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Limononic acid oxidation by hydroxyl radicals and ozone in the aqueous phase Bartlomiej Witkowski, sara jurdana, and Tomasz Gierczak Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04867 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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Environmental Science & Technology

1

Limononic acid oxidation by hydroxyl radicals and ozone in the

2

aqueous phase

3

Bartłomiej Witkowski, 1* Sara Jurdana, 1,2 Tomasz Gierczak 1 1

4 5

University of Warsaw, Faculty of Chemistry,

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

6

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

Keywords: Limononic acid, limonene, secondary organic aerosol, hydroxyl radicals, ozone, relative

20

rate, tandem mass spectrometry

21

*Corresponding author e-mail: [email protected]

22

2

23

Rijeka, Croatia

Permanent address: University of Rijeka, Department of Biotechnology, Radmile Matejčić 2, 51000

1

ACS Paragon Plus Environment


24

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Abstract

25

Kinetics and mechanism of limononic acid, (3-isopropenyl-6-oxoheptanoic acid, LA) oxidation

26

by hydroxyl radicals (OH) and ozone (O3) were studied in the aqueous phase at 298±2 K. These

27

reactions were investigated using liquid chromatography coupled to the electrospray ionization and

28

quadrupole tandem mass spectrometry (LC-ESI/MS/MS). The rate coefficients determined for LA +

29

OH reaction were: 1.3 ± 0.3 × 1010 M-1s-1 at pH=2 and 5.7 ± 0.6 × 109 M-1s-1 at pH=10. The rate

30

coefficient determined for LA ozonolysis was 4.2 ± 0.2 × 104 M-1s-1 at pH=2. Calculated Henry’s Law

31

constant (H) for LA was ca. 6.3 × 106 M × atm-1 thereby indicating that in fogs and clouds with LWC =

32

0.3-0.5 g × m-3 LA will reside entirely in the aqueous-phase. Calculated atmospheric lifetimes due to

33

reaction with OH and O3 strongly indicate that aqueous-phase oxidation can be important for LA

34

under realistic atmospheric conditions. Under acidic conditions, the aqueous-phase oxidation of LA

35

by OH will dominate over reaction with O3, whereas the opposite is more likely when pH ≥ 4.5. The

36

aqueous-phase oxidation of LA produced keto-limononic acid and a number of low-volatility

37

products, such as hydroperoxy-LA and α-hydroxyhydroperoxides.

38 39 40 41 42 43 44 45 46 47 48 49 50 51 2


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52

1. Introduction

53

Secondary organic aerosols (SOA) are formed due to nucleation or partitioning of the semi1, 2

54

volatile oxidation products of VOCs in accordance with the partitioning theory.

55

methane VOCs budget is dominated by biogenic emission of isoprene and monoterpenes: α, β-

56

pinene, and limonene.

57

(BVOC) contributes significantly to the secondary organic aerosols (SOA) formation.

58

because of our limited understanding of the SOA formation mechanisms, impact of the terpene

59

biogenic emission on the climate is still uncertain. 5-10

3, 4

Globally, non-

Gas-phase oxidation of these atmospherically abundant biogenic VOC 5

However,

60

Limonene is a more efficient secondary organic aerosol (SOA) precusor than α and β-pinene

61

because it possess two carbon-carbon double bonds. 11-15 A significant fraction of the first generation

62

oxidation products of limonene will retain the less reactive,

63

Consequently, these unsaturated, semi-volatile compounds can undergo further oxidation in the gas,

64

particle or aqueous phase thereby increasing the overall SOA yield. 14, 15

65

11, 12, 16

exocyclic double bond.

17-20

Recently, it has been recognized that the aqueous-phase processing of the semi-volatile 7, 21-23

66

monoterpene derivatives can contribute to SOA

67

characterized.

68

phase processing of limonene oxidation products.

69

products of limonene can react with ozone (O3) and hydroxyl radicals (OH) in the aqueous-phase. If

70

relevant under atmospheric conditions, these aqueous-phase reaction may lead to aqueous-phase

71

SOA (aqSOA).

