Kinetics of stabilised Criegee intermediates derived from alkene ozonolysis: reactions with SO2, H2O and decomposition under boundary layer conditions.

The removal of SO2 in the presence of alkene-ozone systems has been studied for ethene, cis-but-2-ene, trans-but-2-ene and 2,3-dimethyl-but-2-ene, as a function of humidity, under atmospheric boundary layer conditions. The SO2 removal displays a clear dependence on relative humidity for all four alkene-ozone systems confirming a significant reaction for stabilised Criegee intermediates (SCI) with H2O. The observed SO2 removal kinetics are consistent with relative rate constants, k(SCI + H2O)/k(SCI + SO2), of 3.3 (±1.1) × 10(-5) for CH2OO, 26 (±10) × 10(-5) for CH3CHOO derived from cis-but-2-ene, 33 (±10) × 10(-5) for CH3CHOO derived from trans-but-2-ene, and 8.7 (±2.5) × 10(-5) for (CH3)2COO derived from 2,3-dimethyl-but-2-ene. The relative rate constants for k(SCI decomposition)/k(SCI + SO2) are -2.3 (±3.5) × 10(11) cm(-3) for CH2OO, 13 (±43) × 10(11) cm(-3) for CH3CHOO derived from cis-but-2-ene, -14 (±31) × 10(11) cm(-3) for CH3CHOO derived from trans-but-2-ene and 63 (±14) × 10(11) cm(-3) for (CH3)2COO. Uncertainties are ±2σ and represent combined systematic and precision components. These values are derived following the approximation that a single SCI is present for each system; a more comprehensive interpretation, explicitly considering the differing reactivity for syn- and anti-SCI conformers, is also presented. This yields values of 3.5 (±3.1) × 10(-4) for k(SCI + H2O)/k(SCI + SO2) of anti-CH3CHOO and 1.2 (±1.1) × 10(13) for k(SCI decomposition)/k(SCI + SO2) of syn-CH3CHOO. The reaction of the water dimer with CH2OO is also considered, with a derived value for k(CH2OO + (H2O)2)/k(CH2OO + SO2) of 1.4 (±1.8) × 10(-2). The observed SO2 removal rate constants, which technically represent upper limits, are consistent with decomposition being a significant, structure dependent, sink in the atmosphere for syn-SCI.


Introduction
Atmospheric oxidation processes are central to understanding trace gas atmospheric composition, the abundance of air pollutants harmful to human health, crops and ecosystems, and the removal of reactive greenhouse gases such as methane.The principal atmospheric oxidants responsible for initiating the gas-phase degradation of volatile organic compounds (VOCs), NO x and SO 2 are OH, NO 3 and O 3 , with additional contributions from other species such as halogen atoms.Recent field measurements 1 in a boreal forest have identified the presence of an additional oxidant species, removing SO 2 and producing H 2 SO 4 .The additional SO 2 oxidation observed was substantial (comparable to that due to OH radicals alone), and attributed to a product of alkene ozonolysis -the boreal forest environment being one in which substantial biogenic alkene emissions occur -suggested to be the stabilised Criegee intermediate (SCI).The gas-phase oxidation of SO 2 in the atmosphere is of interest to climate research as it leads to formation of H 2 SO 4 , contributing to new particle formation and sulphate aerosol loading, in competition with condensed phase oxidation. 2A missing mechanism for the conversion of SO 2 to H 2 SO 4 could lead to model underestimation of the sulphate aerosol burden and affect radiative forcing calculations, 3 with corresponding implications for climate predictions.Enhanced SO 2 oxidation in alkene-ozone systems was first reported by Cox & Penkett 4 over forty years ago, however the precise reaction mechanism giving rise to this effect remains elusive.The ''Criegee'' ozonolysis reaction mechanism was first postulated in the 1940s. 5It is now accepted that the ozone molecule adds to the CQC double bond via a concerted cycloaddition to form a primary ozonide, followed by cleavage of the C-C bond and one of the O-O bonds forming a carbonyl molecule and a carbonyl oxide, or 'Criegee intermediate' (CI). 6Ozonolysis derived SCIs are formed with a broad internal energy distribution, to yield chemically activated and stabilised SCIs.SCIs can have sufficiently long lifetimes to undergo bimolecular reactions with H 2 O and SO 2 amongst other species.Chemically activated SCIs may also undergo collisional stabilisation, unimolecular decomposition or isomerisation (Scheme 1).For substituted alkenes, SCIs can undergo a 1,4-Hshift rearrangement through a vinyl-hydroperoxide (VHP) via the so-called ''hydroperoxide channel'' and decompose to yield OH and a vinoxy radical -a substantial non-photolytic source of atmospheric oxidants. 7,8This is the favoured channel for SCIs formed in the syn-configuration.Time resolved studies 9 show that the VHP may persist for appreciable timescales under boundary layer conditions, giving rise to the observed pressure dependence of OH radical yields, 10 and opening the possibility for bimolecular reactions of this species to occur.SCIs formed in the anticonfiguration are thought to primarily undergo rearrangement and decomposition via a dioxirane intermediate (''the acid/ ester channel''), producing a range of daughter products and contributing to the observed overall HO x radical yield. 6,11kene Until recently, it has been thought that the predominant atmospheric fate for SCIs was reaction with water vapour 12,13 leading to a significant source of organic acids and hydroperoxides, suggesting that bimolecular reaction with SCIs is an unimportant oxidation mechanism for trace gas species.This view was recently challenged by direct observation 14 and kinetic studies [15][16][17] of the CH 2 OO and CH 3 CHOO SCIs.
Taatjes and co-workers, 15 directly observing CI kinetics for the first time, found reactions of CH 2 OO with SO 2 and NO 2 to be substantially faster than previously thought, pointing to a potentially important role for this species in atmospheric SO 2 and NO 2 oxidation; subsequent measurements have identified SO 3 (ref.16) and NO 3 (ref.18) as products of these reactions.The key to whether SCIs are indeed significant contributors to gas-phase atmospheric SO 2 oxidation is the ratio of the rate constants for reaction of the SCI with SO 2 (k 2 ), to that with H 2 O (k 3 ) and decomposition (k d ).In laboratory studies where SCIs were produced by the 248 nm laser photolysis of alkyl iodide precursors at 4 Torr total pressure, with the SCI decay monitored by VUV photoionisation in the presence of excess co-reactants, this ratio has recently been reported to be 10 3 -10 4 for the smallest two SCIs (CH 2 OO 15,17 and CH 3 CHOO 16 ), with k 2 on the order of 10 À11 cm 3 s À1 and k 3 on the order of 10 À15 cm 3 s À1 .In contrast, alternative studies of SO 2 oxidation in alkene-ozone systems, performed at atmospheric pressure through detection of the H 2 SO 4 product, find much smaller SCI + SO 2 rate coefficients (by ca.two orders of magnitude). 19Explanations for this apparent discrepancy may include the lifetime of the secondary ozonide (SOZ) adduct formed from the SCI + SO 2 reaction, collisionally stabilised at atmospheric pressure, 20 effects of the presence of multiple SCI conformers with differing reactivity, or contributions from other oxidant species, formed within the ozonolysis system, to SO 2 reaction.Understanding the behaviour of the ozonealkene-SO 2 system in the presence of water vapour is critical to quantifying the impact of SCI chemistry upon atmospheric SO 2 oxidation. 21n additional, potentially important, fate of SCI under atmospheric conditions is unimolecular decomposition (denoted k d in (R4)).For CH 2 OO, rearrangement via a 'hot' acid species represents the lowest accessible decomposition channel, but due to lack of alkyl substituents, the theoretically predicted 298 K rate constant is rather low, 0.3 s À1 . 22Previous studies have identified the hydroperoxide rearrangement as dominant for SCIs with a syn configuration, determining their overall unimolecular decomposition rate. 7,8For syn-CH 3 CHOO recent experimental 23 work yielded a decomposition rate of 3-30 s À1 .Theoretical work 24 has predicted a decomposition rate coefficient of 24.2 s À1 for syn-CH 3 CHOO and 67.2 s À1 for anti-CH 3 CHOO (for which only the ester channel exists), owing to the potential energy release from the higher energy anti-conformer. 23An upper limit to total CH 3 CHOO loss (decomposition and heterogeneous wall losses combined) of o250 s À1 has been reported by Taatjes et al. 16 Earlier experimental work reported decomposition rate constants of r20 s À1 for CH 3 CHOO derived from cis-but-2-ene ozonolysis, 25 and 76 s À1 (accurate to within a factor of three) for CH 3 CHOO derived from trans-but-2-ene ozonolysis. 26For (CH 3 ) 2 COO (derived from 2,3-dimethyl-but-2-ene ozonolysis) a total loss rate of 3.0 s À1 has recently been determined experimentally. 19This value (which represents an upper limit for k d ) is somewhat smaller than but comparable in magnitude to an earlier measurement of 6.4 s À1 (determined at 100 Torr). 27Theoretical estimates of (CH 3 ) 2 COO decomposition rates are higher, at up to 250 s À1 . 22Photolysis loss rates have also recently been reported for CH 2 OO 28 and CH 3 CHOO 29 of 1 s À1 and 0.2 s À1 respectively, calculated for actinic flux values at midday, SZA = 01.
Here, we present results of a series of experiments in which the oxidation of SO 2 during the ozonolysis of ethene, cis-but-2-ene, trans-but-2-ene and 2,3-dimethyl-but-2-ene (tetramethylethylene, TME), was investigated in the presence of varying amounts of water in the European Photochemical Reactor facility (EUPHORE), Valencia, Spain.

