Relative Reactivity Measurements of Stabilized CH2OO, Produced by

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Relative Reactivity Measurements of Stabilized CHOO, Produced by Ethene Ozonolysis, Toward Acetic Acid and Water Vapor Using Chemical Ionization Mass Spectrometry Ryoji Yajima, Yosuke Sakamoto, Satoshi Inomata, and Jun Hirokawa J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05065 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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The Journal of Physical Chemistry

Relative Reactivity Measurements of Stabilized CH2OO, Produced by Ethene Ozonolysis, Toward Acetic Acid and Water Vapor Using Chemical Ionization Mass Spectrometry Ryoji Yajima,a Yosuke Sakamoto,b,c,d Satoshi Inomatad and Jun Hirokawa*e

a

Graduate School of Environmental Science, Hokkaido University, Sapporo, Japan Graduate School of Global Environmental Studies, Kyoto University, Kyoto, Japan c Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, b

Japan d Center for Environmental Measurement and Analysis, National Institute for Environmental Studies, Tsukuba, Japan e Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan. E-mail: [email protected]

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Abstract We investigated the relative reactivity of stabilized CH2OO, produced by ethene ozonolysis, toward acetic acid and water vapor at the temperature of 298 ± 2 K and atmospheric pressure. Hydroperoxymethyl acetate produced through the reaction between stabilized CH2OO and acetic acid was monitored using a chemical ionization mass spectrometer as a function of the acetic acid concentration at different relative humidities. The rate of the reaction between CH2OO and water vapor depended quadratically on the water vapor concentration, suggesting that CH2OO reacted with water dimers in preference to water monomers. We obtained the bimolecular rate constant for the reaction between CH2OO and water dimer relative to the rate constant for the reaction between CH2OO and acetic acid, k3/k1, of (6.3 ± 0.4) × 10−2. The k3 value of (8.2 ± 0.8) × 10−12 cm3 molecule−1 s−1 was derived by combining with k1 of (1.3 ± 0.1) × 10−10 cm3 molecule−1 s−1, which has been previously reported by direct kinetic studies. The k3 value thus obtained is consistent with the absolute rate constants measured directly, suggesting that the reactivity of CH2OO is irrespective of the CH2OO generation method, namely, ethene ozonolysis or diiodomethane photolysis. We indirectly determined the yield of stabilized CH2OO from the ozonolysis of ethene of 0.59 ± 0.17 and 0.55 ± 0.16 under dry and humid (relative humidity 23–24%) conditions,

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respectively, suggesting that the yield is independent of the water vapor concentration. Our results suggest that hydroperoxymethyl acetate is the sole product of the reaction between stabilized CH2OO and acetic acid. The approach presented here can likely be extended to studies of the reactivities of more complicated and atmospherically relevant stabilized Criegee intermediates.

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1. Introduction Atmospheric olefins react with ozone to form a primary ozonide, which then decomposes to a carbonyl compound and a carbonyl oxide called a Criegee intermediate. The Criegee intermediate is generated in its excited state, then part of it decomposes further to produce a reactive species, such as an OH radical, while the other part becomes stabilized after colliding with another molecule. The stabilized Criegee intermediate (sCI) formed may affect chemical processes in the atmosphere by undergoing bimolecular reactions with other trace species.1 A generalized mechanism for the formation and reaction of sCIs is depicted in Scheme 1.

R1

R3 C

R2

R1

O3

C

C

R2 R4

C

R4 O

*

R1

R3 C

O

R2

O

R3

+

O O

O

C R4

Carbonyl oxide Carbonyl compound (Criegee intermediate)

Primary ozonide

M

R1 C

O

R2

O

Unimolecular decomposition/isomerization

Stabilized Criegee intermediate (sCI)

Bimolecular reactions

Unimolecular decomposition/isomerization

Scheme 1. Generalized mechanism for the formation and reaction of stabilized Criegee intermediates (sCIs).

As reviewed in previous publications,2–4 early studies measured sCI yields from the

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ozonolysis by using excess amounts of sCI scavengers. These studies also investigated the reactivities of sCIs indirectly by measuring the products of reactions between the sCIs and the scavengers due to difficulties involved in selectively generating and detecting Criegee intermediates. Neeb et al.5–7 studied reactions of formic and acetic acids with the simplest Criegee intermediate, CH2OO, generated through the ozonolysis of ethene. They reported that the rate constant for the reaction between CH2OO and HCOOH was 14,000 times higher than the rate constant for the reaction between CH2OO and H2O at 293 K. However, they concluded that the reaction between CH2OO and HCOOH may not be important in the troposphere because the HCOOH concentration in the troposphere is 6–7 orders of magnitude lower than the water vapor concentration.7 Criegee intermediates with relatively simple structures have recently been selectively generated through diiodoalkane photolysis, and could be directly detected, allowing absolute rate constants for bimolecular reactions involving these Criegee intermediates to be determined. Welz et al.8 investigated the reactions of CH2OO or C2-Criegee intermediates (syn- and anti-CH3CHOO) with formic and acetic acids at low pressures (4–5 Torr), and found that the bimolecular rate constants were as high as (1.1–5) × 10−10 cm3 molecule−1 s−1. These rate constants were more than one order of

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magnitude higher than rate constants that are used in tropospheric models.9 On the basis of these measured rate constants, they predicted that organic acids can act as important reaction partners for sCIs in the troposphere.8 They assumed that these Criegee intermediates have similar reactivities with larger organic acids, and suggested that reactions between Criegee intermediates and larger organic acids may contribute to the formation of secondary organic aerosols (SOAs) through the formation of low-volatility adduct compounds. The reactions between Criegee intermediates and atmospherically abundant water vapor have also been investigated by direct kinetic measurements. Early studies of the reaction between CH2OO and monomeric water were performed at relatively low water vapor concentrations, and rate constants lower than 4 × 10−15 cm3 molecule−1 s−1 were obtained.10–12 These rate constants were 5–7 orders of magnitude lower than the rate constants for the reactions between CH2OO and organic acids, so it was concluded that reactions with organic acids can compete with the reaction with H2O.8 However, it has been predicted in theoretical studies that CH2OO has high reactivity with the water dimer (H2O)2,13,14 and this was supported by the results of recent experimental studies.15–19 Berndt et al.15 investigated the relative reactivity of CH2OO, produced from the ethene ozonolysis, in the presence of SO2 and water vapor, and found that the rate