72

been focused on the less reactive, saturated cis-pinonic acid (CPA) and other saturated derivatives of

73

α-pinene. 26-29

74

10, 21, 22, 24

7, 14, 21-23

but these processes are very poorly

Also, little attention has been directed to the investigation of the aqueous25

The unsaturated, first-generation oxidation

However, to-date, most studies of the terpene derived aqSOA precursors have

Limononic acid (3-isopropenyl-6-oxoheptanoic acid, LA) is a first-generation product of limonene 12, 17, 30

28

75

oxidation in the gas-phase

76

double bond and the estimated Henry's law constant (H) for this compound is 6.3 × 106 M × atm-1. 31

77

In fog and clouds with LWC = 0.3-0.5 g × m-3 LA will reside entirely in the aqueous-phase. Thus, LA is a

78

good model compound for studying

79

volatile derivatives of monoterpenes. In the gas-phase LA reacts with O3 (k≈5 × 10-19 cm3 × molecule-1

80

s-1)

81

reactions in the aqueous-phase are currently unknown.

11, 12, 15, 16

and also a major product of CPA photolysis.

aqSOA

LA has a exocyclic

formation due to oxidation of the unsaturated, semi-

and OH (k=7 × 1011 cm3 × molecule-1 s-1)

32, 33

but the rate coefficients for these two

82

The aim of this work is to investigate the kinetics and mechanism of LA reaction with OH

83

(reaction 1) and with O3 (reaction 2) in the aqueous-phase. These reactions were studied in a batch

84

reactor using liquid chromatography coupled to the electrospray ionization tandem mass

85

spectrometry (LC-ESI/MS/MS). 3



86 87 88

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2. Experimental List of reagents is provided in section S1 of the Supplementary Information (SI). 2.1. Limononic acid synthesis and purification

89

LA was synthesized by irradiating CPA solution in deionized (DI) water/MeOH with a 254 nm

90

mercury Pen-Ray lamp. To confirm the structure of LA as the major product of CPA Norrish-II

91

isomerization,

92

agreement with the previously published data. 28 Details of this synthesis are provided in section S2.

93

2.2. Kinetics of limononic acid reaction with OH and O3

94 95

28 1

H NMR spectrum was acquired. LA 1H NMR spectrum obtained is in a good

The rate coefficients for reaction (1) reaction (2) in the aqueous-phase were obtained using competition kinetics by monitoring the relative loss of LA and the reference compound. Limononic acid + OH → products 1 Limononic acid + O → products 2

96 97

If LA and the reference compound are lost only due to reaction with OH or O3, the kLA can be calculated using eq. (I) Ln

98 99

[Limononic acid] k ! [Reference] Ln = I [Limononic acid] k "#$ [Reference]

[Limononic acid] and [Reference] are the initial (0) and intermediate (t) concentrations, kLA and kref are the bimolecular rate coefficients for LA and reference compound.

100

Caffeine (kOH=6.9 ± 0.7 × 109 M−1 s−1 at pH=2), phenylalanine (kOH=9 ± 1 × 109 M−1 s−1 at pH=10

101

and 5.7 ± 0.5 × 109 M−1 s−1 at pH=2), p-toluic acid (kOH= 8 ± 0.8 × 109 M−1 s−1 at pH=10) 34 were chosen

102

as the reference compounds to study kinetics of reaction (1) – k1. Gallic acid (kozone= 9.7 × 104 M−1 s−1

103

at pH =2) 27, 38 was used to study kinetics of reaction (2) – k2. It was assumed that one molecule of O3

104

reacts with one molecule of gallic acid and LA. 35

105 106

2.3. Oxidation of limononic acid by OH The aqueous-phase photo-reactor was described in our previous study.

36

Briefly, the photo-

107

reactor consisted of a 50 ml Pyrex vessel and the reaction mixture was stirred with magnetic stirrer.

108

The reaction solution was irradiated with a UVAHAND 250 GS H1/BL lamp (Honle UV technology, 310

109

W) to generate OH by H2O2 photolysis.

