cis-Pinonic Acid Oxidation by Hydroxyl Radicals in the Aqueous

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cis-Pinonic acid oxidation by hydroxyl radicals in the aqueous phase under acidic and basic conditions: kinetics and mechanism Bartlomiej Witkowski, and Tomasz Gierczak Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02427 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 20, 2017

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For Table of Contents Only 47x26mm (300 x 300 DPI)

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Measured OH reaction relative data for aqueous reaction (1) under acidic and basic conditions; symbols are experimental data and straight lines are linear fits to the experimental data 201x140mm (300 x 300 DPI)


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Calculated contributions of H-abstraction from position 1 – 8 to the total rate constant and experimental results 201x140mm (300 x 300 DPI)



Total ion current (TIC) chromatogram for the sample taken from the reaction at t=0 and 60 min and extracted ion current (EIC) chromatograms of the major products 201x140mm (300 x 300 DPI)


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Fragmentation spectra of the reaction (1) products 201x140mm (300 x 300 DPI)



A proposed formation mechanism of the reaction (1) products; the major proton abstraction sites are shown in bold, stable products detected with ESI are shown in blue, all possible isomeric products are not shown for clarity 291x411mm (300 x 300 DPI)


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cis-Pinonic acid oxidation by hydroxyl radicals in the aqueous phase under acidic and basic conditions: kinetics and mechanism

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Bartłomiej Witkowski* and Tomasz Gierczak

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University of Warsaw, Faculty of Chemistry, Al. Żwirki i Wigury 101, 02-089 Warsaw, Poland

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

Keywords: cis-Pinonic acid, aqueous secondary organic aerosol, aqSOA, hydroxyl radicals, liquid

21

chromatography, relative rates

22

*Corresponding author

23

dr Bartłomiej Witkowski, University of Warsaw, Biological and Chemical Research Centre, room 2.27,

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al. Żwirki i Wigury 101, 02-089 Warsaw, Poland, phone: +48(22) 55 26602, email:

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[email protected] 1 ACS Paragon Plus Environment


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Abstract

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Aqueous-phase oxidation of cis-pinonic acid (CPA) by hydroxyl radicals (OH) was studied

28

using a relative rate technique under acidic and basic conditions. Liquid chromatography (LC) coupled

29

to the negative electrospray ionization (ESI) quadrupole tandem mass spectrometry (MS/MS) was

30

used to monitor the concentrations of CPA and reference compounds. The measured second order

31

reaction rate coefficients of CPA with OH were: 3.6 ± 0.3 × 109 M-1s-1 (pH=2) and 3.0 ± 0.3 × 109 M-1s-1

32

(pH = 10) - combined uncertainties are 2σ. These results indicated that the lifetimes of CPA in the

33

atmosphere are most likely independent from the aqueous-phase pH. LC-ESI/MS/MS was also used

34

to tentatively identify the CPA oxidation products. Formation of carboxylic acids with molecular

35

weight (MW) 216 Da (most likely C10H16O5) and MW 214 Da (C10H14O5) was confirmed with LC-

36

ESI/MS/MS. When the initial CPA concentration was increased from 0.3 mM to 10 mM, formation of

37

additional products was observed with MW 187, 200, 204 and 232 Da. Hydroperoxy, hydroxyl and

38

carbonyl-substituted CPA derivatives were tentatively identified among the products. Similar

39

products were formed by the CPA oxidation by OH in the gas-phase, at the air-water interface as well

40

as in the solid phase (dry film). Formation of the stable adduct of CPA and H2O2 was also observed

41

when the reaction mixture was evaporated to dryness and re-dissolved in water. Acquired mass

42

spectrometric data argues against formation of oligomers.

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1. Introduction

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SOA (secondary organic aerosol) formation following the gas-phase monoterpene oxidation is a 1

58

globally occurring phenomena that was studied extensively.

SOA is produced in the gas-phase

59

when semi-volatile oxidation products of the volatile organic compounds (VOC), mainly biogenic VOC

60

(BVOC), nucleate or condense onto preexisting particles (the partitioning theory).

2, 3

However, the

1, 4-7

61

current predictions of the global SOA budget are still largely unsuccessful.

