Competing atmospheric reactions of CH2OO with SO2 and water vapour.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 19130 Received 2nd June 2014, Accepted 29th July 2014

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Competing atmospheric reactions of CH2OO with SO2 and water vapour† Torsten Berndt,*a Jens Voigtla¨nder,a Frank Stratmann,a Heikki Junninen,b Roy L. Mauldin III,bc Mikko Sipila¨,b Markku Kulmalab and Hartmut Herrmanna

DOI: 10.1039/c4cp02345e www.rsc.org/pccp

H2SO4 formation from the reaction CH2OO + SO2 has been measured as a function of the water vapour concentration for close to atmospheric conditions. Second-order kinetics with regard to water indicates a preferred reaction of CH2OO with the water dimer. The obtained kinetic parameters lead to the conclusion that the atmospheric fate of CH2OO is dominated by the reaction with water vapour. A comparison with results from CH3CHOO and (CH3)2COO indicates a structure dependent reactivity of stabilized Criegee intermediates.

Criegee Intermediates (CI) are produced in the atmosphere from the gas-phase ozonolysis of unsaturated compounds. Their ability to oxidize SO2 producing H2SO4, and subsequently H2SO4 aerosol, was first discovered by Cox and Penkett in the seventies.1 Recently, this process attracted new attention due to the observation that probably the reaction of stabilized Criegee Intermediates (sCI) with SO2 can form a substantial fraction of atmospheric H2SO4 beside the main pathway via OH + SO2.2 Furthermore, in laboratory investigations the direct detection of the simplest sCI, CH2OO and CH3CHOO, succeeded at a pressure of 4 Torr by means of tuneable synchrotron photoionization mass spectrometry opening the way for direct kinetic measurements.3,4 This approach uses 248 or 351 nm photolysis of the corresponding diiodoalkanes in the presence of O2 for generating sCI. Resulting absolute rate coefficients of the reaction of sCI with SO2 or other trace gases3–5 are several orders of magnitude higher than those derived so far from alkene ozonolysis at atmospheric conditions applying an indirect way of determination.6–8 Most recent measurements of the kinetics of reaction (1) using the diiodoalkane photolysis technique reveal that k1 is independent of pressure in the range 1.5–450 Torr9,10 confirming the low pressure result by Welz et al.3 k1 = (3.9 0.7) 1011 cm3 molecule1 s1, as well.

a

Institute for Tropospheric Research, TROPOS, Leipzig, Germany. E-mail: [email protected] b Department of Physics, University of Helsinki, Helsinki, Finland c University of Colorado at Boulder, Boulder, CO, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cp02345e

19130 | Phys. Chem. Chem. Phys., 2014, 16, 19130--19136

CH2OO + SO2 - CH2O + SO3

(1)

Stone et al.9 used measurements of the CH2OO decay as well as the CH2O formation for deducing k1. The results from both approaches are in good agreement within experimental uncertainty, k1 = (3.42 0.42) 1011 cm3 molecule1 s1. Clearly higher rate coefficients for CH2OO + SO2 (and for other sCI)3,4,9,10 than thought before do not necessarily imply a substantial higher H2SO4 production via this pathway in the atmosphere. For an assessment of that, also the absolute rate coefficients or rate coefficient ratios regarding k1 of the most important CH2OO reactions in the atmosphere have to be known, i.e. reaction with water vapour (monomer and/or dimer), reactions (2a) and (2b), and the unimolecular sCI reactions (decomposition forming dioxirane, OH and others), reaction (3). CH2OO + H2O - products

(2a)

CH2OO + (H2O)2 - products

(2b)

CH2OO - dioxirane, OH, . . .

