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The reaction between NO3 and R-pinene has been studied in large reaction chambers of 0.5-200 m3 volume, using long path FT-IR, GC-ECD, and GC-FID for ...
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Environ. Sci. Technol. 1997, 31, 2130-2135

Product and Mechanistic Study of the Reaction of NO3 Radicals with r-Pinene I . W A¨ N G B E R G , * I . B A R N E S , A N D K. H. BECKER Physikalische Chemie/Fachbereich 9, Bergische Universita¨t-GH Wuppertal Gauss Strasse 20, 42097 Wuppertal, Germany

The reaction between NO3 and R-pinene has been studied in large reaction chambers of 0.5-200 m3 volume, using long path FT-IR, GC-ECD, and GC-FID for the analyses. The reaction yielded 62 ( 4% pinonaldehyde (3-acetyl-2,2dimethylcyclobutane acetaldehyde) and 3 ( 0.5% pinane epoxide. The total yield of alkylnitrates was estimated to be approximately 14%; two of the nitrates have been identified as 3-oxypinane-2-nitrate, and 2-hydroxy-3nitrate with yields of 3 ( 0.2% and 5 ( 0.4%, respectively. This work represents the first quantitative identification of pinonaldehyde and alkylnitrates from the reaction of NO3 with R-pinene. A thermally stable peroxy acylnitrate was observed to be formed from secondary reactions of pinonaldehyde in the system. This compound has been assigned to 3-acetyl-2,2-dimethylcyclobutane acetylperoxynitrate. Possible implications for the atmospheric NOx chemistry are discussed. From the product data, a mechanism for the NO3 + R-pinene reaction has been constructed.

Introduction The oxidation of unsaturated hydrocarbons, initiated by NO3 addition to carbon-carbon double bonds, constitutes a significant nighttime loss process for biogenically and anthropogenically emitted hydrocarbons (1, 2). Mechanistic studies of these reactions with propene, various butenes, isoprene, and simple cycloalkenes have revealed that the NO3 addition initiates a degradation process leading to the formation of oxygenated organic products such as aldehydes, ketones, and bifunctional nitrooxy-substituted compounds. The large emission strengths of monoterpenes (3), in particular R- and β-pinene, limonene, and ∆3-carene, and their high reactivity toward NO3 radicals make information on this class of reactions especially important for the understanding of nighttime tropospheric chemistry. Information on mechanisms and product yields from reactions between NO3 and monoterpenes is however scarce. For R-pinene, for example, the only product identified is pinonaldehyde (4) (its yield has not been quantified); the other products are unknown. Here we report results from a product study of the reaction between NO3 radicals and R-pinene performed in three different sized reactors using FT-IR , GC-FID, and GC-ECD for the analysis and the thermal decomposition of N2O5 as the source of NO3 radicals. From the product data, a mechanism has been constructed for the reaction of NO3 with R-pinene.

Experimental Section The experiments were carried out in reaction chambers equipped with long path optical mirror systems for sensitive * Author to whom correspondence should be sent. Fax: +49-202439-2505; e-mail: [email protected].

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(ppb range) in situ monitoring of reactants and products using FT-IR spectroscopy, which was the principal analytical method used in the investigations. The decomposition reaction of N2O5 was used as the source of NO3 radicals according to the equilibrium (1/-1).

NO2 + NO3 + (M) S N2O5 + (M)

(1/-1)

Since experimental conditions, such as the initial concentration of the reactants and the volume to surface ratio of the reaction chambers, were seen to be critical parameters, the experiments were performed in three very differently sized reactors. The reactor size, FT-IR facility, and the experimental conditions used in the investigations are summarized in Table 1. In experiments using the 0.5 m3 reactor, N2O5 was prepared directly in the reactor by mixing ozone with excess NO2, according to

