Unimolecular β-Hydroxyperoxy Radical Decomposition with OH

Nov 30, 2009 - PROPHET and AEROBIC97 campaigns, the discrepancy between observed and modeled OH concentrations became more pronounced with ...
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Environ. Sci. Technol. 2010, 44, 250–256

Unimolecular β-Hydroxyperoxy Radical Decomposition with OH Recycling in the Photochemical Oxidation of Isoprene GABRIEL DA SILVA,* CLAIRE GRAHAM, AND ZHE-FEI WANG Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville 3010, Victoria, Australia

Received March 27, 2009. Revised manuscript received September 23, 2009. Accepted November 8, 2009.

A novel process in the photochemical oxidation of isoprene that recycles hydroxyl (OH) radicals has been identified using firstprinciples computational chemistry. Isoprene is the dominant biogenic volatile organic compound (VOC), and its oxidation controls chemistry in the forest boundary layer and is also thought to contribute to cloud formation in marine environments. The mechanism described here involves rapid unimolecular decomposition of the two major peroxy radicals (β-hydroxyperoxy radicals) produced by OH-initiated isoprene oxidation. Peroxy radicals are well-known as key intermediates in VOC oxidation, but up to now were only thought to be destroyed in bimolecular reactions. The process described here leads to OH recycling with up to around 60% efficiency in environments with low levels of peroxy radicals and NOx. In forested environments reaction of the β-hydroxyperoxy radicals with HO2 is expected to dominate, with a small contribution from the mechanism described here. Peroxy radical decomposition will be more important in the unpolluted marine boundary layer, where lower levels of NO and HO2 are encountered.

Introduction Chemistry in the lower atmosphere is controlled by the highly reactive hydroxyl free radical (OH). In daylight hours OH forms by the reaction of water vapor with excited oxygen atoms produced in ozone photolysis. While OH radicals are short-lived, their constant generation in humid sun-lit conditions keeps them present at globally averaged concentrations of around 1 × 106 molecules cm-3 (∼0.05 pptV). Volatile organic compounds (VOCs) in the atmosphere are primarily removed by initial reaction with OH. Large anthropogenic VOC burdens are generally expected to deplete OH levels and thus diminish the oxidation capacity of the troposphere. Isoprene (2-methyl-1,3-butadiene) constitutes the second largest hydrocarbon emission to the atmosphere, behind methane. Isoprene is emitted by biogenic and other sources, and is rapidly oxidized to a range of products including methyl vinyl ketone (MVK), methacrolein (MACR), and formaldehyde (HCHO) (1-3). As well as producing these ozone-forming carbonyls, isoprene oxidation consumes OH radicals, oxidizes NO to NO2 (another ozone precursor), and also leads to secondary organic aerosol (SOA) formation (4-6). Isoprene oxidation governs significant parts of tropospheric chemistry * Corrresponding author e-mail: [email protected]. 250