(3)

For the reaction with water vapour, upper limit estimates of k2a are available from absolute rate coefficient measurements from the diiodomethane photolysis technique together with the rate coefficient of reaction (1) resulting in k2a/k1 o 104 (Welz et al.3) and o2.6 106 (Stone et al.9). Higher rate coefficient ratios k2a/k1 are reported from ethene ozonolysis experiments for different water concentrations using end product analysis, (2.3 1.0) 104 by Suto et al.11 and (8.3 3.6) 104 by Becker et al.12 It should be noted, that there is no experimental indication for a reaction of CH2OO with the water dimer, reaction (2b), up to now, while findings from theoretical kinetics calculations favour reaction (2b) over (2a) for atmospheric conditions.13,14 Welz et al.15 deduced an upper limit k2b o 3 1013 cm3 molecule1 s1 from the experimental data by Stone et al.9 being clearly smaller than results from theoretical considerations, k2b = 7 1011 cm3 molecule1 s1, as recently used in modelling.16 For the unimolecular reaction (3), k3 of about 100 s1 or a few 100 s1 can be deduced from studies

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with absolute rate coefficient measurements3,4,9,10 representing probably an upper limit due to other CH2OO consuming steps in the system. In contrast to that, k3 B 0.3 s1 is stated as a result of theoretical calculations.17 In this work we focus on the reaction of CH2OO with water vapour relative to the reaction with SO2 in order to determine k2a/k1 and/or k2b/k1 for close to atmospheric reaction conditions. Furthermore, at least a rough estimate is expected for the ratio k3/k1 quantifying the relative importance of the unimolecular reactions of CH2OO. The experiments have been performed in the atmospheric pressure flow tube IfT-LFT at 293 0.5 K, using H2SO4 measurements to follow the reaction of CH2OO with SO2 as a function of the water vapour concentration in the carrier gas. The experimental setup was identical to that described in detail elsewhere.18 Therefore, here only a brief description is given. At the top of the flow tube, O3 (from an O3 generator UVP OG-2) diluted in air was added through an inlet to a C2H4–C3H8–SO2– humidified air mixture. C2H4 (99.9%, Linde) premixed in a flask from a gas metering unit, C3H8 (99.95%, Linde), SO2 (calibration gas mixture of 1961 ppmv SO2 in N2, Air Liquide) and air (PSA with further purification by activated 4 Å molecular sieve, charcoal and subsequently by GateKeeper CE-500KF-O-4R, AERONEX) were supplied by calibrated mass flow controllers (MKS 1259/1179). The purity of C2H4 was checked by GC-MS analysis with special attention to the possible presence of other alkenes influencing potentially the H2SO4 production, see ESI.† Reactant concentrations were (unit: molecule cm3): C2H4: 1.5 1013, C3H8 (1.64–8.2) 1015, SO2 (5.6–1600) 1011 and O3: (2.2–2.4) 1011. As SO2 source a calibration gas (Air Liquide) was used with an uncertainty of the SO2 mixing ratio of 2%. The total flow was 30 litre min1 (STP) and the residence time 39.5 s. O3 concentrations were measured by a gas monitor (Thermo Environmental Instruments 49C) and the relative humidity (RH) by a humidity sensor (Vaisala). The stated accuracy of the RH measurement for this instrument is 1% RH. For the lowest RH of ca. 2%, also calculations of the water equilibrium concentration in the water saturator have been performed resulting in RH = 2.2% in good agreement with the sensor measurement of 1.8–2.0%. H2SO4 was detected using a NO3– CI–APi–TOF (chemical ionisation–atmospheric pressure interface–time-of-flight) mass spectrometer (Airmodus, Tofwerk) sampling the centre flow at the tube outlet through a sampling inlet (length: 28 cm, i.d.: 1.6 cm) with a rate of 10 litre min1 (STP). Given H2SO4 concentrations involve the calculated wall loss of 28% in the flow tube and the slight RH-dependence of the calibration factor.18 The uncertainty in the H2SO4 concentration is estimated to be 45%19 affecting the determination of the stabilized CH2OO yield not the relative rate coefficients. Fig. 1 shows an example of a typical measurement. H2SO4 formation was detectable only under conditions of CH2OO (sCI) production from C2H4 ozonolysis in the presence of SO2. Otherwise, H2SO4 concentrations were below 105 molecule cm3. The H2SO4 production from C2H4 ozonolysis in the flow tube was measured as a function of SO2 and water vapour concentration, see Fig. 2. H2SO4 formation can be initiated by the


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Fig. 1 Measured H2SO4 concentration for different composition of the reaction gas from a typical run, [O3] = 2.2 1011, [C3H8] = 1.64 1015, [C2H4] = 1.5 1013 and [SO2] = 1.66 1013 molecule cm3 if used, RH = 25%. Each measurement cycle represents signal average of 6 seconds.

