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A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes, Geochemistry, and Astrochemistry

Effects of Gas-Particle Partitioning on Refractive Index and Chemical Composition of m-Xylene Secondary Organic Aerosol Kun Li, Junling Li, Weigang Wang, Jiangjun Li, Chao Peng, Dong Wang, and Maofa Ge J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b12792 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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The Journal of Physical Chemistry

Effects of Gas-Particle Partitioning on Refractive Index and Chemical Composition of m-Xylene Secondary Organic Aerosol

1 2 3 4 5

Kun Li,†,‡,ǁ Junling Li,†,‡ Weigang Wang,*,†,‡ Jiangjun Li,‡,§ Chao Peng,†,‡ Dong

6

Wang,‡,§ and Maofa Ge*,†,‡

7 8 9 10 11 12 13

†

State Key Laboratory for Structural Chemistry of Unstable and Stable Species, CAS

Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

§

Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of

Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China.

14 15 16

Abstract

17

The formation of secondary organic aerosol (SOA) contains partitioning processes of

18

the oxidation products between gas and particle phase, which could change the

19

particle-phase composition when particles grow. However, the effects of these

20

processes on the optical properties of SOA remain poorly understood. In this study,

21

we performed smog chamber experiments to investigate the effects of gas-particle

22

partitioning (GPP) on the refractive index (RI) and chemical composition of the

23

m-xylene SOA. Here, we show that the GPP processes, as organic mass increases, can

24

increase the proportions of semi- and intermediate volatile organic compounds

25

(SVOCs and IVOCs) in the particle phase and result in the decrease of SOA RI real

26

part for 0.09 ± 0.02 (without seeds) and 0.15 ± 0.02 (with seeds). This indicates that

27

the SOA optical properties are closely related to the total organic mass and

28

molecular-level composition. In addition, the presence of inorganic seeds promotes

29

the GPP to the particle phase and hence further decreases the RI real part for 0.05 ±

30

0.02. As pre-existing aerosols are ubiquitous in the ambient atmosphere, it is

31

suggested that there should be a certain correction when applying the SOA RI of

32

previous laboratory studies to air quality and climate models.

33

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

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Introduction

35

As an important type of atmospheric fine particulate matter, secondary organic

36

aerosol (SOA) plays a vital role in visibility, climate change, and human health.1-2 The

37

formation and evolution of SOA were the foci of atmospheric chemistry in the past

38

few decades.3-8 During the formation of SOA, gas-particle partitioning (GPP, i.e.

39

condensation and evaporation between gas and particle phase) was found to be a

40

traditional and dominant approach.9-12 The partitioning between gas and particle phase

41

of a component i is given by: ∗

42

= 1 + ; = ∑

43

where is the partitioning coefficient, which means the fraction in particle phase,

44

∗ is the effective saturation concentration of the compound, is the

45

concentration of organic aerosol, and is concentration of the compound (in both

46

gas and particle phase).5,

47

increase as well, which will lead to the higher partitioning coefficient according to

48

the equation, so Equation (1) should be applied iteratively. One consequence of this

49

partitioning process is that the organic aerosol composition and oxidation state will be

50

changed as the total organic concentration increases, which has been proved by

51

laboratory chamber studies.13-14 It may have important impacts, because the changing

52

composition could influence the physical properties such as hygroscopic and optical

53

coefficients, which are important parameters related to visibility and aerosol climate

54

effects.

11-12

(1)

If the organic concentration increases, will

55

Due to the complex compositions and various mixing states, the optical properties

56

of SOA and their effects on global radiative balance remain poorly quantified.1, 15-16

57

Refractive index (RI) is one of the most important factors of aerosol optical

58

properties.16 As other optical factors like scattering and extinction efficiencies can be

59

calculated using the RI, quantitatively understanding the RI of SOA is essential.15-17

60

In recent years, some laboratory studies have been carried out to determine the RIs of

61

SOA.17-28 The studied SOA precursors contain both biogenic (monoterpenes, isoprene

62

etc.) and anthropogenic (toluene, xylene, alkanes etc.) volatile organic compounds

63

(VOCs). Some of these studies find that the SOA RI changes as organic mass loading

64

increases (i.e. particles grow).17, 19 Kim et al. found that the RI of limonene SOA

65

increased as particles grow, while the RI of α-pinene SOA didn’t change obviously.19


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66

Li et al. found that SOA generated from m-xylene decreased dramatically among the

67

four studied aromatic precursors.17 However, the driving force of this kind of

68

changing in optical properties is still unclear.