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for the reaction between CH2OO and water vapor is dependent on the square of the water vapor concentration. Direct kinetic studies in which CH2OO was generated through the diiodomethane photolysis have reported the rate constants of (4–7.4) × 10−12 cm3 molecule−1 s−1 for the reaction between CH2OO and (H2O)2.16–18 Berndt et al.19 concluded that the atmospheric fate of stabilized CH2OO is determined by reactions between CH2OO and water dimers. Direct kinetic measurements have been performed in studies of the reactions of other Criegee intermediates, including syn- and anti-CH3CHOO and (CH3)2COO. The results of these studies suggest that the chemical structure of a Criegee intermediate strongly affects its reactivity, especially toward water vapor.20–23 The reactivities of more complicated and atmospherically relevant Criegee intermediates therefore need to be investigated to allow Criegee chemistry in the atmosphere to be assessed. However, direct measurements have not been made because of the difficulties involved in generating complicated Criegee intermediates through the photolysis of diiodoalkanes. On the other hand, indirect reactivity measurements using sCI scavengers have been performed to study complicated sCIs, such as sCIs produced through the ozonolysis of isoprene24,25 and other small alkenes such as cis- and trans-but-2-ene and 2,3-dimethyl-but-2-ene.26,27 These measurements have allowed the relative reactivities

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of sCIs toward atmospherically important species, including water vapor, SO2, and carboxylic acids, to be determined. We have previously performed experiments to investigate the ozonolysis of ethene and isoprene and found that the sCIs were inserted sequentially into a carboxylic acid molecule to give oligomeric hydroperoxides in the gas phase.28–30 The oligomeric hydroperoxides formed can be regarded as potential sources of oxidizing agents in acid-catalyzed reactions in the aerosol phase,31 and high uptake of these species by acidic particles has recently been observed.32 The first step in the formation of an oligomer is the formation of an adduct of the Criegee intermediate and the acid. Information on the kinetics of such reactions is required to allow the importance of oligomeric hydroperoxides in the atmosphere to be assessed. We therefore planned to investigate the reactivities of sCIs with carboxylic acids by analyzing the reaction products, and chose the reaction between stabilized CH2OO and acetic acid as our first target. We chose acetic acid rather than formic acid because formic acid can be formed through the isomerization of CH2OO and/or a reaction between CH2OO and H2O,6,33–37 which would cause uncertainty in the measurements. The reaction between stabilized CH2OO and acetic acid produces hydroperoxymethyl acetate (HPMA) as an adduct product, as shown below:

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CH2OO + CH3COOH → yHPMA CH2(OOH)OC(O)CH3

(HPMA) (R1)

where yHPMA is the yield of HPMA. Neeb et al.5 reported that a part of HPMA produced through (R1) is further decomposed through loss of water to acetic formic anhydride, HC(O)OC(O)CH3. Since adduct formation from the reaction between sCIs and organic acids leads to the formation of oligomer and SOA,8,28–30,32 information on the reaction products of (R1) and their yields is important. However, there is little information regarding the products from (R1) because detection and quantification of hydroperoxides such as HPMA is difficult. Welz et al.8 could not identify any products using photo ionization mass spectrometry in their direct measurements of the rates for the reactions of CH2OO or C2-Criegee intermediates with formic and acetic acids under low pressure conditions. In the study presented here, we used a chemical ionization mass spectrometer (CIMS) to monitor the HPMA produced through (R1) as a function of the acetic acid concentration at different water vapor concentrations. We used these results to derive the rate constant for the reaction between stabilized CH2OO and water dimer relative to the rate constant for the reaction between stabilized CH2OO and acetic acid. From the results at different ethene concentrations, we indirectly determined the sCI yield from ethene ozonolysis. In addition, our results suggested that the HPMA

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production yield is close to unity. These findings demonstrated that our approach, which monitors reaction products between sCI and carboxylic acid, is applicable to studies on the reactivities of other Criegee intermediates that are more complicated and atmospherically relevant.

2. Experimental Section 2.1 Experimental setup for kinetic measurements The experimental setup used in the study is shown in Figure 1. The experiments were performed at atmospheric pressure using a glass flow tube with an inner diameter of 4 cm and a length of 80 cm. Ethene and ozone were introduced at the top of the flow tube along with a pure air carrier gas (a mixture of N2 and O2, and no contaminant at a concentration higher than 1 ppmv; Japan Fine Products, Kawasaki, Japan). This allowed the ozonolysis reaction to be initiated. Acetic acid and, in experiments under humid conditions, water vapor were also introduced into the flow tube. The gas flow rates were regulated using mass flow controllers (models SEC-E40MK3 and B40; HORIBA STEC, Kyoto, Japan). The total flow rate of the gases introduced into the flow tube was kept at 3.0 STP (standard temperature and pressure) L min−1. Thus the experiments were performed under laminar flow conditions at the Reynolds number of ~100. We

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estimated the reaction time to be approximately 30 s from observations of the decay of ozone in the flow tube, as described in the Supporting Information. Part of the gas flowing out of the end of the flow tube was sampled and analyzed using a CIMS and a UV ozone analyzer (model 1150; Dylec, Ibaraki, Japan). The acetic acid, water vapor, and HPMA concentrations were measured using the CIMS, as described below. The flow tube temperature was not regulated, but the room temperature was maintained at 298 ± 2 K.

