Mechanistic studies of the atmospheric oxidation of methyl butenol by OH radicals, ozone and NO3 radicals

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Atmospheric Environment Vol. 32, No. 20, pp. 3547—3556, 1998 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1352–2310(98)00061–2 1352—2310/98 $19.00#0.00

MECHANISTIC STUDIES OF THE ATMOSPHERIC OXIDATION OF METHYL BUTENOL BY OH RADICALS, OZONE AND NO3 RADICALS G. FANTECHI,*-° N. R. JENSEN,- J. HJORTH- and J. PEETERS‡ -The European Commission, Joint Research Centre, TP 272, Environment Institute, Atmospheric Processes Unit, 21020 Ispra (VA), Italy; and ‡Katholieke Universiteit Leuven, Department of Chemistry, Celestijnenlaan 200F, 3001 Heverlee, Belgium (First received 23 September 1997 and in final form 25 January 1998. Published July 1998) Abstract—In a recent field study, strong indications have been obtained that 2-methyl-3-buten-2-ol (methyl butenol, MBO) is a compound emitted in important quantities by some types of vegetation. The atmospheric oxidation products from MBO are not yet well known. In this investigation we studied the reaction mechanisms and products of the reaction of MBO with OH radicals, O and NO radicals. All the 3 experiments were performed in a 480 l Teflon coated Pyrex glass chamber equipped with3a long path length FTIR spectrometer. As products from the reaction between MBO and OH we identified and quantified acetone, glycolaldehyde, formaldehyde, formic acid, CO and CO . From the reaction between MBO and O , the products acetone, formaldehyde, formic acid, CO and CO2 have been identified; also, 2-hydroxy-23 2 on the basis of HPLC and GC-MS methyl-propanal (HMPR) is tentatively put forward as a product analysis of the DNPH derivative. Organic nitrates, peroxynitrates and carbonyl nitrates, together with acetone were observed as products of the reaction between MBO and the nitrate radical. Tentative reaction mechanisms for the oxidation of MBO by OH radicals, O and NO radicals are proposed. ( 1998 3 3 Elsevier Science Ltd. All rights reserved Key word index: 2-methyl-3-buten-2-ol, oxidation, troposphere, acetone, 2-hydroxy-2-methyl-propanal.

INTRODUCTION

MBO (2-methyl-3-buten-2-ol), a C unsaturated alcohol 5 structurally similar to isoprene (2-methyl-1,3-butadiene), has been found at first in emissions from bore holes made by the Ips typographus, the spruce bark beetle, in attacking spruce trees (Birgersson and Bergstro( m, 1989; Lanne et al., 1989). It has been observed also in samples taken from enclosures placed around Lollobly pine branches (Guenther et al., 1990; Zimmerman et al., 1991), but no quantitative analysis had been attempted. Recently, it has been found in samples taken from remote areas such as the foot of Mount Everest and as Spitzbergen Island (Norway) (Ciccioli et al., 1994), but only in trace quantities. In a recent study by Goldan et al. (1993), carried out at Niwot Ridge (Colorado, U.S.), MBO was found to be present in the atmosphere in high abundance. Niwot Ridge is a remote mountain site at 3050 m altitude, covered predominantly by a lodge pole pine forest with some aspen and occasionally some Colora-

*To whom correspondence should be addressed. E-mail: [email protected]. °Also affiliated with Katholieke ºniversiteit ¸euven, Belgium.

do blue spruce. MBO was observed during daylight hours at mixing ratios five to eight times that of isoprene. These high concentrations and the strong similarity between the diurnal trend of MBO and isoprene strongly suggest that MBO is likewise a biogenic VOC, emitted in high quantities by vegetation of the type on the Niwot site. Yet, in other recent studies (Arey et al., 1991; Koenig et al., 1995), several common types of vegetation were found not to emit MBO, or to do so only in small amounts. Therefore, MBO emission is probably specific to only some types of trees. Isoprene together with several monoterpenes such as a- and b-pinene and d-limonene, which are emitted into the atmosphere from vegetative sources, are important examples of biogenic VOCs. These biogenic emissions can dominate over anthropogenic ones on regional and global scales. Recent estimates for example indicate that isoprene accounts for about 80% of the hydrocarbons emitted from deciduous forests and current global emission estimates are as high as 570 Mt yr~1 (Guenther et al., 1995), compared to anthropogenic NMHC emissions of 110 Mt yr~1 (Hough et al., 1991). Generally, VOCs emitted from vegetation into the atmosphere are removed by reaction with atmospheric oxidants such as OH radicals, ozone and