The large

62

discrepancies between bottom-up estimated of the global aerosols budget and the field observations

63

are due, in part, to our limited understanding of the processes leading to SOA formation. 1, 4-7

64

Recently, it was proposed that organic compounds processing in clouds, fogs and wet aerosols

65

may lead to SOA formation in the aqueous-phase (aqSOA). 4, 6, 8-11 When semi-volatile precursors are

66

oxidized in the aqueous-phase, the low-volatility products can form particles after solvent is

67

evaporated. 12 Multiphase processes are also partially responsible for the chemical aging of oligomers

68

detected in monoterpene SOA particles.

69

aqueous-phase processes are becoming the emerging topic of interest in the field of atmospheric

70

chemistry.

71

13

Consequently, the currently poorly characterized

9, 13-15

Gas-phase oxidation of α-pinene by hydroxyl radicals (OH), tropospheric ozone (O3) and nitrate 16, 17

72

radicals (NO3) is a well recognized source of SOA.

73

pinene oxidation products and it is often present in the ambient particles.

74

are in a good agreement with the predicted partitioning of CPA under realistic atmospheric

75

conditions.

76

assuming 5 µg m-3 of organic mass loading at 296 K. 19 The predicted CPA Henry's Law constant (H) ≈

77

2 x 107 M atm-1 14, 20 is also comparable with the effective H of the isoprene oxidation products that

78

have received much study as the aqSOA precursors. 11, 21, 22 Consequently, CPA will be mostly present

79

in the aqueous phase under humid conditions, e.g. in clouds. 9, 20

9, 19

Cis-pinonic acid (CPA) is one of the major α18

The field observations

It was concluded that about 50% of CPA should partition into the particle phase

80

Results of the field measurements also indicate that CPA is subjected to a chemical degradation

81

during summer. 9, 23 According to the most recent estimates, the aqueous-phase oxidation by OH is a

82

dominant removal mechanism of CPA in clouds (assuming liquid water content – LWC - of 0.3 – 0.5 g

83

m-3).

84

only about 6 % in the gas phase. 24 The aqueous-phase processing of CPA is therefore an important

85

source of SOA. 9, 14, 19, 20 The reaction of CPA with OH radicals:

9, 20

SOA yield from CPA oxidation by OH is between 40 and 60 % in the aqueous-phase 9 and

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is here of primary importance.

88

Very recently, a kinetic report describing experiments for the reaction (1) in the liquid phase was

89

published. 9 On-line chemical ionization mass spectrometry (CIMS) was utilized to study reaction (1)

90

kinetics in the aqueous phase under acidic conditions (pH=2). 9 The pH of water-containing particles

91

in the atmosphere ranges from 1 to 9. 10, 25 In cloud water (pH 4-5) 10, 25 CPA (pKa ≈ 4.82) 26 can exist

92

in both dissociated (A-) and protonated forms (AH). It is therefore important to study reaction (1)

93

kinetics under both basic and acidic conditions. 27

94

The objective of this work was to measure the rate coefficient for reaction (1) in acidic and basic

95

environment. For this purpose, relative rate technique (RR) was used. The reaction (1) mechanism

96

was also studied. Liquid chromatography coupled to the electrospray ionization mass spectrometry

97

(LC-ESI/MS), an offline analytical technique, was used in this study.

98

before the mass spectrometric analysis was especially useful for reaction products identification. The

99

currently available literature data indicate that products of reaction (1) were highly-oxygenated

100

1, 28

Analytes separation by LC

derivatives of CPA but detection of the individual isomers was not possible. 9, 14

101

2.

Experimental details

102

2.1. Relative rate method and OH rate coefficient determination

103

Rate coefficient for reaction (1) was measured at 300±2 K using a relative rate technique. The

104

rate coefficient for reaction (1) for acidic and basic pH was obtained by monitoring the relative loss of

105

CPA and the reference compounds (reaction 2) with a well-known OH rate coefficient. 9, 29, 30

106

OH + reference → products

(2)

107

For the unknown rate coefficient determination using a RR technique the compound of

108

interest is mixed with the reference compound in the same reaction vessel. 9, 29, 30 Assuming that CPA

109

and the reference compound are lost only due to reaction with OH in the aqueous phase the rate