MFC MFC MFC

Pure air

pure H2O

Dry air Hg lamp

Glass flow tube Ozone analyzer

Water bath at 18℃ H2O/air Bubbler

MFC MFC Ethene/N2 (0.1 vol.%)

Ozone/air

Splash trap Ethene/N2

Hood

Acetic acid/N2

Stainless steel bottle

CIMS

Hood

Acetic acid/N2 (160～180 ppmv)

Figure 1. Schematic of the experimental setup

Ethene was supplied from a premixed gas cylinder (0.1 vol % in N2; Japan Fine Products). The ethene volume mixing ratio in the flow tube was not measured but was estimated from the flow rates of gases introduced into the flow tube. The typical ethene 11



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mixing ratio was 33 ppmv, but some experiments were performed using an ethene mixing ratio of 17 ppmv or 67 ppmv. Ozone was generated by irradiating pure air with UV light at 185 nm from a low-pressure Hg lamp (model 610-100; JELIGHT, Irvine, CA, USA). The ozone volume mixing ratio before the ozonolysis reaction was 2.9–3.1 ppmv. The ozone concentration was monitored continuously throughout each experiment, but we found that the presence of acetic acid and/or water vapor interfered with the ozone measurements, precluding us from measuring the ozone concentration directly during experiments. We therefore measured the decay in the ozone concentration as a function of the ethene concentration in the absence of water vapor and acetic acid, and used this to estimate the ozone concentration consumed by ozonolysis, ∆[O3]. This concentration was used to determine the stabilized CH2OO yield, as described in section 3.2. Details of the estimation of ∆[O3] are described in the Supporting Information. Acetic acid was introduced as a gaseous mixture of acetic acid and N2, which was prepared in a stainless-steel bottle. The acetic acid volume mixing ratio in the bottle ranged from 159 ± 4 to 177 ± 7 ppmv, determined using the CIMS as described in section 2.2. The error represents 2 standard deviations (2σ). The acetic acid volume mixing ratio in the flow tube before the reaction was 0.1–0.6 ppmv. Water vapor was

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introduced by passing a part of the carrier gas through a water bubbler before introducing it into the flow tube. The water bubbler was immersed in an isothermal bath maintained at 291 K. The water vapor concentration was controlled by changing the ratio of the humidified air flow rate to the dry air flow rate. The water vapor concentration in the flow tube was between 1.8 × 1015 and 3.1 × 1017 molecules cm−3, which corresponded to 0.2%–38% relative humidity (RH) at 298 K. Hereafter, the water vapor concentration is expressed either as the number concentration or as the RH at 298 K. Ion signals attributed to acetic acid and HPMA showed slow response to the change in the acetic acid concentration, probably owing to adsorption/desorption on the reactor wall surface. However, the signals approached a steady state within several minutes. Thus we adopted the steady-state signal intensity for the kinetic analysis. Here, we assume that there were no chemical losses on the wall surface and that physical adsorption-desorption equilibrium was established at the steady state. Therefore, we made no corrections for the wall loss processes. The decomposition of a Criegee intermediate generates OH radicals, which can react with ethene. No OH radical scavengers were used in this study because the OH oxidation products of ethene were unlikely to affect the measurements of the kinetics of

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the reactions of CH2OO with water vapor and acetic acid.

2.2 Chemical ionization mass spectrometer The outlet of the flow tube was connected to a CIMS, which was used to measure the CH3COOH, H2O, and HPMA concentrations. The CIMS has been described in detail previously,28 so only a brief description is given here. In the CIMS analysis, SO2Cl− was used as a reagent ion. In an ion source region, Cl− was generated by the electron impact from a resistively heated filament onto the CH3Cl/Ar gas mixture. Then the reagent ion, SO2Cl−, was produced through the addition of SO2 to Cl−. Analyte molecule was ionized via ion-molecule reactions with SO2Cl− during the residence time of approximately 50 ms in a chemical ionization region at the pressure of 3.3 Torr. Organic acids, such as acetic acid, and organic hydroperoxides, such as HPMA, are ionized through a chloride ion transfer reaction A + SO2Cl− → A Cl− + SO2 where A is an analyte molecule. The product ion, A Cl−, is an adduct of the analyte and a chloride ion. Consequently, acetic acid with a molecular weight of 60 was detected as doublet peaks at mass-to-charge ratios (m/z) of 95 and 97, and HPMA with a molecular weight of 116 was detected at m/z 141 and 143. On the other hand, water vapor forms a

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cluster with the reagent ion H2O + SO2Cl− → SO2Cl− (H2O). Peaks attributed to H2O were therefore observed at m/z 117 and 119. A typical mass spectrum obtained for the reaction between ethene and ozone in the presence of acetic acid and water vapor is depicted in Figure S1 of the Supporting Information. We continuously monitored the ion signal intensities at m/z 95, 117, and 141 for acetic acid, water vapor, and HPMA, respectively, and m/z 101 for the reagent ion. We chose to monitor the minor peak at m/z 101 rather than the major peak at m/z 99 for the reagent ion to minimize the decrease in detector sensitivity that would have occurred if the strong peak at m/z 99 had been continuously monitored.38 The ion signal intensity, which is generally shown as a count rate, will vary over time, so we normalized the signal intensity to the reagent ion signal intensity at m/z 101 to determine the analyte concentrations. The CIMS measurements of the water vapor concentrations were calibrated by comparing the normalized signal intensity at m/z 117, I117, with the RH measured using a hygrometer (model HPM234; Vaisala, Vantaa, Finland). In Figure S2 of the Supporting Information, I117 is plotted against the water vapor concentration, [H2O], obtained from the measured RH and the saturation water vapor pressure, pH2O, the latter of which was estimated using the following equation