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NO . During daylight hours, reactions with OH rad3 icals and ozone are usually the most important removal processes. On the other hand, the degradation of VOCs in the night-time atmosphere is mainly due to the nitrate radical NO , which is formed by the 3 reaction of NO with O . This radical, due to its 2 3 strong absorption coefficient in the visible, is rapidly photolysed in daylight, the product channels being NO#O or NO #O, but at night it builds up and 2 2 establishes the following equilibrium: NO #NO H M N O . (1) 2 3 2 5 During night-time O is also present and therefore 3 can compete with NO radicals in the reactions with 3 unsaturated compounds. But although the concentrations of NO are always much lower than those of O , 3 3 NO reacts with most substances so much faster than 3 its precursor O that the rate of removal by NO is in 3 3 any case greater than that by O . 3 As for isoprene, we expect methyl butenol to react with the main atmospheric oxidants and to lead to the formation of carbonyl compounds. To date, three studies of the reaction of MBO with OH radicals and ozone (Rudich et al., 1995; Grosjean and Grosjean, 1994, 1995a), and two kinetic studies of the reaction of MBO with NO radicals (Hallquist et al., 1996; 3 Rudich et al., 1996) have been reported, but the mechanisms are not adequately understood. MBO reacts rapidly at daytime with the OH radical and reported values of the rate constant k are in the OH range 2.8—5.5]10~11 cm3 molecule~1 s~1 (Grosjean and Grosjean, 1994; Rudich et al., 1996) with a lifetime of 5 h using an OH concentration of 0.04 ppt (Hallquist et al., 1996). The reaction of MBO with O is relatively 3 slow (k "10.0]10~18 cm3 molecule~1 s~1) and the O3 lifetime of MBO derived with an O mixing ratio of 3 40 ppb becomes as long as 30 h. The rate constant of the reaction between MBO and the NO radical has been 3 determined to be in the range 1.2—2.1]10~14 cm3 molecule~1 s~1 which gives a lifetime of 13 h, calculated for 40 ppt of NO (Hallquist et al., 1996). 3 In the present study we aimed to identify the carbonyl products of the oxidation of MBO under simulated atmospheric conditions. The present work is a study of the oxidation of MBO by OH radicals, O and NO 3 3 radicals, in order to achieve a better understanding of the reaction mechanisms. We tentatively propose oxidation pathways which explain the observed formation of products such as acetone, 2-hydroxy-2-methyl-propanal, glycolaldehyde, formaldehyde, formic acid, CO and CO in the case of oxidation by OH and ozone, 2 and products such as organic nitrates, peroxynitrates and carbonyl nitrates, together with acetone in the case of the reaction with NO . 3

EXPERIMENTAL

The experiments were performed in a 480 l evacuable cylindrical Teflon-coated chamber. A multiple reflection

