John J. Orlando, 1 Barbara Nozi re, 2 Geoffrey S. Tyndall, 1 Grazyna E. Orzechowska, 3

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D9, PAGES 11,561-11,572, MAY 16, 2000 Product studies of the - and ozone-initiated oxidation of some monoterpenes John J. Orlando, 1 Barbara Nozi re, 2 Geoffrey S. Tyndall, 1 Grazyna E. Orzechowska, 3 Suzanne E. Paulson, 3 and Yinon Rudich 4 Abstract. The - and O3-initiated oxidation of five monoterpenes (myrcene, terpinolene, A 3- carene, o -pinene, and [3-pinene) has been studied in environmental chambers equipped with either a Fourier transform infrared spectrometer or a gas chromatography/flame ionization detector system. The -oxidation of myrcene and terpinolene is shown to lead to substantial yields of acetone (36 and 39%, respectively), while the acetone yield from the pinene compounds is quite small (4% and -2%, for o - and [3-pinene, respectively). Formaldehyde has been identified as a major product (yields of 20-40%) in the -initiated oxidation of all five species. Formic acid was also observed in the -initiated oxidation of all five monoterpenes, with yields of 2% from [3-pinene and 5-9% from the other specie studied. The production of acetone from the reaction of monoterpenes with ozone in the presence of an scavenger was measured. The yields of acetone for the 03 reactions were (z-pinene, ; [3-pinene, ; A 3- carene, ; myrcene, ; and terpinolene, The mechanism leading to the production of these compounds is discussed, as is the atmospheric relevance of the results. In particular, an estimate of the contribution of monoterpene oxidation to observed atmospheric levels of acetone and formic acid is made. 1. Introduction 1997b], implying lifetimes of less than a day for these species. Thus their oxidation contributes to secondary pollutant formation Monoterpenes are emitted to the atmosphere from vegetative near the source region. Furthermore, some of the terpene sources in amounts believed to exceed 100 Tg/yr [Guenther et reaction products (for example, unsaturated oxygenated species al., 1995]. These emissions are found over many regions of the and aldehydes) are themselves reactive and will further contribute globe, including tropical regions [Zimmerman et a/.,1988' to the degradation of regional air quality. To date, only a handful I ettm$ ½t t. 1998a] ' till... uu IIUUt '' the cununenta... Umtcu :' States of studies of the mechanism of the -initiated oxidation of and Canada [He[mig eta[., 1999a,b; Go[dan eta[., 1995' monoterpenes has been carried out [Arey et al., 1990; Biesentha[ eta[., 1998; He[mig eta[., 1998b], in northern Europe Hatakeyama et al., 1991; Hakola et al., 1994; Aschmann et al., [Hako[a eta[., 1998], and in the Medite anean [Seufert eta[., 1998; Nozi?re et al., 1999a], and the full product distributions 1997, and references therein]. As these compounds are have yet to be characterized for any of these species [Atkinson, unsaturated, they are highly reactive with ozone,, and the 1997b; Calogirou et al., 1999]. Furthermore, discrepancies exist nitrate radical [Atkinson, 1997a,b; Calogirou eta[., 1999] and regarding the yield of some of the major products obtained from thus can play a role in the formation of ozone, other seconda pinene oxidation, nopinone from -pinene and pinonaldehyde pollutants, and aerosols [Trainer et al., 1987' Jacob and Wofsy, from c -pinene [Arey et al., 1990; Hakola et al., 1994; 1988' Roselie et al., 1991' Fehsenfe[d et al., 1992; Andreae and Hatakeyama et al., 1991; Nozibre et al., 1999a]. Crut en, 1997]. The oxidation of certain te enes also leads to Reactions of ozone with monoterpenes are also quite rapid the formation of acetone [Gu et al., 1984; Aschmann et al., 1998; [Atkinson, 1997b; Calogirou et al., 1999], generally falling in the Reisse[[ eta[., 1999' Ca[ogirou eta[., 1999], a compound which range cm 3 molecule -1 s -1. For the fastereacting is now believed to be an important source of HO in the free monoterpenes the ozonolysis reaction is able to compete with troposphere [Singh eta[., 1994, 1995]. reaction as an important daytime loss process. Reactions of Rate coefficients for reactions of with most te enes e monoterpenes with ozone compete with their reaction with NO3 rather large, typically >10 - cm molecule - s - [Atkinson, at night and are important both as a destruction mechanism for the terpene itself and because of the potential for the production latmospheric Chemistry Division, National Center for Atmospheric of radicals during the evening and nighttime [Paulson and Research, Boulder, Colorado. Orlando, 1996; Makar et al., 1999]. The ozonolysis of 2Advanced Studies Program, National Center for Atmospheric Research, Boulder, Colorado. monoterpenes has been studied [e.g., Hatakeyama et al., 1989; 3Department of Atmospheric Sciences, University of California, Los Atkinson et al., 1992; Grosjean et al., 1992, 1993; Hakola et al., Angeles. 1993, 1994; Alvarado et al., 1998; Paulson et al., 1998; Reissell 4Department of Environmental Sciences, Weizmann Institute of et al., 1999], both for measurements of the yields and for the Science, Rehovot, Israel. identification of carbon-bearing products. As is the case with Copyright 2000 by the American Geophysical Union. Paper number 2000JD /00/2000JD ,561, these studies are far from complete [Atkinson, 1997b; Calogirou et al., 1999]. In this paper, results of environmental chamber studies of monoterpene oxidation are described. The -initiated

2 11,562 ORLANDO ET AL.: - AND O3-INITIATED OXIDATION OF MONOTERPENES oxidation of five monoterpenes ( z- and [ -pinene, terpinolene, et al., 1999]. In most cases, these reactions were negligibly slow. A3-carene, and myrcene) has been studied in the presence of NOx, However, in the case of myrcene (a conjugated diene), the while the ozonolysis of these species has been studied under NOx-free conditions. This suite of monoterpenes was selected because of their potential to make significant contributions to the reaction with NO2 is faster and needed to be accounted for. The rate coefficient for reaction of NO2 with myrcene was measured to be ( ) x cm 3 molecule - s 'l, in excellent agreement global acetone budget (the pinenes and A3-carene because of their with previously reported values, 2.6 and 2.9 x cm 3 high atmospheric source strengths and myrcene and terpinolene because their structure implies that large acetone yields are possible). Products of the oxidation are identified and quantified and are used to make inferences regarding the oxidation mechanisms. Comparisons with previous work are