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given by Tetens:39 pH2O / hPa = 6.11 × 10[7.5(T − 273.15)/(T − 35.85)] where T is the absolute temperature. As is shown in Figure S2, I117 increased linearly as [H2O] increased. The sensitivity of the CIMS to the water vapor concentration, S117, was determined as (5.7 ± 0.4) × 10−19 cm3 molecule−1 from the slope of the I117 vs. [H2O] plot. It should be noted that I117 is non-dimensional, so the unit for S117 is the reciprocal concentration. Similarly, we calibrated the acetic acid concentration using the normalized signal intensity at m/z 95, I95. The primary standard gas used to calibrate the system was prepared by diluting gaseous acetic acid from a diffusion tube (D-10; GASTEC, Kanagawa, Japan) with pure nitrogen. The calibration experiments were performed twice, and gave a detection sensitivity, S95, of (1.06 ± 0.02) × 10−14 cm3 molecule−1. By using the S95 value thus determined, the acetic acid concentration in the mixture of acetic acid and nitrogen prepared in the stainless-steel bottle described above was determined. The S95 value may have varied from day to day, so we determined S95 for each experiment using the mixture of acetic acid and nitrogen in the bottle as a secondary standard. The sensitivity obtained in this way ranged from (1.14 ± 0.05) × 10−14 to (1.26 ± 0.03) × 10−14 cm3 molecule−1, irrespective of the presence of water

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vapor for all the experiments. The uncertainty in the S95 value was estimated by taking into consideration the systematic error of the acetic acid concentration in the bottle and statistical uncertainties (2 standard errors) of the I95 measurements. The CIMS could not be calibrated for HPMA measurements because a standard HPMA gas could not be prepared. In section 3.1, we analyze the data and assess the kinetics of the reaction of CH2OO in the presence of acetic acid and water vapor by assuming that I141 is proportional to the HPMA concentration. In section 3.2, we prove indirectly that I141 linearly depends on the HPMA concentration.

2.3 Kinetic analysis of CH2OO To determine the relative rate constants for the reaction involving CH2OO, we investigated the normalized signal intensity, I141, as a function of the acetic acid concentration on the assumption that I141 is proportional to the concentration of HPMA produced by (R1). From the derivation described in detail in the Supporting Information, we obtain the following relation between I141 and the initial concentration of acetic acid, [CH3COOH]0, as I141 =

k1 [CH 3 COOH] 0 max I 141 , k1 [CH 3 COOH] 0 + k L

(1)

where k1 is the bimolecular rate constant for (R1) and kL is the pseudo-first-order rate

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constant for all the reactions with rates that do not depend on the acetic acid concentration. I141max stands for the asymptotic value of I141 at a sufficiently high acetic acid concentration, when k1[CH3COOH]0 >> kL. Inverting both sides of equation (1), we obtain equation (2).

1 I141

kL

=

k1 I 141

max

1 1 . + [CH 3 COOH ] 0 I 141 max

(2)

The reciprocal of I141 is therefore linearly proportional to the reciprocal of the acetic acid concentration. We can derive equation (2) under the assumption that the acetic acid concentration is unaffected by the reaction with stabilized CH2OO. Actually, however, the change in the acetic acid concentration by the reaction with stabilized CH2OO could not be neglected. At the minimum acetic acid concentration of 2.6 × 1012 molecules cm−3, up to 70% of the acetic acid was found to be consumed by the reaction. We therefore calculated the average of the acetic acid concentration in the presence and absence of stabilized CH2OO, [CH3COOH]ave, and used the following equations in which [CH3COOH]0 is replaced by [CH3COOH]ave: I141 =

1 I141

=

k1[CH 3 COOH] ave max , I 141 k1[CH 3 COOH] ave + k L kL k1 I 141

max

1 1 + . [CH 3 COOH] ave I 141max

(3) (4)

We plotted the reciprocal of I141 measured by CIMS against the reciprocal of

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[CH3COOH]ave to obtain a linear relation, then determined I141max and kL/k1 from the y-intercept and slope/y-intercept, respectively, on the basis of equation (4). The validity of using [CH3COOH]ave instead of [CH3COOH]0 in our analysis was confirmed by performing simple model simulations, which is described in the Supporting Information. We used an assumed kL/k1 value, and obtained [HPMA] by performing a simulation using the same conditions as were used in the experiments. We then obtained kL/k1 from reciprocal plots and compared it with the assumed value. When we used an assumed kL/k1 value of 5.0 × 1012 molecules cm−3, for example, the model simulation gave the [HPMA] shown in Figure S3(a) of the Supporting Information. Each kL/k1 value was obtained by dividing the slope by the y-intercept for each reciprocal plot line shown in Figure S3(b) of the Supporting Information. The [HPMA]−1–[CH3COOH]0−1 plot (the black line in Figure S3(b)) gave a kL/k1 value of 5.6 × 1012 molecules cm−3, and the [HPMA]−1–[CH3COOH]ave−1 plot (the red line in Figure S3(b)) gave a kL/k1 value of 5.0 × 1012 molecules cm−3. Considering that typical uncertainties in the reported kL/k1 values are 10%, we concluded that the [HPMA]−1–[CH3COOH]ave−1

plot

provided

more

accurate

[HPMA]−1–[CH3COOH]0−1 plot.

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results

than

the


3. Results and Discussion 3.1 Determination of the relative rate constants Plots of I141 against [CH3COOH]ave at RHs of 0.7%, 13%, and 38%, with an initial C2H4 mixing ratio of 33 ppmv, are shown in Figure 2. The dependence of HPMA on the acetic acid concentration can be seen. The I141 values shown are the values after the background signals had been subtracted.

0.007 0.006 0.005

I141

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0.004 0.003 0.002

RH 0.7% RH 13% RH 38%

0.001 0 0

0.3

0.6

0.9

1.2

1.5

[CH3COOH]ave/1013 molecules cm−3

Figure 2. Plots of I141 against [CH3COOH]ave at relative humidities (RHs) of 0.7% (circles), 13% (squares), and 38% (diamonds). The initial C2H4 volume mixing ratio was 33 ppmv.