Fig. 2 H2SO4 formation as a function of SO2 and water vapour concentration (relative humidity, RH); [O3] = 2.2 1011 and [C2H4] = 1.5 1013 molecule cm3, t = 39.5 s. Open symbols represent data in presence of C3H8, full symbols in absence of C3H8 (OH scavenger). Lines are results from regression analysis according to eqn (II). The error bars represent the 45% uncertainty of the H2SO4 measurement. Error bars of the measurements at RH = 10% and 25% are omitted for more clarity.

reaction of OH radicals with SO2 or by stabilized Criegee Intermediates (CH2OO) via reaction (1). Both oxidants are formed by C2H4 ozonolysis, reaction (4). y1 stands for the formation yield of prompt OH radicals.20 O3 + C2H4 - y1 OH + y2 sCI(CH2OO) + . . .

(4)

The presence of C3H8 in excess ensured that more than 98% of formed OH radicals reacted with C3H8 allowing to separate H2SO4 formation arising from CH2OO + SO2. A H2SO4 molar formation yield of unity from CH2OO + SO2 in the presence of water vapour is assumed according to reaction (1). In this process, the conversion of SO3 to H2SO4 via the reaction with water vapour is complete within 50 milliseconds21 even for the lowest water

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vapour concentration of 1.2 1016 molecule cm3 (RH: ca. 2%) as used here. For conditions of CH2OO titration by SO2 (high SO2 concentration, see Fig. 2), the measurements in absence of C3H8 showed only slightly higher H2SO4 concentrations compared with the runs in presence of C3H8 for OH radical scavenging (additional [H2SO4 ] = (4–8) 106 molecule cm3). This fact indicates a small formation yield of prompt OH radicals (y1). A detailed analysis yields y1 = 0.06 0.03 (see ESI†) qualitatively in line with predictions from theoretical calculations.20,22 The data in presence of C3H8 as given in Fig. 2 were analysed considering reactions (1)–(3). The experimental conditions allow to neglect in data analysis other bimolecular steps of CH2OO (self reaction, reaction with CH2O and H2SO4) as well as the wall loss of CH2OO. A modelling study shows that the reactions CH2OO + CH2OO, CH2OO + CH2O (CH2O product from O3 + C2H4) and CH2OO + H2SO4 account for less than 0.01% and the diffusion-limited wall loss for less than 1% of CH2OO conversion, even for low SO2 concentrations and without any contribution of water vapour for CH2OO depletion, see ESI.† According to reactions (1)–(3), the fraction of stabilized CH2OO producing H2SO4, CH2OOH2SO4/CH2OOtotal, is equal to the reaction rate of CH2OO + SO2, reaction (1), divided by the total reaction rate of CH2OO: CH2 OOH2 SO4 k1 ½SO2 ¼ CH2 OOtotal k1 ½SO2 þ k2a ½H2 O þ k2b ðH2 OÞ2 þ k3

(I)

The terms of reactions (2a), (2b) and (3) can be summarized to kloss for a given water vapour concentration (or RH), kloss = k2a[H2O] + k2b[(H2O)2] + k3. The SO2-dependent H2SO4 concentration formed during an experiment is given by ½H2 SO4 ¼

1 ½H2 SO4 CH2 OO kloss 1þ k1 ½SO2

(II)

where [H2SO4]CH2OO (equal to CH2OOtotal) stands for [H2SO4] from CH2OO titration by SO2, i.e. all CH2OO is converted to H2SO4 in the presence of high SO2 concentrations via reaction (1). The formation yield of stabilized CH2OO from reaction (4) y2 can be obtained from the knowledge of reacted [C2H4] and [H2SO4]CH2OO assuming again a H2SO4 formation yield of unity from reaction (1). y2 ¼

½H2 SO4 CH2 OO reacted½C2 H4

(III)