made when molecule - s - [Atkinson et al., 1984; Reissell et al., 1999]. Corrections to the myrcene consumption for this reaction were made, using measured NO2 concentrations in the chamber. The corrections to the myrcene loss were between 2 and 25%, depending on the extent of reaction and on the method of available. The contribution of the oxidation of the monoterpenes measurement. For most myrcene experiments, spectra were to the atmospheric abundances of acetone and formic acid is discussed. recorded while photolysis was occurring, thus minimizing the importance of the dark reaction of NO2 with myrcene. When experiments were conducted in this fashion, reaction of the myrcene with NO2 accounted for <5% of its consumption. The 2. Experimental Methods acetone yield from reaction of NO2 with myrcene was found to be (11+3)%, and acetone yields in the experiments were Initiated Studies corrected using this value. Monoterpenes are also known to react rapidly with O(3p) The -initiated oxidation of the monoterpenes was carried atoms [Atkinson, 1997b], which can be formed in our system out in a stainless steel environmental chamber, which is described by Shetter et al. [1987]. The chamber is 2 m in length, from the photolysis of NO2. To check for interferences from with a volume of 47 L, and is interfaced to a Bomem DA3.01 O(3p) reactions, some myrcenexperiments were conducted in 700 torr 02, where the levels of O(3p) should be considerably Fourier transform infrared spectrometer via a set of modified lower due to their recombination with 02. Product yields were Hanst multipass optics, which provided a total IR pathlength of 32.6 m. Spectra were obtained from the coaddition of indistinguishable from those obtained in experiments conducted interferograms recorded over the range cm - at a in air, indicating that no significant O(3P) chemistry was spectral resolution of 1 cm - interfering with the yield determinations. Experiments involved the photolysis of static mixtures of 2.2. ethyl nitrite or isotopically labelled methyl nitrite (13CH3ONO), Ozone Reactions used as the source, the monoterpene under study, and NO, in Experiments were performed in 240 L Teflon chambers at 02 and N2 diluent, at a total pressure of torr. Ethyl nitrite or 13CH3ONO was used to avoid production of 12CH20, a K and 750 torr in the dark (described in more detail by Kramp and Paulson [2000]). For most experiments, reactants possible product of the terpene oxidations. Concentrations were evaporated into a stream of purified air as the chamber was (molecule cm -3) employed were in the following ranges: organic filled. For experiments performed in 02 or N2 the chambers were nitrite, (7-28) x 1014; monoterpene, (4.5-30) x 1014; NO, (2-22) x rinsed twice and then filled with the respective gas (boil-off from 10 TM. Most experiments were conducted with 150 torr of 02 LN2 or LO2 cylinders). Reactions of 03 with monoterpenes were present, though some test experiments in 700 torr 02 were also carried out. Photolysis was carried out using the output of a Xecamed out in an excess of cyclohexane or di-n-butyl ether to scavenge radicals. The amount of the scavenger added was arc lamp, filtered to provide radiation between 240 and 400 nm. sufficient to react with 92-99% of the radicals. The initial In most cases, multiple irradiations of each gas mixture were carried out (usually about four photolyses, each of 3-5 min monoterpene concentrations were (1-75) x 10 TM molecule cm -3. Aliquots of 03 were added every half hour until at least 70% of duration), with an IR spectrum recorded after each photolysis the initial concentration of the monoterpene had reacted, with an period. In some experiments, the mixture was continuously average of six additions of 03 for each experiment. Acetone, photolyzed, with IR spectra recorded during the course of the photolysis. Methyl and ethyl nitrite were prepared from the dropwise addition of sulfuric acid to a saturated solution of sodium nitrite monoterpene, and scavenger concentrations were measured with a Hewlett Packard 5890 gas chromatograph/flame ionization (GC/FID) system, equipped with either a DB-1 (0.32 mm ID, 1 gm film, 30 m) column or a DB-624 (0.32 mm ID, 1.8 gm film, in either methanol or ethanol [Taylor et al., 1980]. The 30 m ) column (J&W Scientific). The GC was calibrated daily monoterpenes (all from Aldrich) were purified and degassed by repetitive freeze-pump-thaw cycles before use. NO and 02 (both ultrahigh purity) were used as received. with a 4.9 _+ 0.1 ppm cyclohexane standard (Scott Specialty Gases). Concentrations of the monoterpenes and cyclohexane were calculated based on the per carbon FID response factor Products identified in these experiments were formaldehyde, normalized to the cyclohexane standard. This provides acetone, and formic acid. Quantitative yield information on these species was obtained from a comparison with standard spectra of known amounts of these species, which had previously been obtained in our laboratory. All reported yield data have been corrected for secondary losses of the reaction products via their reaction with, using methods described by Meagher et al., [1997]. Monoterpenes, and other unsaturated species, are known to undergo slow reactions with NO2 [Atkinson et al., 1984; Reissell concentrations to within 4% for compounds that contain only carbon and hydrogen. We performed a calibration of our GC/FID system to determine the FID response, or effective carbon number (ECN), for acetone. Known volumes of liquid acetone and cyclohexane (internal standard) were introduced to Teflon chambers in a stream of purified air. Calibration curves for acetone and cyclohexane were obtained for eight and five different concentrations, respectively, in the ppm range. The cyclohexane (internal standard) concentrations were