I141 increased as [CH3COOH]ave increased, and eventually approached its maximum

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value, I141max. The I141 values at a RH of 0.7% were high even at the minimum acetic acid concentration, 2.1 × 1012 molecules cm−3, and became asymptotic to I141max as the acetic acid concentration increased. The I141 values at RHs of 13% and 38% were much lower than the I141 values at a RH of 0.7% at the lowest acetic acid concentration, and increased gradually as [CH3COOH]ave increased. This was consistent with the behavior expected from equation (3) presented in section 2.3. Then we plotted the reciprocal of I141 against the reciprocal of [CH3COOH]ave to analyze the data based on equation (4). The I141−1–[CH3COOH]ave−1 plots shown in Figure 3 correspond to the I141–[CH3COOH]ave plots shown in Figure 2. As expected from equation (4), a linear relationship between the reciprocal of I141 and the reciprocal of [CH3COOH]ave can be seen in Figure 3. According to equation (4), the y-intercept corresponds to the reciprocal of I141max and the slope corresponds to kL/(k1 I141max). The slope increased as the RH increased, which suggests that kL increased as the water vapor concentration increased.

21



1000 RH 0.7% RH 13% RH 38%

800

I141−1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

600 400 200 0 0

1.0

2.0

3.0

[CH3COOH]ave

4.0

−1/10−13

5.0

6.0

7.0

cm3 molecule−1

Figure 3. Plots of I141−1 against [CH3COOH]ave−1 at relative humidities (RHs) of 0.7% (circles), 13% (squares), and 38% (diamonds). The initial C2H4 volume mixing ratio was 33 ppmv.

We drew plots similar to those shown in Figure 3 for the results obtained in each experimental run, and obtained the I141max and kL/k1 values from the y-intercept and the slope/y-intercept, respectively, of each linear plot. The kL/k1 values thus obtained are plotted against [H2O] in Figure 4(a). Each error bar in the figure indicates two standard deviations associated with the linear fitting of the plot.

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2.5

(a) 2.0 1.5 1.0 0.5 0 0

0.5

1.0

1.5

2.0

2.5

3.0

[H2O]/1017 molecules cm−3 kL/k1 /1013 molecules cm−3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kL/k1 /1013 molecules cm−3

Page 23 of 44

2.5

(b) 2.0 1.5 1.0 0.5 0 0

0.5

1.0

1.5

2.0

[(H2O)2]/1014 molecules cm−3

Figure 4. Dependence of kL/k1 on (a) [H2O] and (b) [(H2O)2]. The solid lines are the regression lines.

It can be seen from Figure 4(a) that kL/k1 had a nonlinear response to the water vapor concentration. We can classify the processes contributing to kL into the following three reactions in the presence of water vapor: CH2OO + H2O → products

(R2)

CH2OO + (H2O)2 → products

(R3)

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CH2OO → products

Page 24 of 44

(R4)

It should be noted that (R4) includes all the reactions involving CH2OO except for those with acetic acid and water vapor. Therefore, it may include bimolecular reactions between CH2OO and other species, such as ethene, ozone, formaldehyde, and CH2OO itself, as well as unimolecular decomposition/isomerization of CH2OO. Using second-order rate constants for (R2) and (R3), the pseudo-first-order rate constant for (R4), and the concentrations of water monomer and dimer, kL can be written as kL = k2[H2O] + k3[(H2O)2] + k4.

(5)

Therefore, the plot shown in Figure 4(a) was fitted to a quadratic polynomial. Least-squares fitting weighted with the uncertainties gave the equation kL/k1 = (1.4 ± 0.3) × 10−22 × [H2O]2 + (−1.3 ± 5.4) × 10−6 × [H2O] + (5.3 ± 0.7) × 1011, where errors represent 2 standard deviations. This strongly suggests that kL/k1 depended quadratically rather than linearly on the water vapor concentration. It should be noted that the data point for a kL/k1 of 2.1 × 1013 molecules cm−3 at [H2O] = 2.4 × 1017 molecules cm−3 deviated strongly from the trend line but had a large standard deviation, so its contribution to the weighted least-squares fitting process was negligible. Recent indirect15 and direct16–18 studies of the reactivity of CH2OO have shown that CH2OO is highly reactive toward the water dimer, (H2O)2. Our results are consistent with the

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results of those studies. We therefore neglected the contribution of the reaction between CH2OO and water monomers, and plotted kL/k1 as a function of the water dimer concentration [(H2O)2]. In the same way as performed by Berndt et al.,15 we estimated [(H2O)2] from the equilibrium constant Kp (T) for the dimerization of water molecules, which was expressed by Scribano et al.40 as a function of temperature, as shown in the following equation:  1851.09  Kp (T) / atm−1 = 4.7856 × 10 − 4 × exp − 5.10485 × 10 −3 × T  .  T 

The plot of kL/k1 against [(H2O)2] shown in Figure 4(b) demonstrates that kL/k1 almost linearly depended on [(H2O)2]. A regression line was fitted to the data using the linear least-squares fitting method. The slope of the regression line gave k3/k1 = (6.3 ± 0.4) × 10−2, showing that k1 is 16 times higher than k3. We estimated the k3 value of (8.2 ± 0.8) × 10−12 cm3 molecule−1 s−1 using the k1 value of (1.3 ± 0.1) × 10−10 cm3 molecule−1 s−1 determined by direct kinetic measurements.8 We compared our estimated k3 value with those previously reported by the indirect and direct kinetic measurements as shown in Table 1.

