Kinetics of Limonene Secondary Organic Aerosol Oxidation in the

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Kinetics of limonene SOA oxidation in the aqueous phase Bartlomiej Witkowski, Mohammed Al-sharafi, and Tomasz Gierczak Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02516 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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Environmental Science & Technology

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Kinetics of limonene SOA oxidation in the aqueous phase

2

Bartłomiej Witkowski, 1* Mohammed Al-sharafi, 1 Tomasz Gierczak 1 1

3 4

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University of Warsaw, Faculty of Chemistry,

Al. Żwirki i Wigury 101, 02-089 Warsaw, Poland

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Keywords: Limonene, secondary organic aerosol, aqueous-phase, relative rates, hydroxyl radicals,

22

ozone

23

*Corresponding author e-mail: [email protected]

1 ACS Paragon Plus Environment

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Abstract

25

Twenty semi-volatile organic compounds that contribute to limonene secondary organic

26

aerosol (SOA) were synthesized in the flow-tube reactor. Kinetics of the aqueous-phase oxidation of

27

the synthesized compounds by hydroxyl radicals (OH) and ozone (O3) were investigated at 298±2 K

28

using the relative rate method. Oxidized organic compounds identified as the major components of

29

limonene SOA were quantified with liquid chromatography coupled to the electrospray ionization

30

and quadrupole tandem mass spectrometry (LC-ESI/MS/MS). The bimolecular rate coefficients

31

measured for the oxidation products of limonene are: kOH = 2-5 × 109 M-1s-1 for saturated and kOH = 1-

32

2 × 1010 M-1s-1 for unsaturated compounds. Ozonolysis reaction bimolecular rate coefficients

33

obtained for the unsaturated compounds in the aqueous phase are between 2-6 × 104 M-1s-1. The

34

results obtained in this work also indicate that oxidation of limonene carboxylic acids by OH was

35

about factor of 2 slower for the carboxylate ions than for the corresponding carboxylic acids while

36

the opposite was true for the ozonolysis. The data acquired provided new insights into kinetics of the

37

limonene SOA processing in the aqueous-phase. Ozonolysis of limonene SOA also increased the

38

concentration of dimers, most likely due to reactions of the stabilized Criegee intermediates with the

39

other, stable products. These results indicate that aqueous-phase oxidation of limonene SOA by OH

40

and O3 will be relevant in clouds, fogs and wet aerosols.

41 42 43 44 45 46 47 48 49 50 51



52

1.

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Introduction

53

Secondary organic aerosols (SOA) are produced by oxidation of the volatile organic

54

compounds (VOC) that are abundant in the atmosphere. 1 Because of high impact of aerosols on the

55

climate and human health,

56

atmospheric scientists. However, the incomplete understanding of SOA formation is still a source of

57

significant uncertainty in the atmospheric models. 3 Recently, more attention has been directed to

58

the aqueous-phase processes 4, 5 in order to improve the performance of the traditional atmospheric

59

models that tend to significantly underestimate the amount of SOA in the atmosphere. 6

60

2, 3

SOA sources and formation mechanisms are of great interest to the

Monoterpenes, mainly α and β-pinene as well as limonene are the second largest group of 7

61

VOC in the atmosphere.

62

monoterpene SOA, including functionalized carboxylic acids, carbonyls as well as non-volatile

63

oligomers. 8-13 In fogs and clouds with LWC = 0.3-0.5 g × m-3 these compounds will reside entirely in

64

the aqueous-phase as inferred from the estimated Henry's law solubility coefficients (H) for such

65

molecules.

66

monoterpene-derived compounds can lead to aqSOA. 15-19

67

14

A large number of semi-volatile organic compounds contribute to

More importantly, recent studies also indicate that the aqueous-phase oxidation of

However, despite some recent advantages,

15, 17, 19-21

the current database of the aqueous22

68

phase reaction of the monoterpene oxidation products is still exceedingly sparse.

69

these reactions are still not represented in the current 3D models. 6 Thus, further studies are needed

70

to expand the current understating of the aqueous-phase processing of monoterpene SOAs.