3 ORLANDO ET AL.' - AND O3-INITIATED OXIDATION OF MONOTERPENES 11,563 within 10% of those calculated based on the liquid volume and the calibration standard. The ECN for acetone was _ 0.11 calculated based on the measured cyclohexane concentration, which is in excellent agreement with values reported by Scanlon and Willis [1985] and E. Apel [personal coinmunication, 1999]. An ECN of 7.6 was used to calculate concentrations of di-n-butyl ether scavenger [Kramp and Paulson, 2000]. For the ozonolysis studies, (1R)-(+)-ct-pinene (99+%), (1S)- (-)-[3-pinene (99+%), (1S)-(+)-A3-carene (99%), cyclohexane (99.5%), di-n-butyl ether (99.3%), and acetone (99+%-high performance liquid chromatography grade) were purchased from Aldrich. Myrcene (supra-99+%) and terpinolene (supra-99+%) were generously donated by Millennium Specialty Chemicals. Ozone was generated by flowing pure oxygen ( L min 'l) through a mercury lamp 03 generator (Jelight, model 6000). 3. Results and Discussion Initiated Oxidation The use of FTIR as the analytical tool for the study of the mechanism of the -initiated oxidation of the monoterpenes limits the suite of products that can be identified and quantified Table 1. Molar Product Yields From the -Initiated Oxidation of the Terpenes Studied in This Work Acetone Formaldehyde Formic acid This Work Myrcene Terpinolene Acetone 39+5 Acetone co-product Formaldehyde 29+6 Formic acid 8+2 C 0 Ring-opening product -Pinene Acetone Formaldehyde Formic Acid Nopinone A- -Carene Acetone Formaldehyde Formic Acid C 0 Ring-opening product a'-pinene Acetone Formaldehyde Formic acid Pinonaldehyde Other Studies 41 }, a 26 b 8.5,6 c 54 e 27 b, 79e,, h 15 a 34 11,9 c'tt 29 ' 28_89b,d,e& h ß i i i i MYRCENE CONSUMED (1014 molecule C!TI '3) Figure 1. Corrected product concentrations obtained in the initiated oxidation of myrcene. Solid circles, HCO; open squares, CH20' solid triangles, acetone. to those with readily distinguishable infrared spectra. In this study, only formaldehyde, acetone and formic acid could be quantified. These products were formed in the oxidation of all five monoterpenes studied, as summarized in Table 1. Sample data (showing amount of product formed versus monoterpene consumed) are shown in Figures 1 (for myrcene) and 2 (for terpinolene). All product yields were linear with the degree of monoterpene consumption (which was varied between 3 and 70%), within experimental uncertainties. However, this does not preclude the possibility of a small amount of acetone, formaldehyde, or formic acid formation from the reaction of with primary products, rather than from the parent terpene itself. Some of the reaction products anticipated in relatively large yields (such as pinonaldehyde from ct-pinene, caronaldehyde from A3-carene, 4- methyl-cyclohex-3-ene-l-one from terpinolene) likely react with with approximate rate constants of (5-15) x 10 - cm 3 z O 0.5 Z O.4 z' o ø ( O E 0.2 Yields in percent. ' Reissell et al. [ 1999]. bhakola et al. [ 1994]. CAschmann et al. [1998]. dnozi ret al. [1999a]. ehatakeyama et al. [ 1991 ]. tarey et al. [ 1990]. ran upper limit due to possible interference from other carbonyl compounds. hsee also Calogirou et al. [1999] for a summary. o o A ß TERPINoLENE CONSUMED (10 4 molecule cm '3) Figure 2. Corrected product concentrations obtained in the initiated oxidation of terpinolene. Solid squares, HCO; open triangles, CH20: solid circles, acetone. i i

4 11,564 ORLANDO ET AL.: - AND O3-INITIATED OXIDATION OF MONOTERPENES molecule - s 'l, about half as fast as the rate constants of with the parent monoterpene. Thus, after 70% of the monoterpene has multifunctional products. Since the likelihood of acetone production due to addition at other than these two sites is been converted, -35% of these reaction products will themselves negligible and the yield of acetone from attack at these two sites have been consumed. The worst possible case appears to be pinonaldehyde, where the rate coefficient for its reaction is nearly equal to that of with the parent (x-pinene[nozi re et al., 1999b], and the yield of CH20 from pinonaldehyde [Nozii re et al., 1999a] is thought to be quite large. Nonetheless, from an examination of product data at low (x-pinene conversion, it is apparenthat CH20 formation with a yield of at least 15% arises from the (x-pinene itself (compared to the measured 19% yield). Production of formaldehyde, acetone, or formic acid from secondary chemistry is likely of even lower significance in the [3- pinene, myrcene, A3-carene, or terpinolene cases. The largest acetone yields are seen in the oxidation ot myrcene (36+5%) and terpinolene (39_+5%), as might be expected from their structures (see Figures 3a and 3b for reaction schemes I and II). The yields obtained from both species agree very well with those recently reported by Reissell et al. [1999]. is likely high, the 36% acetone yield should be fairly close to, but slightly less than, the fractional attack at this isolated C=C oond. This is in very good agreement with calculated positions of attack of on myrcene given by the structure-activity relationships (SARs) of Kwok and Atkinson [1995], which predict 45% of the reaction to occur at the isolated C=C double bond and 54% to occur at the conjugated double bonds. SARs for addition to unsaturated hydrocarbons have also been reported by Peeters et al. [1994]. Their data lead to a very similar prediction, 46% at the isolated double bond and 54% to the conjugated system.?eeters et al. also predict that pathway B of scheme I will dominate over pathway A by a factor of 2. Reaction of with terpinolene should again occur predominantly by addition, with four different sites available. Addition to either side of the double bond external to the ring should lead to acetone production (see Figure 3b), while addition Reaction of with myrcene should occur almost of to the double bond within the ring is unlikely to lead to exclusively by addition [Kwok and Atkinson, 1995] and can occur at six different locations, two of which lead to acetone (see acetone. The 39% yield of acetone is somewhat lower than the predicted extent of reaction at the exo double bond (a branching scheme I). The expected carbon-bearing coproduct of acetone, fraction of 56% is predicted from both the Kwok and Atkinson CH2=CH-C(=CH2)-CH2CH2CHO, could not be conclusively [1995] and Peeters et al. [1994] parameterizations). A small identified in our experiments. In addition to the chemistry shown portion of this difference is probably accounted for by organic in Figure 3a, the formation of organic nitrates is also expected. nitrate production. The coproduct of acetone is likely to be 4- Also, isomerization via a 1,5-H shift is likely to be a minor, but not insignificant, process [Atkinson, 1997c; Kwok et al., 1996; Vereecken et al., 1999] for one of the [3-hydroxyalkoxy radicals (that formed in pathway B of scheme I), leading to methyl-3-cyclohexene-l-one, while attack at the double bond in the ring is likely to produce a substituted heptanal species, neither of which is uniquely identifiable in our spectra or available commercially. The acetone yield from terpinolene Scheme I oh,02 o + HO 2 / B,O 2 NO d comp. 02 ' (o + // O!-i Figure 3a. Scheme I, mechanism for the production of acetone from the -initiated oxidation of myrcene.