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Table 1 The k3/k1 and k3 values obtained in this study and relative and absolute rate constants from the literature. Error limits represent 2 standard deviations.

k3/k1 or k3/kSO2

/10

k3 −12

cm3

Conditionsa

Reference

−1 −1

molecule s

Indirect kinetic studies (CH2OO from C2H4 ozonolysis) k3/k1 = (6.3 ± 0.4)×10−2 k3/kSO2 = 0.29 ± 0.01 k3/kSO2 = (1.4 ± 1.8)×10−2

8.2 ± 0.8b

298 ± 2 K

This study

11 ± 2c

293 ± 0.5 K

Berndt et al.15

0.56 ± 0.70c

298–303 K

Newland et al.27

Direct kinetic studies (CH2OO from CH2I2 photolysis) 298 K 6.5 ± 0.8

Chao et al.16

100–500 Torr 294 K 4.0 ± 1.2

Lewis et al.17

50–400 Torr 298 K 7.4 ± 0.6

Smith et al.18

500–600 Torr a

Experiments were conducted at atmospheric pressure unless otherwise noted.

b

Estimated from the k3/k1 value found in this study and the k1 value of (1.3 ± 0.1) ×

10−10 cm3 molecule−1 s−1 reported in the direct kinetic study.8 c

Estimated from the k3/kSO2 values and the kSO2 value of (3.9 ± 0.7) × 10−11 cm3

molecule−1 s−1 reported in the direct kinetic study.10

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Berndt et al.15 and Newland et al.27 determined the k3 value relative to the rate constant for the reaction between CH2OO and SO2 (kSO2), and found that k3/kSO2 = 0.29 ± 0.01 and (1.4 ± 1.8) × 10−2, respectively. We converted these k3/kSO2 values to k3 by assuming kSO2 = (3.9 ± 0.7) × 10−11 cm3 molecule−1 s−1 reported by Welz et al.10 The k3 value estimated in this study was within the range of the previously reported k3 values. This suggests that our k3/k1 value was valid and implied that k1, which was determined at low pressures,8 would be applicable at atmospheric pressure. On the other hand, we obtained k4/k1 = (5.2 ± 0.7) × 1011 molecules cm−3 from the y-intercept of the regression line fitted to the data shown in Figure 4(b). From the k1 value reported by Welz et al.,8 we estimated that the k4 value was 70 ± 10 s−1. This was much higher than the k4 values of (9 ± 2) s−1 and (−8.8 ± 13) s−1 estimated by indirect kinetic studies reported by Berndt et al.15 and Newland et al.,27 respectively, and the upper limit of 11.6 ± 8.0 s−1 for the rate constant for unimolecular loss of CH2OO reported by direct kinetic studies.41 Quantum chemical calculations estimated the rate constant of 0.33 s−1 for the unimolecular decomposition/isomerization of CH2OO.42 Berndt et al.19 very recently reported a CH2OO unimolecular decay rate constant of 0.19 ± 0.07 s−1. Taking these values into consideration, it is likely that the k4 value we obtained may have been affected by other CH2OO loss processes, such as bimolecular

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reactions with other species and heterogeneous decomposition on the reactor surfaces. In our experimental system, CH2OO could have reacted with the reactants, ethene and ozone, and reaction products, formaldehyde and formic acid. CH2OO may also have undergone self-reactions. In the Supporting Information, we estimated possible contributions of bimolecular reactions of CH2OO with individual reaction partners to k4 on the basis of the reported rate constants. According to the estimation, it is suggested that the bimolecular reactions of CH2OO with ozone, formaldehyde, and formic acid may contribute to some extent to k4. It should be noted that the kL value obtained under dry conditions, which was therefore equivalent to k4, did not systematically depend on the initial ethene mixing ratio between 17 and 67 ppmv. We therefore concluded that the k4 value obtained in this study could be affected by unimolecular heterogeneous loss of CH2OO on the inner walls of the flow tube as well as bimolecular reactions of CH2OO with other species.

3.2 Estimation of the sCI yield The I141−1–[CH3COOH]ave−1 plots shown in Figure 3 gave not only the kL/k1 value, but also the I141max value. The sensitivity of the CIMS to the HPMA concentration, S141, can be used to relate I141max to [HPMA]max using the equation

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I141max = S141 [HPMA]max.

(6)

As shown in the Supporting Information, [HPMA]max can be related to the concentration of the reacted ozone, ∆[O3], as [HPMA]max = yHPMA ysCI ∆[O3].

(7)

Combining equations (6) and (7) gives the equation I141max = S141 yHPMA ysCI ∆[O3].

(8)

We therefore plotted the experimentally determined I141max values against the estimated ∆[O3] values from the experiments performed using three different ethene concentrations under dry (RH 0.4%–0.7%) and humid (RH 23%–24%) conditions. The ∆[O3] values were estimated using the initial ozone concentration, [O3]0, measured using the ozone monitoring instrument described in the Supporting Information. The plots of I141max against ∆[O3] are shown in Figure 5. I141max linearly correlated with ∆[O3] in both plots, as expected from equation (8). Estimating the S141 yHPMA value allowed ysCI to be obtained from the slopes of the plots shown in Figure 5.

29



0.014 0.012 0.010

I141max

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0.008 0.006 0.004 0.002 0 0

1.0

2.0

∆[O3

]/1012

3.0

4.0

molecules

5.0

6.0

cm−3

Figure 5. Plots of I141max against ∆[O3] determined under dry (relative humidity 0.4%–0.7%, in black) and humid (relative humidity 23%–24%, in red) conditions.

As mentioned in section 2.2, a HPMA standard could not be prepared, so S141 could not be directly obtained. Instead, we attempted to estimate the S141 yHPMA value using the acetic acid concentration consumed in (R1), ∆[CH3COOH], which was obtained using the S95 value and the difference between the m/z 95 signal intensities when ozone was absent and present. [HPMA] can be related to ∆[CH3COOH] using the equation [HPMA] = yHPMA ∆[CH3COOH],

(9)

and thus the relationship between I141 and ∆[CH3COOH] can be obtained as I141 = S141 yHPMA ∆[CH3COOH].

(10)

We therefore examined the correlation between I141 and ∆[CH3COOH]. A plot of I141 against ∆[CH3COOH] at an initial ethene mixing ratio of 67 ppmv and RH 24% is

30


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shown in Figure 6. A linear relationship was found, and the slope gave the S141 yHPMA value. Although linear relationships were found for other runs under different experimental conditions, the plots at lower ethene mixing ratios were too scattered to give precise S141 yHPMA values. Moreover, the linear relationship could only be obtained for a narrow range of ∆[CH3COOH] under dry conditions, causing a large uncertainty in the S141 yHPMA value determined from the slope. We therefore used S141 yHPMA = (4.0 ± 1.2) × 10−15 cm3 molecule−1 obtained from the slope of the plot shown in Figure 6 to determine ysCI at RH 23%–24% and at RH 0.4%–0.7%. Here, we estimated the uncertainty as ±30% by taking into account day-to-day variation of S141 and uncertainty associated with the acetic acid concentration.