71

Moreover, the kinetic data previously acquired strongly indicate that the aqueous-phase oxidation of

72

semi-volatile organic compounds produced following gas-phase oxidation of α-pinene and limonene

73

should be relevant under realistic atmospheric conditions. 15, 17, 20, 21

74

Consequently,

In this work we direct our attention to the aqueous-phase oxidation of limonene SOA by two 22

75

tropospheric oxidants: ozone (O3) and hydroxyl radicals (OH).

76

reactions of the limonene oxidation products since some of these molecules retain the less reactive,

77

exocyclic double bond of the precursor.

78

products of limonene will be more reactive towards OH and O3 than the oxidation products of α-

79

pinene that are mostly saturated molecules.

80

unsaturated oxidation products of limonene can also yield low-volatility compounds thereby

81

contributing to aqSOA formation. 20 However, to date, the aqueous-phase oxidation of limonene SOA

82

wasn’t investigated.

12, 23

We investigate the aqueous-phase

Consequently, first generation, unsaturated oxidation

12, 17, 20

Following oxidation in the aqueous-phase, the

83

Previously, limononic acid (3-isopropenyl-6-oxoheptanoic acid, LA) was synthesized by

84

photolysis of the commercially available cis-pinonic acid and purified with the semi-preparative liquid 3 ACS Paragon Plus Environment

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20, 24

85

chromatography.

86

ozonolysis of limonene.

87

products since a separate synthesis and purification of each of these compounds would be very

88

complicated and time-consuming. Afterwards, the oxidation products of limonene (LimSOA) that were

89

produced in the flow-tube reactor were extracted into water and oxidized by OH (reaction 1) and O3

90

(reaction 2) in two batch reactors. The aim of this work was to provide detailed insights into kinetics

91

of limonene SOA aging in clouds, fogs and wet aerosols. Reactions 1 and 2 were investigated with

92

liquid chromatography coupled to the electrospray ionization tandem mass spectrometry (LC-

93

ESI/MS/MS). The chromatographic (LC) separation combined with the selective and sensitive

94

detection method (MS/MS) allowed for the following a behavior of the individual LimSOA during the

95

aqueous-phase oxidation experiment. Consequently, it was possible to investigate the kinetics of

96

reactions (1) and (2) for each LimSOA while working with the mixture of these compounds; SOA

97

sample from the flow-tube reactor.

98

2.

99

In this work, a flow-tube reactor was used to generate SOA by gas-phase 25

Flow-tube reactor was used to generate twenty limonene oxidation

Experimental Materials and reagents are listed in section S1 of the Supplementary Information (SI). LA was

100

synthesized as previously described.20

101

2.1.

102

Synthesis of the limonene oxidation products The flow reactor was used to synthesize the compounds that were later used in the aqueous25

103

phase experiments.

104

mixing plunger where it was mixed with limonene to initiate the reaction. The reaction conditions

105

were: [O3] = 3 ppm, [limonene] = 10 ppm, pressure = 1 atm, T = 298 K (room temperature), relative

106

humidity ≈ 5%, total flow through the tube = 1.3 L/min, Reynolds number ≈ 14, 26 average residence

107

time = 10 min; no OH scavenger was used. Under these conditions both first and second/third

108

generation products of limonene oxidation were formed.

109

(TX40H120WW, Pall) and extracted into a buffered solution of the reference compounds (see Table

110

1) by mechanical agitation to avoid artifacts formation that may be formed during ultrasound

111

extraction. This solution was passed through a 0.22 µm PTFE syringe filter. The pH of the extract was

112

adjusted and checked with pH-meter (HI 221, Hanna Instruments) before and after each aqueous-

113

phase oxidation experiment (see sections 2.3 and 2.4). The aqueous-phase oxidation was carried out

114

immediately after the extraction and the estimated concentrations of the individual LimSOA were

115

several µM.

116

2.2. Relative rate method

Ozone was added to the 1.5 m x 12 cm Pyrex glass tube through a movable

25

SOA was collected on a filter



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117

Relative rate method was used to study kinetics of reactions (1) and (2) for LimSOA. Reference

118

compounds used to measure the bimolecular rate coefficients (kOH and kOzone) for reactions (1) and

119

(2) are listed in Table 1.