O3 + NO2 f NO3 + O2

(2)

followed by reaction 1. Ozone was produced in a stream of pure oxygen by a silent discharge ozone generator. After preparation of N2O5, R-pinene was then introduced into the reactor in a flow of pure nitrogen. In experiments with the 1 m3 reactor, N2O5 was produced by mixing high concentrations of O3 with excess NO2 in a glass flask of 11 dm3 volume. The glass flask was connected to the 1 m3 reactor, and aliquots of the N2O5/NO2 mixture were transferred into the reactor via a valve. A number of experiments were performed in the largescale (200 m3) EUPHORE photoreactor facility in Valencia, Spain (7). The following procedure was used to prepare N2O5/ NO2 mixtures for use in this reactor. Pure oxygen from a gas flask was passed through a mass flow controller and into an silent discharge ozone generator yielding about 3000 ppm ozone in oxygen at a flow rate of 925 mL min-1. An NO2/ N2O4/air mixture containing 15% NO2 (calculated as CNO2 ) [NO2] + 2[N2O4]) was passed through a mass flow controller set to 42-50 mL min-1. The ozone and nitrogen oxide flows were then mixed in a Pyrex flow reactor prior to inlet into the 200 m3 reactor. The time needed to reach a concentration of N2O5 corresponding to 8 × 1012 molecules cm-3 was 20 min. Experiments with the 1 and 200 m3 reactors where initiated either by adding the concentrated N2O5/NO2 mixture into the reactor and then introducing R-pinene or vice versa. The mixing times of reactants in the 0.5 and 1 m3 reactors were less than 1 min. In the Valencia chamber, the time needed to mix the reactants was 4-20 min, depending on the order in which R-pinene and the N2O5/NO2 mixture were added. Analytical Methods. Infrared absorption cross sections of low volatile reference compounds were obtained by simultaneously measuring the pressure and recording infrared spectra when samples of the reference compounds were evaporated into the evacuated 0.5 m3 reactor. The pressure was measured using a Baratron (type 128) pressure gauge, with a resolution of 0.01 Pa. The following peak infrared absorption cross sections were obtained: 3-oxopinane-2nitrate at 1654.6 cm-1, (1.94 ( 0.09) × 10-18 cm2 molecule-1; 2-hydroxypinane-3-nitrate at 1662.1 cm-1, (1.33 ( 0.07) × 10-18 cm2 molecule-1; pinane epoxide at 2924.8 cm-1, (6.54 ( 0.14) × 10-19 cm2 molecule-1. Pinonaldehyde was measured using the infrared absorption cross section of (3.51 ( 0.14) × 10-19 cm2 molecule-1 at 1725.5 cm-1. This value has been obtained using the same calibration technique as described above (8). All cross sections are expressed as base 10, and stated precisions are the statistical errors at the 95% confidence level. Gas samples (1 cm3) for direct GC-ECD analysis

S0013-936X(96)00958-3 CCC: $14.00

 1997 American Chemical Society

TABLE 1. Summary of Reactors Used in the Experiments, FT-IR Systems, and the Experimental Conditions Employed in Investigationsa reactor 0.5

m3

reactor in Wuppertal

FT-IR setup 1

cm-1

resolution, 52 m optical path length

1 m3 reactor in Wuppertal

1 cm-1 resolution, 492 m optical path length

EUPHORE reactor, 200 m3

1 cm-1 resolution, 327 m optical path length

a

experimental conditions room temperature 1000 mbar synthetic air typical starting concentrations: [R-pinene] ) 1.5 × 1014 molecules cm-3 (6000 ppb) [N2O5] ) 1 × 1014 molecules cm-3 (4000 ppb) [NO2] ) 1.6 × 1014 molecules cm-3 (6640 ppb) room temperature 1000 mbar synthetic air typical starting concentrations: [R-pinene] ) 1.5 × 1013 molecules cm-3 (600 ppb) [N2O5] ) 1 × 1013 molecules cm-3 (400 ppb) [NO2] ) 7.5 × 1012 molecules cm-3 (300 ppb) room temperature 1000 mbar purified air typical starting concentrations: [R-pinene] ) 1.2 × 1013 molecules cm-3 (480 ppb) [N2O5] ) 8 × 1012 molecules cm-3 (320 ppb), [NO2] ) 3 × 1012 molecules cm-3 (120 ppb)