0.01 0.008 0.006

I141

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.004 0.002 0 0

0.5

1.0

1.5

∆[CH3COOH]/1012

2.0

molecules

2.5

cm−3

Figure 6. Plot of I141 against ∆[CH3COOH] at a relative humidity of 24%.

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According to equation (8), we divided the slopes obtained from the plots shown in Figure 5 by the S141 yHPMA value to give ysCI values of 0.59 ± 0.17 and 0.55 ± 0.16 for dry (RH 0.4%–0.7%) and humid (RH 23%–24%) conditions, respectively. The ysCI values at different water vapor concentrations agreed with each other, taking the uncertainties into account, suggesting that ysCI was insensitive to the water vapor concentration. Alam et al.43 reported a stabilized CH2OO yield of 0.54 ± 0.12 from studies using CO as a sCI scavenger. As they summarized, ysCI values of 0.35–0.52 had been reported based on studies using different sCI scavengers before they published their findings. More recently, Berndt et al.15 and Newland et al.27 reported ysCI values of 0.40 ± 0.18 and 0.37 ± 0.04, respectively, by investigating the reaction between stabilized CH2OO and SO2. Our values are higher than those previously reported but are consistent with several of them, taking the uncertainties into account. In most of the previous studies, ysCI was determined in the presence of excess amounts of sCI scavengers on the assumption that almost all the stabilized CH2OO is scavenged. On the other hand, we determined ysCI from the asymptotic value, I141max, at infinite acetic acid concentration. This may be why our obtained ysCI values are slightly higher than the reported values. It should be noted that Berndt et al.15 found that the sCI yield was not dependent on the

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water vapor concentration, which is consistent with our conclusions. We obtained RH-independent sCI yields that agreed with the yields reported previously by assuming that the S141 yHPMA value obtained at RH 24% was applicable under dry conditions. This implied that neither S141 nor yHPMA depended on the water vapor concentration. We have found that the sensitivities of SO2Cl−–CIMS to HONO38 and HCOOH30 are independent of the water vapor concentration. The sensitivity of the CIMS to the acetic acid concentration, S95, was also not affected by the presence of water vapor, as mentioned in section 2.2. It is therefore likely that the sensitivity of the CIMS to HPMA, S141, was also independent of the water vapor concentration. The RH-independence of the yHPMA value has implications for the products of reaction (R1). Neeb et al.5 found that HPMA produced through (R1) was partially decomposed to acetic formic anhydride. On the other hand, Welz et al.,8 who measured absolute rate constants for the reaction of CH2OO with formic and acetic acids, could not identify the reaction products. Since adduct formation between a sCI and an organic acid may contribute to aerosol formation through the formation of oligomeric hydroperoxide, it is important to identify the products of reaction (R1) and determine their yields. No information on the time constant for the decomposition of HPMA is available, but Thamm et al.44 investigated the unimolecular decomposition of

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hydroperoxymethyl formate produced through the reaction between stabilized CH2OO and formic acid, and found half-lives of 35 min under dry conditions and 8 min in the presence of 1 mbar of water vapor. Assuming that the unimolecular decay rate of HPMA might also depend on RH in a manner similar to the decay of hydroperoxymethyl formate, the HPMA yield under humid conditions should be smaller than that under dry conditions. The lack of dependence of the yHPMA value on the water vapor concentration suggests that most of the HPMA produced was not decomposed in our study, even at RH 24% (corresponding to a water vapor pressure of 7 mbar), leading us to conclude that yHPMA was close to unity. This was consistent with no peaks assignable to acetic formic anhydride being observed when we performed CIMS measurements using SO2Cl− as the reagent ion.

4. Atmospheric Implications Welz et al.8 assessed the importance of the reactions between CH2OO and organic acids under atmospheric conditions using their k1 values in comparison with the reactivity of CH2OO toward water vapor. However, they did not take into account the high reactivity of CH2OO toward water dimers. More recently, Berndt et al.19 evaluated the atmospheric fate of CH2OO by using bimolecular rate constants of CH2OO with SO2,

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water monomer and dimer, and organic acids. They concluded that the fate of CH2OO is mostly governed by the reaction with (H2O)2, and the reaction with organic acids accounts for ~1.8% of the reacted CH2OO at RH 25% in typical remote areas where the atmospheric organic acid concentration is approximately 1 × 1011 molecules cm−3. Considering that the k3/k1 value obtained in the present study is consistent with the literature values used for the evaluation by Berndt et al.,19 a similar conclusion would be reached from our value. However, in urban regions, where the acetic acid volume mixing ratio occasionally exceeds 10 ppbv (2.5 × 1011 molecules cm−3 at 298 K),45 the reaction between CH2OO and acetic acid may become more important. Using the k3/k1 value of 6.3 × 10−2 obtained in the present study, we estimated that the reaction with acetic acid would account for ~22% and ~ 4% at RH 10% and RH 25%, respectively, at 10 ppbv of acetic acid. The production of HPMA from the reaction between stabilized CH2OO and acetic acid may have a role in atmospheric chemistry in an urban environment. Although HPMA itself will not be partitioned into the aerosol phase because of its high volatility, it can further react with sCIs to form oligomeric hydroperoxides with low volatility. More importantly, as Welz et al.8 pointed out, the reaction of CH2OO with larger carboxylic acids can directly produce adduct compounds with low volatility.