4


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110

The reaction mixture consisted of 25 ml of 17 mM H2O2 solution in DI water. Concentrations of

111

LA and the reference compounds were ca. 30 µM. Solution pH was adjusted by adding HCl or NaOH.

112

OH concentration in the absence of other reactants was ca. 5 × 10-12 M.

113

concentrations were used in some experiments to study the mechanism and identify reaction (1)

114

products. Aliquots of the reaction mixture (100 µL) were sampled from the reactor, incubated with

115

the equal volume of the catalase solution (≈ 0.1 mg/ml) and injected into LC/MS.

17

Slightly higher reactants

116

2.4. Ozonolysis of limononic acid

117

Kinetics of reaction (2) was studied by adding different volumes of the ozone solution (ca. 6

118

µM, determined with the indigo method 37) to the aqueous solution of LA and gallic acid (2 µM each).

119

38

120

(pH=2). 38, 39 The samples always contained excess reactants relative to O3 and the tert-butyl alcohol

121

(t-BuOH) was used as OH scavenger (10-20 mM). 38, 39

Ozone solution was prepared by bubbling the ozone through phosphoric acid solution in DI water

122

Mechanism of reaction (2) was studied by slowly bubbling the ozone (≈ 5 ml/min) through

123

solution of LA (50 µM) in DI water. 150 µL aliquots of the reaction mixture were sampled from the

124

bubbler and subjected to the LC/MS analysis.

125

2.5. Liquid chromatography coupled to the mass spectrometry

126

LC/MS analysis were carried out with LC20A liquid chromatograph (Shimadzu) coupled to the 17

127

QTRAP 3200 (AB Sciex) triple quadrupole mass spectrometer as previously described.

128

products identification a 55 min gradient elution program was used. Shorter gradient elution

129

program was used for the kinetic analysis (section S3).

For the

130

Mass spectra were acquired in the scan mode in the mass range 50-700 m/z and in the

131

multiple reaction monitoring (MRM) mode. The ESI conditions were as follows: capillary voltage was -

132

4.5 kV and 5.5 kV in the negative and positive ionization modes. Ion source temperature was 450 °C,

133

nitrogen was used as curtain (3 × 105 Pa), auxiliary (3 × 105 Pa) and collision gas. Fragmentation

134

spectra of reactions (1) and (2) products were acquired during chromatographic run, after the

135

analytes were separated by LC. Ion lenses voltages for the MRM mode were adjusted by directly

136

injecting the analytes into the ESI ion source.17

137

2.6.

138

Control experiments and error analysis Control experiments were carried out to search for systematic errors.

17

Briefly, UV-vis

139

irradiation, H2O2, phosphoric acid and t-BuOH by themselves did not caused any measureable

140

decomposition of LA and the reference compounds within the time-scale of the experiment as 5



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141

described in section S4. Unfortunately, reaction (2) kinetics was only investigated under acidic

142

conditions due to some experimental problems (data not shown).

143

It was also impossible to avoid some minor repartitioning of LA into the gas-phase under

144

acidic pH conditions. To account for the non-OH losses of LA under acidic pH conditions, two Pyrex

145

vessels were placed in front of the lamp after carefully adjusting their positions. The reaction mixture

146

volumes and compositions were identical in both bottles but H2O2 was added only to one. During

147

these experiments, the reaction solutions were sampled simultaneously from both bottles. In the

148

second set of control experiments, the reaction vessel without the H2O2 was place at exactly the

149

same distance from the lamp immediately after the photo-oxidation experiment (with the H2O2

150

added to the reaction mixture) was completed. The results from both sets of these control

151

experiments were averaged. During the experiments under acidic conditions, LA was lost due to

152

reaction (1) and also due to repartitioning into the gas phase. Using the first order loss rates (min-1),

153

it was possible to account for the non-OH losses of LA as shown in Fig. S8.