110

coefficient for reaction 1 can be calculated using eq. (I). Ln

Pinonic acid k Ref Ln I Pinonic acid k Ref


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[cis-Pinonic acid] and [Ref] are concentrations of CPA and the reference compound before

112

turning on the lamp (time=0) and during the experiment (time=t). Caffeine (kOH=6.9 ± 0.7 × 109 M−1

113

s−1), suberic acid (kOH= 4.8 ± 0.4 × 109 M−1 s−1) and pimelic acid (kOH=3.5 ± 0.4 × 109 M−1 s−1) at acidic

114

pH conditions as well as 4-chlorobenzoic acid (kOH= 5 ± 0.5 × 109 M−1 s−1), p-toluic acid (kOH= 8 ± 0.8 ×

115

109 M−1 s−1) and phenylalanine (kOH=9 ± 1 × 109 M−1 s−1) at basic pH conditions were used as a

116

reference compounds to measure the reaction (1) rate coefficient. 31

117

The experimental apparatus (see Fig. S1 in the Supplementary Information, SI) consisted of a

118

150 ml Pyrex glass vessel. The reaction mixture was constantly mixed with a magnetic stirrer. OH

119

radicals were generated in-situ by hydrogen peroxide (H2O2) photolysis. The UVAHAND 250 GS H1/BL

120

lamp (Honle UV technology, 310 W) was used to irradiate the reaction mixture. This lamp emitted a

121

broad UV-Vis spectrum and the shortest UV radiation wavelengths (< 300 nm) were filtered-out by

122

the Pyrex glass wall of the photo-reactor. 9 The photo-reactor temperature was kept constant using

123

a fan. The temperature was approx. 300±2 K; the temperature increase inside the photo-reactor was

124

most likely due to irradiation by a high-power lamp.

125

For the kinetic studies, 100 ml of water, 100 µl of H2O2 and ca. 0.5 mg of CPA and a reference

126

compound were mixed. Therefore, the reactants concentrations were ca. 10 mM of H2O2 and ca. 30

127

µM of both CPA and a reference compound. The OH concentration in the absence of the reactants

128

was ca. 5 × 10-12 M; this value was obtained from the simulation using a box model (see section S1 in

129

the SI).

130

were used previously. 9, 34 HCl or NaOH was added to the reaction mixture until the desired pH was

131

achieved – the solution pH was checked with a pH-meter before every experiment. For the reaction

132

(1) products identification CPA was 0.3, 1, 2, 5 and 10 mM and the H2O2 concentration was increased

133

accordingly to evaluate if the initial precursor concentration had any impact on the products

134

distribution. After each photo-oxidation experiment, the reaction solution was also evaporated to

135

dryness and re-dissolved in the same amount of water to check for any non-radical reactions.

32, 33

The above mentioned reaction conditions are considered atmospherically-relevant and

136

The reaction progress was monitored by sampling the solution (100 µl) from the reactor at

137

different time intervals and the total reaction time was 2.5 - 4 h - see page S2 in the SI for more

138

detailed description of the sampling procedure. The reaction solution aliquots were mixed with the

139

equal volume of catalase solution (≈ 0.1 mg/ml) to neutralize the leftover H2O2. Catalase was

140

dissolved in 50 mM ammonium acetate buffer (pH = 7 or 5) to neutralize the reaction mixture pH,

141

maintain the enzyme activity and to avoid damaging the C18 stationary phase since most C18

142

columns operate in the pH range between 2 and 8. 32 The sample was incubated with the enzyme at

143

25 °C for ≈ 15 min in a dry heating block. Afterwards, 50 µl of ACN was added, the solution was 5 ACS Paragon Plus Environment


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filtered through PTFE syringe filter (0.2 µm pore size) and subjected to the chromatographic analysis

145

(section 2.2).

146

2.2. HPLC/MS analysis

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HPLC/MS experiments were carried out with LC20A liquid chromatograph (Shimadzu)

148

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

149

out using a reverse phase Luna (Phenomenex) C18 column (100 mm × 2.1mm, 3 µm, 100 Å) kept at

150

30°C. The column was equipped with the security guard cartridge with a 2mm ID C18 pre-column.