Descriptions of the 0.5, 1, and 200 m3 reactors can be found in refs 5-7, respectively.

of 2-hydroxypinane-3-nitrate were taken from the 0.5 m3 reactor. A specially designed syringe was used to prevent this compound from sticking to the surface of the syringe. The following procedure was employed. The syringe was connected to the reactor via a septum port. By help of excess pressure in the reactor of about 20 mbar over the ambient pressure, the surface of the syringe could be conditioned by flushing the sample gas through it at a gas flow rate of 1.5 mL s-1. After 5 min of such conditioning, the gas phase inside the syringe was considered as being representative of that in the reactor. The syringe was removed from the reactor and the needle replaced. The sample was then transferred to the GC instrument (Helwett Packard 5890) for split injection on a 60 m, i.d. ) 0.53 mm RTX-200 (Restek) column. Air samples for GC-FID analysis of pinane epoxide, although not as sticky as that of 2-hydroxypinane-3-nitrate, were handled in a similar manner; however, 2 mL gas samples were injected on a 30 m, i.d. ) 0.53 mm Stabilwax column (Restek). To convert GC peak areas to concentration, pure reference compounds were introduced into the 0.5 m3 volume reactor, and samples for GC analysis were taken, using the described method, while the concentration was simultaneously monitored by FT-IR. Chemicals. The chemicals used and their purities were as follows: (1S)-(-)-R-pinene (99%) was obtained from Fluka, trans-R-pinane epoxide (98%) was obtained from Aldrich, 2-hydroxypinane-3-nitrate was synthesized by nitration of (1S,2S,3R,5S)-(+)-pinane-2,3-diol (Aldrich), and 3-oxo-pinane-2-nitrate was synthesized by nitration of (1S,2S,5S)-2hydroxy-3-pinanon (Aldrich). A sample of pinonaldehyde was obtained from M. Hallquist, Inorganic Chemistry Department of the University of Go¨teborg, who prepared it by ozonolysis of (1S)-(-) R-pinene (8). All synthesized reference compounds were analyzed by recording twodimensional 1H/13C NMR and DEPT spectra and found to be pure stereo isomers of g95% purity. Infrared spectra of 2-hydroxypinane-3-nitrate and 3-oxopinane-2-nitrate are shown in Figure 1. These compounds are colorless solids with melting points between 25 and 30 °C and have to our knowledge not been described previously. The infrared spectra of 2-hydroxypinane-3-nitrate show three strong ONO2 group absorptions at 1662, 1289, and 849 cm-1; a typical O-H stretching band at 3634 cm-1 is also seen. The corresponding ONO2 vibrations of 3-oxopinane-2-nitrate are at 1655, 1290, and 846 cm-1, and the carbonyl band is at 1750 cm-1.

FIGURE 1. FT-IR spectra of 2-hydroxypinane-3-nitrate (top) and 3-oxopinane-2-nitrate (bottom), recorded in the spectral range 3700700 cm-1.

Results The yields of the products identified in the reaction between NO3 and R-pinene are listed in Table 2. In all cases, the yields are the maximum yields observed and have not been corrected for loss due to further chemical reaction, loss to the wall, or loss to aerosol surfaces. Product Identification and Yields. Figure 2, trace a, shows an infrared spectrum from an experiment with the 1 m3 reactor, recorded after all the N2O5 had reacted. Figure 2, trace b, is a reference spectrum of 160 ppb pinonaldehyde (3-acetyl-2,2-dimethylcyclobutane acetaldehyde). From a visual comparison between traces a and b, it is evident that pinonaldehyde is a major product of the reaction. Its yield has been determined by spectral subtraction of the aldehydic hydrogen stretching bands at 2715 and 2814 cm-1 using a calibrated reference spectrum. From experiments performed in the 0.5 m3 volume reactor, the pinonaldehyde yield was (40.7 ( 2)%. However, at about 10 times lower initial concentrations of the reactants in the 1 m3 volume reactor, the yield increased to (52.6 ( 5)%. In experiments in the 200 m3 volume reactor, a pinonaldehyde yield of (62 ( 4)% was obtained. Indications of aerosol formation were observed in experiments made with high initial concentrations of reactants, i.e., broad absorptions increasing with reaction time were observed in the FT-IR spectra in the range from 2000 to 4000 cm-1, which is indicative of aerosol formation. Indication of aerosol formation was however observed in all experiments,

