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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)
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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)
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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
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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
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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
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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.
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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
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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
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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
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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.
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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
147
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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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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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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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
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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
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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
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365 366 367 368 369
Figures
370 371
Figure 1
372
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373 Figure 2
374
375 376
Figure 3 14 ACS Paragon Plus Environment
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377 378
Figure 4
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379 380
Figure 5
381 382 16 ACS Paragon Plus Environment
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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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