151

Eluent A was formic acid solution in water (pH = 2.8 and eluent B was acetonitrile (ACN) and the

152

mobile phase was delivered at a flow rate of 0.2 mL/min; the injection volume was 5 µl. Mass spectra

153

(in the negative ionization mode) were acquired in both total ion current (TIC) in the mass range 50-

154

700 m/z, selected reaction monitoring (SRM) modes and MS2 mode. In the MS2 mode the selected

155

precursor ion was subjected to collision induced dissociation (CID). The ESI conditions were as

156

follows: capillary voltage was -4.5 kV, source temperature 450 °C and nitrogen was used as curtain

157

gas (3 × 105 Pa), auxiliary gas (3 × 105 Pa) and collision gas. Analytes concentrations were monitored

158

in the SRM mode and the individual ion-lenses voltages were optimized for each Q1/Q3 transition.

159

Selected reaction monitoring (SRM) mode conditions were optimized by directly introducing the

160

analytes solution into the mass spectrometer ion source using a Harvard Apparatus pump at a flow

161

rate of 10 μL/min – see section 3.1.

162

In order to ensure the linear response of the MS detector, calibration was carried out using

163

standards solutions (Fig. S3). HPLC-ESI/MS/MS analysis method parameters for each analyte are

164

listed Table S2. The analytes were identified based on the characteristic retention times and Q1/Q3

165

transitions. For each compound linear regression coefficients (R2) with values more than 0.99 were

166

obtained using integrated peak areas as dependent variables and concentrations (mg/ml) as

167

independent variables, respectively. Gradient elution programs were different for reaction (1)

168

kinetics analysis and products identification:

169

Gradient elution program for the reaction (1) kinetics analysis:

170

0–5 min 5% B, 5–7 min linear gradient to 25% B, 7–11 isocratic 25% B, 11-16 min linear gradient

171

to 95% B, 16-18 min 95% B, 18-18.5 min linear gradient to 5% B. Afterwards, the column was re-

172

equilibrated at 5% B and the analysis was completed in 25 min.

173

Gradient elution program for the reaction (1) products identification


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0–0.5 min 5% B, 0.5–35 min linear gradient to 30% B, 35–38 linear gradient to 90% B, 38-42 min

175

95% B, 42-42.5 min linear gradient to 5% B. Afterwards, the column was re-equilibrated at 5% B and

176

the analysis was completed in 55 min.

177

2.3. Materials

178

cis-Pinonic acid (≥ 98.0 %), caffeine (≥ 98.0 %), suberic acid (≥ 99.5 %), pimelic acid (≥ 99.5

179

%), azelaic acid (≥ 99.5 %), p-toluic acid (≥ 98.0 %), 4-chlorobenzoic acid (≥ 98.5 %), phenylalanine (≥

180

98.5%), catalase from bovine liver 2,000-5,000 units/mg protein, as well as LC/MS grade solvents and

181

eluent additives: acetonitrile (≥ 99.9%), formic acid (≥ 99.5%) and 25% ammonia solution in water

182

were all purchased from Sigma - Aldrich. Deionized (DI) water (18 MΩ×cm-1) was prepared using

183

Direct - Q3 Ultrapure Water System (Millipore).

184

3. Results and discussion

185

3.1. The rate coefficient, k1, of the cis-pinonic acid with OH radicals measurement

186

The rate coefficients of reaction (1) were studied under acidic and basic conditions using the

187

relative rate technique as described in section 2.1.

188

work is summarized in Fig. 1

9, 27

Kinetic data for reaction (1) acquired in this

189

As shown in Fig. 1 the rate coefficients for reaction (1) were measured using seven reference

190

compounds. Bimolecular rate coefficients for reaction (1) were measured for neutral (pH=2) and

191

dissociated (pH=10) forms of CPA. The k1/kref (see eq. I) values were obtained using linear least squares

192

regression. The results for rate coefficient measurements are summarized in Table 1.

193

As listed in Table 1, the obtained k1 in the aqueous phase under acidic conditions was 3.6 ±

194

0.3 × 109 M−1 s−1. The value obtained here is in a good agreement with the k1 reported by Aljawhary

195

at al. 9: 3.3 ± 0.5 × 109 M−1 s−1 for the undissociated form of CPA (pH=2). For the dissociated form of

196

CPA (pH =10), k1 obtained was 3.0 ± 0.3 × 109 M−1 s−1. As listed in Table 1 k1 calculated for all

197

reference compounds was very consistent, ranging from 3.5 to 3.7 × 109 M−1 s−1 for pH=2 and 2.8 to

198

3.0 × 109 M−1 s−1 for pH=10. However, the measured k1 values for pH=2 and pH=10 were the same

199

within the uncertainty of the experiment.