Very low reactant consumption (o1%) allowed to calculate the reacted [C2H4] from the initial concentrations, i.e. reacted [C2H4] = k4[C2H4][O3]t with k4 = 1.58 1018 cm3 molecule1 s1.23 The results for kloss/k1 and [H2SO4]CH2OO due to eqn (II) as obtained from four measurement series for different RH along with the yields of stabilized CH2OO are given in Table 1. The rate coefficient ratio kloss/k1 for the lowest water vapour concentration (RH: ca. 2%) can be considered an approximation for the upper limit of k3/k1 because in kloss = k2a[H2O] + k2b[(H2O)2] + k3 the terms including the water monomer and dimer have their lowest values, i.e. k3/k1 r 2.4 1011 molecule cm3. From the relative rate coefficients


Table 1 Results according to eqn (II) from nonlinear regression analysis. Given error limits for kloss/k1 and [H2SO4]CH2OO represent 2s data of statistical errors. The error limit of the stabilized CH2OO yield includes the uncertainty of H2SO4 calibration. Reacted [C2H4] = 2.06 108 molecule cm3

RH (%)

kloss/k1 (molecule cm3)

ca. 2 10 25 50

(2.36 (2.59 (1.55 (5.79

0.32) 0.10) 0.04) 0.20)

1011 1012 1013 1013

[H2SO4]CH2OO (molecule cm3) (7.38 (8.58 (8.18 (8.36

0.04) 0.03) 0.03) 0.07)

107 107 107 107

y2, stabilized CH2OO yield 0.36 0.42 0.40 0.41

0.16 0.19 0.18 0.18

given by Suto et al., k3/k1 = 5.2 1011 molecule cm3 follows about a factor of two higher than our upper limit.11 The corresponding rate coefficients from the experiments using the diiodomethane photolysis technique yield k3/k1 = 2.9 1012 molecule cm3 (assuming k3 = 100 s1)3,5,10 more than a factor of 10 higher than our k3/k1 ratio. The smaller k3/k1 obtained in our study points to a lower k3 than 100 s1 at 293 K. It is to be noted also here, that k3 = 100 s1 from diiodomethane photolysis experiments3,5,10 presents probably an upper limit due to other CH2OO consuming steps under these reaction conditions. It might be that the value of k3 (or k3/k1) from the present experiment can be affected by additional CH2OO consuming steps not considered in the reaction scheme, i.e. reactions (1)–(3). Very recently, Vereecken et al. published rate coefficients for the reaction of CH2OO with O3 and C2H4 from theoretical calculations.16 Resulting first-order rate coefficients for the disappearance of CH2OO are 0.22 s1 (O3) and 0.32 s1 (C2H4) for the reactant concentrations applied here. These loss processes are not distinguishable from the unimolecular steps via reaction (3) under our conditions and could have the same magnitude like k3 B 0.3 s1 as given by Olzmann et al.17 A possible contribution of the O3 and C2H4 reaction for the CH2OO decay can lead to an overestimation of k3 (or k3/k1) but does not influence the other parameters obtained in this work. The yield of stabilized CH2OO calculated according to eqn (III), average: y2 = 0.40 0.18, is well in line with results from other studies applying different scavenger techniques, see Table 2. The good agreement with literature data also indicates the validity of a H2SO4 formation yield of unity from CH2OO + SO2, reaction (1). The values of kloss/k1 for different water vapour concentrations (or RH) were plotted versus the water monomer concentration showing a clear square dependence in [H2O] while the plot versus [H2O]2 or the water dimer gives a straight line, see Fig. 3. Water dimer concentrations were calculated from the water monomer concentration using the literature value of Kp(T).29 The good correlation of kloss/k1 with the water dimer concentration favours distinctly reaction (2b) over (2a), i.e. k2b[(H2O)2] c k2a[H2O] in kloss = k2a[H2O] + k2b[(H2O)2] + k3. This finding is in agreement with results from theoretical kinetics calculations.13,14 Ryzhkov and Ariya14 reported that the rate of reaction (2b) is about 100 times the rate of reaction (2a) at 293 K and a relative humidity of 60–100%. Neglecting k2a[H2O] in kloss, linear regression analysis of the data in Fig. 3, kloss/k1 vs. [(H2O)2], yields k2b/k1 = 0.29 0.01 and for the intercept k3/k1 = (4.3 3.0) 1011 molecule cm3.