69

In addition, when investigating the refractive index of SOA, previous studies

70

mostly focus on the SOA particles in the absence of aerosol seeds,15-26 though it has

71

been found that seeds could influence the GPP process and enhance the SOA

72

yields.29-31 Pre-existing aerosols can provide the surface for organic materials to

73

condense on, and increase the proportion of more volatile compounds in the particle

74

phase. They are ubiquitous in the ambient atmosphere; hence the impacts of

75

pre-existing aerosol seeds on SOA optical properties should be further studied. A

76

recent study investigated the effects of seeds on optical properties of SOA derived

77

from long-chain alkanes, a type of intermediate VOC (IVOC), attempting to relate the

78

chemical composition to RIs.27 However, as the more classical and widely studied

79

precursors of anthropogenic SOA, the aromatics, the effects of aerosol seeds on

80

optical properties of their SOA remain unknown.

81

In this study, we performed smog chamber experiments to investigate the effects

82

of the GPP process with and without inorganic seeds on the RI of SOA generated by

83

photo-oxidation

84

High-resolution electrospray ionization mass spectrometry was used to analyze the

85

molecular composition of the SOA particles. By combining the high-resolution

86

chemical analysis with optical detection, we try to find the relationship between

87

chemical composition and RI. We show the impacts of GPP and pre-existing

88

inorganic seeds on composition and RI of m-xylene SOA, and discuss their

89

atmospheric implications.

of

m-xylene,

a

typical

anthropogenic

aromatic

precursor.

90 91

Experimental

92

The experiments discussed here have been described in detail in a previous

93

publication.28 Briefly, the experiments were performed with a dual-reactor smog

94

chamber. The combined light sources including 340 nm broadband lamps and 365 nm

95

narrowband black lamps are used.32 The temperature was maintained at 293 ± 1K.

96

The RH in the reactor was controlled at < 5%. The size distribution of aerosol was

97

measured by the scanning mobility particle sizer (SMPS; Model 3936, TSI). The NOx

98

and O3 concentration were monitored by trace gas analyzers (T200UP and T400,



99 100

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Teledyne API). The concentration of m-xylene was measured by the quadrupole proton transfer reaction mass spectrometry (PTR-MS; HS500, Ionicon).

101

Zero air was injected into the chamber before each experiment. NOx was

102

introduced into the chamber from a standard gas cylinder, and the concentration in the

103

reactor was 300 ± 50 ppb. A known volume of m-xylene (≥ 99%, Sigma-Aldrich) was

104

injected into a glass U-tube, and then it was introduced into the chamber with the zero

105

air flow. The concentration of m-xylene in the reactor was around 4 ppm. For the

106

seeded condition, the atomizer which contained inorganic salts (namely ammonium

107

sulfate, AS and sodium chloride, NaCl; ≥ 99%, Sigma-Aldrich) solution was used to

108

generate the inorganic aerosols. The seed particles were dried by a diffusion dryer and

109

then introduced into the reactors without size selecting. The seeds are solid under the

110

< 5% RH in the chamber, which can exclude the influence of liquid phase reactions

111

on compositions and optical properties.28 The number concentration of seeds in the

112

reactors is (1.6 ± 0.3) × 104 cm-3. The non-sizing particles have a mode of 63 ± 4 nm,

113

with geometric standard deviation (σg) of 1.8 ± 0.05. The UV lights were turned on 30

114

min after the NOx, m-xylene, seeds were introduced into the chamber to make sure

115

that they are well mixed. The representative reaction profiles are shown in Fig. S1.