EUPHORE
EUPHORE is a 200 m 3 simulation chamber used for studying reaction mechanisms under atmospheric boundary layer conditions.In general, experiments comprised time-resolved measurement of the removal of SO 2 in the presence of an alkene-ozone system, as a function of humidity.SO 2 and O 3 abundance were measured using conventional fluorescence and UV absorption monitors, respectively; alkene abundance was determined via FTIR spectroscopy.The precision of the SO 2 and O 3 monitors were 0.25 and 0.47 ppbv respectively (evaluated as 2 standard deviations of the measured value prior to SO 2 or O 3 addition).The chamber is fitted with large horizontal and vertical fans to ensure rapid mixing (three minutes).Further details of the chamber setup and instrumentation are available elsewhere. 30,31xperiments were performed in the dark (i.e. with the chamber housing closed; j(NO 2 ) r 10 À6 s À1 ), at atmospheric pressure (ca.1000 mbar) and temperatures between 296 and 303 K, on timescales of ca.20-30 minutes.Chamber dilution was monitored via the first order decay of an aliquot of SF 6 , added prior to each experiment.Cyclohexane (ca.75 ppmv) was added at the beginning of each experiment to act as an OH scavenger, such that SO 2 reaction with OH was calculated to be r1% of the total chemical SO 2 removal in all experiments.

Experimental approach
Experimental procedure, starting with the chamber filled with clean air, comprised addition of SF 6 and cyclohexane, followed by water vapour, O 3 (ca.500 ppbv) and SO 2 (ca.50 ppbv).A gap of five minutes was left prior to addition of the alkene, to allow complete mixing.The reaction was then initiated by addition of the alkene (ca.500 ppbv for ethene, 200 ppbv for cis-and transbut-2-ene and 400 ppbv for TME).The chamber was monitored for an hour subsequent to the addition of ethene and forty five minutes for cis-and trans-but-2-ene and TME.The rate of alkeneozone consumption is dependent on k 1 .Roughly 25% of the ethene was consumed after an hour, while for cis-and trans-but-2ene and TME 90% of the alkene was consumed within roughly 25 minutes, 20 minutes and 6 minutes respectively.Each experiment was performed at a constant humidity, which was increased in a step-wise manner for consecutive runs to cover the range 1.5-21% RH.Measured increases in [SO 2 ] agreed with measured volumetric addition across the SO 2 and humidity range used in the experiments.