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Table 2

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Yields of stabilized CH2OO (sCI) from different scavenger techniques

Applied scavenger

Measured signal

sCI yield

SO2 Formic acid (CF3)2CO H2O Formic acid H2O SO2

H2SO4 Hydroperoxymethyl formate Consumed (CF3)2CO Hydroxymethyl hydroperoxide Hydroperoxymethyl formate Hydroxymethyl hydroperoxide H2SO4

0.39 0.44 0.52 0.38 0.39 0.42 0.40

Fig. 3 kloss/k1 from the individual measurement series (Table 1) as a function of the water dimer concentration. The error bars represent 2s data of statistical errors.

Although k3/k1 is affected with a quite large uncertainty, it is not in contradiction with k3/k1 r 2.4 1011 molecule cm3 as found from the SO2-dependent measurements for the lowest water vapour content, see explanations before. A direct comparison of our ratio k2b/k1 with other experimental data is only possible in the case of the upper limit of k2b o 3 1013 cm3 molecule1 s1 deduced by Welz et al.15 from the diiodomethane photolysis experiments by Stone et al.9 in connection with their k1 value, i.e. k2b/k1 o 8.8 103. This upper limit for k2b/k1 is about a factor of 30 lower than our k2b/k1 value using C2H4 ozonolysis. A possible reason for this discrepancy cannot be given. Generally, results from C2H4 ozonolysis experiments seem to point to a more rapid reaction of CH2OO with water vapour than those from diiodomethane photolysis. Unfortunately, former ozonolysis studies varied the water vapour concentration only in a relatively small range and assumed in the data analysis the reaction of CH2OO with the water monomer via reaction (2a). Suto et al.11 spectroscopically followed the H2SO4 aerosol formation and observed an inhibiting effect by water vapour that allowed to estimate k2a/k1. Using a more direct way, Becker et al.12 measured H2O2 as a product of the reaction of CH2OO with water vapour. The ratio k2a/k1 was determined from decreasing H2O2 formation with rising [SO2] using water partial pressures of 2–4.7 Torr. Neeb et al.28 deduced the rate coefficient ratio k(CH2OO + HCOOH)/k2a = 1.4 104 from C2H4 ozonolysis experiments (fixed water mixing ratio: 16 000 ppmv) by measuring the respective reaction products as a function of converted C2H4.


0.05 0.11 0.11 0.18

Ref. Hatakeyama et al.24 Neeb et al.25 Horie et al.26 Hasson et al.27 Hasson et al.27 Neeb et al.28 This study

Applying k(sCI + HCOOH)/k(sCI + SO2) = 2–3 being in good consensus between photolysis15 and ozonolysis measurements,30 k2a/k1 = (1.4–2.1) 104 follows for the data by Neeb et al.28 in good agreement with the finding by Suto et al.11 Setting for the fixed water vapour content [(H2O)2] = 3.7 1014 molecule cm3 according to the H2O/(H2O)2 equilibrium,29 the ratio k2b/k1 = 0.15– 0.22 can be obtained supporting the k2b/k1 value of this study. On the other hand, Welz et al.3 probed CH2OO directly and observed no water effect on the CH2OO decay for water concentration of up to 3.1 1016 molecule cm3 resulting in an upper limit k2a o 4 1015 cm3 molecule1 s1. Under these experimental conditions, probably the water dimer concentrations of r2.3 1012 molecule cm3(ref. 29) were too small and reaction (2b) was not competitive with the other CH2OO consuming steps. Stone et al.9 followed the CH2O generation from CH2OO + H2O at 200 Torr and water concentrations of up to 1.7 1017 molecule cm3 close to the conditions of our experiment. No change in the CH2O formation (produced in multiple paths in the photolysis system) in the presence of water addition led to the conclusion k2a o 9 1017 cm3 molecule1 s1.9 Furthermore, using also the diiodomethane photolysis technique, Ouyang et al.31 measured spectroscopically the formation of NO3 and N2O5 in total from CH2OO + NO2 for varying water content and deduced for k2a a value of (2.5 1) 1017 cm3 molecule1 s1. A re-evaluation of the Ouyang et al. results31 by Stone et al.9 yielded k2a = 5.4 1018 cm3 molecule1 s1. It should be noted that as a result of end product studies a water concentration of about 1017 molecule cm3 (or the corresponding dimer) was sufficient to ensure complete conversion of CH2OO to peroxides, mainly hydroxymethyl hydroperoxide.27 However, assuming k2a = 2.5 1017 or 5.4 1018 cm3 molecule1 s1 (ref. 9 and 31), reaction (2a) with [H2O] = 1017 molecule cm3 cannot be competitive with the unimolecular reaction (3) using k3 of about 100 s1.3,5,10 Applying k3 B 0.3 s1 as given by Olzmann et al.17 reaction (2a) becomes much more important relative to reaction (3) and could explain the peroxide measurements by Hasson et al.27 for k2a = 2.5 1017 cm3 molecule1 s1 (ref. 31) or higher k2a values. In the literature it is generally accepted from experimental findings,27,28,32,33 theoretical work13,14,34 and modelling studies16,35–37 that the atmospheric fate of CH2OO is governed by the reaction with water vapour (or water dimer) primarily forming hydroxymethyl hydroperoxide. CH2OO + (H2O)n - HO–CH2–OOH + (n 1)H2O; n = 1, 2 (5)