116

The light extinction and RIs of aerosols are detected through a custom-built

117

cavity ring-down spectrometer (CRDS), see previous work for details.33 The CRDS

118

provides the decay time τ directly, and the RI is gained by programs based on Bohren

119

and Huffman.34 The detailed process and method are introduced elsewhere.17 Briefly,

120

The RI of each diameter is calculated by minimizing the following merit function (χ): = (, − ,!"!" (#, $))&

(2)

121

where n is the real part of RI, k is the imaginary part of RI, and Qext is the extinction

122

efficiency. Qext,measured is determined using the following equation:

123

, = (

'

+

( −, )

)*) !" ,

-

(3)

124

where Stot is the total surface area that measured by SMPS, L is the length of the

125

cavity, c is the velocity of light, l is the length filled with aerosol and τ0 is the decay

126

time when the cavity is filled with zero air. This paper and a previous publication28

127

share the same dataset of SOA optical properties. While that paper focuses on effects

128

of multiphase reactions, this study mainly investigates the effects of GPP process.


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129

The morphology of m-xylene SOA particles was determined via atomic force

130

microscope (AFM). The aerosol particles were collected by impacting on silicon

131

wafers and then were analyzed by a tapping mode AFM (NT-MDT Ntegra) at room

132

temperature (~298 K) and ambient RH (~30%). The SOA particles were also collected

133

by PTFE membrane filters with a pore size of 0.22 µm (FGLP047, Millipore), and

134

then were dissolved in 5 mL methanol. The solution was then analyzed by

135

ultra-high-resolution electrospray ionization time-of-flight mass spectrometry

136

(UHR-ESI-TOF-MS; Impact HD, Bruker) in positive ion mode to get the mass

137

spectra.

138 139

Results and discussions

140

The morphology and structure of aerosols in the presence of inorganic seeds are

141

important to retrieve the RI, as particle shape and mixing state can influence the

142

application and calculation of Mie theory. The studied inorganic salts are not spherical,

143

which may introduce some deviation when applying Mie theory. Hence, we at first

144

investigate the morphology and mixing state of SOA in the presence of inorganic

145

seeds.

146

The AFM image of SOA particles in the presence of AS seeds is illustrated in Fig.

147

1. It is indicated that the particles are in the core-shell structure, of which the organic

148

coating is very soft and was impacted to be a very thin layer on the silicon wafer

149

(height < 50 nm). As a result, we speculate that the organic aerosols are likely liquid,

150

and they have an approximately spherical shape when suspended in the air.17, 35 This

151

finding suggests that there is little deviation when using the Mie theory to calculate RI

152

of seeded SOA, which is similar to our previous study about the non-seeded aromatic

153

SOA.17

154



c Height (nm)

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200

section 1

section 2

100

0 0

155

300

600

900

1200

1500

0

300

600

900

1200

1500

Width (nm)

156

Figure 1 AFM images of the m-xylene SOA sample in the presence of AS seeds. a. Height image

157

with two cross sections; b. phase image, in which core-shell can be observed; c. two cross sections

158

in the height image.

159 160

Same as our previous studies, negligible light absorption was found for m-xylene

161

SOA at 532 nm, so only the real part of RI are considered here.17, 28 For brevity, “RI”

162

represents the real part in the following text. The RIs of seeded experiments are

163

retrieved by Mie theory, assuming that the particles are in the core-shell structure.

164

However, as the seed is much smaller than the whole particle, there seems to be little

165

differences between core-shell assumption and homogeneous assumption (see Fig.

166

S2). The RIs of m-xylene SOAs under non-seeded and seeded conditions are shown in

167

Fig. 2. It is found that the SOA is generated earlier in the presence of seeds, which

168

agrees with previous studies.29 The RIs decrease as time goes on, which is the same

169

under both non-seeded and seeded conditions. In addition, at the same reaction time,

170

the RIs are smaller for seeded experiments compared with non-seeded experiments. It

171

should be noted that the RI data in the companion paper28 are from the period when

172

RIs are nearly stable, in this case after 200 min.