Analysis approach
The following sections describe (i) a common analysis applied to all systems, but which features several approximations, and (ii) more detailed consideration of each chemical system in turn, to address potential contributions from the water dimer, and multiple criegee species within each system (e.g.contrasting reactivity of different SCI conformers).
From the chemistry presented in reactions (R1)-(R4) it is assumed that SCI will be produced in the chamber from the reaction of the alkene with ozone at a given yield, f.The SCI produced can then react with SO 2 , with H 2 O, with other species or decompose under the experimental conditions applied.The rate at which SO 2 is lost, compared with the total production of SCI, is determined by the fraction, f, of the total SCI produced which reacts with SO 2 , compared to the sum of the total loss processes of the SCI (eqn (E1)): Here, L is the sum of any other pseudo-first order chemical loss processes for SCI in the chamber, after correction for dilution.(E2) neglects other (non-alkene) chemical sinks for O 3 , such as reaction with HO 2 -also produced directly during alkene ozonolysis, 11 but indicated through model calculations to account for o2% of ozone loss under all the experimental conditions of this work.Eqn (E1) and (E2) treat the SCIs formed as a single species -this is the case for (e.g.) ethene and TME, but is an approximation for the 2-butenes; considered further below.
2.3.1 SCI yield calculation.Values for f SCI were determined for each ozonolysis reaction from experiments performed under dry conditions (RH o 1%) in the presence of excess SO 2 (ca.1000 ppbv), such that SO 2 scavenges the overwhelming majority of the SCI.From eqn (E2), regressing dSO 2 against dO 3 (corrected for chamber dilution), assuming f to be unity (i.e.all the SCI produced reacts with SO 2 ) determines the value of f min , a lower limit to the SCI yield.Fig. 2 shows the experimental data, from which f min was derived, for all four alkene ozonolysis systems studied.
The lower limit criterion applies as in reality f will be less than one, at experimentally accessible SO 2 levels, as a small fraction of the SCI will still react with any H 2 O present, or undergo decomposition.The actual yield, f, was determined by combining the results from the high-SO 2 experiments with those from the series of experiments performed at lower SO 2 , as a function of [H 2 O], to determine k 3 /k 2 and k d /k 2 (see Section 2.3.2),through an iterative process to determine the single unique value of f SCI which fits both datasets.It is important to note that the SCI yield is to an extent an operationally defined quantity -for example, OH formation from alkene ozonolysis is known to proceed over at least hundreds of milliseconds 9 following the alkene-ozone reaction, and so the corresponding CI population must also be evolving with time.In this work, SCI yields reflect the amount of SCI available to oxidise SO 2 on timescales of seconds to minutes.
2.3.2 k(SCI + H 2 O)/k(SCI + SO 2 ) and k d /k(SCI + SO 2 ).To determine k 3 /k 2 and k d /k 2 , a series of experiments were performed for each alkene, in which the SO 2 loss was monitored as a function of [H 2 O].From eqn (E2), regression of the loss of ozone (dO 3 ) against the loss of SO 2 (dSO 2 ) (both corrected for dilution, measured through the removal SF 6 , added at the start of each experiment and monitored via FTIR) for an experiment determines the factor fÁf at a given point in time.This quantity will vary through the experiment as SO 2 is consumed, and other potential SCI co-reactants are produced, as predicted by eqn (E1).A smoothed fit was applied to the experimental data for the cumulative consumption of SO 2 and O 3 , DSO 2 and DO 3 , (Fig. 1) to determine dSO 2 /dO 3 (and hence fÁf) at the start of each experiment, for use in eqn (E3).The start of the experiment (i.e. when [SO 2 ] B 50 ppbv) was used as this corresponds to the greatest rate of production of the SCI, and hence largest experimental signals (O 3 and SO 2 rate of change) and is the point at which the SCI + SO 2 reaction has the greatest magnitude compared with any other potential chemical loss processes for either species (see discussion below).
The value [SO 2 ]((1/f) À 1) can then be regressed against [H 2 O] for each experiment to give a plot with a gradient of k 3 /k 2 and an intercept of (k d + L)/k 2 (eqn (E3)).Our data cannot determine absolute rate constants (i.e.values of k 2 , k 3 , k d ) in isolation, but is limited to assessing their relative values, which may be placed on an absolute basis through use of an (external) reference value.
Eqn (E1)-(E3) as presented above assume that only a single SCI species is present in each ozonolysis system.While this is the case for the ethene and TME systems, for the but-2-ene systems this is an approximation as two conformers of the CH 3 CHOO SCI (syn and anti) are produced.Further analysis is performed in Section 3.3.2 in which the SO 2 loss in the but-2ene systems is treated as having two components, related to the different SCI.