5 ORLANDO ET AL.' - AND O3-INITIATED OXIDATION OF MONOTERPENES 11,565 Scheme II +, 02 NO.. decomp. o H H, Hch NO d,omp HO 2 H H Figure 3b. Scheme II, mechanism for the production of acetone from the -initiated oxidation of terpinolene. oxidation of Reissell et al. [1999] appears to increase from a value of 32% at low terpinolene conversion to -42% at higher conversion. While our terpinolene measurements were done at significantly lower conversions (<15%) to minimize secondary reactions and no variation of the acetone yield with extent of conversion is evident in our data, the precision of the measurements does not preclude some curvature. The acetone yields of-40% from this work and Reissell et al. [1999] are broadly consistent with the 26+6% yield of the acetone coproduct, 4-methyl-3-cyclohexene-l-one, reported by Hakola et al. [1994]. As expected from their structures, the yield of acetone from the -initiated oxidation of c -pinene (5+2%), -pinene (2+2%), and A3-carene (15+3%) were considerably lower than from myrcene or terpinolene. While the acetone yield from A 3- carene agrees well with that reported by Reissell et al. [1999], the values for the two pinenes are somewhat lower than those previously reported [Aschmann et al., 1998; Nozibre et al., 1999a]; see Table 1. At present, the reason for these discrepancies are not known. Calibration differences are unlikely given the good agreement seen with myrcene and terpinolene. Given the chemical stability of acetone, it is difficult to explain why too little acetone would be seen in our system. Given the presence of >C(CH3)2 groups in all three species (ot-pinene, [3-pinene, and A3-carene), acetone production is certainly plausible in small quantities. Atkinson and coworkers [Reissell et al., 1999; Aschmann et al., 1998] proposed mechanisms involving isomerizations of the -hydroxyalkoxy radicals formed subsequent to addition, or chemistry occurring subsequent to abstraction pathways, all of which seem possible. Note, however, that due to the constraints of the bicyclo skeleton, 1,5-H shifts across the ring may not actually be the most favorable. In fact, unique isomerization pathways (1,6- H shifts, for example, see below) may be possible for certain stereoisomeric forms of the various hydroxyalkoxy radicals produced. Nozibre et al. [1999a] proposed an opening of the four-membered ring following initial attack, though similar processes involving four-membered rings appear to be slow [Newcomb, 1993]. Other possible pathways to acetone production from c -pinene will be discussed below. The largest yields of CH20 were obtained from the initiated oxidation of -pinene (45+8%) and myrcene (30+6%), as might be expected given the presence of a terminal (i.e.,=ch2) double bond in these compounds (see Figure 3c for -pinene). As discussed above, addition to the two conjugated double bonds in myrcene is expected to account for % of the reaction. The addition to this conjugated system likely leads to a large (but not 100%) yield of CH20 (note that the CH20 yield from the very similar conjugated double bond system isoprene system is -60%) [Paulson and Seinfeld, 1992]. Tautomerization of the various radicals formed in this conjugated system will likely lead to a number of hydroxycarbonyl species by mechanisms analogous to those recently suggested for isoprene [Kwok et al., 1995]. In the case of -pinene, attack at either end of the C=C double bond should lead (at least in part) to the 'formation of CH20 and nopinone (see Figure 3c). Our CH20 yield, 45%, is similar to that reported by Hatakeyama et al. [1991], 54+5%, but somewhat higher than the nopinone yield reported by Hakola et al. [1994], 27+4%. Even so, it appears that CH20 and nopinone account for no more than half of the reaction products, while the remaining products are completely unaccounted for by this or any other study. Attack of at the =CH2 group (pathway A of scheme III) is expected to be favored by about a factor of 10 over addition of the at the ring (pathway B) [Peeters et al., 1994]. While CH20 and nopinone are likely obtained in high yield via pathway B, cleavage of the six-membered ring in the - hydroxyalkoxy radical formed via pathway A is a distinct possibility. Note that elimination of R2CH radicals from [3- hydroxyalkoxy radicals of the form R2CH-CR(O')-CH2 has been shown to be significant in the case of some substituted 1- butenes [Atkinson et al., 1995, 1998]. Relieving the strain of the bi-cyclo system may make this type of C-C bond breaking even more favorable in the [3-pinene case. Chemistry occurring

6 11,566 ORLANDO ET AL.: - AND O3-INITIATED OXIDATION OF MONOTERPENES Scheme III,O 2 oo I A NO,02 B H d øømp'o2'no / + CH2 NO 1 0 CH20 + HO2 isom.,o2,no d comp. 1 H 02 I o +CH20 ß HO ß isom., O2,NO H.\ ß + HO2 HC 02 + HO 2 Figure 3c. Scheme III, mechanism for the production of formaldehyde from the -initiated oxidation of 13-pinene. subsequent to ring opening leads to a variety of multifunctional five terpenes is potentially of great interest. The production of species; one possibility, involving isomerizations of the oxy formic acid is often found in chamber systems via the reaction of radical formed following ring cleavage is shown in Figure 3c. CH20 with HO2 [Suet al., 1979]. However, this chemistry The low observed yield of acetone precludes it from being a usually leads to formic acid yields that increase with increasing major product in this chemistry. hydrocarbon consumption (since both CH20 and HO2 The observation of substantial yields of CH20 from the concentration tend to build up), while the HCO production oxidation of o -pinene (19+5%), terpinolene (29+6%), and A 3- observed in these experiments is clearly linear. Furthermore, carene (21+4%) was not anticipated. Though methyl groups are experiments in which the initial [NO] was varied by a factor of present in all three species, no obvious route to their elimination 10 (which should affect the steady state [HO2]) had no presents itself. The 13-hydroxyalkoxy formed in all three cases is discernibleffect on the measured formic acid yield. Formic acid very unlikely to eliminate CH3 directly, given the presence of two is also believed to be generated in some chamber systems via better leaving groups in all cases. CH20 (yield of 29+10%) has heterogeneous processes, though evidence for this phenomenon also been observed from the -initiated oxidation of o -pinene has not been observed in our system. One myrcene oxidation by Nozi?re et al. [1999a]. Possible routes to the formation of experiment was conducted with molecule cm -3 H20 added CH20 from o -pinene are illustrated in Figure 3d (involving a 1,6- to the reaction mixture in an effort to change the condition of the H shift, as alluded to earlier) and 3e (via formic acid formation, wall surface; no effect on the HCO yield was observed. In see below). Mechanisms to explain the yields of CH20 from addition, photolyzed reaction mixtures were left in the dark for either terpinolene or A3-carene not immediately obvious. periods of 10 rain or more. No HCO build up was observed. The observation of small, but clearly measurable and These observations all point to the possibility of the production consistent formic acid yields in the -initiated oxidation of all of HCO as a primary product in the gas phase oxidation of the