154

The total uncertainty in the bimolecular rate coefficients listed in Table 1 is a sum of

155

precision of the kLA/kref data fits obtained from linear regression analysis (standard error) as well as

156

uncertainties of the kref values (±10%). 34, 36 The uncertainties of the analytes measurements was ±5%

157

as estimated from the reproducibility of the integrated MRM peak areas (2σ). Thus, the combined

158

uncertainty for k1 value calculated for basic pH conditions was ±10% and ± 20 % for reaction (1)

159

under acidic conditions.

160

3. Results and discussion

161

3.1.

162

Determination of the reaction rate coefficients with OH and O3 Relative kinetic plots for reaction (1) are shown in Fig. 1.

6


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163 164

Figure 1 Relative kinetic data for reaction (1)

165

Linear regression analysis (straight lines in Fig. 1) was used to calculate k1/kref ratios listed in

166 167

Table 1. Table 1 Measured k1 and k2

Reference

168

R2

Slope, k1/krefa Caffeine 0.996 1.45 ± 0.04 Phenylalanine 0.994 3.00 ± 0.09 p-Toluic acid 0.993 0.72 ± 0.02 Phenylalanine 0.995 0.62 ± 0.02 a ± standard uncertainty; b listed by Buxton et al. 34

kref (109 M−1 s−1)b 6.9 ± 0.7 5.7 ± 0.5 8.0 ± 0.8 9.0 ± 0.9

pH 2 10

Average k1 (1010 M−1 s−1) k1 for pH=2 1.3 ± 0.3 k1 for pH=10 0.57 ± 0.06

169

As shown in Fig. 1, the kinetic plots were linear: R2 > 0.99. Calculated k1 values listed in Table

170

1 for pH=10 were very consistent for both reference compounds, and the average value of 5.70 ±

171

0.06 × 109 M−1 s−1 was obtained. Because there was a need to correct for a non-OH losses of LA under

172

acidic conditions (section 2.6), the uncertainty of k1 obtained for pH=2 was higher. For the

173

undissociated LA, an average k1 value of 1.3 ± 0.3 × 1010 M−1 s−1 was obtained.

174

For CPA, a saturated compound, considerably lower k1 values of 3.6 ± 0.3 and 3.3 ± 0.5 × 109

175

M−1 s−1 for pH=2 and 3.0 ± 0.3 × 109 M−1 s−1 for pH = 10 were recently reported.

176

increased reactivity is most likely due to the OH addition to the double bond. Using aqueous-phase

177

structure–activity relationship (SAR)

178

will react with OH via addition mechanism (section S5).

17, 18

29, 36

Thus, this

it can be roughly estimated that ca. 70 % of LA molecules

7



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179

It is difficult to discuss the pH-dependence of k1, especially since the current literature

180

database for keto-acids oxidation by OH is rather limited. 34, 36 Similar results were presented in our

181

previous study for OH reaction with CPA, where the apparent lack of pH dependence on the kOH was

182

associated with the unexpectedly high acidity of the hydrogen atom in α position to the carbonyl

183

group. 36

184

For reaction (2), k2 = 4.2 ± 0.4 × 104 M−1 s−1 was obtained as shown in Fig. S12. As expected,

185

ozonolysis of the exocyclic double bond of LA was significantly slower than the ozonolysis of

186

endocyclic double bonds. 40, 41

187 188 189

The data acquired was used to calculate atmospheric lifetimes of LA - section 3.3. 3.2. Mechanisms of limononic acid oxidation by OH and O3 The products of reaction (1) and (2) detected using LC/MS as shown in Fig 2. 2

190 191 192

Figure 2 Chromatograms (total ion current): red - reaction mixtures at t=0 and, blue – 60 min after reactions initiation

193

Fragmentation spectra of the products detected with LC/MS were used to propose the

194

mechanisms of reactions (1) and (2). Molar yields (section S7) were estimated using integrated peak

195

areas for the [M-H]- ions. Since the lack of standards the yields calculated using this method are most

196

likely underestimated. The ESI response increases together with the percentage of volatile organic

197

modifier (ACN), 42 hence it was lower in the beginning of the gradient run - see section S3. Thus, the

8


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198

highly oxygenated compounds that were eluted earlier than limononic acid (tr =27.5 min) were

199

ionized less efficiently. 3.2.1. Aqueous phase oxidation of limononic acid by OH

200 201 202

Extracted ion chromatograms (EICs) and fragmentation spectra of the reaction (1) products are shown in Fig. 3.