120

Table 1 List of the reference compounds and rate coefficients, kref, for their reactions with OH

121

and ozone

Reference compounds

kref for oxidation by OH (M−1 s−1) × 10-9 pH =2

pH=5

pH=10

6.9

6.9

-

Phenylalanine

5.7

6.2

a

p-Toluic acid

-

-

Caffeine

Reference compounds

122

9.0 8.0

kref for oxidation by O3 (M−1 s−1) × 10-5 pH =2

pH=7 and 8

Gallic acid

0.97

4.7

Cinnamic acid

0.31b

3.8

Phenylalanine

-

0.19

a

b

calculated using kOH for caffeine (probably pH-independent) calculated using kOzone for gallic acid-see SI

The kref values listed in Table 1 were taken from the literature.

123

22, 30

22, 27-29

124

discrepancies in kozone values are reported for cinnamic acid.

125

104 M−1 s−1 listed in Table 1 for pH=2 was measured in this work (section S3.3).

126

Note that significant

For this reason, the value of 3.1 ×

Reaction (2) was studied at pH =7 and 8 to avoid dissociation of the hydroxyl groups of gallic 31

and also to avoid decomposition of this compound under strongly basic pH conditions. 29 Note

127

acid

128

that pH = 7 and 8 was sufficient to convert the monoterpene acids into carboxylate ions. 32, 33

129 130

Assuming that a given LimSOA and ref are removed only by reacting with a single oxidant, the unknown bimolecular rate coefficient is calculated using eq. (I). 17, 21 Ln

k [Lim ] [Ref] Ln

=

(I) [Lim ] k [Ref]

131

[LimSOA] and [Ref] are the initial (0) and intermediate (t) concentrations, kLimSOA and kref are the

132

bimolecular rate coefficients for LimSOA and the reference compound. Note that ozone reacts with

133

gallic and cinnamic acids as well as with phenylalanine in 1:1 molar ratio. 29, 30, 34

134

2.3. Oxidation by hydroxyl radicals


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135

The photooxidation experiments were carried out as previously described.

20, 21

pH of the

136

filter extract was adjusted by adding small amount of HCl (pH=2), NaOH (pH=10) or a diluted

137

phosphate buffer (pH=5). The filtered reaction mixture (ca. 15 ml) was placed in a small Pyrex glass

138

bottle and OH was generated by H2O2 (15 mM) photolysis with a 310 W lamp (UVAHAND 250 GS

139

H1/BL, Honle) equipped with a Pyrex glass UVA filter. The solution was mixed with a magnetic stirrer.

140

Aliquots of the reaction mixture (100 µl) were quenched with the catalase solution to stabilize the

141

sample before the LC/MS analysis.

142

2.4. Oxidation by ozone

143

Aqueous-phase ozonolysis was carried in a miniaturized bubble column reactor shown in Fig.

144

S3. The reaction mixture (ca. 15 ml) consisted of limonene SOA solution in DI water with the addition

145

of H3PO4 (pH=2) or H3PO4/Na3PO4 buffer (pH=7) and (pH=8) and ca. 15 mM of t-butyl alcohol (OH

146

scavenger). 35 Ozone was generated by photolysis of pure O2 using a UV generator (UVP SOG-2, 3ppm

147

at 1 L/min version). O2/O3 mixture was bubbled though the solution at a flow rate of 10 ml/min. 100

148

µl aliquots were sampled from the bubbler and incubated with 50 µl of the quenching buffer: 2 µM

149

of indigotrisulfonate and a solution of 0.001 mg/ml of catalase to stabilize the sample. Afterwards, 30

150

µl of ACN was added and the sample was analyzed with LC/MS.

151

2.5. Liquid chromatography coupled to the mass spectrometry

152

Chromatographic analyses were carried out with LC20A liquid chromatograph (Shimadzu) 25

153

coupled to the QTRAP 3200 (AB Sciex) triple quadrupole mass spectrometer.

154

C18 column (100 mm × 2.1 mm, 3 µm, 100 Å) with the security guard cartridge with a 2 mm ID C18

155

pre-column was used for analytes separation. Eluent A was 0.03 % formic acid solution in water (pH =

156

2.8) and eluent B was acetonitrile (ACN), flow rate of the mobile phase was 0.2 mL/min. Gradient

157

elution programs are provided in section S2.