200

The rate coefficients listed in Table 1 are a sum of the specific rate coefficients for the individual 24, 35, 36

201

hydrogen atoms abstraction by OH.

The individual rate coefficients were calculated using

202

aqueous-phase structure-activity relationship (SAR). 35, 36 Fig. 2 source data is provided in Table S3.

203

As shown in Fig. 2, positions 3 and 4 were the most reactive proton-abstraction sites, followed

204

by positions 6 and 7. 36 According to the SAR predictions, dissociation of the carboxylic group should 7 ACS Paragon Plus Environment


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205

increase the overall CPA reactivity towards OH as reflected by the calculated k1 values listed in Fig. 2.

206

35, 36

207

were acquired since some reversible aqueous-phase processes, such as hydration of the carbonyl

208

group, will also affect the CPA reactivity towards OH. 35, 36 The results presented in Fig. S4-S6 strongly

209

indicate that the hydration equilibrium constant (Keq) for CPA was very low. The 1H NMR spectra also

210

revealed the dissociation of the methyl group hydrogen atoms (carbonyl group α-position) in basic

211

solution. Since the measured k1 values were the same for both acidic and basic conditions, the

212

increased reactivity of CPA due to carboxylic group deprotonation was most likely counteracted by

213

deprotonation of the CH3 in position 1. Unfortunately, to-date, aqueous SAR is unable to account for

214

the neighbouring effects of the dissociated aliphatic hydrogen atoms.

In addition to the SAR calculations, CPA 1H NMR spectra in neutral, acidic and basic solutions

215

As already discussed in the introduction, reaction (1) is most likely a dominant CPA removal

216

mechanisms in clouds with LWC between 0.3-0.5 g×m-3. 9, 20 The data summarized in Table 1 confirms

217

that the k1 is most likely independent from the aqueous medium pH. Consequently, CPA aqueous-

218

phase lifetimes calculated using k1 obtained for pH=2 9 are probably accurate for acidic, neutral and

219

basic conditions.

220

3.2. Error analysis and control experiments

221

The absolute uncertainty in the rate coefficient ratios for CPA is a combination of the uncertainty

222

in the sample and reference compound measurements, the precision of the k1/kref data fits

223

(measurement precision), and the uncertainties reported for the reference compounds

224

coefficients (kref).

31

Based on the literature compiled by Buxton et al.

31

37-39

rate

10% is a reasonable estimate

225

of the rate coefficients uncertainty for the reference compounds.

226

and reference compound measurements inferred from the reproducibility of the integrated SRM

227

peak areas (see section S2) was generally ca. 5%. As it could be seen in Table 1, for the experiments

228

under acidic and basic conditions squared linear regression coefficients are more than 0.99.

229

Therefore, the precision for majority of k1/kref data fits is ca. 5%. On the basis of combined

230

uncertainties listed above we estimate the rate coefficient 2σ uncertainty to be ca. 12%.

231 232

The uncertainty in the sample

Several control experimental test were carried out in search for systematic errors. Control experiments are listed in Table 2.

233

The first two sets of control experiments were carried out in order to ensure that the observed

234

decay of the CPA and the reference compounds was solely due to reaction with OH. Initially, we

235

checked if CPA and the reference compounds can be directly photolyzed under the experimental

236

conditions described in section 2.1. The results of these control experiments (reported in Fig. S7) 8 ACS Paragon Plus Environment

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237

strongly indicated that no noticeable decomposition of the analytes listed in Table S2 was observed if

238

no H2O2 was added to the reaction mixture. Tyrosine was initially considered as a reference

239

compound but it wasn’t used due to considerable photolysis.

240

In the second set of the control experiments listed in Table 2 H2O2 was added to the solution

241

containing CPA or one of the reference compounds. The lamp was kept off and the reaction mixture

242

was mixed for 4h. Results of the second set of the control experiment also indicated that addition of

243

H2O2 did not caused any measurable decomposition of the analytes (data not shown).