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Table 3

Kinetic data used in the modelling. A rate coefficient ratio k6/k1 = 315,30 was applied for all sCI

T (K) CH2OO

n.s. 298 293 0.5 293 0.5 293 0.5

CH3CHOOc (CH3)2COO


n.s.: not stated.

k2a/k1

a

Upper limit.

b

k3/k1 (molecule cm3)

k2b/k1 4

2.3 10 1.0 104 a — 8.8 105 4 106 a

— — 0.29 — —

5.2 2.6 2.4 1.2 4.2

11

10 1012 b 1011 a 1012 1012

Ref. Suto et al.11 Welz et al.3 This work Berndt et al.18 Berndt et al.18

k3 = 100 s1. c For syn- and anti-conformer in total.

Further reactions of HO–CH2–OOH lead to the formation of HCOOH, H2O2 and CH2O.12,28,32 The small effects of water vapour on the CH2OO decay as deduced by Stone et al.9 and Ouyang et al.31 (in combination with the order of magnitude of k3 from a series of experiments3,5,10) seems to be significantly different to the former observations from literature13,14,27,28,32–34 and the result of the present study. The reason for this discrepancy is not clear at the moment. More experimental effort is needed for clarification. A very simple modelling study was undertaken describing the fraction of sCI forming H2SO4 as a function of RH utilizing kinetic data given in Table 3. Beside the reaction of sCI with SO2 and organic acids,15,30 reaction (6), only the main loss processes in the atmosphere via the reaction with water vapour (water monomer or dimer), reactions (2a) and (2b), and the unimolecular reactions, reaction (3), are considered. CH2OO + org.acid - products

(6)

This scenario assumes SO2 and total organic acid concentrations of 1010 and 1011 molecule cm3, respectively, typically for remote areas.37 The rate coefficient ratio k6/k1 was set at 3 for all sCI.15,30 Calculations have been performed according to eqn (IV). CH2 OOH2 SO4 CH2 OOtotal ¼

k1 ½SO2 k1 ½SO2 þ k2a ½H2 O þ k2b ðH2 OÞ2 þ k3 þ k6 ½org:acid (IV)

In the RH range of 10–90% the results for CH2OO using parameters by Suto et al.11 Welz et al.3 and from the present study agree within a factor of five, see Fig. 4a. These three studies point to a very small sCI fraction forming H2SO4 being o0.001 for moderate and relatively high RH. Additionally, in Fig. 4a also the calculated sCI fraction forming H2SO4 for CH3CHOO and (CH3)2COO is depicted using data from our former study.18 For CH3CHOO, the water vapour reaction was described via sCI + H2O for the syn- and anti conformer in total, analogous to reaction (2a). In the case of (CH3)2COO only an upper limit can be given for the rate coefficient of sCI + H2O.18 The comparison shows that the sCI fraction reacting with SO2 is clearly dependent on the sCI structure for identical reaction conditions. In Fig. 4b the CH2OO fraction reacting with water vapour via reaction (2a) or (2b), CH2OOwater/CH2OOtotal, eqn (V), is plotted as a function of RH.