203 204 205

Figure 3 EICs and fragmentation spectra of reaction (1) products

206

As it is shown in Fig. 3, the major product of reaction (1) was keto-LA – see section S7. Keto-

207

LA was also identified using the data reported in our previous study of limonene SOA composition. 17

208

The small quantities of compounds with MW 234 Da were most likely produced due to the non-

209

radical addition of H2O2 to LA as it could be inferred from the retention time of these peaks – see

210

section S8.

211

To further investigate the non-radical addition of H2O2 to LA, a series of experiments was carried

212

out, using linear carboxylic acids, carboxylic acids containing hydroxyl and carbonyl groups as well as

213

dicarboxylic acids – see section S8. These carboxylic acids were dissolved in the H2O/H2O2 mixture,

214

evaporated to dryness and re-dissolved in pure water. H2O2 only reacted with the keto-acids (Table

215

S3), including LA. As shown in Fig. 3, LA + H2O2 reaction generated products detected as negatively 9



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216

charged ions with m/z 233. Retention times of some of these ions overlapped with the products of

217

reaction (1), as shown in Fig.3. Interestingly, H2O2 did not cause any measurable decay of LA during

218

control experiments but small peaks as m/z 233 ions were detected in these experiments. Perhaps a

219

substantial amount of H2O2 was produced during the photooxidation experiments, thereby increasing

220

the yields of these non-radical products.

221

The other chromatographic peaks for the m/z 185 ion are most likely a result of the in-

222

source fragmentation as m/z 233 ions (the same retention times of the chromatographic peaks). As

223

shown in Fig. 3, compounds detected as m/z 233 ions readily produced m/z 185 fragmentation ion

224

following collision induced dissociation (CID). A number of minor products of reaction (1) were also

225

detected as shown in Fig. 4.

226 227

Figure 4 EICs and fragmentation spectra of the additional products of reaction (1)

228

As shown in Fig. 4, minor products of reaction (1) were detected as m/z 215 (two isomers),

229

201 (three isomers) and 197 (two isomers). Compounds detected as m/z 215 and 197 ions are most

230

likely carboxylic acids as indicated by the neutral loss of 44 Da (CO2). Neutral losses of 18 Da (H2O)

231

and 62 Da (CO2 + H2O) from m/z 215, 201 as well as from 197 ions indicate that these compounds

232

could be hydroxy carboxylic acids. These compounds were most likely produced by the abstraction

233

mechanism that is shown Fig. 5

234

The proposed mechanism for reaction (1) is shown in Fig. 5.

235 10


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236 237 238 239

Figure 5 Proposed mechanism of LA oxidation by OH, products detected with LC/MS are shown in blue As shown in Fig.5 addition mechanism is expected to dominate over H atom abstraction as 17, 18

240

already discussed in section 3.1 and section S5.

241

experimental data, since keto-LA (major product detected) is formed by conversion of the terminal

242

double bond to a carbonyl.

This assumption is consistent with the

243

As shown in Fig. 5, the two peroxy radicals (RO2) are produced due to OH addition to the

244

double bond and reaction of the hydroxylalkyl radicals with O2. The formation of peroxy radical (1)

245

should be favored because it is produced from a tertiary alkyl radical.

246

with each other to form the unstable tetroxides (the water-cage effect).

247

decompose to primary or tertiary alkoxy radicals. Keto-LA can be produced following •CH2OH

248

elimination from the tertiary alkoxy radical – this pathway is most likely favored. 43, 44 Alternatively,

249

after elimination of formaldehyde from the primary alkoxy radical a C9 α-hydroxyalkyl radical is

250

formed. The C9 α-hydroxyalkyl radical can react with O2 thereby yielding keto-LA by eliminating HO2

43

The two RO2 radicals react 44

Both tetroxides can

11



251 252

45

or to produce α-hydroxyalkyl alkoxy radical.

alkoxy radical by OH elimination.