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(7) due to its ubiquitous nature and high reactivity toward OH, and is particularly important above pristine rainforests in daytime hours, where isoprene fluxes are large and OH concentrations are relatively high. Isoprene is also emitted from the oceans (8, 9), where NOx, HO2, and RO2 levels are typically low. Marine isoprene is linked to phytoplankton blooms, and it has been proposed that aerosols formed in the photooxidation of this isoprene affects cloud formation above the oceans (10). Kinetic models for the photooxidation of isoprene (11-13) and other important VOCs are widely used to model important atmospheric phenomena, and the availability of accurate, realistic models is vital to the success of these efforts. According to our current understanding (14), VOCs react with OH to form organic free radicals (R•), either by abstraction or addition pathways. Organic radicals rapidly associate with O2 to form alkylperoxy radicals (RO2) which then undergo bimolecular reactions with NO, HO2, and with other RO2 radicals, ultimately forming a range of oxygenated organics. In isoprene oxidation, OH addition occurs at an unsaturated carbon atom, forming four alkyl-hydroxy radicals. The subsequent addition of O2 produces six alkylhydroxyperoxy radicals which react with NO and other species to form oxidation products that include MVK and MACR. In addition to OH-initiated reactions, isoprene reacts with ozone, NO3, and halides, but at significantly reduced rates (13). This study is concerned with the two major alkylhydroxyperoxy radicals formed in isoprene oxidation, termed ISOP-OH-OO I and II. These are structurally similar β-hydroxyperoxy radicals, with the methyl group substituted at a different carbon atom. Experiment (15) and theory (16) shows these radicals to account for around two-thirds of the total peroxy radical yield in OH-initiated isoprene oxidation, with 41% as ISOP-OH-OO I and 23% as ISOP-OH-OO II. Field studies conducted over the past decade point to a major shortfall in our current understanding of VOC oxidation chemistry in pristine environments. Measured OH concentrations in forested, low-NOx environments are consistently greater than predicted by atmospheric chemistry models, and the deviation between measured and predicted concentrations correlates with isoprene levels. Conversely, OH levels are generally well-predicted above the planetary boundary layer and in areas affected by polluted urban air. Higher than predicted OH concentrations in forested regions have been reported from the PROPHET (17), AEROBIC97 (18), and LBA-CLAIRE-2001 (19) field campaigns. In the PROPHET and AEROBIC97 campaigns, the discrepancy between observed and modeled OH concentrations became more pronounced with decreasing levels of NO [in the PROHET campaign, the median measured NO value was 65.8 pptV. NO peaked in the early morning (median value of ∼140 pptV), dropping to around 30-40 pptV in the afternoon. Measured OH and isoprene values respectively peaked at around midday and in the mid-afternoon. With the AEROBIC97 campaign NO levels were consistently at or below the detection limit of 100 pptV]. In 2008 the INTEX-A campaign reported that OH concentrations were systematically underpredicted in the continental boundary layer over North America (20). Areas of increased OH corresponded to forested regions with high isoprene emissions, and the ratio of observed to modeled OH increased sharply with increasing isoprene levels above 500 pptV. Higher than expected OH levels in forested regions have now been definitively confirmed by direct, airborne OH measurements in the forest boundary layer over the Amazon rainforest in Guyana, Guyane, and Suriname (the GABRIEL campaign) (21, 22). It 10.1021/es900924d

 2010 American Chemical Society

Published on Web 11/30/2009

was proposed that low-NOx oxidation of isoprene naturally recycles OH radicals by some as-yet unknown mechanism, and in order to reproduce field observations this mechanism needs to recycle 40-80% of the OH consumed in isoprene oxidation (21, 22). A subsequent report of a field campaign in China’s Pearl River Delta found that an OH source of around 30 ppbV/hour was required to explain high OH levels under low-NOx conditions, and that OH was being generated in a process that avoids the production of HO2 (23). Using first-principles quantum chemistry we have discovered a reaction in the OH-initiated oxidation of isoprene that recycles OH. This process involves the unimolecular thermal decomposition of the β-hydroxyperoxy radicals ISOP-OH-OO I and II. In the combustion of olefins, addition of OH and then O2 is known to produce aldehydes +OH via the Waddington mechanism (24). While such decomposition reactions of peroxy radicals are important in low-temperature combustion phenomena, they are not considered in atmospheric chemistry because of the large energy barriers typically involved. It has been shown by Vereecken and Peeters (25) that unimolecular reactions of peroxy radicals formed in isoprene and monoterpene oxidation can proceed with barriers low enough to be competitive with bimolecular reactions at ambient temperatures, similar to ring-closing reactions that are known to take place in the atmospheric oxidation of aromatic hydrocarbons (26). However, up to now, unimolecular peroxy radical reactions leading to new dissociated products have, to our knowledge, been excluded from atmospheric chemistry.