Introduction
Table 1 shows the resulting SCI yields obtained for the ozonolysis of ethene, cis-but-2-ene, trans-but-2-ene and tetramethylethylene (TME); uncertainties are AE2s, and represent the combined systematic (estimated measurement uncertainty) and precision components.Also shown are past literature values, obtained under various conditions using a range of different SCI scavengers.
The yield of CH 2 OO from ethene ozonolysis obtained in this work is 0.37 (AE0.04).This yield has been investigated in many previous studies, with values ranging from 0.34-0.50determined -a more detailed review is available elsewhere. 31The yield obtained in this work is at the lower end of but within the envelope of these estimates, and is in excellent agreement with the current IUPAC recommendation of 0.37.
The yield of CH 3 CHOO from cis-but-2-ene ozonolysis obtained in this work is 0.38 (AE0.05), with that from transbut-2-ene being 0.28 (AE0.03).These values fall within the range of reported literature values of 0.18-0.43 and 0.13-0.53for cisbut-2-ene and trans-but-2-ene respectively (Table 1).cis and trans-but-2-ene both yield (differing) mixtures of syn and anti conformers of CH 3 CHOO, the relative amounts of which are not well known, and which are treated here initially as a single SCI species (this approximation is discussed further in Section 3.3).Berndt et al. 32 recently reported a yield of 0.49 (AE0.22) for the CH 3 CHOO produced from trans-but-2-ene ozonolysis (also treating both syn and anti conformers as a single SCI species).
The yield of (CH 3 ) 2 COO from TME ozonolysis obtained in this work is 0.32 (AE0.02).This again falls within the (wide) range in the literature of 0.10-0.65,with Berndt et al. 32 most recently reporting a yield of 0.45 (AE0.20).20%)) for all four alkenes.This trend would be expected from the understood chemistry ((R1)-(R4)), as there is competition between SO 2 , H 2 O, and decomposition for reaction with the SCI.
Fig. 3 shows a fit of eqn (E3) to the data for each alkene, giving a slope of k 3 /k 2 , and an intercept of (k d + L)/k 2 .The results appear to show a generally linear relationship; however, for cisand trans-but-2-ene and TME, the data point at the highest relative humidity accessible in this work ([H 2 O] = 1.5-2.0Â 10 17 cm À3 ) appears to deviate from this relationship.These data points lie outside the 95% confidence intervals defined by all the other (lower relative humidity) data for each alkene.For the analysis to determine k 3 /k 2 and (k d + L)/k 2 presented in Table 2, the points at the highest RH are excluded and the kinetic parameters are derived from a linear fit to the measurements from all other experiments.Extended analyses to account for the non-linearity observed for CH 3 CHOO and (CH 3 ) 2 COO are presented in the following sections.
One potential explanation for the observed curvature in the CH 3 CHOO and (CH 3 ) 2 COO data is measurement error.DSO 2 is relatively small at high [H 2 O] compared to the precision of the measurements; however, even allowing for associated uncertainties, the points at high RH do not fit the linear relationship successfully applied to the remaining data points.Moreover, any systematic error in the measurement of O 3 , SO 2 or H 2 O would also be expected to affect the results for the ethene system (and to a greater extent, given the slow ethene-ozone reaction rate and consequent lower overall chemical SO 2 loss observed), suggesting that the cause lies in contrasting chemical behaviour.In terms of experimental factors, H 2 O was measured using multiple approaches (two dew-point hygrometers in addition to a solid state probe) with no evidence for any divergence with RH.SO 2 monitors can exhibit humiditydependent interferences (quenching of the SO 2 signal), commonly of the order of a few percent, observed at very high RH, and corrected through incorporation of a nafion dryer (fitted in this case); in addition the monitor-derived SO 2 concentration increments were in agreement with those calculated from the measured SO 2 addition and chamber volume, across the relative humidity range studied.
It should be noted that the k d values reported here represent upper limits, as a consequence of possible further chemical losses for the SCI within our experimental system (as represented by L in eqn (E3), notwithstanding the approach of extrapolating to the start of each experiment to minimise these).Other potential     fates for SCIs include reaction with ozone, 42,43 other SCI, 43 carbonyl products, 44 acids, 45 or with the parent alkene 43 itself.
Sensitivity analyses indicate that the reaction with ozone could be significant, as predicted by theory 42,43 with a possible contribution of up to 10% of SCI loss for (CH 3 ) 2 COO at 2% RH, while total losses from reaction with SCI (self-reaction), carbonyls and alkenes are calculated to account for o1% of the total SCI loss under the experimental conditions applied.2).
These relative rates can be placed on an absolute basis using absolute measurements of k 2 (SCI + SO 2 ).In Table 3 we apply the absolute k 2 values reported by Welz et al., 15 obtained using direct methods at reduced pressure (4 Torr), to the relative rates shown in Table 2. Using this method, the value obtained for k 3 (CH 2 OO + H 2 O) is 1.3 (AE0.4)Â 10 À15 cm 3 s À1 .This is consistent with the recent determination by Welz et al., 15 that k 3 o 4 Â 10 À15 cm 3 s À1 , but is (ca.14-50 times) greater than the recent estimates of Ouyang et al. 18 (k 3 = 2.5 (AE1) Â 10 À17 cm 3 s À1 ) and Stone et al. 17  The derived (k d + L) value for CH 2 OO using this method is À8.8 (AE13) s À1 , i.e. zero within uncertainty.Theoretical work 22 has predicted k d (CH 2 OO) to be small (B0.3 s À1 ), in agreement with the experimentally derived value reported here.
3.2.2CH 2 OO + (H 2 O) 2 .Recent experimental work 46 has reported the reaction of CH 2 OO with the water dimer, (H 2 O) 2 , (reaction (R5)) to be very fast (1.1 Â 10 À11 cm 3 s À1 -assuming k 2 = 3.9 Â 10 À11 cm 3 s À1 (ref.15)), in broad agreement with theoretical predictions, 47 but in contrast to other experimental work. 45 The CH 2 OO data from this study appear to be well described by a linear fit under the experimental conditions applied (a fast reaction of CH 2 OO with (H 2 O) 2 would be manifested as a significant upward curvature in Fig. 3).However, this does not mean the results are inconsistent with reaction of CH 2 OO with (H 2 O) 2 .
In Fig. 4, eqn (E4) (an expanded version of eqn (E3), including the SCI + (H 2 O) 2 reaction (R5)) is applied to the data, now expressed in terms of (H 2 O) 2 , calculated for each RH via the equilibrium constant. 48 The value for k 3 /k 2 (water monomer) derived from the fit shown in Fig. 4 is 2.5 (AE0.7)Â 10 À5 .It is seen that this value is rather insensitive to the inclusion of the (H 2 O) 2 term in eqn (E4) as the value is within the uncertainties of the linear fit to the data presented in Fig. 3 -see also Table 2. Converting this value to an absolute value using the k 2 from Welz et al. 15 gives k 3 = 9.9 (AE2.9)Â 10 À16 cm 3 s À1 .The derived value of k d /k 2 is À6.4 (AE66) Â 10 10 cm À3 , which, using the Welz et al. 15 k 2 gives an absolute value for k d of À1.5 (AE16) s À1 .This is again indistinguishable from zero, within uncertainty, as is the k d determined from eqn (E3) (Fig. 3).Note the large uncertainties in k d , resulting from allowing three parameters to vary in the optimisation; consequently k d was fixed to zero in eqn (E4) to determine the k 3 /k 2 and k 5 /k 2 values.
The resulting value of k 5 /k 2 (water dimer) is 1.4 (AE1.8)Â 10 À2 .Converting this to an absolute value using the k 2 from Welz et al. 15 gives k 5 = 5.6 (AE7.0)Â 10 À13 cm 3 s À1 .This is roughly a factor of twenty smaller than the value derived by Berndt et al., 46 but within a factor of two of the upper limit for k 5 deduced by Welz et al. (o3 Â 10 À13 cm 3 s À1 ) (ref.45) from the data presented by Stone et al. 17 The inset plot in Fig. 4 also shows two additional fits generated using eqn (E4) with k 3 /k 2 fixed to 9.9 Â 10 À16 and k d fixed to zero.One fit line uses the k 5 value reported by Berndt et al. 46 (blue dashed line).This is seen to overestimate the presented data.The green dotted line shows a fit to the upper limits of the uncertainties of the measured data.This yields a k 5 /k 2 value of 0.10 (AE0.01),giving an upper limit k 5 value of 3.9 (AE0.39)Â 10 À12 cm 3 s À1 .O] (B2.5-7.5 Â 10 17 molecules per cm À3 ), greater than was accessible in this study, reaction with (H 2 O) 2 could become the dominant sink for CH 2 OO.In this case using just the H 2 O monomer kinetics in models would considerably underestimate the total effect of water on removal of CH 2 OO in the atmosphere.3 for both cis and trans-but-2-ene appear to be well described (within the uncertainties) by a linear fit to eqn (E3), with the exception of the experiment at the highest RH ([H 2 O] = 1.8-1.9Â 10 17 cm À3 ) in both cases.The kinetic parameters derived from a linear fit to the data (Fig. 3), using eqn (E3) (which treats the system as producing a single SCI), excluding the highest RH experiments, are shown in Table 2. Very similar results are obtained for k 3 /k 2 for CH 3 CHOO derived from both cis-and trans-but-2-ene ozonolysis, with values of 26 (AE10) Â 10 À5 and 33 (AE10) Â 10 À5 respectively.The (k d + L)/k 2 obtained for CH 3 CHOO from cis-but-2ene ozonolysis is 13 (AE43) Â 10 11 molecule cm À3 and from transbut-2-ene ozonolysis À14 (AE31) Â 10 11 molecule cm À3 .Berndt et al. 32 reported the k 3 /k 2 ratio from trans-but-2-ene ozonolysis to be 8.8 (AE0.4)Â 10 À5 (also assuming a single SCI system), a factor of 3.75 smaller than that reported here.
The relative rate constants (Table 2) can be placed on an absolute basis using the measurements of k 2 (SCI + SO 2 ) reported by Taatjes et al. 16 (derived using the same methodology as for CH 2 OO) (Table 3).As eqn (E3) treats the SCI produced as a single SCI, we use an average of the syn and anti conformer rates presented in Taatjes et al., 16 4.55 Â 10 À11 cm 3 s À1 .Using this method, the value obtained for k 3 (CH 3 CHOO + H 2 O) from cis-but-2-ene ozonolysis is 12 (AE4.5)Â 10 À15 cm 3 s À1 and from trans-but-2-ene ozonolysis is 15 (AE4.5)Â 10 À15 cm 3 s À1 .Taking a mean of the k 3 values reported for the two CH 3 CHOO conformers by Taatjes et al. 16 gives a value of 7.0 Â 10 À15 cm 3 s À1 , while Sheps et al. 49 give a mean value of 12 Â 10 À15 cm 3 s À1 .The values obtained for (k d + L) are 59 (AE196) s À1 from cis-but-2-ene and À64 (AE141) s À1 from trans-but-2-ene.Clearly there is a large uncertainty associated with the k d determined from this analysis.Fenske et al. 26 have reported k d (CH 3 CHOO) from trans-but-2-ene ozonolysis to be 76 s À1 (accurate to within a factor of three).
3.2.2Two conformer system.In Fig. 1 it is evident that dSO 2 /dO 3 falls rapidly with increasing [H 2 O] for all but-2-ene experiments as RH is initially increased, but that the experiments at higher RH all appear to display a similar dSO 2 /dO 3 , i.e. the trend in decreasing SO 2 removal with increasing H 2 O levels off.From this observation it appears that there may be competing H 2 O dependencies to the SO 2 loss present.This is manifested in Fig. 3 as a curving over of the data at high RH.We propose two possible explanations for this observation: firstly that it arises from differing kinetics of the two CH 3 CHOO conformers formed in but-2-ene ozonolysis; secondly that the behaviour may reflect the presence of an additional oxidant being formed in the ozonolysis system that reacts with SO 2 but is less sensitive to H 2 O.The first of these possibilities is discussed below, the second is discussed in relation to (CH 3 ) 2 COO in the following section.