244

The third set of experiments was performed to monitor the stability of the CPA oxidation

245

products in the HPLC autosampler. Literature data and the results obtained here strongly indicate

246

that some of the products of reaction (1) contain hydroperoxy group. However, the samples taken

247

out from the reactor were mixed with the catalase to decompose any leftover of H2O2. 10, 11 Thus, it

248

was important to investigate if the products of reaction (1) are stable when mixed with the enzyme.

249

These experiments were carried out as follows: initially CPA was oxidized by OH for ca. 1h to

250

generate sufficient amount of the oxidation products (see section 3.3). Afterwards, the sample was

251

taken out from the reactor, mixed with the catalase solution and subjected to the HPLC/MS analysis.

252

The stability of the reaction (1) products in the HPLC autosampler was monitored by performing 9

253

injection of the same sample for ca. 4.5 h. As shown in Fig. S8, no measurable decomposition of the

254

major reaction (1) products was observed within the time scale of the control experiments.

255

3.3. Mechanism aqSOA formation from cis-pinonic acid reaction with OH

256

Mechanism of the reaction (1) in the aqueous phase was studied in order to evaluate its SOA

257

formation potential. The extracted ion chromatograms (EIC) for the reaction (1) products detected

258

with ESI (negative ionization mode) are shown in Fig. 3.

259

The compounds detected as: m/z 215 and m/z 213 ions were the major products of reaction

260

(1) produced in the bulk reactor. When the initial CPA concentration was increased from 0.5 mM to

261

10 mM additional products were detected, as m/z 231, 203 and 199 and 187 ions, as shown in Fig. 3.

262

Previously published data indicates that when CPA was oxidized by OH at the air-water interface the

263

initial precursor concentration had little impact on the products distribution.

264

initial CPA concentration was increased from 50 µM to 500 µM, the yields of products detected as

265

m/z 213 and 215 ions were also higher.

266

reaction mechanism at the air-water interface as compared to the bulk reactor used here. 14 It is also

267

possible that CPA concentration inside the microdroplets generated in the ESI nebulizer, that were

268

later exposed to OH, was much higher than in the initial solution due to rapid solvent evaporation

14

14

In fact, when the

The reason for these discrepancies might be a different,



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269

inside the ESI chamber. 14 Results shown in Fig. 3 also confirmed that stable oligomers (MW>300 Da)

270

were not present in the reaction mixture, even when the initial CPA concentration was 10 mM.

271

Fragmentation spectra of the reaction (1) products are shown in Fig. 4.

272

As shown in Fig. 4 compounds detected as m/z 215 ion (tr ≈ 17.8 min) produced intense

273

fragmentation ions: m/z 181 (neutral loss of H2O2) and m/z 137 (neutral loss of H2O2 and CO2); these

274

three compounds were most likely pinonic hydroperoxides.

275

second group of compounds with MW = 216 Da (tr = 16.5 – 17.5 min) were characterized by intense

276

fragment ions: m/z 171 (44 Da, CO2 elimination), 153 (62 Da, neutral loss of CO2 + H2O) which

277

indicates presence of carboxylic group and absence of the hydroperoxide moiety. Product detected

278

as m/z 213 ion also contained carboxylic group as indicated by the formation of m/z 169 ion (CO2

279

elimination), and hydroxyl as well as carbonyl group; m/z 195 as well as m/z 151 ions formed by the

280

neutral loss of H2O from m/z 213 and 169 ions, respectively. Fragmentation spectra of the m/z 199

281

ions indicated that these three isomers (see Fig. 3) were different hydroxyl-substituted CPA

282

derivatives. Fragmentation spectrum of the m/z 187 ion indicated presence of one carboxylic group

283

(m/z 143 ion, neutral loss of CO2) and possibly two carboxylic groups (m/z 99 ion) and

284

hydroxyl/carbonyl groups (neutral loss of H2O). Fragmentation spectra of the two m/z 203 ions were

285

similar to the fragmentation spectrum of 3-methyl-1, 2, 3-butanetricarboxylic acid (MBTCA, MW =

286

204 Da) and its isomer. 23 Note that even when the initial CPA concentration was high, intensity of

287

the m/z 203 ions was relatively low, most likely reflecting a low yield of MBTCA from reaction (1),