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TABLE 2. Formation Yields of Products Observed in Reaction of N2O5/r-Pinene/Air Reaction Mixturesa yields (%)

products pinonaldehyde

62 ( 4 O

pinane epoxide

FTIR

O

3 ( 0.5

O

[ ] 5 ( 0.4

2-hydroxypinan3-nitrate

OH

3-oxopinan-2nitrate

ONO2 O

total alkylnitrates sum of product yields

analytical method

ONO2

GC-FID

FTIR/GC-ECD

3 ( 0.2

FTIR

14 ( 0.7 79 ( 4

FTIR

FIGURE 3. FT-IR product spectra recorded in the spectral range 1770-1600 cm-1. Trace a shows the asymmetric (O-N-O) absorptions in a product spectrum from the same experiment as shown in Figure 2. Absorptions due to 2-hydroxypinane-3-nitrate have already been subtracted. Structures at 1610-1640 cm-1 in trace a are remains from subtraction of NO2. Trace b is a reference spectrum of 3-oxopinane-2-nitrate.

a All product yields are the maximum observed yields and have not been corrected for losses due to secondary chemistry or loss to aerosol and wall surfaces.

FIGURE 2. FT-IR product spectra recorded in the spectral range 3200-700 cm-1. Trace a is a product spectrum from an experiment where N2O5 was added to r-pinene. The spectrum was recorded after all N2O5 had been consumed. Absorptions due to NO2, r-pinene, HNO3, and water have already been subtracted. Trace b is a reference spectrum of an authentic sample of 160 ppb pinonaldehyde. The presence of absorptions from pinonaldehyde in spectrum a is evident. Spectrum a also contains absorptions due to alkylnitrate groups. but appeared to decrease with decreasing start concentration. It is very likely that the difference in the measured yield of pinonaldehyde in the different reactors is lost from the gas phase to an aerosol. This problem can be compounded by deposition of pinonaldehyde at the reactor walls. The higher yield of pinonaldehyde in the 200 m3 volume reactor may then be due to the much smaller surface to volume ratio of this reactor. Intense infrared bands due to nitrates are also present in the product spectrum shown in Figure 2, trace a. Two alkylnitrates, 2-hydrox-pinane-3-nitrate and 3-oxopinane2-nitrate, were identified by GC-ECD and in situ FT-IR, respectively, and their yields are given in Table 2. FT-IR spectra of the strongest nitrate band (O-N-O asymmetric stretching) centered at about 1655 cm-1 is shown in Figure 3, trace a, in which contributions from 2-hydroxypinane-3nitrate have already been subtracted. Figure 3, trace b, is a reference spectrum of 3-oxopinane-2-nitrate. The comparison shows that 3-oxopinane-2-nitrate can be identified by its sharp Q branch. The hydroxy- and oxopinane nitrates were,