White-type mirror system was included in the reactor, adjusted to give a total optical pathlength of 81 m, and coupled to a Bruker IFS 113 V FT spectrometer for on-line Fourier Transform Infrared Spectroscopy (FTIR). The spectra were obtained by co-adding 50 scans recorded at 1 cm~1 instrumental resolution. Reactants were mixed in purified air at 295$2 K and 740$2 Torr total pressure. OH radicals were formed by the in situ photolysis of hydrogen peroxide, with irradiation provided by a set of three 253.7 nm mercury lamps. Ozone was externally produced by passing pure O through a Sander ozonisator and then added to the 2 compounds in the dark. NO levels in the smog chamber x ranged from 10 to 100 ppbV. In a typical experiment, the initial concentrations of MBO, H O and O were, respec2 2 3 tively, 8—11, 50 and 15—30 ppmV. H O was introduced into the reaction chamber by using 2 2 a double-filter system in a Teflon tube: a known amount of H O (about 50 kl) was placed on the first filter and the 2 2 compound was then introduced into the reaction chamber by a stream of purified air. The reaction mixture was subsequently irradiated for up to 2 h and the reactants and products monitored by FTIR spectroscopy. In some of the ozonolysis experiments cyclohexane or methylcyclohexane were also added to the reaction mixture in order to scavenge the OH radicals that may possibly be formed during the process. The nitrate radical NO was generated in situ from N O 3 2 5 N O H M NO #NO (2) 2 5 2 3 and N O was generated in the chamber by mixing O and 2 5 3 NO , according to the reaction 2 O #NO PNO #O (3) 3 2 3 2 and the reverse of reaction (2). Initial mixing ratios of MBO and N O were in the range 2 5 10—14 ppmV. The product spectra were analysed by successively subtracting the absorptions of the known compounds using calibrated reference spectra, which were obtained by introducing known amounts of authentic samples (2-methyl3-buten-2-ol 98% pure, Aldrich, and acetone 99% pure, Carlo Erba) in the 480 l reaction chamber. Integrated band intensities (cm]molecule~1) of relevant bands for MBO (2829—3140 cm~1), acetone (1175—1261 cm~1), CO (2022— 2246 cm~1) and CO (2283—2391 cm~1) were determined in 2 this work to be 1.70]10~17, 1.15]10~17, 4.84]10~18 and 2.02]10~21, respectively; those for CH O, HCOOH, 2 glycolaldehyde, HNO , N O , NO and O , were taken 3 2 5 2 3 from the literature (Cantrell et al., 1985; Hjorth et al., 1988; E. C. Tuazon-private communications; Hjorth et al., 1987; Jensen et al., 1991). The yields on a molar basis of the primary products of reaction were derived from the initial slopes of the plots of the products concentrations vs time and the uncertainty is given by p. The carbonyl compounds formed from MBO during the reaction were also sampled as their 2,4-dinitrophenylhydrazones by pumping 10 l of air directly from the reaction chamber through 2,4-dinitrophenylhydrazine (DNPH) coated C cartridges. The DNPH cartridges were then 18 eluted with 1 ml of acetonitrile and an aliquot was injected into the HPLC using a 20 kl injection loop. The DNPH—carbonyl derivatives were separated on a Bischoff 3 km C column, 250]4.6 mm. Since no 2-hydroxy-218 methyl-propanal (HMPR) standard was available, the concentration of the hydroxyaldehyde was derived assuming that the resulting hydrazones have the same absorption cross section and taking the CH O—DNPH derivative as the stan2 dard. The identification of the DNPH derivatives was further done by gas chromatography-mass spectrometry (GCMS) in the electron impact (70 eV) mode using a HP 5890/5970 GC-MSD system with a 60 m OV1 (0.25 mm internal diameter) column.

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Table 1. Product yields derived from initial slopes in the MBO—O reaction 3

RESULTS AND DISCUSSION

¹he ozone—MBO reaction

Products

The ozonolysis of MBO was carried out in both absence and presence of an OH radical scavenger, cyclohexane or methylcyclohexane. The products formed during the reaction of MBO with ozone include acetone, formaldehyde, formic acid, carbon monoxide and carbon dioxide. Since no reference standard is available for HMPR, this compound was tentatively identified as a product of reaction following HPLC and GC-MS analysis of reaction mixture samples on DNPH cartridges. The yields of each primary product on a molar basis, and derived from initial slopes, for the three experiments carried out without OH radical scavengers are listed in Table 1. Note that these yields are not corrected for OH and O (3P) reactions. Note that the acetone yield at longer reaction times is higher than that derived from the initial slope. The yield of acetone on a molar basis, derived from the acetone concentration vs MBO reacted after 30 min of reaction, is 17—18%. Formic acid appears to be mainly a secondary product of the reaction and its concentration constantly increases to reach a molar yield of 3% at the end of the reaction. In the case where cyclohexane or methylcyclohexane were added in order to minimise interferences from OH radicals, the initial MBO and O concentra3 tions were, respectively, in the range 8—11 ppmV and 5—60 ppmV. No substantial differences were observed and the product yields listed in Table 2 are similar to those obtained when no scavenger was added to the reaction mixture. Again, in the case when scavengers were added, formic acid is formed as a secondary product with a molar yield of 3%. Cyclohexanone and cyclohexanol, which are the major products of the OH-cyclohexane reaction, were not found amongst the products during the analysis of the FTIR spectra. If part of the MBO was oxidised by a secondary reaction with OH radicals formed during the O —ole3 fin reaction, one would have expected a more significant difference in the acetone and formaldehyde yields (see Table 4 and Fig. 5). Moreover, in the absence of

Product yields (%)