Computational Methods Molecular structures were located with the B3LYP/631G(2df,p) density functional theory (DFT) method. Lowest energy conformations were ensured by systematic internal rotor scans at the B3LYP/6-31G(d) level. Vibrational frequency calculations on the lowest-energy conformers confirmed them to be minima or transition states by virtue of their number of imaginary vibrational frequencies. Intrinsic reaction coordinate scans were also conducted to confirm transition state reactants and products. Composite G3SX (27) and CBS-QB3 (28) calculations were performed to obtain accurate molecular enthalpies. Additional DFT calculations are also reported, to help verify activation and reaction enthalpies. The M05 (29), BMK (30), and BB1K (31) DFT methods were selected, which are all broadly accurate for kinetics and thermochemistry. Geometries were optimized with each method using the 6-31+G(d) basis set, followed by single-point energy calculations with the large aug-ccpVTZ basis set. All calculations were performed using the Gaussian 03 package (32). For all energies in the G3/99 test set the G3SX method is accurate to 0.95 kcal mol-1, whereas for the smaller G2 test set, the CBS-QB3 method is accurate to 0.87 kcal mol-1 (mean absolute deviations). The two respective composite model chemistries are accurate to 0.57 and 1.62 kcal mol-1 for the DBH24/08 database of barrier heights (33). The M05, BMK, and BB1K DFT methods reproduce these barrier heights with mean unsigned errors of 2.43, 1.22, and 1.20 kcal mol-1 (with the MG3S augmented triple-zeta split valence basis set). The use of theoretical methods accurate for both thermochemistry and kinetics is important in this study due to the small height of the transition state above the product energies. Average G3SX/CBS-QB3 barrier heights and reaction enthalpies are used in our rate constant calculations, and these numbers are expected to be accurate to (2 kcal mol-1 (95% confidence limits). This uncertainty estimate is supported by the results of the DFT calculations (vide infra). Standard enthalpies of formation (∆fH°298, kcal mol-1) are reported for all species and transition states at the G3SX and

CBS-QB3 levels, from atomization calculations. Entropies and heat capacities (S°298 and Cp(T), cal mol-1 K-1) are evaluated from statistical mechanical principles, using the B3LYP/6-31G(2df,p) level moments of inertia and vibrational frequencies. Vibrational modes are treated using the rigidrotor-harmonic-oscillator (RRHO) approximation. Elementary rate constants in the high-pressure limit are calculated from canonical transition state theory. Rate constants were corrected for quantum mechanical tunnelling through the potential energy surface via Eckart theory (34), based upon the 0 K forward and reverse barrier heights and the imaginary vibrational frequency of the transition state. Rate constants between 250 and 350 K were fit to the Arrhenius equation, providing the pre-exponential factor A (s-1) and the activation energy Ea (kcal mol-1). All statistical mechanics and transition state theory calculations were performed using ChemRate (35). This contribution is principally concerned with the elementary hydroxy to peroxy hydrogen-shift reactions in the lowest-energy conformations of the isoprene β-hydroxyperoxy radicals. In reality, higher-energy conformations of the peroxy radicals will be populated to some extent (likely at equilibrium), affecting the apparent rate of peroxy radical decomposition. A further complication is the competing dissociation to a hydroxyalkyl radical plus O2, which may act to deplete vibrationally hot peroxy radicals.

Results Structures for all stationary points reported in this study are depicted in Figure 1, including critical interatomic distances in Å (coordinates are available as Supporting Information (SI)). Standard enthalpies of formation have been calculated for all C5O3H9 isomers, transition states, and decomposition products using the G3SX and CBS-QB3 methods, and are listed in the SI. The G3SX and CBS-QB3 enthalpies are in good agreement for all stable species (mean absolute deviation of 0.5 kcal mol-1). The G3SX transition state enthalpies, however, are consistently 1-2 kcal mol-1 above those calculated at the CBS-QB3 level. Calculated heats of formation also agree with literature values, where available (∆fH°298 ) -27.4 (36), -25.43 (37), -26.05 (38), and 8.91 (39) kcal mol-1 for MVK, MACR, HCHO, and OH, respectively).