Luna (Phenomenex)

158

The mass spectrometer was equipped with the electrospray (ESI) ion source and was operating

159

in the multiple reaction monitoring (MRM) and scan modes: 50-700 m/z. The ESI conditions were as

160

follows: spray voltage: 4.5 kV (negative ionization mode) and 5.5 kV (positive ionization mode), N2

161

was curtain (3 × 105 Pa), auxiliary (3 × 105 Pa) and collision gas. The MRMs for the LimSOA were

162

published previously. 25 MRM conditions and the sample chromatogram of the reference compounds

163

are provided in section S2, Fig. S1 and S2 and Table S1.

164

2.6. Control experiments and uncertainty



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165

Validation of the bubble reactor included: calculations of the ozone mass transfer through the

166

gas-liquid interface to ensure that these experiments were carried out in the kinetic regime (section

167

S3.2) and comparison of the kOzone values obtained for limononic and cinnamic acid under static and

168

dynamic conditions (section S3.3). Control experiments were carried out by turning off the ozone

169

generator to check if the analytes are purged from the solution by the gas flow – Fig. S4-S6.

170

LimSOA and the reference compounds decomposition in the aqueous solution of H2O2 and due to

171

UV-Vis irradiation was checked by carrying out appropriate control experiments – Fig. S8-S10. Some

172

repartitioning of LimSOA into the gas-phase was observed under acidic pH as shown in Fig. S8. To

173

correct for these non-OH losses a control experiment was always carried out simultaneously with the

174

photooxidation as previously described.

175

using a smaller bottle with a reduced headspace volume.

20

However, it was possible to minimize this problem by

176

The uncertainties of the kOH and kOzone values obtained depends on the: precision of the

177

kLimSOA/kref (slope of the linear fits to the experimental data), uncertainty of the kref values that are

178

reported in the literature and also uncertainty of the LC/MS measurements. All of these factors were

179

taken into account and the cumulative uncertainties obtained for kOH and kOzone values were between

180

10-20%.

181

3.

182

3.1. Kinetics of the aqueous-phase oxidation of limonene SOA

Results and discussion

183

In our previous study, 25 limonene SOA composition was investigated with LC-ESI/MS using both

184

low and high-resolution mass spectrometers. The high-resolution fragmentation spectra together

185

with the data reported in the literature was used for structural identification of the semi-volatile

186

organic compounds that contribute to limonene SOA. These tentatively identified structures of the

187

individual LimSOA are shown in Table 2. Detailed discussion regarding structural identification and

188

possible formation pathways of the compounds listed in Table 2 is provided elsewhere. 25

25

As

189

previously demonstrated,

dimers can be unambiguously distinguished from the “monomers” due

190

to chromatographic separation of these higher and lower-MW molecules before introducing the

191

sample into the ESI ion-source of the mass spectrometer.

192

Sample relative kinetic plots for limonic acid (compound 5) and dimer identified as ester of

193

limonic acid and 7-hydroxylimononic (compound 18) are shown in Fig. 1; temporal profiles for the

194

compounds 1-20 and the reference compounds are provided in Fig. S11-S16.


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195 196 197

Figure 1 The plots for relative kinetic experiments for the selected LimSOA, red lines are linear fits to the experimental data (blue squares)

198

As shown in Fig. 1, the relative kinetic plots were linear. Therefore the kOH and kOzone values were

199

calculated under different pH conditions using the experimental data acquired. Similar results were

200

obtained for the rest of the LimSOA using the reference compounds listed in Table 1. Average kOH and

201

kOzone values that were calculated with eq. I are listed in Table 2.



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Table 2 Bimolecular rate coefficients measured for the LimSOA

202

Compound

Namea

number

1

Structureb

MRM (Q1/Q3

kOH (M−1 s−1) × 10-10

m/z, Da)

Limononic acid

183/139

kOzone (M−1 s−1) × 10-4

Neutral

Carboxylate

Neutral

Carboxylate

1.3 ± 0.1

0.54 ± 0.1

2.6 ± 0.2

5.7 ± 0.3

0.20

0.21

(SAR)

(SAR)

O OH

O

2

Isomers of keto-limononic acid

185/115 O

3

OH

Stable

O O

4

Keto-limononic acid

5

Limonic acid

Increase O

185/141

OH

1.3 ± 0.2

0.39 ± 0.06

0.11

0.17

(SAR)