173

These findings may be due to the following reasons. Particle nucleation occurs in

174

the absence of seed, and according to the Köhler theory,36 the gaseous components for

175

nucleation need relative lower volatility than heterogeneous condensation on the

176

surface of seeds. Under non-seeded condition, longer gas-phase oxidation period is

177

needed to form lower volatile organic compounds that nucleate to form new particles,

178

so the particles are formed later. Previous studies show that the presence of

179

pre-existing seeds promotes the condensation of organic vapors.29-30 Therefore, at the

180

same reaction time, the particle diameter under the seeded condition is throughout

181

larger than the diameters under the non-seeded condition and the aerosol mass is also

182

larger (see the color map in Fig. 2). Hence, products with higher volatility could

183

condense on the particles under the seeded condition. Likewise, as reaction goes on,


Page 7 of 28

184

total organic concentration increases and particles getting larger, so more products

185

with higher volatility will condense on the particles. The higher volatility compounds

186

possibly have lower RIs, which lead to the lower bulk RIs of SOA under the seeded

187

condition, and the decrease of RIs as diameter increases. In addition to the reason

188

above, the ongoing oxidation and aging process may change the chemical

189

composition of the particles as well, which is another possible explanation for the

190

decreased RI with increasing photochemical time.

Non-seeded 1# Non-seeded 2# Seeded 1# Seeded 2# Non-seeded fit Seeded fit

1.60

Real part of SOA RI

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1.55

1.50

Dsm (nm) 420

400.0

280

280.0

140

160.0

1.45

1.40 100

150

200

250

300

191

t (min)

192

Figure 2 Refractive indices (RIs) of SOA under non-seeded and seeded conditions. The X-axis is

193

the time after lights were turned on. Particle surface mean diameters (Dsm) are in the range of

194

140-420 nm and are shown in the color map.

195 196

To verify the hypothesis above, the SOA particles are collected by filters and are

197

analyzed by ultra-high-resolution electrospray ionization time-of-flight mass

198

spectrometry (UHR-ESI-TOF-MS). The particles are collected at two different times

199

for both seed conditions: the first one is when SOA RIs are just detectable and

200

particles are relatively small (~110 min for seeded experiments; ~170 min for

201

non-seeded experiments); another one is at the end of an experiment when the

202

particles stop growth (~300 min). The mass spectra of these four SOAs are shown in

203

Fig. 3. In general, the mass spectra are similar at different sizes, no matter for

204

non-seeded or seeded experiments. However, if carefully analyzing the mass spectra

205

(Fig. S3), one will find that the bigger particles have more abundant species with

206

lower molecular weight (mostly < 200 Da). In addition, the mass spectra are very

207

different between non-seeded and seeded experiments, especially for the products



208

with higher molecular weight (> 400 Da). The non-seeded SOA contains more species

209

with molecular weight of > 400 Da, which are likely the highly oxygenated

210

multifunctional compounds (HOMs) formed during new particle formation.37-38

211

Though HOMs have been observed mainly from the oxidation of biogenic VOCs,37-40

212

recent studies show that they could also be formed from aromatics.41-42 It is noticed

213

from Fig. 2 and Fig. S1 that the particles grow about 1 h later under non-seeded

214

conditions, during when the HOMs may be generated by multi-step gas phase OH

215

oxidation (i.e. autoxidation).41, 43 For seeded experiments, the large molecular weight

216

products of > 400 Da are much less than those of non-seeded experiments, even

217

negligible. The reason may be that the semi- or low volatile compounds mostly

218

condensed into particle phase in the presence of seeds before they could be oxidized

219

to HOMs in the gas phase.

220

221 222

Figure 3 Mass spectra of SOA under non-seeded and seeded conditions with different diameters. a.

223

the non-seeded condition, of which the black and red sticks are at the onset of particles and the

224

end of the experiments, namely ~120 nm and ~250 nm. b. the seeded condition, of which the


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225

black and red sticks are at the onset of particles and the end of the experiments, namely ~150 nm

226

and ~390 nm. The enlarged view of this figure is shown in Fig. S3.

227 228

To clearly see the change of chemical compositions when particles grow or when

229

with pre-existing seeds, the subtraction plots of mass spectra are shown in Fig. 4.

230

These three MS are the differences between large particles and small particles under

231

non-seeded condition (a); the differences between large particles and small particles

232

under seeded condition (b); and the differences between seeded and non-seeded

233

particles at the end of the experiments (c). As discussed above, seeded SOA has more

234

low molecular weight products and less high molecular weight products (Fig. 4c). It is

235

also found that the most obvious difference when particles grow is the molecules of

236

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