Fig. 4 (a) Modelling results of the H2SO4 formation from sCI + SO2 for different sCI according to eqn (IV), [SO2] = 1010 and [org. acid] = 1011 molecule cm3, room-temperature conditions. Applied kinetic data are summarized in Table 3. Data in black are from this work (CH2OO) or from ref. 18. (b) Modelling results of the CH2OO fraction reacting with water vapour either via reaction (2a) or (2b), eqn (V) and the kinetic data in Table 3, [SO2] = 1010 and [org. acid] = 1011 molecule cm3. The open circles show the results assuming k2b/k1 = 8.8 103 (ref. 9 and 15) instead of 0.29 (k3/k1 from this work).

CH2 OOwater CH2 OOtotal ¼

k2a ½H2 O þ k2b ðH2 OÞ2 k1 ½SO2 þ k2a ½H2 O þ k2b ðH2 OÞ2 þ k3 þ k6 ½org:acid (V)


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The resulting curves using parameters by Suto et al.11 and from this study are nearly identical showing that for RH 4 20% ([H2O] 4 1017 molecule cm3 at 293 K) more than 95% of CH2OO is reacting with water vapour. This is in line with experimental results applying water titration for the determination of the sCI yield from CH2OO.27,28 The data by Welz et al.3 indicate a somewhat lower importance of the water vapour reaction applying their upper limit of k2b/k1 and neglect the water dimer reaction, reaction (2b). Assuming the upper limit of k2b/k1 = 8.8 103 (ref. 9 and 15) instead of 0.29 in the modelling (k3/k1 = 2.4 1011 molecule cm3 from this work, reaction (2a) is neglected), the importance of the reaction of CH2OO with water vapour is clearly pushed back and only for RH Z 90% more than 90% of CH2OO are converted in the reaction with water vapour. In conclusion, the formation of H2SO4 from the reaction CH2OO + SO2 has been quantified as a function of the water vapour concentration for close to atmospheric conditions at 293 0.5 K. The kinetic analysis allows to conclude that the reaction of CH2OO with water vapour is second order in [H2O] for high enough water vapour content relevant in the atmosphere. This fact points to a reaction of CH2OO with the water dimer as favoured from theoretical kinetics calculations. The rate coefficient ratio of the reactions CH2OO + (H2O)2 relative to the reaction CH2OO + SO2, k2b/k1 = 0.29 0.01 has been determined. Measurements at the lowest relative humidity (RH ca. 2%) yield an upper limit of the rate coefficient ratio (unimolecular CH2OO reaction/CH2OO + SO2) of k3/k1 r 2.4 1011 molecule cm3. The results of this experiment indicate a predominate reaction of CH2OO with water vapour for atmospheric conditions in agreement with a series of former studies27,28,32,33 using C2H4 ozonolysis for CH2OO formation under close to atmospheric conditions. It is to be noted, that other recent studies applying the diiodomethane photolysis technique9,31 do not support the findings of this work, i.e. a clearly less efficient reaction of CH2OO with water vapour was reported. More effort is needed to clarity the discrepancy. A comparison of the sCI reactivity toward water vapour for CH2OO, CH3CHOO and (CH3)2COO shows a distinct sCI-structure dependent behaviour.

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Acknowledgements ¨fe and A. Rohmer The authors thank K. Pielok, S. Richters, R. Gra for technical assistance.

References 1 R. A. Cox and S. A. Penkett, Nature, 1971, 230, 321. ¨, P. Paasonen, 2 R. L. Mauldin III, T. Berndt, M. Sipila ¨ja ¨, S. Kim, T. Kurte ń, F. Stratmann, V.-M. Kerminen T. Peta and M. Kulmala, Nature, 2012, 488, 193. 3 O. Welz, J. D. Savee, D. L. Osborn, S. S. Vasu, C. J. Percival, D. E. Shallcross and C. A. Taatjes, Science, 2012, 335, 204. 4 C. A. Taatjes, O. Welz, A. J. Eskola, J. D. Savee, A. M. Scheer, D. E. Shallcross, B. Rotavera, E. P. F. Lee, J. M. Dyke,


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