Discussion Decomposition Mechanism and Kinetics. The two major peroxy radical isomers formed in the photochemical oxidation of isoprene are found to decompose to OH plus HCHO and one of either MVK or MACR. Energy diagrams for the proposed reaction mechanisms are depicted in Figure 2, using average G3SX and CBS-QB3 reaction enthalpies (relative to the reactants). Both peroxy radicals undergo low-energy intramolecular hydrogen shifts from the hydroxyl group to the peroxy radical site, via six-membered ring transition states (TS1 and TS3). Intrinsic reaction coordinate scans reveal that these transition states lead to the formation of weakly bound radical intermediates (denoted MVK-HCHO-OH and MACR-HCHO-OH) at the B3LYP/6-31G(2df,p) level of theory. The two intermediates lie just below the transition states in energy, and are accordingly similar in structure. Both reaction intermediates dissociate with essentially no barrier (TS2 and TS4), recycling OH and forming HCHO + MVK/MACR. The calculated barrier heights for hydroxy to peroxy hydrogen transfer are very low for an intramolecular hydrogen abstraction, and it is this unexpected result that makes these reactions possible at ambient temperatures. A series of DFT calculations have been performed to further investigate the magnitude of these barrier heights. Activation and reaction enthalpies calculated using the M05, BMK, and BB1K DFT methods are reported in Table 1, compared to the G3SX and VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Structures for stationary points in the ISOP-OH-OO I and II unimolecular decomposition mechanisms, at the B3LYP/ 6-31G(2df,p) level.

FIGURE 2. Energy diagrams for unimolecular decomposition of the ISOP-OH-OO I and II radicals. Values are the average of G3SX and CBS-QB3 298 K enthalpies.

TABLE 1. Activation Enthalpies (∆‡H°298) and Reaction Enthalpies (∆rxnH°298) for β-Hydroxyperoxy Radical Decomposition (kcal mol-1) G3SX CBS-QB3 M05a BMKa BB1Ka ISOP-OH-OO I ∆‡H°298 ∆rxnH°298 ISOP-OH-OO II ∆‡H°298 ∆rxnH°298

21.8 20.8 21.7 20.4

20.8 20.3 20.1 19.5

19.7 20.4 18.6 19.2

20.3 20.0 19.2 18.5

21.6 21.4 21.0 20.2

a Single-point aug-cc-pVTZ energies using 6-31+G(d) optimized structures and enthalpy corrections.

CBS-QB3 results. The BB1K activation enthalpies are similar to the G3SX results, whereas the M05 and BMK values are all somewhat smaller than the CBS-QB3 numbers. The DFT results are generally similar to (BB1K) or smaller than (M05, BMK) the average G3SX/CBS-QB3 numbers, but fall within the estimated 2 kcal mol-1 uncertainty. Formation of a weakly bound reaction intermediate from the isoprene β-hydroxyperoxy radicals appears to facilitate their decomposition. To our knowledge such intermediates are unreported in the literature, and are seemingly stabilized by the presence of a vinyl group adjacent to the peroxy group [similar structures also occur when the vinyl group is adjacent to the hydroxyl group (i.e., when the OH and OO groups are interchanged), as in two of the relatively unimportant isoprene-OH-OO isomers]. When the vinyl group is 252