One explanation for the observed non-linearity at high RH apparent in Fig. 3 is the differing reactivities of the syn-and anti-conformers of CH 3 CHOO produced in the ozonolysis of cis-and trans-but-2-ene.It has been predicted 50 that the anticonformer reacts with H 2 O several orders of magnitude faster than the syn-conformer, while the rate constant for the SCI reaction with SO 2 has been determined experimentally 16 to be about a factor of three greater for the anti-conformer than the syn-conformer.The fraction of each conformer that is lost to reaction with SO 2 can be considered in the same way as illustrated in eqn (E2), leading to eqn (E5) and (E6) below, plus simplifications outlined in the following text.The total loss of SO 2 to CH 3 CHOO is then the sum of the fractional loss to each conformer, multiplied by the relative SCI yield (g) (i.e.j syn /j) of that conformer (eqn (E7)).
Eqn (E8) can then be fitted to the data presented in Fig. 3 for cis-and trans-but-2-ene (Fig. 5).
Here we make two assumptions to reduce the degrees of freedom and hence make the problem tractable with the dataset available.First it is assumed that k syn 3 [H 2 O] may be neglected, in keeping with theoretical predictions 24 predicting k syn d to be over three orders of magnitude greater than k syn 3 [H 2 O].Further theoretical work 50 predicts a rate constant for syn-CH 3 CHOO + H 2 O of 2.39 Â 10 À18 cm3 s À1 , and recent experimental work 49 yields an upper limit for k syn 3 of o2 Â 10 À16 cm 3 s À1 .Hence with k syn d expected to be relatively fast, given the decomposition rate presented here for (CH 3 ) 2 COO (a syn-conformer) and the facile decomposition route available via the hydroperoxide mechanism for the syn-conformer, it can be assumed that k syn d c k syn 3 [H 2 O] at the experimental conditions reported here.Second it is assumed that k anti  4).  is comparable to (a factor of two greater than) that reported by Taatjes et al. 16 (1.0(AE0.4)Â 10 À14 ).Novelli et al. 23 have recently reported k syn d to be an order of magnitude smaller (3-30 s À1 ) based on direct observation of OH formation during trans-but-2-ene ozonolysis at atmospheric pressure.
The point of interception in Fig. 6 also determines the relative yields of the two conformers, g syn and g anti (which in turn has been used to derive the optimised fits shown in Fig. 5).For cis-but-2-ene these are determined as 0.45 and 0.55 for g syn and g anti respectively.For trans-but-2-ene they are determined as 0.25 for g syn and 0.75 for g anti .The analysis performed in this section has implications for the determination of the SCI yield.Using the relative rate constant k 3 /k 2 (anti-CH 3 CHOO) obtained, as shown in Table 4, it is calculated that B90% of the anti-CH 3 CHOO produced in the SCI yield experiments would react with SO 2 .From the determined k d /k 2 (syn-CH 3 CHOO) it is calculated that B67% of the syn-CH 3 CHOO produced in the SCI yield experiments would react with SO 2 .Applying these (with the corresponding syn and anti yields shown in Table 4) corrections to j min determines total SCI yields of 0.29 for transbut-2-ene and 0.42 for cis-but-2-ene.These values both lie within the uncertainties in the SCI yields presented in Table 1 for the two but-2-ene systems.
It is not practicable to assess the possible contribution of the water dimer to the SCI loss for CH 3 CHOO because of the Fig. 5 Fits of eqn (E8) to the cis-but-2-ene and trans-but-2-ene data shown in Fig. 3. g syn and g anti are 0.45 and 0.55 for cis-but-2-ene and 0.25 and 0.75 for trans-but-2-ene (see Fig. 6).
Fig. 6 The ranges of k anti 3 /k anti number of free parameters that would result for a small dataset.However, theoretical predictions 47 suggest that this may be less important for CH 3 CHOO than for CH 2 OO, indicating k(H 2 O) 2 / k(H 2 O) to be two orders of magnitude smaller for anti-CH 3 CHOO than for CH 2 OO.
Berndt et al. 32 have recently reported the k 3 /k 2 ratio for (CH 3 ) 2 COO to be o0.4Â 10 À5 (i.e.approximately a factor of 22 lower than the relative rate reported in this study).Theoretical predictions 50 also suggest k 3 to be very slow, 3.9 Â 10 À17 cm 3 s À1 .No measured values have been reported for k d ((CH 3 ) 2 COO), but a more facile overall decomposition than for CH 2 OO or the mean of the CH 3 CHOO isomers might be anticipated as the vinylhydroperoxide isomerisation channel 26 is always available.
3.4.2Additional oxidant.In the case of the SCI formed from TME ozonolysis, (CH 3 ) 2 COO, there is always a methyl group in a syn position to the carbonyl oxide moiety, thus the analysis presented in Section 3.3.2for the CH 3 CHOO isomers does not apply.A possible alternative explanation for the observed behaviour (and possible contributor to the behaviour observed in the but-2-ene systems) is that there is a further oxidant (X) of SO 2 , in addition to the SCI, being formed during the ozonolysis reaction.If this oxidant reacts relatively slowly with H 2 O, it could give rise to the apparent 'two component' nature of the observations seen in Fig. 3.It may also provide an alternative explanation of the observed nature of the SO 2 loss from the but-2-ene experiments or could be occurring in addition to the effects of differing conformer reactivities.
Eqn (E9) (below) is an expanded version of (E2), in which we consider the contribution from a second SO 2 oxidant, making the approximation that this species does not react appreciably with water vapour.In eqn (E9), f is the sum of f SCI (the fraction of SCI reacting with SO 2 ) and f x , each multiplied by the relative yield of the total oxidant (i.e.SCI + X) g SCI and g x .Following the assumption of negligible H 2 O reactivity, (dSO 2 /dO 3 ) x in eqn (E9) can be derived from the SO 2 loss at the highest RH experiments (i.e. when all the SO 2 loss is attributed to X + SO 2 ) of B10 ppbv.Therefore, loss of SO 2 from reaction with X, relative to the loss of O 3 (dSO 2 /dO 3 ) x is approximately 0.025.j represents the total oxidant yield (i.e.j SCI + j x ).Assuming that g x is not dominant (o0.5), then j, as calculated from correcting j min as in Section 3.1, changes little (0.31-0.34) from the value presented in Table 1.Eqn (E10) is then an expanded version of eqn (E3) that includes the additional oxidant term.However, Fig. 7 does demonstrate that a two-oxidant system, as represented by eqn (E10), is able to describe the data within uncertainty.
Fig. 7 also includes a linear fit (i.e.eqn (E3)) to the full (CH 3 ) 2 COO dataset (including the highest RH experiment).While it seems unlikely that the curvature observed in the data is a result of measurement error (as described in Section 3.1), this must be considered as a possibility for (CH 3 ) 2 COO in light of the two conformer explanation not being applicable.The linear fit in Fig. 7 gives a k 3 /k 2 value of 3.8 (AE3.2) Â 10 À5 cm 3 s À1 , a factor of two smaller than the k 3 /k 2 value presented in Table 2.
Table 4 Kinetic parameters derived for syn-CH 3 CHOO and anti-CH 3 CHOO from two-component fits to the trans-but-2-ene (T2B) and cis-but-2-ene (C2B) experiments (see Fig. 5).Also relative (g) and absolute (j) yields of the two conformers Taatjes et al. 16 The most obvious candidate for an additional oxidant present to consume SO 2 is OH.OH radicals are produced in the chamber, primarily (in the absence of sunlight and NO x ) through the alkene + ozone reaction. 11However cyclohexane was added in excess at the beginning of each experiment to act as an OH scavenger, such that SO 2 reaction with OH was calculated to be r1% of the total chemical SO 2 removal in all experiments.Other potential candidates for this oxidant species include the (stabilised) vinyl hydroperoxide (VHP) intermediate, a secondary ozonide (formed through an SO 2 -SCI cyclic adduct), and dioxirane (Scheme 1).
Drozd et al. 10 have presented evidence for substantial VHP stabilisation (derived from the (CH 3 ) 2 COO CI) at pressures of a few hundred Torr, with a lifetime of the order of a few hundred milliseconds with respect to decomposition to form OH, providing scope for bimolecular reactions of this species to occur.This would be consistent with the kinetic observations presented here: a small yield in the systems with syn-SCIs, while for ethene, no VHP intermediate is available, and the standard chemistry ((R1)-(R4)) can be used to satisfactorily reproduce the observations.However, no significant SO 2 reactivity is known for the peroxide or alkene functionalities present in the closed shell VHP in isolation, hence it may be surprising if this species reacted rapidly with SO 2 , although theoretical studies have suggested that the VHP may react with H 2 O. 23 Secondary ozonide species, formed as an adduct from the SCI + SO 2 reaction, have been suggested to account for the observed isotopic exchange in alkene-ozone-SO 2 systems, 25 and are suggested to have lifetimes of seconds or longer. 19Such a secondary ozonide could react with a further SO 2 molecule (i.e. a two-component secondary ozonide catalysed oxidation route for SO 2 to SO 3 conversion), however a substantial humidity dependence to the overall process may still be anticipated (on the basis of SCI removal through the SCI + H 2 O reaction), which is not observed here.
The 'hot acid/ester channel' (rearrangement and decomposition via a dioxirane intermediate) is the dominant isomerisation route available for anti-SCIs.Although the hydroperoxide channel is the principal isomerisation route for syn-SCIs, the ester mechanism is also available, 25 and it is likely that a small proportion of the syn-CI will isomerise through this channel to form a dioxirane.Dioxiranes are known to be highly reactive and selective oxidising agents, 51,52 and have a particular affinity for sulphur compounds. 53Additional oxidation of SO 2 by the dioxirane, over and above that arising directly from reaction with the SCI, could then explain the observed behaviour of SO 2 in the TME experiments (and contribute to the behaviour observed for the but-2-ene systems).For the small CI CH 2 OO, formed in the ethene system, it has been predicted that the dioxirane formed is considerably less stable than the methyl substituted dioxiranes formed from but-2-ene and TME ozonolysis, and furthermore will decompose promptly due to chemical activation.
The possibility of non-CI products of the ozonolysis system being responsible for some of the observed SO 2 oxidation has been suggested previously as an alternative (or additional) explanation for the observed behaviour of SO 2 in the atmosphere. 1,54,55In their laboratory study Berndt et al. 19 note that their data were not perfectly described by a model in which a single (SCI related) SO 2 oxidation process was assumed, and commented that the SO 2 oxidation in ozonolysis systems may in fact be more complex.Taatjes et al. 56 have recently suggested that these data 19 are consistent with a two-component oxidation system, either two processes removing SO 2 in parallel (as described above), or through a sequential two-step SO 2 removal mechanism, such as the secondary ozonide route outlined above.However, this latter mechanism cannot (in isolation) account for the observations presented here.
The presence of an additional oxidant would have implications for the role of alkene ozonolysis in oxidation of trace gases in the atmosphere.If an oxidant is being formed that reacts slowly with H 2 O then this, perhaps in addition to SCI, may contribute to the additional (non-OH) SO 2 oxidation observed in recent field experiments. 1Further investigation of this possibility is needed.
As for CH 3 CHOO, the analysis performed in this section has implications for the determination of the SCI ((CH 3 ) 2 COO) yield.Using the value of k d derived from the linear fit to all the data shown in Fig. 7 would indicate an SCI yield of 0.33.Using the value of k d derived from the 'additional oxidant fit in Fig. 7, and taking into account that B10 ppb of the SO 2 loss in the high SO 2 experiment would be attributed to reaction with the additional oxidant rather than the SCI, determines a slightly lower j min of 0.23, and a corrected yield of 0.32, very similar to that shown in Table 1.Both of these possible alternative SCI yields lie within the uncertainties in the SCI yield from TME ozonolysis presented in Table 1.
As for CH 3 CHOO, it is not possible to quantitatively consider the contribution of the water dimer to the SCI loss given the limited data, but theory 53