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Determination of the reactivity toward larger acids and identification of the products will be necessary. In addition, the reactivity of a Criegee intermediate strongly depends on its chemical structure. It has been reported that alkyl-substituted Criegee intermediates show different reactivities toward water vapor.20–23 Very recently, Liu et al.46 reported that the alkyl-substituted Criegee intermediates form isomeric vinyl hydroperoxides through carboxylic acid-catalyzed tautomerization. It is important to investigate the reactivities of more complicated and atmospherically relevant Criegee intermediates. The ozonolysis of isoprene, a biogenic volatile organic compound with the highest emission rate on a global scale, generates CH2OO and C4-Criegee intermediates.

The results of recent studies have suggested that different

isomers/conformers of the C4-Criegee intermediates have different reactivities toward H2O.24,30,47 The relative rate measurements used in our study will provide insights into the reactivities of such atmospherically important Criegee intermediates.

5. Conclusions We investigated the ozonolysis of ethene in the presence of acetic acid and water vapor using a flow tube reactor. The reaction between CH2OO and the water dimer was found to be much more important than the reaction between CH2OO and the water

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monomer, and the bimolecular rate constant for the reaction between CH2OO and the water dimer, k3, relative to the bimolecular rate constant for the reaction between CH2OO and acetic acid, k1, was determined experimentally. The k3/k1 value we obtained was consistent with the values found in previous studies. The rate constant for unimolecular processes of CH2OO, k4, relative to k1 was also determined, but the high value we found for k4/k1 suggests that bimolecular reactions of CH2OO with other species and the heterogeneous loss of CH2OO on the inner walls of the reactor affected the magnitude of k4. From the k3/k1 values we obtained, we estimated the relative contribution of the reaction between CH2OO and acetic acid to the total loss of CH2OO under atmospheric conditions. We determined the yield of stabilized CH2OO produced during the ozonolysis of ethene, ysCI, from the decrease in the acetic acid concentration by the reaction with CH2OO. These ysCI values were slightly higher than but agreed with the previously reported values, within the relevant uncertainties. In addition, our results suggest that the yield of HPMA from (R1) is close to unity. In summary, we used a flow tube reactor coupled with a CIMS to monitor the products of the reactions of stabilized sCIs with organic acids to investigate the reactivity of CH2OO. This experimental approach will be applicable to studies of the reactivities of other sCIs. The results of recent studies have suggested that the reactivity

37



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of a Criegee intermediate will strongly depend on its chemical structure, so our approach is expected to provide useful information on the reactivities of more complicated and atmospherically relevant sCIs.

Associated Content Supporting Information Estimation of the ozone concentration consumed by ozonolysis reaction Derivation of the equations used in the kinetic analysis Description of the model simulation Possible bimolecular reactions contributing to k4 Figure S1. Typical CI mass spectrum Figure S2. Plot of I117 against [H2O] Figure S3. (a) Plots of [HPMA], obtained using a model, against [CH3COOH]0 and [CH3COOH]ave. (b) Plots of [HPMA]−1 against [CH3COOH]0−1 and [CH3COOH]ave−1. Figure S4. Plot of ln([O3]/[O3]0) against [C2H4]0

Acknowledgements This work was supported by the joint research program of the Institute for

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Space-Earth Environmental Research (ISEE), Nagoya University.

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3

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compounds in the troposphere. Chem. Soc. Rev. 2008, 37, 699−716. Donahue, N. M.; Drozd, G. T.; Epstein, S. A.; Presto, A. A.; Kroll, J. H. Adventures in ozoneland: down the rabbit-hole. Phys. Chem. Chem. Phys. 2011, 13, 10848−10857.

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Neeb, P.; Horie, O.; Moortgat, G. K. Gas-phase ozonolysis of ethene in the presence

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of hydroxylic compounds. Int. J. Chem. Kinet. 1996, 28, 721−730. Neeb, P.; Sauer, F.; Horie, O.; Moortgat, G. K. Formation of hydroxymethyl hydroperoxide and formic acid in alkene ozonolysis in the presence of water vapour.

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Phys. Chem. A 1998, 102, 6778−6785. Welz, O.; Eskola, A. J.; Sheps, L.; Rotavera, B.; Savee, J. D.; Scheer, A. M.; Osborn, D. L.; Lowe, D.; Booth, A. M.; Xiao, P.; et al. Rate coefficients of C1 and C2 Criegee intermediate reactions with formic and acetic acid near the collision limit: direct kinetics measurements and atmospheric implications. Angew. Chem., Int. Ed.

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2014, 53, 4547−4550. Vereecken, L.; Harder, H.; Novelli, A. The reaction of Criegee intermediates with NO, RO2, and SO2, and their fate in the atmosphere. Phys. Chem. Chem. Phys. 2012, 14, 14682−14695.

10 Welz, O.; Savee, J. D.; Osborn, D. L.; Vasu, S. S.; Percival, C. J.; Shallcross, D. E.; Taatjes, C. A. Direct kinetic measurements of Criegee intermediate (CH2OO) formed by reaction of CH2I with O2. Science 2012, 335, 204−207. 11 Ouyang, B.; McLeod, M. W.; Jones, R. L.; Bloss, W. J. NO3 radical production from the reaction between the Criegee intermediate CH2OO and NO2. Phys. Chem. Chem. Phys. 2013, 15, 17070−17075. 12 Stone, D.; Blitz, M.; Daubney, L.; Howes, N. U. M.; Seakins, P. Kinetics of CH2OO

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23 Lin, L.-C.; Chang, C.-H.; Chao, W.; Smith, M. C.; Chang, C.-H.; Lin, J. J.; Takahashi,

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and

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reactions

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TOC Graphic O OH CO CH 3 k 1

C 2H 4

O3

(H2O)2

CH2OO

k3

ysCI = 0.55−0.59

[HPMA] =

k4

H 2C

OCCH3

OOH (HPMA) yHPMA ~ 1

products

products

k1[CH3COOH] k1[CH3COOH] + k3[(H2O)2] + k4

[HPMA]max

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Relative Reactivity Measurements of Stabilized CH2OO, Produced by

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