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not present, as in the β-hydroxyethylperoxy radical (CH2(OH)CH2OO•), the intramolecular hydrogen shift requires a larger barrier (25-35 kcal mol-1) (40-44), and is reported to lead to the formation of a conventional hydroperoxyalkoxyl radical (CH2(O•)CH2OOH). This is despite the β-hydroxyethylperoxy radical being relatively similar in structure to the ISOP-OH-OO I and II radicals (43). The proposed products of β-hydroxyperoxy radical decomposition are consistent with those observed in isoprene photooxidation experiments in the absence of NO. OHinitiated isoprene oxidation has been studied in systems devoid of NO by several groups (45-48), and the major products are universally seen as HCHO, MVK, and MACR. Generally, MVK and MACR yields are similar, and sum to about the HCHO concentration, suggesting a mechanism that forms MVK + HCHO and MACR + HCHO in similar quantities. A recent environmental chamber study (49) has found evidence for a prompt isoprene photooxidation pathway that forms MACR/MVK along with OH, again supporting our hypothesized reaction process. Additionally, the mechanism described here can potentially explain OH regeneration with avoided formation of HO2 (23), since the direct formation of MACR/MVK + HCHO + OH from isoprene + OH + O2 bypasses the HO2-generating oxyl radical intermediates common to conventional isoprene oxidation mechanisms. Rate constants (k, s-1) have been calculated for decomposition of the two isoprene-derived β-hydroxyperoxy radicals, from transition state theory. Quantum mechanical tunnelling corrections have been applied using Eckart theory, although they are relatively small due to the near-absence of an intrinsic reaction barrier; inclusion of tunnelling increases the 300 K rate constant for decomposition of isomer I from 3.9 × 10-4 to 5.8 × 10-4 s-1 [If TS1 and TS3 bypass the proposed products MACR-HCHO-OH and MVKHCHO-OH (as suggested by the CBS-QB3 energies of TS2 and TS4), leading straight to the dissociated products, then quantum tunnelling significantly increases the rate of peroxy radical decomposition, particularly at lower temperatures]. For the rate constant calculations the forward dissociation reaction of the complex intermediate to products was assumed to be instantaneous, as it proceeds in both cases with a barrier below that for formation of the intermediate. For ISOP-OH-OO I the decomposition rate constant is given by the expression k ) 2.38 × 1012e(-21400(2000)/1.987T s-1, whereas for ISOP-OH-OO II k ) 1.27 × 1012e(-21000(2000)/1.987T

TABLE 2. Lifetimes for Thermal Decomposition of the Two β-Hydroxyperoxy Radicals That Form in OH-Initiated Isoprene Oxidation lifetime (s) 290 K 300 K 310 K ISOP-OH-OO I f MVK + HCHO + OH 6 000 1 700 ISOP-OH-OO II f MACR + HCHO + OH 5 000 1 500

540 480

s-1 (calculated and fitted rate constants are listed in the SI). Peroxy radical lifetimes (1/k, s) at temperatures of 290, 300, and 310 K are listed in Table 2. The rate of decomposition is strongly temperature-dependent, due to the exponential effect of the energy barrier; the peroxy radical lifetimes decrease by around an order of magnitude from 290 to 310 K. Daytime air temperatures above tropical rainforests average around 310 K, where the β-hydroxyperoxy radicals decompose with lifetimes on the order of several minutes. In colder climates peroxy radical decomposition will be slower, and this mechanism could exhibit a seasonal effect. Since this work was submitted, rapid intramolecular hydrogen shift reactions in the isoprene β-hydroxyperoxy radicals have also been reported by another group (50). This new study calculates β-hydroxyperoxy radical lifetimes that are similar to, but somewhat smaller than, those calculated here (ca. 100-200 s at 303 K). This is likely due to the relatively small tunnelling effect incorporated into our calculations [see note on TS1 and TS3 bypass above]. This new study also found that intramolecular hydrogen shifts can take place with low barriers in the isoprene δ-hydroxyperoxy radicals (from the Z conformations), in an overall process that consumes OH, generating HO2 along with a hydroperoxide. While these δ-hydroxyperoxy radicals are far less populous than the β ones, it was proposed that rapid peroxy radical interconversion favors the δ channel. Recent chamber studies, however, fail to detect large quantities of the proposed δ-hydroxyperoxy reaction products, while they do detect prompt formation of MACR/MVK along with OH (49). Furthermore, the δ-hydroxyperoxy radical chemistry achieves the opposite effect of that observed in the field, where OH radicals are generated at the expense of HO2 (23). Further work is required to accurately characterize the kinetics of peroxy radical interconversion, while consideration must also be paid to the possibility that repeated formation and dissociation of the β-hydroxyperoxy radicals could increase the yield of MACR/MVK + HCHO + OH via a chemically activated mechanism, given the similar energy levels of the entrance and exit channels. Atmospheric Importance. In the atmosphere, the peroxy radical decomposition reactions identified here are in direct competition with bimolecular reactions, particularly with NO, HO2, and other RO2 radicals. These reactions are second order, and peroxy radical lifetimes will depend on both radical concentrations and reaction rate constants. The rate constant for reaction of the isoprene β-hydroxyperoxy radicals with NO is taken as 9 × 10-12 cm3 molecule-1 s-1 (51, 52), assuming that all isoprene peroxy radicals are equally reactive. Making a similar assumption, the rate constant for reaction of the β-hydroxyperoxy radicals with HO2 is set as 1.74 × 10-11 cm3 molecule-1 s-1 (53) [rate constants for reaction of the ISOP-OH-OO radicals with NO and HO2 are at 298 K. Both reactions demonstrate a small negative temperature dependence, which is ignored in this study]. Given these rate constants, competition between unimolecular and bimolecular reactions of the ISOP-OH-OO radicals are considered below, for typical unpolluted conditions in forest and marine environments. There is more uncertainty regarding the peroxy radical reactions with other RO2 radicals, both in terms of rate constants and typical RO2 concentrations. Rate