(CH ) -C"O 32 CH O 2 CO CO 2

8.3$5.0 46.7$5.5 34.3$2.4 23.3$1.5

scavengers, there is no formation of glycolaldehyde, a major product of the OH-initiated oxidation. Therefore, all the evidence indicates that OH production in the ozonolysis of MBO is small. Figure 1 shows a HPLC chromatogram of the DNPH derivatives of the carbonyl products of the ozonolysis reaction in the absence of OH scavengers. Assuming that DNPH derivatives of C —C hy3 5 droxycarbonyls elute between the DNPH reagent and CH O—DNPH and/or between CH O—DNPH and 2 2 CH CHO—DNPH (Grosjean and Grosjean, 1995b) 3 and given that HMPR is a relatively polar molecule, one can tentatively identify peak U1 or U2 as the HMPR—DNPH derivative. Peak U3 corresponds to an unidentified product, while peak U4 can be tentatively identified as the cyclohexanone— DNPH derivative coming from a system contamination since no cyclohexane was added to the reaction mixture in the experiments used for this sampling. Table 3 summarises the results of the HPLC analysis. A GC-MS analysis of these carbonyl-DNPH derivatives allows a further identification of the hydroxyaldehyde. Figure 2 shows the GC-MS spectrum for the same sampling of Fig. 1. The third peak of Fig. 2a corresponds to a molecular ion peak at 250 m/z (Fig. 2b) and can be assigned to the HMPR—DNPH derivative which has lost a molecule of water. Fragments of 233, 203, 188 and 173 m/z show further losses of OH, NO, CH and CH , 3 3 respectively. Peak U4 corresponds to the cyclohexanone—DNPH contaminant (molecular ion at m/z"278).

Table 2. Comparison between the product yields of MBO’s ozonolysis in the absence and presence of different concentrations of OH radical scavengers. The OH radical scavengers concentrations are expressed in ppm Product yields (%) Presence of OH scavenger Cyclohexane Products (CH ) -C"O CH 3O2 CO2 CO 2

Methylcyclohexane

Absence of OH scavenger

40

700

40

700

8.3 46.7 34.3 23.3

13.6 49.8 56 30

12.5 48 29.5 44

12.5 59 43 21

18.2 57 50 29.5

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Fig. 1. HPLC chromatogram of the products of the MBO!O reaction after sampling on DNPH 3 cartridges.

Table 3. Retention time and concentration of main products using HPLC at sampling time, after 20—30 min of reaction Retention time (min) 5.02 8.38 10.03 14.46 18.10

Product

Product concentration ppbV

U1 U2 CH O-DNPH 2 (CH ) CO-DNPH 32 U3

372.9 456.9 938.2 *1852 Unidentified

hydroperoxide channel is not open to the Criegee intermediates from MBO. ¹he hydroxyl radical—MBO reaction The reaction with the OH radical is the most important daylight removal pathway of most known VOCs. The hydrogen peroxide photolysis offers a way to produce OH radicals in situ, following the equation: H O P )l 2 OH. 2 2

¹he ozone—MBO oxidation mechanism Reactions between O and olefinic organics are 3 believed to proceed through electrophylic addition of O on the carbon—carbon double bond, formation of 3 an energy-rich complex, a molozonide, and decomposition of the trioxolane (by rupture of the carbon—carbon bond and either one of the oxygen— oxygen bonds) to form a stable carbonyl compound plus an energy-rich Criegee biradical (Fig. 3). The fate of Criegee intermediates has not totally been elucidated yet, but one can suppose that they either undergo further transformations or will be collisionally stabilised. A speculative explanation here could be direct unimolecular decomposition of the biradical to form acetone (Grosjean and Grosjean, 1995a). The apparently negligible yield of OH is in accord with the accepted OH formation mechanism (Atkinson, 1997); due to the absence of H atoms on the alkanic carbon atom adjacent to the double bond, the

(4)

A series of MBO—H O —air irradiations were carried 2 2 out. In all cases, we observed acetone, glycolaldehyde (hydroxyacetaldehyde), formaldehyde, formic acid, carbon monoxide and carbon dioxide as major products of the OH radical initiated reaction of MBO. Table 4 lists the yields (derived from initial slopes) of each product. Formic acid is a secondary product and its molar yield reaches a maximum of 6—8% when all the MBO has reacted. Note that the yields in both CO and CO are not 2 reported in Table 4 because these are products which are also formed by irradiation of the chamber containing only purified air. CO and CO apparently 2 come from the Teflon coating. Experiments were carried out by irradiating the Teflon for up to 2 hs, and the concentrations of these products varied from 1 ppmV to nearly 3 ppmV. Due to this, a measure of the CO and the CO concentrations effectively for2 med during an experiment can only be given as an order of magnitude and cannot be derived exactly. Given these considerations, one can expect that the