(SAR)

1.3 ± 0.2

0.50 ± 0.10

2.3 ± 0.2

4.6 ± 0.3

O

OH

6

Keto-limonic acid

O

187/143

OH

O O OH

7

Keto-limonic acid isomer

187/115

O

Increase

OH O HO OH HO O O

O

O

8

Dicarbonyl derivative of limononic acid

197/153

O O

4.0 ± 0.2

6.4 ± 0.3

O OH

9


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9

Dicarbonyl derivative of

197/153

O O

limononic acid

10

1.1 ± 0.1

1.1 ± 0.1

4.1 ± 0.2

6.4 ± 0.3

1.2 ± 0.3

0.8 ± 0.1

2.9 ± 0.3

5.3 ± 0.4

0.35

0.48 ± 0.05

O OH

OH

7-hydroxy limononic acid

O

O

OH

11

199/181 Carbonyl-substituted keto-

O

O O

12 13

O

O

O

O

OH

O

limononic acids

Hydroxy keto – keto-limononic

O

215/97

acid

(SAR)

OH

O OH O

Stable

0.20 ± 0.05

0.81 ± 0.1

0.27

0.30

(SAR)

(SAR)

1.3 ± 0.2

0.69 ± 0.14

3.2 ± 0.3

7.1 ± 0.4

2.2 ± 0.4

0.69 ± 0.06

10 ± 1

11 ± 1

1.2 ± 0.2

0.67 ± 0.2

HO

14

Dihydroperoxy limononic acid

O

233/183

OH O

OH O OH

15

Aldol reaction producs or a

337/319

O

O O

hemiacetal

OH O O O

HO OH O

16

339/115

O

O

Esters/aldol reaction products

O

Increase

OH O OH

17

339/115

O

O

1.3 ± 0.1

0.90 ± 0.1

OH O O O

10



18

367/185

Page 12 of 20

1.2 ± 0.2

0.70 ± 0.04

3.8 ± 0.5

5.9 ± 0.7

1.6 ± 0.4

1.2 ± 0.2

10 ± 2

10 ± 1

1.4 ± 0.2

1.2 ± 0.1

5.1 ± 1

6.0 ± 1

O OH

O

19 20

Esters of limonic and 7hydroxylimononic acid

O

HO

367/185

O

O OH

367/185

O

O O

HO

O O

203

a

tentatively identified in our previous work and/or based on the literature data available b If known or one of the possible isomers– see below

204

11


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205


As expected, the kOH values that are listed in Table 2 are within diffusion limit 22

36

due to high

206

reactivity of unsaturated compounds towards OH.

207

good agreement with the kinetic data reported for the ozonolysis of unsaturated compounds with

208

the terminal C=C bonds. 22, 35, 37 The pH-dependence of the kOH and kOzone is discussed in section 3.2.

The kOzone values listed in Table 2 are also in a

209

Concentrations of compounds 4, 6 and 7 increased during reactions (1) and (2) since their

210

formation (oxidation of the first generation products) was faster than decomposition (oxidation of

211

saturated molecules by OH). Therefore, compounds 4, 6 and 7 were identified as second-generation

212

products (with the terminal C=C bonds converted into C=O). This conclusion is in a good agreement

213

with the previously proposed structures of these compounds. 9, 25, 38, 39

214

Concentrations of compounds 2, 3, 11-13 decreased during reactions (1) and (2) in some

215

experiments but were stable or had a local maximum (initial increase followed by decomposition) in

216

other as shown in Fig. S11-12 and S14-15. Therefore, compounds 2, 3, 11-13 were identified as the

217

third-generation products. Previously, compounds 11-13 were incorrectly identified as the first

218

generation products and their revised structures are presented in Table 2. A detailed discussion

219

about elucidation of the structures of compounds 11-13 is provided in section S6.

220

When it was possible (concentration decreased exponentially in some but not in all experiments),

221

kOH values for the saturated LimSOA were estimated with eq. (I). The rest of kOH values for the

222

saturated LimSOA was estimated with the structure-activity relationship (SAR) parameters.