44

Page 12 of 20

Keto-LA is then produced from the α-hydroxyalkyl

44

253

Minor products of reaction (1) are most likely produced by addition pathway and subsequent

254

oxidation of the terminal double-bonds of the first generation products. Hydroperoxy (MW = 216

255

Da), hydroxyl (MW = 200 Da) and carbonyl (MW = 198 Da) – substituted LA derivatives are produced

256

due to reaction of the peroxy radicals with HO2 or RO2. 36 Formation mechanism of hydroperoxy-LA

257

was most likely similar to the formation of hydroperoxy-CPA that was previously described. 36, 46, 47

258

Estimated LA conversion yield to keto-LA is rather low (ca. 10%). Clearly other products of

259

reaction (1) are formed, and these additional compounds were most likely not detected with LC/MS

260

(e.g. carbonyl compounds and lower-MW carboxylic acids). This might be a result of the

261

fragmentation of the alkoxy radicals. In case of carboxylic acids, it has been recently demonstrated

262

that fragmentation of alkoxy radicals is enhanced for the carboxylate ions. 48 Clearly, there is a need

263

to quantify the lower-MW products of reaction (1) for LA and similar compounds to further

264

investigate the impact of the carboxylic acid/carboxylate equilibrium on the alkoxy radicals

265

fragmentation.

266 267

3.2.2. Aqueous phase ozonolysis of limononic acid EICs and fragmentation spectra of the reaction (2) products are shown in Fig. 6.

268 269

Figure 6 EIC chromatograms and fragmentation spectra of the reaction (2) products

12


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270

As shown in Fig. 6, the major product of reaction (2) is keto-LA; only a single chromatographic

271

peak was detected for m/z 185 ion. Fragmentation spectra and retention time of keto-LA were

272

consistent with data reported in our previous study.

273

and 34 Da (H2O2) from the m/z 201 ion indicated that the second product most likely contains

274

hydroperoxide moiety. This characteristic fragmentation mechanism confirms that at least the

275

compounds with MW 202 Da might be acyloxyhydroperoxy aldehydes. Fragmentation of the second

276

m/z 201 ion was very difficult (low intensity of the fragmentation ions), which might be consistent

277

with the formation of secondary ozonide (data not shown).

278

17

As shown in Fig.6 neutral losses of 18 (H2O)

A proposed mechanism for reaction (2) is shown in Fig. 7.

279 Figure 7 Proposed mechanism of LA ozonolysis, products detected with LC/MS are shown in

280 281 282

blue As it is shown in Fig. 7, when O3 adds to the terminal double bond, the unstable primary 40, 41, 49

283

ozonide (POZ) is formed.

284

formaldehyde (strongly favored) 49 or C1 CI and keto-LA (minor, not shown). 40 In the aqueous-phase,

285

the C9 CI is efficiently stabilized and a stabilized CI (SCI) is formed. 47, 57 In the laboratory reactor, the

286

C9 SCI can react with water to produce the α-hydroxyhydroperoxide (MW 220 Da). The α-

287

hydroxyhydroperoxides are unstable as confirmed by the very low intensity of the chromatographic

288

peak for the m/z 219 ion (data not shown) and decompose to keto-LA and H2O2. 40 The experimental

289

results also indicated that small fraction of the C9 SCIs can self-react, thus two products with MW 202

290

Da are observed as shown in Fig 7.

291

3.3. Atmospheric implications

POZ can decompose to C9 Criegee intermediate (CI) and

13



Page 14 of 20

292

The following data was used to calculate the gas-phase lifetimes of LA that are shown in Fig. 8:

293

kOH = 7 × 10-11 cm3 × molecule-1 s-1, (calculated with SAR) 32, 33 and kozone = 5 × 10-19 cm3 × molecule-1 s-1

294

(ozonolysis reaction rate coefficient for the limonene exocyclic double bond), 11, 12, 16 [OH] = 2 × 106

295

molecule × cm-3 and [O3] = 2 × 1012 molecule × cm-3.