7 ORLANDO ET AL.: - AND O3-INITIATED OXIDATION OF MONOTERPENES 11,567 Scheme IV, 02, NO o isomcrizati n multi-funotional products + CH20 I 02, N 0 Figure 3d. Scheme IV, possible mechanism for the production of formaldehyde in the -initiated oxidation of -pinene. oh2c; terpenes studied here. We propose that the origin of the formic acid stems from the occurrence of an addition reaction between 02 and radicals of the type R-CH, where R is a long organic chain [Atkinson et al., 1995]: RCH > RCHO + HO2 (R 1 a) R-CH()O2 (Rlb) R-CH()O2 + NO _ R-CH()O + NO2 (R2) R-CH()O _ R + HCO (R3) Acetone was observed as a product of the reaction of O3 with all monoterpenes considered in this study, as is summarized in Table 2 and Figure 4. To calculate the acetone yields reported here, we made very small corrections to account for generation of acetone from reaction of the monoterpenes with ; is generated by the ozone reactions and was scavenged so that only a few percent of reacted with the monoterpene. These corrections change the acetone yields by <0.5% in all cases. Acetone formation is lowest for the pinenes, followed by A 3- carene, and highest for the monoterpenes with a =C(CH3)2 moiety (myrcene and terpinolene). This general trend is in agreement with data reported by Reissell et al. [1999] and Ruppert et al. [1997] (Table 2). Our yields for the terpenes that generate lesser amounts of acetone ( -pinene,[3-pinene, and A3-carene) are lower than those of Reissell et al. measured with a GC/FID. Reissell et al. also report measurements made with a GC/MS, and these values for c - and [3-pinene are lower and in better R-CH()O RCO + HO2 (R4) agreement with our values. In an effort to determine the source of the difference between our measurements and the others, we Radicals possessing this structure are expected in high yield from generated a calibration curve for acetone with concentrations as c -pinene, A3-carene, and terpinolene (following ring opening of low as 260 ppb (limited by the volume of the Teflon chamber and the cyclic [3-hydroxyalkoxy radicals), as well as from myrcene the microsyringe). The GC detection limit for acetone is -12 (following the elimination of acetone from the [3-hydroxyalkoxy radical, for example). ppb. The calibration curve fit with a straight line had a correlation coefficient of 0.994, a zero intercept, with no trend in With formic acid yields of 5-8% from these species it is the residuals; there is no indication that small amounts of acetone evident that 02 addition need only constitute a 10-15% branch of are lost in our system. The reason for the discrepancy remains to (R1). In the case of c -pinene (see Figure 3e), chemistry be determined but may be related to the different amounts of the subsequent to the production of HCO could also generate various peroxy radicals present due to the use of different acetone and two CH20, thus accounting for the bulk of the yields scavengers. of these three minor products. While similar pathways to The acetone yield for terpinolene in our experiment (50%) is HCO are possible for myrcene, terpinolene, and A3-carene, equal to the value reported by Reissell et al. [1999], while our this pathway cannot as easily explain the yields of the other value for myrcene (25%) is also in reasonable agreement with the minor products, e.g., the large CH20 yield observed from Reissell et al. value (33%) and the Ruppert et al. [1997] value terpinolene. A lower formic acid yield from [3-pinene than from (29%). Acetone from both of these compounds is anticipated and the other terpenes is consistent with the mechanism presented above, as no major pathway to an R-CH radical is evident in likely arises from decomposition of the primary ozonide formed the case of [3-pinene. from attack at the (CH3)2C= moleties and is roughly consistent with the expected partitioning of ozone attack among the double 3.2. O3-Initiated Oxidation bonds [Reissell et al., 1999]. Our acetone yields for both (z- and [3-pinene are lower than other measurements [Reisse[l et al., 1999; Ruppert et al., 1997; Grosjean et al., 1993]; see Table 2. Some acetone from c -pinene is expected from one of the RO2 radicals resulting from one of the elimination channels, see Figure 3f [Reissell et al., 1999];

8 11,568 ORLANDO ET AL.: - AND O3-INITIATED OXIDATION OF MONOTERPENES Scheme V O, 02, NO 02 + HO 2 d omp., 02, NO o //02 CH(O )00 HCO + oq(o )o O d oomp., 02, NO + CH20 d omp, 0, NO O O deoomp., 02 (O + CH3CO CH20 + CO 2 \ + (CH )2C=O Figure 3e. Scheme V, possible mechanism for the production of formic acid, formaldehyde, and acetone in the -initiated oxidation of (x-pinene. in this case an alkoxy radical may form on one of the tertiary carbons that make up the four-membered ring. Acetone formation via this pathway will probably be small since it involves two steps where RO2 radicals must be converted to RO radicals (steps c and e), a process that is not completely efficient and depends on the concentrations of other RO2 and HO2 radicals and the degree of RO2-RO2 radical disproportionation. What is more, decomposition of the first alkoxy radical to produce the tertiary alkyl radical (step d) is probably not competitive with elimination of acetyl. A similar pathway to form acetone can be drawn for [3-pinene. We measured a 10% acetone yield from A3-carene, which is about half that of Reissell et al. [1999] (22%). As with several other acetone yields reported here, the reason for the discrepancy is not known. A 10% acetone yield indicates that a channel leading to its formation must be reasonably favorable. A Scheme VI + ct-pinene o o V D 02 ' RO 2 \c. / o (d) o= 02, RO 2 &(C) Figure 3f. Scheme VI, mechanism for the generation of acetone from the ozonolysis of o:-pinene.