223

reactivity of the second and third generation products towards OH is most likely caused by the

224

deactivating effects of the carbonyl and carboxylic groups 49, 50 and also by the relatively low number

225

of aliphatic hydrogen atoms in these molecules. 40, 41

226

40, 41

Low

Ozonolysis of the saturated compounds is too slow to compete with their oxidation by OH,

22

227

therefore, in this case, we did not estimate the kOzone values for the second and third generation

228

products.

229

Reaction (1) also leads to the degradation of dimers, which is in a good agreement with the 15

230

results reported by Zhao et al.

231

neither OH nor O3 is selective towards dimers. More importantly, compounds 16 and 17 were

232

generated during reaction (2) as listed in Table 2 and shown in Fig. S14-S15. In the aqueous solution,

233

the Criegee intermediates are efficiently stabilized 47, 57 thus they can react with the stable molecules

234

(mainly carboxylic acids) to generate higher-MW products. 42, 43 Note that aqueous-phase ozonolysis

235

can dominate over oxidation by OH as a result of the pH-dependence of kOH and kOzone values – see

236

section 3.3.

However, the kinetic data summarized in Table 2 indicate that

237 12 ACS Paragon Plus Environment


238

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3.2. pH-dependence of kOH and kOzone

239

For the LimSOA that are shown in Table 2, the carboxylic groups are most likely not adjacent to

240

the terminal double bond. Consequently, the rate coefficient of reaction (2) is only moderately

241

enhanced for the carboxylate ions due to the electrophilic character of ozone, as previously

242

concluded.

243

literature database for the carboxylic acid/carboxylate equilibrium impact on the reaction (1)

244

mechanism, especially for the unsaturated carboxylic acids. 22, 27, 45

22, 44

The pH-dependence of the kOH is however difficult to explain due to very limited

245

Kinetics of reaction (1) were also studied at pH=5 (see Table 1) to compare the

246

experimentally obtained and predicted kOH values. The kOH values listed in Table 2 were used to

247

calculate the cumulative rate coefficients at the intermediate pH with eq. (II) and eq. (III). 17 α=!

[H # ] ' (II) [H # ] + Ka

k ( )**+, - = k ( × α + k / × (1 − α) (III) 248

α –fraction of the non-dissociated molecules: AH

249

Ka- estimated carboxylic acid/carboxylate equilibrium constant, 10-pKa - see Table 3

250

[H+] – concentration of the H+ ions (M), 10-pH

251

k OH AH = rate coefficients measured for the protonated acids (AH)

252

k OH A- = rate coefficient measured for the carboxylate ions (A-)

253

For the reaction (1) the calculated pH-dependence of the kOH values were compared with the

254

experimental data - the kOH values measured at pH=5 - as listed in Table 3. Only compounds with the

255

kOH values obtained experimentally (first generation products - see Table 2) are included in Table 3.

256

Table 3 Estimated and experimentally obtained kOH values at pH=5

Compound number pKaa

kOH (M−1 s−1) × 10-9 Calculated Experimental

1

4.76

8.2

6.2 ± 1.0

5

4.17

6.0

5.2 ± 1.0

4.91

8

4.49

6.9

10.0 ± 1.4

9

4.49

9.9

11.0 ± 1.0 13


Page 15 of 20


10

4.10

14

4.35

b

7.3 ± 1.0

8.0

6.6 ± 1.1

-

15

16 17

4.3

18

8.8 ± 1.2 7.6

5.8 ± 0.8

7.6

5.4 ± 0.9

5.4

6.7 ± 1.0

19

3.90

9.0

7.7 ± 1.1

20

4.50

7.0

6.1 ± 0.8

a

257

8.4

b

estimated with Marvin software Not an acid

258

As listed in Table 3, the measured and predicted kOH values at pH=5 are in a reasonably good

259

agreement. These results indicate that eq. II and III can be used to calculate the kOH values between

260

pH 2 and 10 using the experimental data acquired. Consequently, the rate coefficients obtained were

261

used to estimate the atmospheric lifetimes of LimSOA as a function of pH of the aqueous medium as

262

presented in section 3.3.

263

3.3. Atmospheric implications

264

Average lifetimes calculated for saturated and unsaturated LimSOA are presented in Fig. 2.

265

Values of kOH and kOzone under intermediate pH conditions were estimated as described in section 3.2.