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Fig. 2. (a) GC chromatogram of the DNPH derivatives of the carbonyl products of the MBO#O 3 reaction. (b) The mass spectrum corresponds to peak HMPR—DNPH in the chromatogram.

reaction between OH radicals and MBO results in a CO yield and a CO yield of approximately 5% 2 each on a molar basis. In order to check for the presence of HMPR amongst the products of reaction, an HPLC analysis of the 2,4-dinitrophenylhydrazones derivatives of the carbonyl compounds was also performed for this reaction. Figure 4 shows the corresponding HPLC chromatogram. Peaks at retention times 5.05 and 8.40 min correspond to peaks U1 and U2 of Fig. 1 and therefore we can conclude that HMPR is also formed in the oxidation of MBO by OH radicals, even if to a smaller extent than in the ozonolysis.

Table 5 summarises the results of the HPLC analysis. ¹he hydroxyl radical—MBO oxidation mechanism The formation of acetone, glycolaldehyde, 2-hydroxy-2-methyl-propanal, formaldehyde, formic acid, carbon monoxide and carbon dioxide can be rationalised by the reaction scheme shown in Fig. 5, where OH adds to either one of the two carbon atoms of the double bond, favouring the formation of the most substituted radical. Note that the addition to the internal carbon atom may also be sterically hindered.

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Fig. 3. The proposed mechanism for the reaction of MBO with O . 3 Table 4. Product yields derived from initial slopes in the MBO—OH reaction Products OHC-CH OH 2 (CH ) -C"O 32 CH O 2

Product yields (%) 28.0$2.8 14.1$0.2 9.3$3.3

¹he NO radical—MBO reaction 3 A series of MBO—NO experiments were carried 3 out and showed similar patterns to those of previous

studies with other alkenes. Build-up of the organic nitrate bands (840, 1285, 1683 cm~1) (Fig. 7) and of the peroxynitrate bands (793 cm~1) were observed (Fig. 6); peroxynitrate bands subsequently decay and stable products such as acetone and carbonyl nitrates are formed. The acetone yield of the reaction is found to be (68.7$7.1)%. The yield on a molar basis of organic nitrate compounds was estimated to be in the range 12.5—13.5% of reacted MBO adopting an integrated band intensity (IBI) value of 3.0]10~17 cm]molecule~1 for the band at 840 cm~1. This IBI value had been obtained

Fig. 4. HPLC chromatogram of the products of the MBO—OH reaction after sampling on DNPH cartridges.

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Table 5. Retention time and concentration of main products using HPLC at sampling time, after 20 min of reaction Retention time (min) 5.05 8.40 10.06 14.52 18.04

Product

Product concentration (ppbV)

U1 U2 CH O—DNPH 2 (CH ) CO—DNPH 32 U3

56.9 33.0 377.2 1323 Unidentified

Fig. 5. The proposed mechanism for the reaction of MBO with OH radicals.

Fig. 6. Build-up of one of the peroxynitrate bands (793 cm~1) in the first 30 min of reaction. Afterwards the band decays.

as an average for the nitrate species derived from 2-butene (Hjorth et al., 1990). Figure 7 shows the FTIR absorption spectrum of one of the runs corresponding to a reaction time of approximately 3 h and 50 min after subtraction of the known absorptions of NO , HNO , N O and 2 3 2 5

acetone. The formation of hydroxy-aldehyde nitrates can be evidenced by the aldehyde absorption features between 2700 and 2850 cm~1 and by a very weak OH spectral feature (O—H stretch) observed in the spectral range 3500—3650 cm~1. However, in the FTIR spectra that were recorded, the region

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Fig. 7. Product spectrum of the reaction of MBO with NO . 3