9 ORLANDO ET AL.: - AND O3-INITIATED OXIDATION OF MONOTERPENES 11,569 Scheme VII I 0--0 \ O- O H I + 3 -carene. o o H..k. + c. c o, (d) O2,RO2 (c) / o c j c.,, o (c') /O' O Figure 3g. Scheme VII, mechanism for the generation of acetone from the ozonolysis of A3-carene. proposed pathway is shown in Figure 3g. The key reaction in this mechanism is the isomerization of an alkyl radical (step c) that results after elimination from a primary carbonyl oxide (steps a-b). Alkyl radicals rapidly add 02 (step c'), such that other reaction pathways cannot normally compete; for a secondary radical such as that formed in step b, the rate for this addition is about 5 x 107 s -1[Atkinson, 1997a,b]. A3-carene, however, likely produces a structure containing a substituted cyclopropylcarbinyl radical, and these radicals are known to rapidly rearrange (step c). The rate of ring opening for the Table 2. Formation Yields of Acetone for 03 Reactions With c and [3-Pinenes, A3-Carene, Myrcene, and Terpinolene in a Presence of an Scavenger Acetone Yield, % Monoterpene Scavenger This Work This Work Reissell et al., Other Studies Initial Concentration, Initial Concentration, Average [ 1999] 1014 molecule cm molecule cm -3 ct-pinene butyl ether cyclohexane b _ [3-pinene butyl ether " cyclohexane 4 b A3-carene butyl ether cyclohexane b butyl ether 1.81 in N in O, in myrcene butyl ether b terpinolene butylether O ag wjean et at. [1993] bruppert et al. [1997]

10 11,570 ORLANDO ET AL.' - AND O3-INITIATED OXIDATION OF MONOTERPENES r-i i-i MONOTERPENE CONSUMED (1014 molecule cm '3) Figure 4. Observed acetone production as a function of monoterpene consumed in the reaction of 03 with the monoterpenes' solid circles, o -pinene' open squares, [3-pinene; open triangles, A3-carene; solid diamonds, myrcene; inverted open triangles, terpinolene. unsubstituted cyclopropylcarbinyl is -8 x 107 s -] [Horner et al., 1998]. The effect of the carbonyl group adjacento the radical is expected to be about neutral due to a competition between stabilization of the radical and a lowering of the transition state energy [Newcomb et al., 1999]. In contrast to the pinenes, this pathway to acetone requires formation of only one alkoxy radical (step d). This alkoxy radical, formed at the tertiary carbon, can eliminate acetone and form a secondary alkyl radical (shown), or it can form a larger carbonyl and eliminate a methyl radical. Clearly, the pathway leading to acetone formation is preferred. Further, unlike the pinenes, two of the primary carbonyl oxides likely produce a precursor to acetone formation; thus the acetone unit. Also, acetone yields from the ozone reactions may be somewhat higher since the intermediate alkoxy radicals may be more efficiently generated in the presence of NO. Most other terpenes (i.e., those not containing a-c=c(ch3) 2 linkage) lead to substantially lower acetone yields [Reissell et al., 1999]. Thus, in an assessment of the contribution of terpene oxidation to acetone formation only the major emitted species (the pinenes and limonene, for example) and those leading to substantial acetone yields need be considered. Assessing the overall contribution of terpene oxidation to the global acetone budget is difficult, owing to large uncertainties the speciated emissions inventory. As a starting point, speciated formation from A3-carene is expected to be much higher than monoterpene emissions inventories for North America [Guenther from the pinenes. Experiments in 98+% N 2 or 02 were et al., 2000] were employed. Acetone yields from the various performed to investigate the possibility of a competition between monoterpenes were calculated using the data obtained in this 02 addition (step c') and rearrangement of the work, rate constants for the reactions of the monoterpenes with cyclopropylcarbinyl radical (step c). In N 2 the acetone yield was unchanged, while in 02 the acetone yield decreased to 7.5%, consistent with the proposed mechanism and the idea that in this rare instance, 02 addition in air does not shut down the chemistry of the alkyl radical. and O3 [Atkinson, 1997b], and assumed average [] and [03] of 106 and 10 ]2 molecule cm -3, respectively. Yield data for three monoterpenes not studied here, sabinene, camphene, and ocimene, were taken from Reissell et al. [1999], since they appear to contribute significantly (-25% in total) to the acetone budget calculation. An overall yearly acetone production from 4. Atmospheric Impact monoterpenes released in North America of 0.5 Tg yr -] is obtained. This amounts to only about 1% of the global acetone As alluded to in Section 1, one of the goals of these experiments was to determine the influence of terpene oxidation budget [Singh et al., 1994, 1995], which includes direct emissions from anthropogenic [Singh et al., 1994] and biogenic on the global acetone budget. It is clear that terpenes possessing [Goldan et al., 1995] sources, secondary production from the a-c=c(ch3) 2 component, including terpinolene and the acyclic species, myrcene and ocimene, will all lead to large (-40%) acetone yields. Note also that higher overall acetone yields may result, since acetone is expected as a major product of the some of the products formed in the initial oxidation step. For example, the C9 compounds formed as coproducts of CH20 in the initiated oxidation of myrcene still contain the intact-c=c(ch3) 2 oxidation of anthropogenic and biogenic compounds, and a biomass burning source [Singh et al., 1994]. However, global monoterpenemissions are believed to be -15-fold larger than the North American emissions, and assuming a similar spectrum of emissions as is present in North America, the global acetone source from monoterpen emissions can be scaled accordingly. Thes estimated sources (0.5 Tg yr - in North America, 7.5 Tg

11 ORLANDO ET AL.: - AND O3-INITIATED OXIDATION OF MONOTERPENES 11,571 yr - globally) are somewhat smaller than similar estimates made Atkinson, R., S. M. Aschmann, J. Arey, and B. Shorees, Formation of by Reissell et al., largely because of the lower acetone yields radicals in the gas phase reactions of ozone with a series of terpenes, d. Geophys. Res., 97, , obtained from the pinenes herein. Atkinson, R., E. C. Tuazon, and S. Aschmann, Products of the gas-phase Gas phase observations of formic acid in forested regions as reactions of a series of 1-alkenes and 1-methylcyclohexene with the high as a few ppbv have been reported [Andreae et al., 1988; radical in the presence of NO, Environ. Sci. Technol., 29, Talbot et al., 1990]. The origin of this formic acid has not been 1680, Atkinson, R., E. C. Tuazon, and S. Aschmann, Products of the gas-phase firmly established, though direct emissions [Kesselmeier et al., reaction of the radicals with 3-methyl-1-butene in the presence of 1997] have been observed and the oxidation of isoprene has also NO, Int. d. Chem. Kinet., 30, , been postulated [Jacob and Wofsy, 1990]. Assuming an Aumont, B., S.,Madronich, I. Bey, and G. S. Tyndall, Contribution of pinene mixing ratio of 100 pptv, the 7% formic acid yield secondary VOC to the composition of aqueous atmospheric particles: measured above, and using published rate coefficients for A modeling approach, J. Atm. Chem., 35, 59-75, Biesenthal, T. A., J. W. Bottenheim, P. B. Shepson, S.