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FIGURE 4. FT-IR spectrum recorded during the addition of an amount of N2O5 corresponding to 1.0 × 1013 molecules cm-3 to 1.4 × 1013 molecules cm-3 of r-pinene. Features due to NO2, N2O5, H2O, pinonaldehyde, and r-pinene have already been subtracted. The strong residual -O2NO2 and -ONO2 absorption bands have been assigned to 2-nitroperoxypinane-3-nitrate and 3-nitroperoxypinane2-nitrate. however, not the only nitrates being formed. After subtraction of the two identified nitrates, absorptions due to one or more unidentified alkylnitrate remained in the infrared spectrum. Apart from the nitrate bands, absorption likely to be due to carbonyl compounds was also present. The total yield of alkylnitrates has been estimated to be approximately 14%. This estimation is based on the integrated absorption of the alkylnitrate band of the product spectra in the range from 1635 to 1700 cm-1 using the average integrated cross section, (2.32 ( 0.13) × 10-17 cm molecule-1 (base 10), of 2-hydroxypinane-3-nitrate and 3-oxopinane-2-nitrate. Formation of pinane epoxide was measured by GC-FID, and the average yield from five measurements, using the 0.5 m3 reactor, is given in Table 2. The yield of pinonaldehyde given in Table 2 is the average yield obtained from four experiments made in the 200 m3 reactor. The yields of 2-hydroxypinane-3nitrate, 3-oxopinane-2-nitrate, and the estimated total yields of alkylnitrates are the average from three experiments made in the 1 m3 reactor.

Discussion Reaction Mechanism. Under the experimental conditions, i.e., mixing N2O5/NO2 and R-pinene with R-pinene in excess, most of the N2O5 was reacted within the time of mixing the reactants. Figure 4 shows an FT-IR spectrum recorded during the addition of N2O5 to R-pinene in an experiment using the

FIGURE 5. Tentative reaction mechanism leading to pinonaldehyde and bifunctional alkylnitrates in the reaction of NO3 radical with r-pinene. 1 m3 reactor. During this mixing period bands due to -O2NO2 and -ONO2 groups are evident. These bands have been assigned to 2-nitroperoxypinane-3-nitrate, which is likely to be formed via reactions 3, 5, and 6 as shown in Figure 5. The nitrate radical will predominately add to carbon number 3 forming a tertiary alkyl radical. Addition to carbon 2 forming a less stable secondary alkyl radical may, however, also occur to some extent. The alkyl radical will in the presence of oxygen form a peroxy radical, which in turn reacts with NO2 forming 2-nitroperoxypinane-3-nitrate and some 3-nitroperoxypinane-2-nitrate. Reaction 4, yielding pinane epoxide (low yields of which are observed) is also conceivable. Epoxide formation from NO3 addition to alkenes has been reported for several compounds (9); however, high yields are generally only obtained under conditions of low pressure and low oxygen concentration. Under atmospheric conditions the reported epoxide yields are low, which is in line with the present result. The alkyl peroxynitrate decomposes back to the peroxy radical via reaction -6 and thus reaction 6 and -6 constitute an equilibrium. Despite the presence of relatively high NO2 concentrations, the infrared peroxynitrate bands disappeared within minutes as soon as all N2O5 had reacted. This transient behavior indicates that the peroxynitrate is a thermally very unstable compound, which would be expected for a peroxynitrate with this structure (10). Removal of the peroxynitrate is likely to occur via self-reaction of the peroxy radicals according to reaction 7 or through reaction between the peroxy radical and NO3 according to reaction 8. The importance of reaction 8 in the system can be assessed since reaction 8 consumes an additional NO3 molecule, and thus the ratio ∆[R-pinene]/∆[N2O5] will be lower than unity if this reaction path is operative. The average ∆[R-pinene]/∆[N2O5] ratio from 12 experiments was 0.99 ( 0.07, where the precision is expressed as 1 σ(n - 1). Accordingly, no significant differences in this ratio were seen between experiments where N2O5 was added to R-pinene or vice versa. This result suggests