FIGURE 3. Branching ratio to unimolecular decomposition versus bimolecular reaction with NO and HO2, for the isoprene β-hydroxyperoxy radicals at 310 K. Shaded areas represent uncertainty. constants for reaction of the isoprene peroxy radicals with RO2 are expected to be less than for their reaction with HO2, however, total RO2 concentrations often exceed HO2 levels in the forest boundary layer. We do not consider these reactions further, but estimate that they will be of similar or lesser importance than reaction with HO2. Branching ratios to β-hydroxyperoxy radical decomposition are plotted in Figure 3 as a function of NO and HO2 concentration, at 310 K. Similar results are available in the SI for other temperatures. Peroxy radical decomposition is predicted to be the dominant mechanism at concentrations of around 9 pptV (∼2 × 108 molecules cm-3) NO and below, and 4-5 pptV (∼1 × 108 molecules cm-3) HO2 and below. Uncertainties in the branching ratios are relatively large, due to the exponential effect of the 2 kcal mol-1 uncertainty in the activation energies. Typical NOx (NO + NO2) concentrations in remote forested environments are on the order of 10-100s of pptV, and in the forest boundary layer NO is significantly oxidized to NO2 by O3 and peroxy radicals. The GABRIEL campaign measured NO concentrations of 20 ( 20 pptV in the forest boundary layer (21), and other field studies in which OH recycling has been observed report similar concentrations. NO levels generally peak at midmorning, and steadily drop into the afternoon as sunlight decreases. At 20 pptV (4.9 × 108 molecules cm-3) the β-hydroxyperoxy radical lifetimes for reaction with NO are 227 s. At 310 K thermal decomposition lifetimes are 540 and 480 s, which results in 31% of the combined peroxy radical population undergoing OH-recycling unimolecular decomposition (ignoring reaction with HO2 and RO2). Uncertainty in the decomposition rate constants places the peroxy radical lifetimes anywhere from 4 h to 20 s, and this uncertainty VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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clearly needs to be refined in order to appreciate the potential impact of the proposed mechanism. Further field studies providing accurate spatial and temporal NO levels throughout the forest boundary layer will also be of considerable value. In marine environments, outside of significant pollution events, NO levels are much lower than those found above forests. Concentrations of NO in the range of 1-10 pptV are common (for example, see ref 54, corresponding to peroxy radical lifetimes of around 8-75 min. Unimolecular decomposition of the isoprene β-hydroxyperoxy radicals is therefore expected to be of importance in the marine boundary layer, relative to reaction with NO. In the unpolluted free troposphere, daytime HO2 levels are typically around 10 pptV (2.5 × 108 molecules cm-3) (17, 55). Comparatively, total peroxy radical concentrations (HO2 + RO2) are often found to be in the range of 10-100 pptV (55-57). At these conditions the isoprene β-hydroxyperoxy radical lifetimes toward HO2 are on the order of 200-300 s. Similar HO2 levels are encountered in marine environments, and coupled with single-digit pptV levels of NO, β-hydroxyperoxy radical decomposition is expected to play a role in the photooxidation of marine isoprene. Isoprene is released into the marine boundary layer in locally large quantities by blooms of phytoplankton (8, 9). Recycling of OH in the oxidation of marine isoprene, via the mechanism described here or otherwise, should facilitate rapid oxidation. Further processing of the isoprene photooxidation products in the absence of NOx yields SOA in large yields (5), and it has been proposed that these aerosols act as cloud condensation nuclei, facilitating cloud formation above the oceans (10). Within the forest boundary layer HO2 levels can be significantly higher than in the free troposphere, leading to reduced peroxy radical lifetimes. In the GABRIEL campaign mean daytime HO2 levels were (1.05 ( 0.27) × 109 molecules cm-3 in the forest boundary layer, compared to around 5 × 108 molecules cm-3 in the free troposphere, and this leads to forest boundary layer peroxy radical lifetimes toward HO2 of only 57 s. Under these conditions peroxy radical decomposition is predicted to be responsible for 10% of β-hydroxyperoxy radical loss, recycling 7% of the OH consumed in isoprene oxidation. At these conditions total peroxy radical levels are also expected to be relatively high, and self- and cross-reactions of the β-hydroxyperoxy radicals may further diminish the importance of their decomposition reactions. During the AEROBIC97 campaign Carslaw et al. (18) measured mean daytime HO2 concentrations of around 5 × 108 molecules cm-3 on two of the days under observation, consistent with HO2 levels calculated using a photochemical box model, and typical of values reported elsewhere. Under these conditions we predict that 17% of the β-hydroxyperoxy radicals are consumed via unimolecular decomposition (ignoring reaction with NO). However, on two other days of the campaign HO2 levels anomalously dropped to about 7 × 107 molecules cm-3, and under these conditions peroxy radical decomposition is expected to be more important than reaction with HO2. On these two days of study NO levels were reported to be 93 and 97 pptV; this would result in reaction with NO being the dominant channel for peroxy radical removal, although the uncertainty in these NO measurements is considerable since they are below the 100 pptV detection limit. Again, accurate experimental rate constants for peroxy radical decomposition are required to better ascertain the importance of this mechanism relative to reaction with HO2, although our current estimates indicate that it will have some importance at HO2 concentrations typically encountered in the forest boundary layer. Furthermore, similar mechanisms operating in other VOCs may lead to additional OH recycling. 254