Atmospheric implications
The derived values for k 3 reported in Table 3 correspond to loss rates for reaction of SCI with H 2 O in the atmosphere of 650 s À1 for CH 2 OO, B3500 s À1 for CH 3 CHOO and B1050 s À1 for (CH 3 ) 2 COO Fig. 7 Data for TME ozonolysis (as shown in Fig. 3), with fit to eqn (E10), and linear fit to the full dataset.
(assuming [H 2 O] = 5 Â 10 17 molecules per cm À3 , equivalent to an RH of 65% at 298 K).Comparing this to the derived k d values it is seen that reaction with H 2 O is predicted to be the main sink for SCI in the atmosphere, but also that loss through decomposition cannot be neglected for some SCIs -contributing on the order of 0-1% for CH 3 CHOO and 13% for (CH 3 ) 2 COO.
An estimate of a mean steady state SCI concentration in the background atmospheric boundary layer can then be calculated using eqn (E11).
Using the data given below, a steady state SCI concentration of 1.7 Â 10 3 molecules per cm À3 is estimated for an ozonolysis source (noting that other potential atmospheric sources of SCI exist, e.g.photolysis of alkyl-iodides in the marine boundary layer, and sinks, e.g.reaction with NO and NO 2 ).This assumes an ozone mixing ratio of 40 ppbv, an alkene mixing ratio of 2 ppbv, j of 0.35, and mean reaction rate constants k 1 (alkene-ozone) of 1 Â 10 À16 cm 3 s À1 ; k 2 (SCI + SO 2 ) of 3.5 Â 10 À11 cm 3 s À1 , However, in the case of CH 3 CHOO the data shown in Fig. 3, and the discussion above, indicate contributions from multiple species -syn-and anti-conformers with contrasting behaviour.It is clear that the burden of CH 3 CHOO in the atmosphere would be better described by considering these two fractions of SO 2 loss separately.Eqn (E12) expands eqn (E11) to treat the two conformers separately, where [SCI] = [anti-SCI] + [syn-SCI], making the same assumptions as for the analysis of CH 3 CHOO in eqn (E5) and (E6).k anti 3 is estimated to be 1 Â 10 À14 cm 3 s À1 (taking into account that CH 2 OO is considered as an anti-SCI in this analysis and that the derived k 3 (CH 2 OO) is more than an order of magnitude smaller than the derived k 3 (CH 3 CHOO) of 2.3 Â 10 À14 cm 3 s À1 ), k syn d is assumed to be 200 s À1 and j anti = j syn = 0.175.Additionally the anti-SCI + water dimer reaction is also considered, using a value of 5.6 Â 10 À13 cm 3 s À1 as derived for CH 2 OO in this work.
Using these values in eqn (E12) determines [anti-SCI] = 164 molecules per cm À3 and [syn-SCI] = 4.4 Â 10 3 molecules per cm À3 .The formation of an additional oxidant during alkene ozonolysis would be expected to have a similar effect to the two component contribution presented in eqn (E12) based on the apparent yields from the experiments presented here.From this analysis the atmospheric SCI burden is seen to likely be dominated by syn-SCI since this term is at least an order of magnitude greater than the anti-SCI term.
A typical diurnal loss rate of SO 2 to OH (k OH Â [OH]) is 9 Â 10 À7 s À1 , 15 while the SO 2 loss rate due to reaction with SCI, using the values derived from eqn (E12), would be 1.2 Â 10 À7 s À1 .This suggests, for the conditions and assumptions given above, the loss of SO 2 to SCI to be about 13% of loss to OH.This analysis neglects additional chemical sinks for SCI, which would reduce SCI abundance but are unlikely to be competitive with the two main SCI loss processes identified herein.SCI concentrations are expected to vary greatly depending on the local environment, e.g.alkene abundance may be considerably higher (and with a different reactive mix of alkenes) in a forested environment, compared to a rural background environment.The majority of the SCI burden, particularly in forested regions, is likely to be dominated by larger SCI derived from (C 5 ) isoprene and (C 10 ) monoterpenes.The chemistry of these species could differ greatly from the small SCI reported here (which we have found to be structure specific, even for small alkene systems), especially for tethered SCI derived from ozonolysis of internal double bonds within (for example) some monoterpenes.It is clear that the total SCI loss rate is dependent upon SCI identity and configuration, and that further work is required to quantify speciated SCI in the atmosphere, and to accurately calculate SCI concentrations for use in atmospheric modelling.