that reaction 8 is of little importance in the present reaction system. Both reactions 7 and 8 yield the same alkoxy radical, which will largely decompose forming pinonaldehyde according to reaction 9a. 3-Oxopinane-2-nitrate and 2-hydroxypinane-3-nitrate are assumed to be formed through cross-reaction between isomeric peroxy radicals according to reaction 10b, where the second peroxy radical in reaction 10b is thought to be formed from NO3 addition to carbon number 2 of R-pinene. One can compare the formation yields of 3-oxopinane-2nitrate and 2-hydroxypinane-3-nitrate with those of oxo and hydroxynitrates observed from the reactions between NO3 and cyclohexene and NO3 and 1-methylcyclohexene (11). The oxo- and hydroxynitrate formation yields from cyclohexene were both about 25% each, whereas the yields of the corresponding compounds from 1-methylcyclohexene were only about 10% each. Reactions such as eq 10b require that at least one of the peroxy radicals contains a -CH(O2)- group, since this reaction involves a hydrogen transfer from one of the reactants to the other. The reason for the lower yield of nitrates in the case of 1-methylcyclohexene was attributed to the preferred formation of tertiary peroxy radicals in the addition of NO3 to 1-methylcyclohexene. The reason for the low yield of oxo- and hydroxynitrates from R-pinene is likely to be the same, and in addition, the complicated bicyclic structure of R-pinene may also give rise to steric hindrance. Influence from steric hindrance is even more conceivable when considering the presumed complicated nature of this type of reaction (12). Reaction 10a forming two isomeric alkoxy radicals is probably the principal product channel of reaction 10. The second alkoxy radical formed in reaction 10a, a 2-nitrooxypinane-3-alkoxy radical, will also decompose to form pinonaldehyde and NO2, similar to reaction 9a. The decomposition reaction may, however, occur in competition with hydrogen abstraction by O2, forming the 3-oxopinane2-nitrate. Experiments were made where the oxygen concentration was varied from 3 to 21%, but no difference in the

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FIGURE 6. FT-IR product spectra recorded in the spectral range 1760-1550 cm-1. Trace a is a product spectrum from an experiment in the 0.5 m3 volume reactor showing two nitrate bands in the asymmetric -ONO2 stretching region. Traces b and c are liquid phase spectra of 3-oxopinane-2-nitrate and 2-hydroxypinane-3-nitrate, respectively.

yield of 3-oxopinane-2-nitrate was observed. This implies that the alkoxy radical is too unstable to undergo further reactions other than decomposition. Decomposition of the alkoxy radical forming nopinon3-nitrate and a methyl radical according to reaction 9b is suggested as one possible route to the formation of another nitrate-containing compound. Because of the lack of a reference compound, the formation of nopinon-3-nitrate cannot be proven. As mentioned in the results section, infrared bands due to alkyl nitrates and carbonyl groups remain in the product spectrum after subtraction of all identified compounds. In the presence of methyl radicals, there is another possible reaction channel forming 2-hydroxypinane-3-nitrate, i.e., reaction 11. The occurrence of this reaction would be consistent with the yield of 2-hydroxypinane-3-nitrate being slightly higher than the yield of 3-oxopinane-2-nitrate, in addition, the reactions also yielded small amounts of formaldehyde. One interesting question is to what extent OH radical formation may take place in this reaction system. As the NO concentration is negligible, OH radicals can only be formed from reactions between NO3 and the hydroperoxy radical (1):

NO3 + HO2 f OH + NO2 + O2

(12)

Hydroperoxy radicals, in turn, are formed from hydrogen abstraction reactions between alkoxy radicals and oxygen. The question is then to what extent alkoxy radicals with an abstractable hydrogen will occur. In reaction 10a such an alkoxy radical is formed, but as mentioned above, the product data support that it is more likely to decompose rather than undergo hydrogen abstraction. Fragmentation such as can occur in reaction 9b may also produce hydroperoxy radicals and consequently OH radicals via reaction (12). That is, hydroperoxy radical formation is dependent on to what extent carbon-carbon bonds, except the double bond, are broken. The products support that at least 70% of the R-pinene oxidation occurs through reaction paths that do not produce hydroperoxy radicals. Thus, implying that the OH formation is limited. Apart from the broad absorptions observed in the 20004000 cm-1 region discussed in the results section, another indication of aerosol formation at high initial reactant concentrations was observed. FT-IR spectra from experiments made in the 0.5 m3 volume reactor showed two infrared bands in the range of 1600-1700 cm-1 as shown in Figure 6, trace a. Palen et al. (13) have recorded infrared spectra of liquid aerosol particles containing carbonyl and alkylnitrates