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The proposed mechanism is expected to be most relevant in the early afternoon, when NO and HO2 concentrations decrease from their morning peaks but temperatures remain high, facilitating isoprene emission and peroxy radical decomposition. The present study has focused solely on isoprene, the major biogenic VOC, but the OH-recycling oxidation mechanism identified here is expected to be important for other VOCs. Particularly, this mechanism should occur in the oxidation of VOCs containing a conjugated diene structure. The conjugated dienes 1,3-butadiene and 2,4hexadiene are important VOCs, emitted by biogenic and anthropogenic sources. Several biogenic terpenes, such as ocimene and myrcene, also possess conjugated diene structures.

Acknowledgments Computational resources provided in part by the Victorian Partnership for Advanced Computing (VPAC).

Note Added in Proof The work of Frost et al. (58) on the near-IR photolysis of organic peroxy radicals has recently come to our attention. Efficient near-IR photolysis of the isoprene β-hydroxyperoxy radicals via the low-energy exit channels identified here provides a mechanism for OH recycling in the unpolluted forest boundary layer and could explain why this phenomenon has been identified in the field but not in the laboratory.

Supporting Information Available Structures, energies, and enthalpies of formation for all reported stationary points. Calculated and fitted rate constants as a function of temperature. Branching ratios to peroxy radical decomposition as a function of temperature, [NO], and [HO2]. This material is available free of charge via the Internet at http://pubs.acs.org.

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