Conclusions
It has been shown that at relatively low [H 2 O] (o1 Â 10 17 cm À3 ) the loss of SO 2 in the presence of four ozone-alkene systems: ethene, cisbut-2-ene, trans-but-2-ene and 2,3-dimethyl-but-2-ene significantly decreases with increasing water vapour.This is consistent with production of a stabilised Criegee intermediate from the ozonolysis reaction and subsequent reaction of this species with SO 2 and H 2 O. Competition between H 2 O and SO 2 for reaction with the SCI leads to the observed relationship which is sensitive to water vapour abundance over a relatively narrow range of RH.Derived kinetic data for these ozonolysis systems shows that the reaction rates are dependent on the structure of the SCI.At [H 2 O] 4 1 Â 10 17 cm À3 the SO 2 loss in the presence of cis-and trans-but-2-ene, and 2,3-dimethylbut-2-ene appears to show a reduced dependence upon H 2 O.The results suggest that there is an H 2 O dependent and an H 2 O independent fraction to the observed SO 2 loss in these systems.These two fractions may be attributable to differing kinetics of the two conformers produced in but-2-ene ozonolysis or to other oxidant products of the alkene ozonolysis reaction.This observation means that SCI structure must be considered in atmospheric modelling of SCI production from alkene ozonolysis, and suggests that the atmospheric SCI burden (and hence the oxidation of trace gases) will be dominated by syn-SCI.
This work provides constraints on the behaviour of SCI formed through alkene ozonolysis under conditions relevant to the atmospheric boundary layer, but also highlights the complex nature and incomplete current understanding of the ozonolysis system.Further research is needed to definitively quantify the impact of this chemistry upon atmospheric oxidation.