3500—4000 cm~1 is characterised by a very poor signal-to-noise ratio and this might be the reason of the poor evidence of the O—H stretch band. The weak but sharp absorption peak observed at 3566 cm~1 could be attributed to the intramolecularly bonded hydroxyl band of an hydroxyketone (see Fig. 9), which is capable of intramolecular bonding as shown in Fig. 8. Table 6 summarises the spectral features and their tentative assignment to the reaction products. ¹he NO radical—MBO oxidation mechanism 3 In a general way, two main kinds of initial steps can be taken into account: hydrogen abstraction NO #RHPHNO #R 3 3

(5)

or addition to unsaturated bonds (6) For olefinic compounds, addition is generally much faster than hydrogen abstraction. No significant formation of HNO was observed other than that 3 known to come from the reaction between N O 2 5 and H O in the chamber. The adduct formed in reac2 tion (6) can add O and undergo further transforma2 tions. Another less important fate of the adduct in equation (6) can be the elimination of NO to form an 2 epoxide. In the case of MBO, NO will add to either one of 3 the carbon atoms of the double bond, favouring the most substituted radical (Fig. 9).

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The possibility of the formation of an epoxide species was not rejected because epoxides have characteristic absorption bands between 1200 and 1250 cm~1 and Fig. 7 shows a feature at 1240 cm~1. The reaction scheme shown in Fig. 9 accounts for the products observed in the reaction chamber. Fig. 8. Intramolecularly bonded hydroxyketone. CONCLUSIONS AND ATMOSPHERIC IMPLICATIONS

Table 6. The most important spectral features and their tentative assignment to the products of the MBO—NO 3 reaction —ONO 2 (cm~1) 840, 1285, 1683

—CHO (cm~1)

'CO (cm~1)

—OH (cm~1)

2821

1710—1774

3500—3650

The alkylperoxy radical should be reduced by its self-reaction to the alkoxy radical which can then follow different reaction pathways, such as the reaction with NO to form a dinitrate, the reaction with 2 O to form a carbonyl nitroxy species, or other reac2 tions to form an unsubstituted carbonyl species (acetone) or an a-hydroxy aldehyde (2-hydroxy2-methyl-propanal). However, the formation of the latter could be excluded because no formaldehyde formation has been observed.

The reaction between OH radicals and MBO has been studied and the major products have been identified. These proved to be acetone, glycolaldehyde, formaldehyde, formic acid, CO and CO . 2 2-hydroxy-2-methyl-propanal was also tentatively identified as a minor reaction product. A plausible reaction mechanism for the oxidation of MBO by OH radicals has been proposed to explain the experimental observations. The oxidation of MBO by O was investigated in both the presence and 3 absence of OH scavengers. Two different alkanes were used for this purpose, cyclohexane and methylcyclohexane, which have the property of reacting uniquely with OH radicals and not with the reactants themselves. In both oxidation reactions the major products have been quantified using FTIR, with the exception of 2-hydroxy-2-methyl-propanal which was tentatively identified and quantified by HPLC and GC-MS.

Fig. 9. The proposed mechanism for the reaction of MBO with NO radicals. 3

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It has been shown that MBO reacts with NO 3 radicals in air following similar reaction pathways to other alkenes, and a reaction mechanism for the oxidation of MBO by NO has been proposed. The NO 3 3 radicals react with MBO to form acetone, nitroxycarbonyl compounds, nitroxyalcohols and possibly dinitrates. No other unsubstituted carbonyl compound could be detected. The atmosphere is effectively a very large reaction chamber where conditions can vary drastically and all of the three oxidation reactions studied can be relevant sinks for MBO. All of the oxidation pathways investigated in this study give significant yields of acetone, which is believed to have an important atmospheric role. Oxygenated species are relevant in atmospheric chemistry because of their ability to photolyse producing free radicals. One of the most abundant oxygenated compounds found in the remote atmosphere is precisely acetone, which is believed to be an important precursor of peroxyacetylnitrate (PAN) (Singh et al., 1995). The main sinks of acetone are photolysis and reaction with OH radicals (Singh et al., 1994), while its average lifetime has been estimated to be of the order of 16 d (Singh et al., 1995). More measurements should be carried out to achieve a better understanding of the origin of MBO and of its role as a source of acetone in the troposphere. Acknowledgements—The authors gratefully acknowledge the help rendered by Mrs. C. Brussol during the HPLC and GC-MS analysis and Dr B. Larsen for helpful discussions. The authors would also like to thank Mr Ottobrini for the experimental work. G. Fantechi acknowledges the European Commission for the Ph.D. grant.

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