288

which is consistent with the literature data. 9, 24

14

The fragmentation spectra of the

289

When the reaction mixture was dried without neutralizing the leftover of H2O2, formation of

290

the non-radical reaction product detected as m/z 233 ion was observed (see Fig. S9). CID of the m/z

291

233 ion revealed the formation of: m/z 183 and m/z 139 fragmentation ions. Very similar compound

292

with the elemental formula C10H18O6 (MW = 234 Da) was detected in our previous study of gas-phase

293

limonene ozonolysis and it was labeled as unknown, highly oxidized derivative of limononic acid.40

294

This product was most likely formed by addition of hydrogen peroxide molecules to the CPA carbonyl

295

group, as shown in Fig. 5. Even if CPA hydration is low (as inferred from the 1H NMR spectra, see SI),

296

the Keq for the α-hydroxyhydroperoxide formation is significantly higher

297

most likely increased during the solvent evaporation. Another possibility is a peracid formation

298

together with addition of a single H2O2 molecule to the CPA carbonyl group. Note that these

299

reactions may be also important under realistic atmospheric conditions; it was argued that H2O2 and

300

organic hydroperoxides signifincantly contribute to the monoterpene SOA mass.

301

peroxide is also produced by aqueous-phase ozonolysis of methacrolein and methyl vinyl ketone. 45

10, 41

and H2O2 concetration

42-44

Hydrogen


Page 17 of 26


Possible mechanism leading to the formation of the reaction (1) products is shown in Fig. 5.14,

302 303

24

304

As shown in Fig. 5, the initially formed alkyl radical reacts with molecular oxygen to yield

305

peroxy radicals (MW = 215 Da). These peroxy radicals can subsequently react with HO2 or RO2

306

radicals to produce hydroperoxy (MW = 216 Da)46, 47 or hydroxyl-substituted CPA (MW = 200 Da). 48

307

The intensity of the second disproportionation product was relatively low (m/z 197 ion, data not

308

shown), perhaps due to lower ionization efficiency of the carbonyl-substituted CPA or subsequent

309

decomposition due to reaction with OH. The formation of products with MW 216 and 214 is also

310

possible via ring-opening rearrangement of the alkoxy radical followed by disproportionation of the

311

resulting peroxy radical. The formation of product with MW = 232 Da from reaction (1) in the gas-

312

phase was reported by Muller et al.

313

may be formed by subsequent oxidation of the first generation products with MW = 216 Da. The

314

second possible formation pathway involves H-shift of the alkoxy radical and formation of

315

dicarboxylic acid with MW 232 Da due to reaction with HO2 and molecular oxygen.47

24

but no structural information were presented. This product

316

MBTCA formation mechanism was previously proposed by Muller et al. 24 for the gas-phase

317

CPA oxidation by OH. 9, 24 Here, MBTCA formation by-product proposed by Enami and Sakamato was

318

observed (MW = 188 Da).

319

interface MBTCA is produced via similar pathway, this mechanism is shown in Fig. 5.

14

Thus, it is more likely that in the bulk reactor and at the air-water

19

14

MBTCA

320

formation (MW = 204 Da) was also observed as a result of CPA dry film oxidation by OH.

Evidently,

321

higher precursor concentration and longer reaction times are needed to observe polycarboxylic acids

322

formation in the CPA/OH system, since MBTCA and similar compounds are most likely the higher-

323

generation products. 24

324

Results presented in this section indicate that the aqSOA production from reaction (1) is

325

mainly due to addition of different, highly oxygenated functional groups to CPA 19 rather than due to

326

formation of oligomers. This mechanism of aqSOA formation in the CPA/OH system is significantly

327

different as compared with the aqueous-phase oxidation of the smaller, water-soluble molecules. 1, 4,

328

12, 49

329

aqueous-phase oxidation by OH when higher precursor concentrations were used. 48

Glyoxal

32

and methyl vinyl ketone

33

produced large quantities of oligomers following the

330

New insights into kinetics and mechanism of CPA reaction with OH in the aqueous phase

331

presented in this work underline that HPLC-ESI/MS/MS is an important method for studying

332

multiphase processing of the semi-volatile, monoterpene-derived aqSOA precursors.