266

It was assumed that only unsaturated compounds will react with O3 (kOzone for the terminal C=C bond

267

≈ 5 × 10-19 cm3 × molecule-1 s-1 in the gas-phase).

268

estimate kOH values for the reaction of LimSOA with OH – see Table S4. 23, 46, 47

269

concentrations were assumed:

270

molecule × cm-3 and 2 × 10-9 M. 22, 32, 50

23, 46, 47

Gas-phase SAR parameters were used to

6

48, 49

The following oxidants

-3

[OH] ≈ 2 × 10 molecule × cm and 2 × 10-14 M, [O3] = 2 × 1012

271 14 ACS Paragon Plus Environment


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272

Figure 2 The average lifetimes for the saturated and unsaturated LimSOA as a function of pH,

273

uncertainty bars represent the data range, data points are the averaged results for saturated,

274

unsaturated compounds and dimers (na rysunku słowo oxidation jest napisane niepoprawnie

275

Estimated lifetimes for unsaturated LimSOA (as well as for the dimers) due to oxidation by OH in

276

the gas and aqueous-phase are of similar order as shown in Fig. 2. The unsaturated oxidation

277

products of limonene can also react with O3 in addition to the reaction with OH whereas aqueous-

278

phase ozonolysis is not important for the saturated molecules, as expected.

279

unsaturated limonene acids can also compete with their oxidation by OH under mildly acidic and

280

basic pH-conditions due to higher reactivity of the carboxylate ions towards ozone as discussed in

281

section 3.2.

22

Ozonolysis of the

282

The kinetic data acquired, strongly indicate that the reactions studied here can occur in clouds

283

and fogs which agrees well with the previously published data about the aqueous-phase processing

284

of monoterpene SOA.

285

present entirely in the aqueous-phase as inferred from their H values. 14 Consequently, whether or

286

not the LimSOA will be oxidized in aqueous-phase depends on the partitioning of the individual

287

compounds contributing to limonene SOA.

17, 20, 24

When LWC ≈ 0.3-0.5 g × m-3 the molecules listed in Table 2 will be

288

As previously demonstrated, aside from limonene, gas-phase oxidation of the other

289

atmospherically abundant terpenes yields semi-volatile products and these molecules will often

290

retain the less reactive double bond of the parent hydrocarbon. 51-53 Results presented here strongly

291

indicate that such molecules will be highly susceptible to the aqueous-phase oxidation by OH and O3

292

under realistic atmospheric conditions. Aqueous-phase ozonolysis of the compounds with the

293

terminal double bonds appears to be less important than their oxidation by OH. However,

294

generation of the carboxylate ions can sufficiently enhance the rate of aqueous-phase ozonolysis

295

thus it should still be considered as the potential oxidation mechanisms of the unsaturated terpene

296

acids.

297

Both types of reactions that were studied here can yield low-volatility products as previously

298

demonstrated, 17, 19-21 thereby potentially contributing to aqSOA formation under certain conditions.

299

However, it is very difficult to speculate about the aqSOA yields following aqueous-phase oxidation of

300

limonene SOA using the data obtained here. For this reason, a more quantitative approach is

301

necessary to estimate the branching ratios of the unstable by-products as well as yields of highly

302

oxidized products of reactions (1) and (2).

303

Supplementary data


Page 17 of 20

304


Supporting Information available, this information is available free of charge via the Internet at

305

http://pubs.acs.org.

306

Acknowledgments

307

This project was founded by the polish National Science Centre: grant number

308

2014/13/B/ST4/04500. We thank the Structural Research Laboratory (SRL) at the Department of

309

Chemistry of University of Warsaw for the LC/MS measurements. SRL was established with financial

310

support

311

1.4.3./1/2004/72/72/165/2005/U. We thank dr Dagmara Tymecka for LA purification with a semi-

312

preparative HPLC. The study was carried out at the Biological and Chemical Research Centre,

313

University of Warsaw, established within the project co-financed by European Union from the

314

European Regional Development Fund under the Operational Programme Innovative Economy, 2007

315

– 2013. We thank the anonymous reviewers for very helpful, constructive and insightful comments

316

and suggestions.

317

References

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Kinetics of Limonene Secondary Organic Aerosol Oxidation in the

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