Fig. 1
Fig. 1 Cumulative consumption of SO 2 and O 3 , DSO 2 versus DO 3 , for the ozonolysis of four alkenes in the presence of SO 2 at a range of relative humidities from 1.5-21%.Open symbols are experimental data, corrected for chamber dilution.Solid lines are smoothed fits to the experimental data.

Fig. 2
Fig. 2 DSO 2 vs. DO 3 during the excess SO 2 experiments, to determine the minimum SCI yield for the four alkenes.

Fig. 1
Fig.1shows the cumulative consumption of SO 2 relative to that of O 3 , DSO 2 versus DO 3 (after correction for dilution), as a function of [H 2 O] for each experiment for the four alkenes studied.A fit to each experiment, extrapolating the experimental data to evaluate dSO 2 /dO 3 at t = 0 (start of each experimental run) for use in eqn (E1)-(E3), is also shown.The overall change in SO 2 , DSO 2 , is seen to decrease substantially with increasing humidity (over a relatively narrow range of RH (1.5-20%)) for all four alkenes.This trend would be expected from the understood chemistry ((R1)-(R4)), as there is competition between SO 2 , H 2 O, and decomposition for reaction with the SCI.Fig.3shows a fit of eqn (E3) to the data for each alkene, giving a slope of k 3 /k 2 , and an intercept of (k d + L)/k 2 .The results appear to show a generally linear relationship; however, for cisand trans-but-2-ene and TME, the data point at the highest relative humidity accessible in this work ([H 2 O] = 1.5-2.0Â 10 17 cm À3 ) appears to deviate from this relationship.These data points lie outside the 95% confidence intervals defined by all the other (lower relative humidity) data for each alkene.For the analysis to determine k 3 /k 2 and (k d + L)/k 2 presented in Table2, the points at the highest RH are excluded and the kinetic parameters are derived from a linear fit to the measurements from all other experiments.Extended analyses to account for the non-linearity

3. 3
CH 3 CHOO 3.3.1 Single SCI Approach.The CH 3 CHOO data shown in Fig.

Fig. 4
Fig. 4 Application of eqn (E4) to derive rate constants for reaction of CH 2 OO with H 2 O (k 3 /k 2 ) and (H 2 O) 2 (k 5 /k 2 ) relative to that of CH 2 OO with SO 2 .Inset: eqn (E4) as shown in the main figure (red line), (E4) applied using the dimer reaction rate (k 5 ) reported by Berndt et al.45 (1.1 Â 10 À11 cm 3 s À1 ) (dashed line) and a fit of (E4) to the upper limits of the uncertainties in the ethene data (solid green line).

d { k anti 3 [
H 2 O] under the experimental conditions used.If the kinetics derived from treating the but-2-ene data in Fig. 3 as representing a single SCI are dominated by the anti-conformer then the k d derived from these kinetics, which is indistinguishable from zero within the uncertainties, suggests that k anti d is small.Taatjes et al. 16 report k anti 3 to be 1.0 (AE0.4)Â 10 À14 cm 3 s À1 , while Sheps et al. 49 report a value of 2.4 (AE0.4)Â 10 À14 cm 3 s À1 .Thus, even at the lowest [H 2 O] considered here (B1 Â 10 16 cm À3 ), loss of anti-CH 3 CHOO H 2 O would be 4100 s À1 , and decomposition negligible in comparison (including a k anti d of 50 s À1 changes the derived k anti 3 and k syn d values by o5%).Fitting eqn (E8) to the data shown in Fig. 5 derives a range of values for k anti 3 and k syn d dependent on the values of g anti and g syn used.As the CIs, once formed and thermalised, are expected to show the same kinetic behaviour in these experiments irrespective of their precursor alkene, the measurements from the cis-but-2-ene and trans-but-2-ene experiments can be used in combination to constrain k anti 3 , k syn d and also g syn and g anti from each alkene.Fig. 6 plots the k anti 3 vs.k syn d determined at different g syn and g anti from cis-but-2-ene and trans-but-2-ene.Where these two lines intercept represents the unique solution for both k anti3 and k syn d and for g syn and g anti (Table

Fig. 6 determines k anti 3 /k anti 2 to be 3 . 5 (
AE3.1) Â 10 À4 and k syn d /k syn 2 to be 1.2 (AE1.1)Â 10 13 cm À3 .The 2s uncertainties presented are, unsurprisingly, large as there are two free parameters.Using the relevant values of k 2 for the syn and anti-CH 3 CHOO conformers from Taatjes et al. 16 to place the relative rate constants on an absolute basis gives a value for k anti 3 of 2.3 (AE2.1)Â 10 À14 cm 3 s À1 and for k syn d of 288 (AE275) s À1 .This k anti

Fig. 7
Fig.7shows eqn (E10) fitted to the TME data from Fig.3.It is not possible to determine unique values for the parameters included in eqn (E10) due to the degrees of freedom vs. the limited data set.The fit shown in Fig.7uses values of j = 0.34, g SCI = 0.88, k 3 /k 2 = 6.7 Â 10 À4 and k d /k 2 = 1.2 Â 10 12 cm À3 .However, Fig.7does demonstrate that a two-oxidant system, as represented by eqn (E10), is able to describe the data within uncertainty.Fig.7alsoincludes a linear fit (i.e.eqn (E3)) to the full (CH 3 ) 2 COO dataset (including the highest RH experiment).While it seems unlikely that the curvature observed in the data is a result of measurement error (as described in Section 3.1), this must be considered as a possibility for (CH 3 ) 2 COO in light of the two conformer explanation not being applicable.The linear fit in Fig.7gives a k 3 /k 2 value of 3.8 (AE3.2) Â 10 À5 cm 3 s À1 , a factor of two smaller than the k 3 /k 2 value presented in Table2.

Table 3
Comparison of SCI relative rate constants derived in this work to relative and absolute values from the literature.Uncertainty ranges (AE2s) indicate combined precision and systematic measurement error components SCI 10 5 k 3 /k 2 10 15 k 3 (cm 3 s À1 ) 1 0 À11 k d /k 2 (cm À3 ) k d (s À1 ) The contribution of (H 2 O) 2 to the removal of CH 2 OO increases in relation to that of H 2 O as [H 2 O] increases.Hence at typical atmospheric [H 2 3 CHOO 10 4 k 3 /k 2 10 14 k 3 (cm 3 s À1 ) 10 À13 k d /k 2 (cm À3 ) k d (s À1 ) predicts k(H 2 O) 2 /k(H 2 O) to be three orders of magnitude smaller than that for CH 2 OO suggesting that reaction with the water dimer would be unimportant for (CH 3 ) 2 COO at typical atmospheric boundary layer [H 2 O].