333



Page 18 of 26

334

Acknowledgments

335

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

336

2014/13/B/ST4/04500. The authors would like to thank the Structural Research Laboratory (SRL) at

337

the Department of Chemistry of University of Warsaw for making LC/MS measurements possible. SRL

338

has been established with financial support from European Regional Development Found in the

339

Sectoral Operational Programme “Improvement of the Competitiveness of Enterprises, years 2004 –

340

2005” project no: WPK_1/ 1.4.3./1/2004/72/72/165/2005/U. The study was carried out at the

341

Biological and Chemical Research Centre, University of Warsaw, established within the project co-

342

financed by European Union from the European Regional Development Fund under the Operational

343

Programme Innovative Economy, 2007 – 2013. The authors would like to thank dr Marcin Wilczek

344

(University of Warsaw, Faculty of Chemistry, Nuclear Magnetic Resonance Laboratory) for the 1H

345

NMR measurements. The authors would like to thank the anonymous reviewers for very helpful,

346

constructive and insightful comments.

347 348

Supporting Information Available, this information is available free of charge via the Internet at http://pubs.acs.org.

349 350

Figures captions

351

Figure 1 Measured OH reaction relative data for aqueous reaction (1) under acidic and basic

352

conditions; symbols are experimental data and straight lines are linear fits to the experimental data

353

Figure 2 Calculated contributions of H-abstraction from position 1 – 8 to the total rate constant and

354

experimental results

355

Figure 3 Total ion current (TIC) chromatogram for the sample taken from the reaction at t=0 and 60

356

min and extracted ion current (EIC) chromatograms of the major products

357

Figure 4 Fragmentation spectra of the reaction (1) products

358

Figure 5 A proposed formation mechanism of the reaction (1) products; the major proton abstraction

359

sites are shown in bold, stable products detected with ESI are shown in blue, all possible isomeric

360

products are not shown for clarity

361 362 363 364 12 ACS Paragon Plus Environment

Page 19 of 26


365 366 367 368 369

Figures

370 371

Figure 1

372



Page 20 of 26

373 Figure 2

374

375 376

Figure 3 14 ACS Paragon Plus Environment

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377 378

Figure 4



Page 22 of 26

379 380

Figure 5

381 382 16 ACS Paragon Plus Environment

Page 23 of 26

383


Tables Table 1 OH reaction rate coefficients in aqueous phase for cis-pinonic acid in acidic and basic

384 385

conditions Reference compound

R2

pH

k1/kref

kref *

Calculated k1

(109 M−1 s−1) Caffeine

0.54 ± 0.02

6.9 ± 0.7

3.7 ± 0.3

0.996

0.74 ± 0.02

4.8 ± 0.4

3.6 ± 0.3

Pimelic acid

0.997

1.01 ± 0.02

3.5 ± 0.4

3.5 ± 0.3

4-Chlorobenzoic acid

0.996

0.61 ± 0.01

5 ± 0.5

3.0 ± 0.3

p-Toluic acid

0.992

0.36 ± 0.01

8 ± 0.8

2.9 ± 0.3

0.998

0.45 ± 0.01

6.9 ± 0.7

3.1 ± 0.3

0.991

0.31 ± 0.01

9±1

2.8 ± 0.3

Caffeine

2

10

Phenylalanine

386 387

(109 M−1 s−1)

0.994

Suberic acid

*Taken from Buxton et al.

k1 for cis-pinonic acid,

k1 for pH = 2:

3.6 ± 0.3

k1 for pH = 10:

3.0 ± 0.3

31

Table 2 Experimental conditions for the control experiments UV lamp

H2O2

Yes

No

No

Yes

Reaction mixture composition

Experiment type Direct CPA photolysis?

DI water + cis-pinonic acid or reference compounds

Direct degradation of the

with only one compound present at the time, 3-4 h

substrates by H2O2?

the duration time of test

Yes

Yes

DI water + cis-pinonic acid after 1.5 h of irradiation,

Stability of the major

sample was mixed with the catalase solution and

reaction (1) products in

kept for 4.5 h in the HPLC autosampler at 10 °C to

the HPLC autosampler?

study stability of the reaction products 388 389 390 391 392

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cis-Pinonic Acid Oxidation by Hydroxyl Radicals in the Aqueous

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