assumptions of

440

$ estimation. In this study, the estimated effective vapor pressures are 8.07 × 10–8

441

atm for glyoxal species, and 2.77 × 10–7 atm for methylglyoxal species. These values

442

agree well with the measured volatility behaviors of glyoxal and methylglyoxal

443

precursor/product mix formed under cloud-relevant conditions in the laboratory,52,53

444

where the estimated effective vapor pressures are ∼1 × 10−7 atm for the glyoxal

445

mixture and 6 × 10−7 atm for the methylglyoxal mixture. The estimated values for

446

measured formaldehyde, acetaldehyde, acetone, and propionaldehyde species are 6.76

447

× 10–7 atm, 7.20 × 10–7 atm, 1.63 × 10–6 atm, and 5.66 × 10–7, respectively. To our

448

knowledge, this is the first report on the overall effective vapor pressures of these

449

carbonyl species in ambient particles, and we think that these values represent the real

450

volatility of these reversible carbonyl species during the observation.

19



451

We compare the estimated values with their corresponding hydrates. The

452

estimated values of methylglyoxal species in this study is about two orders of

453

magnitude lower than that of methylglyoxal diols (about 10–5 atm).54 This suggests

454

that products with volatility lower than its diols, such as methylglyoxal oligomers,

455

may exist in ambient particles. Similarly, the estimated values of formaldehyde,

456

acetaldehyde, and acetone species are also more than one magnitude lower than the

457

vapor pressure of their hydrates estimated theoretically (4.47 × 10–6 atm, 8.91 × 10–5

458

atm, and 3.09 × 10–4 atm, respectively).55 The estimated value here is lower than that

459

estimated by Odabasi et al.29 in a suburban area; where the estimated vapor pressure

460

of formaldehyde species agrees well with formaldehyde hydrates. The higher

461

temperature and RH during their observation may be helpful for hydrates formation.

462

Note that carbonyls could incorporate into aerosol phase via reacting with other

463

compounds; thus, it is difficult to estimate the exact content of each form accurately.

464

However, we can confirm that carbonyl species might be dominated by different

465

forms in various conditions, and reversible products with volatility lower than their

466

hydrates, such as oligomers, do exist in ambient particles. This also provides

467

supporting evidence for the RH dependence of in Section 3.4.

468

4 ATMOSPHERIC IMPLICATIONS

469

We simultaneously measured the gas and particle phase concentrations of six

470

carbonyls (formaldehyde, acetaldehyde, acetone, propionaldehyde, glyoxal, and

471

methylglyoxal) at an urban site in Beijing, China, and systemically reported their

472

gas-particle partitioning in the ambient atmosphere for the first time. The

473

field-derived gas-particle partitioning coefficients ( ) for these carbonyls are 105–

474

106 times larger than the values predicted by Pankow’s absorptive theory, providing

475

field evidence of the importance of particle-phase reactions. Additionally, as major

476

carcinogenic and genotoxic compounds in urban areas, unexpected highly

477

particle-phase carbonyls could increase the exposure to toxicity from inhalation.

478

A Henry’s law-type partitioning process alone cannot represent the ambient 20


Page 20 of 30

Page 21 of 30


479

gas-particle partitioning of these small carbonyls neither.56 This is reasonable

480

considering that aerosol water solution generally represents a concentrated solution,

481

particularly at low-to-moderate RH as in this study, while the application of Henry’s

482

law is usually valid for dilute water droplets such as clouds and fog. The carbonyl

483

oligomers formed in highly concentrated aerosols could be helpful to explain this

484

deviation. This has important implications for model simulations of carbonyl SOA

485

formation in ambient particles. Although the measured particle-phase carbonyls

486

account for only a small proportion of the total particle mass (

Gas-Particle Partitioning of Carbonyl Compounds in the Ambient

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