-M. Li, and P. C. reaction with c -pinene and formic acid (5 x cm 3 molecule - Brickell, The chemistry of biogenic hydrocarbons at a rural site in s - and 4 x cm 3 molecule - s - respectively) [Atkinson, eastern Canada, d. Geophys. Res., 103, 25,487-25,498, b; DeMore et al., 1997], a steady state HCO mixing ratio Calogirou, A., B. R. Larsen, and D. Kotzias, Gas-phase terpene oxidation of 1 ppbv is obtained, consistent with the available field data. products: A review, Atmos. Environ., 33, , DeMore, W. B., S. P. Sander, D. M. Golden, R. F. Hampson, M. J. Clearly, this is a potentially important source of formic acid (and Kurylo, C. J. Howard, A. R. Ravishankara, C. E. Kolb, and M. J. possibly larger organic acids), and further work to confirm the Molina, Chemical kinetics and photochemical data for use in mechanism proposed here is in order. stratospheric modeling, Evaluation No. 12, NASA JPL Publ. 97-4, Finally, it is clear from an examination of the reaction Fehsenfeld, F., et al., Emissions of volatile organic compounds from schemes that a variety of midsized carbonyls, C3-Cs, will be vegetation and the implications for atmospherichemistry, Global formed from the oxidation of the terpenes (for example, Biogeochem. Cycles, 6, , HC(O)CH2CHO of scheme V). While these species are probably Goldan, P. D, W. C. Kuster, F. C. Fehsenfeld, and S. A. Montzka, not unique markers for terpene oxidation, as is the case with the Hydrocarbon measurements in the southeastern United States: The C7-C 9 species, their chemistry requires further consideration. Rural Oxidants in the Southern Environment (ROSE) Program 1990, J. Geophys. Re& 100, 25,945-25,963, This chemistry will likely involve competition between gas-phase Grosjean, D., E. L Williams II, and J. H. Seinfeld, Atmospheric oxidation oxidation processes and heterogeneouscavenging, which will of selected terpenes and related carbonyls: Gas-phase carbonyl increase with the degree of oxygen substitution [Aumont et al., products, Environ. Sci. Technol., 26, , ]. Grosjean, D., E. L. Williams, E. Grosjean, J. M. Andino, and J. H. Seinfield, Atmospheric oxidation of biogenic hydrocarbons: reaction of ozone with [3-pinene, A-limonene and trans-caryophyllene, Environ. Sci. Technol., 27, , Gu, C. L., C. M. Rynard, D. G. Hendry, and T. Mill, radical Acknowledgments. The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under the sponsorship of the National Science Foundation. Acknowledgment is made to the National Science Foundation (grant ATM ) and to the Environmental Protection Agency for partial support of the research at UCLA. Y.R. is incumbent of the William Z. and Eda Bess Novick career development chair. Helpful discussions with Alam Hasson (UCLA), Kenneth Houk (UCLA), and Martin Newcomb (Wayne State University) are gratefully acknowledged. The authors would also like to thank Eric Apel and Jim Greenberg, both of NCAR, for their constructive comments on this work. oxidation of ot-pinene, reporto U.S. Environ. Prot. Agency, SRI Int., Menlo Park, CA, Guenther, A., et al., A global model of natural volatile organic compound emissions, d Geophy. Res., 100, , Guenther, A., C. Geron, T. Pierce, B. Lamb, P. Harley, and R. Fall, Natural emissions of non-methane volatile organic compounds, CO and oxides of nitrogen from North America, Atmos. Environ., in press, Hakola, H., B. Shorees, J. Arey, and R. Atkinson, Product formation from the gas-phase reactions of radicals and 03 with [3- References phellandrene, Environ. Sci. Technol., 27, , Hakola, H., J. Amy, S. M. Aschmann, and R. Atkinson, Product Alvarado, A., E. C. Tuazon, S. M. Aschmann, R. Atkinson, and J. Arey, Products of the gas-phase reactions of O(3P) atoms and O3 with pinene and 1,2-dimethyl-l-cyclohexene, d. Geophys. Res., 103, formation from the gas-phase reactions of radicals and O3 with a series of monoterpenes, J. Atmos. Chem., 18, , Hakola, H., J. Rinne, and T. Laurila, The hydrocarbon emission rate of 25,541-25,551, tea-leafed willow (Salixphyliciœolia), silver birch (Betula pendula) and Andreae, M., and P. J. Crutzen, Atmospheric aerosols: Biogeochemical sources and role in atmospherichemistry, Science, 276, , Andreae, M., R. W. Talbot, T. W. Andreae, and R. C. Harriss, Formic and acetic acid over the central Amazon region, Brazil 1. Dry season, d. Geophys. Res., 93, , Arey, J., R. Atkinson, and S. M. Aschmann, Product study of the gasphase reactions of monote penes with the radical in the presence ofnox, d. Geophys. Res., 95, 18,539-18,546, Aschmann, S. M., A. Reissell, R. Atkinson, and J. Arey, Products of the gas phase reactions of the radicals with ct- and [3-pinene the presence of NO, J. Geophys. Res., 103, 25,553-25,562, Atkinson, R., Gas-phase tropospherichemistry of organic compounds, J. Phys. Chem. Ref Data Monogr. 2, 1-216, 1997a. Atkinson, R., Gas-phase tropospheric chemistry of volatile organic compounds: 1. Alkanes and alkenes, d. Phys. Chem. Ref. Data, 26, , 1997b. European aspen (Populus tremula), Atmos. Environ., 32, , Hatakeyama, S., K. Izumi, T. Fukuyama, and H. Akimoto, Reactions of ozone with ot-pinene and 3-pinene in air: Yields of gaseous and particulate products, J. Geophys. Res., 94, 13,013-13,024, Hatakeyama, S., K. Izumi, T. Fukuyama, H. Akimoto, and N. Washida, Reactions of with ot-pinene and 3-pinene in air: Estimate of global CO production from the atmospheric oxidation of terpenes, J. Geophys. Res., 96, , Helmig, D., B. Balsley, K. Davis, L. R. Kuck, M. Jensen, J. Bognar, T. Smith Jr., R. V. Arrieta, R. Rodriguez, and J. W. Birks, Vertical profiling and determination of landscape fluxes of biogenic nonmethane hydrocarbons within the planetary boundary layer in the Peruvian Amazon, J. Geophys. Res., 103, 25,519-25,532, 1998a. Helmig, D., J. Greenberg, A. Guenther, P. Zimmerman, and C. Geron, Volatile organic compounds and isoprene oxidation products at a temperate deciduous forest site, J. Geophys. Res., 103, 22,397- Atkinson, R., Atmospheric reactions of alkoxy and [3-hydroxyalkoxy 22,414, 1998b. radicals, Int. J. Chem. Kinet., 29, , 1997c. Atkinson, R., S. M. Aschmann, A.M. Winer, and J. N. Pitts, Gas phase reactions of NO2 with alkenes and dialkenes, Int. J. Chem. Kinet., 16, , Helmig, D., L. F. Klinger, A. Guenther, L. Viefling, C. Geron, P. Zimmerman, Biogenic volatile organic compound emissions (BVOCs) I. Identifications from three continental sites in the U.S., Chemosphere, 38, , 1999a.