296

[OH]= 2 × 10

-14

-9

M, [O3] =2 × 10 M,

25

32

In the aqueous-phase, it was assumed that

and pKa of LA ≈ 4.76. 50

297 298

Figure 8 Calculated LA atmospheric lifetimes in the gas and aqueous phase

299

As shown in Fig. 8, in the gas-phase LA primarily reacts with OH due to very slow ozonolysis

300

of the terminal double bond. 11, 12, 16 The data acquired also indicate that LA lifetime in the aqueous-

301

phase is similar to the estimated lifetime of this compound in the gas-phase. Moreover, since both k1

302

(Fig. S16) and k2 are pH-dependent, reaction (2) may dominate over reaction (1) for LA and related

303

compounds when pH ≥ 4.5 due to dissociation of the terpene-derived carboxylic acids under mildly

304

acidic and basic conditions.

305

carboxylate ion of LA,

306

marine aerosols but not in the acidic aerosol particles (Fig. 8).

307

ozonolysis of monoterepene-derived, unsaturated carboxylic acids are needed.

25

50, 51

Given a reasonable assumption that k2 is factor of 2-4 higher for

reaction (2) can compete with reaction (1) in e.g. clouds, fogs and some 25

Therefore, further studies of the

308

When LWC ≥ 0.05 g × m-3 LA will reside entirely in the aqueous-phase (Fig. S17), therefore

309

reactions (1) and (2) can be relevant under realistic atmospheric conditions. However, in clouds the

14


Page 15 of 20


310

mechanisms of these reactions and products distribution will be different than in the laboratory

311

reactor.

312

The RO2 radicals that are formed due to reaction (1) will most likely react with the other RO2

313

radicals that are more abundant in clouds rather than self react. Consequently, the distribution of

314

products will depend on the branching ratios of the “mixed” tetroxides. Moreover, inside the water-

315

containing particles, atmospherically abundant carboxylic acids (formic, acetic etc.) 52, 53 will rapidly 54

316

scavenge the C9 SCIs that are produced folloging reaction (2) to yield the relatively stable,55 non-

317

volatile α-acyloxyhydroperoxy aldehydes.

318

likely remain in the particle phase after water evaporation thereby contributing to SOA. 56 Previously,

319

it was demonstrated that even when a low amounts of a non-volatile products are formed (such as

320

methyl-1,2,3-butanetricarboxylic acid, MBTCA from the CPA + OH reaction) the aqSOA relative yield

321

can be as high as 50%.

322

atmosphere. Both reactions (1) and (2) also produce keto-LA, a well-known limonene SOA tracer that

323

was identified in the ambient particles. 58-60

324 325

29

5, 56, 57

These α-acyloxyhydroperoxy aldehydes will most

Thus, reactions (1) and (2) can most likely contribute to

aqSOA

in the

Acknowledgments This

work

was

founded

by

the

Polish

National

Science

Centre:

grant

number

326

2014/13/B/ST4/04500. Sara Jurdana was supported by the EU Foundation for the Development of

327

the Education System Erasmus+ program 2014-2020. We thank the Structural Research Laboratory

328

(SRL) at the Department of Chemistry of University of Warsaw for making LC/MS measurements

329

possible. SRL has been established with financial support from European Regional Development

330

Found in the Sectoral Operational Programme “Improvement of the Competitiveness of Enterprises,

331

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

332

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

333

co-financed by European Union from the European Regional Development Fund under the

334

Operational Programme Innovative Economy, 2007 – 2013. We thank dr Dagmara Tymecka for LA

335

purification with a semi-preparative HPLC. We thank dr Marcin Wilczek for the

336

measurements. We thank the anonymous reviewers for very helpful, constructive and insightful

337

comments.

338 339

1

H NMR

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

340 341

Literature

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