12 11,572 ORLANDO ET AL.: - AND O3-1NITIATED OXIDATION OF MONOTERPENES Helmig, D., L. F. Klinger, A. Guenther, L. Vierling, C. Geron, P. Zimmerman, Biogenic volatile organic compound emissions (BVOCs) II. Landscape flux potentials from three continental sites in the U.S., Chemosphere, 38, , 1999b. Homer, J., N. Tanaka, and M. Newcomb, Kinetics of cyclopropylcarbinyl radical ring openings measure directly by laser flash photolysis, J. Am. Chem. Soc., 120, 10,379-10,390, Jacob, D. J., and S.C. Wofsy, Photochemistry of biogenic emissions over the Amazon forest, J. Geophys. Res., 93, , Kesselmeier, J., et al., Emission of short chained organic acids, aldehydes, and monoterpenes, from Quercus ilex L. and Pinus pinea L. in relation to physiological activities, carbon budget and emission algorithms, At nos. Environ., 31, Suppl. l, S119-S133, Kramp, F., and S. E. Paulson, The gas phase reaction of ozone with butadiene: Formation yields of some toxic products, Atmos. Environ., 34, 35-43, Kwok, E. S. C., and R. Atkinson, Estimation of hydroxyl radical reaction rate constants for gas-phase organic compounds using a structurereactivity relationship: an update, Atmos. Environ., 29, , Kwok, E. S. C., R. Atkinson, and J. Arey, Observation of hydroxycarbonyls from the radical-initiated reactions of isoprene, Environ. Sci. Technol., 29, , Kwok, E. S. C., R. Atkinson, and J. Arey, lsomerization of [3- hydroxyalkoxy radicals formed from the radical-initiated reactions of C4-C8 l-alkenes, Environ. Sci. Technol., 30, , Makar, P. A., J. D. Fuentes, D. Wang, R. M. Staebler, and H. A. Wiebe, Chemical processing of biogenic hydrocarbons within and above a temperate deciduous forest, J. Geophys. Res., 104, , Meagher, R. J., M. E. Mcintosh, M. D. Hurley, and T. J. Wallington, A kinetic study of the reaction of chlorine and fluorine atoms with HCOF at K, lnr J. Chem. Kinet., 29, , Newcomb, M., Competitive methods and scales for alkyl radical reaction kinetics, Tetrahedron, 49, , Newcomb, M., S. Choi, and J. Homer, Adjusting the top end of the alkyl radical kinetic scale. Laser flash photolysis calibrations of fast radical clocks and rate constants for reactions of benzeneselenol, J. Org. Chem., 64, I, Nozibre, B., I. Barnes, and K.-H. Becker, Product study and mechanisms of the reactions of ot-pinene and of pinonaldehyde with radicals, J. Geophys. Res., 104, 23,645-23,656, 1999a. Nozi re, B., M. Spittier, L. Ruppert, I. Barnes, and K.-H. Becker, Kinetics of the reactions of pinonaldehyde with radicals and with CI atoms, Int. J. Chem. Kinet., 31, , 1999b. Paulson, S. E., and J. J. Orlando, The reactions of ozone with alkenes: An important source of HOx in the boundary layer, Geophys. Res. Lett., 23, , Paulson, S. E., and J. H. Seinfeld, Development and evaluation of a photooxidation mechanism for isoprene, J. Geophys. Res., 97, 20,703-20,715, Paulson, S. E., M. Chung, and G. Orzechowska, Measurement of radical formation from the reaction of ozone with several biogenic alkenes, J. Geophys. Res., 103, 25,533-25,539, Peeters, J., W. Boullart, and J. Van Hoeymissen, Site-specific partial rate constants for addition to alkenes and dienes, in The Proceedings of EUROTRAC Symposium '94, edited by P.M. Borrell et al., PSB Academic, The Hague, Netherlands, pp , Reissell, A., C. Harry, q. M. Aschmann, R. Atkinson, and J. Arey, Formation of acetom )m the radical- and O3-initiated reactions of a series of monoterpenes, J. Geophys. Res., 104, 13,869-13,879, Roselle, S. J., T. E. Pierce, and K. L. Schere, The sensitivity of regional ozone modeling to biogenic hydrocarbons, J. Geophys. Res., 96, , Ruppert, L., et al., Product and aerosol formation from the ozonolysis of monoterpenes, paper presented at the Workshop on Biogenic Hydrocarbons in the Atmospheric Boundary Layer," American Meteorological Society, Charlottesville, VA, Aug Scanlon, J.T., and D. E. Willis, Calculation of flame ionization detector relative response factors using the effective carbon number concept, J. Chromatogr. Sci., 23, , Seufert, G., et al., An overview of the Castelporziano experiments, Atmos. Environ., 31, Suppl. 1, S5-S17, Shetter, R. E., J. A. Davidson, C. A. Cantrell, and J. G. Calvert, Temperature variable long path cell for absorption measurements, Rev. Sci. Instrum., 58, , Singh, H. B., D. O'Hara, D. Herlth, W. Sachse, D. R. Blake, J. D. Bradshaw, M. Kanakidou, and P. J. Crutzen, Acetone in the atmosphere: Distribution, sources and sinks, J. Geophys. Res., 99, , Singh, H. B., M. Kanakidou, P. J. Crutzen, and D. J. Jacob, High concentrations and photochemical fate of oxygenated hydrocarbons in the global troposphere, Nature, 378, 50-54, Su, F., J. G. Calvert, and J. H. Shaw, Mechanism of the photooxidation of gaseous formaldehyde, J. Phys. Chem., 83, , Talbot, R. W., M. O. Andreae, H. Berresheim, D. J. Jacob, and K. M. Beecher, Sources and sinks of formic, acetic, and pyruvic acids over central Amazonia 2. Wet season, J. Geophys. Res., 95, 16,799-16,811, Taylor, W. D., T. D. Allston, M. J. Moscato, G. B. Fazekas, R. Kozlowski, and G. A. Takacs, Atmospheric photodissociation lifetimes for nitromethane, methyl nitrite, and methyl nitrate, lnt. J. Chem. Kinet., 12, 231, Trainer, M., et al., Models and observations of the impact of natural hydrocarbons on rural ozone, Nature, 329, , Vereecken, L., J. Peeters, J. J. Orlando, G. S. Tyndall, and C. Ferronato, Decomposition of [3-hydroxypropoxy radicals in the -initiated oxidation of propene: A theoretical and experimental study, J. Phys. Chem. A, 103, , Zimmerman, P. R., J.P. Greenberg, and C. E. Westberg, Measurements of atmospheric hydrocarbons and biogenic emission fluxes in the Amazon boundary layer, J. Geophys. Res., 93, , B. Nozibre, Advanced Studies Program, NCAR, 1850 Table Mesa Drive, Boulder, CO (noziere@ucar.edu) J. J. Orlando and G. S. Tyndall, Atmospheric Chemistry Division, NCAR, 1850 Table Mesa Drive, Boulder, CO (orlando@ acd.ucar.edu: acd.ucar.edu) G. E. Orzechowska and S. E. Paulson, Department of Atmospheric Sciences, University of California, 405 Hilgard Avenue, Los Angeles, CA (paulson@ atmos.ucla.edu) Y. Rudich, Department of Environmental Sciences, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel. wise. weizmann. ac.il) (Received August 16, 1999; revised December 20, 1999; accepted December 30, 1999.)