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FIGURE 7. FT-IR spectrum of 3-acetyl-2,2-dimethylcyclobutaneacetylperoxynitrate in the range 3300-700 cm-1 recorded at 1 cm-1 resolution. Typical -OONO2 bands are evident at 794, 1301, and 1736 cm-1. The carbonyl band at 1829 cm-1 is due to the -C(O)O2NO2 group.

using FT-IR microscopy. They sampled aerosols formed in NOx photooxidation experiments with isoprene and β-pinene. The asymmetric stretching alkylnitrate absorptions gave raise to a narrow band centered in the range 1629-1634 cm-1, which is 20-40 cm-1 lower in comparison to the absorption of alkylnitrates in the gas phase. Traces b and c in Figure 6 are pure liquid phase spectra of 3-oxopinane-2-nitrate and 2-hydroxypinane-3-nitrate. Their centers of absorption at 1628 and 1632 cm-1, respectively, are lowered by 27-30 wavenumbers in comparison to the gas phase absorptions. Since the band at around 1630 cm-1 does not appear in experiments made with low starting concentrations of reactants, it is likely that the absorption at 1630 cm-1 is due to absorption of alkylnitrates trapped in a liquid aerosol matrix, which probably also contains pinonaldehyde. The yields of alkylnitrates given in Table 2 are from experiments made with low starting concentrations of the reactants, which gave somewhat higher yields than those obtained in the high concentration experiments. The total yield of identified products is 73% and increases to 79% on inclusion of the estimated total yields of alkylnitrates. The origin of the missing products is not known, but they may partly be due to carbonyl compounds, as weak absorption in the carbonyl stretching range remains after subtraction of the identified products. It is mentioned above that the infrared bands from 2-nitroperoxypinane-3-nitrate disappeared within minutes as soon as all N2O5 had reacted. In some experiments, however, small absorptions that seemed to belong to a stable alkyl peroxynitrate remained. In an experiment with a higher starting concentration of N2O5 than of R-pinene, the strength of these absorptions were enhanced. The appearance of a band at 1829 cm-1 suggested that the peroxynitrate bands are due to an acyl peroxynitrate. In an experiment where pinonaldehyde was reacted with an N2O5/NO2 mixture in synthetic air, pinonaldehyde seemed to be quantitatively transformed to an acetyl peroxynitrate. An infrared spectrum of this compound is shown in Figure 7 and has been assigned to 3-acetyl-2,2-dimethylcyclobutane acetylperoxynitrate. The spectrum in Figure 7 is identical to that of the acyl peroxynitrate compound found in the R-pinene + NO3 experiments and supports that 3-acetyl-2,2-dimethylcyclobutane acetylperoxynitrate is being formed. The suggested route to this compound is hydrogen abstraction of pinonaldehyde by NO3 as shown in reaction 13. The acyl radical will further react with oxygen and NO2 according to reactions 14 and 15.

the thermal stability of 3-acetyl-2,2-dimethylcyclobutane acetylperoxynitrate should be similar to that of PAN, its formation along with other analogous peroxynitrates from similar terpenes could represent an appreciable reservoir and a means of transport for nitrogen oxides in the troposphere.

Acknowledgments Mattias Hallquist (Department of Inorganic Chemistry, Go ¨ teborg) is acknowledged for providing a sampe of pinonaldehyde. This work was supported by the Bundesminister fu ¨r Bildung, Wissenschaft, Forschung und Technologies (BMBF) and the EC (European Commission). I.W. thanks the EC for a Human Capital and Mobility grant.

Literature Cited Preliminary results indicate that the atmospheric stability of this compound is comparable with that of peroxyacetylnitrate (PAN), which is what would be expected from the structure of the peroxynitrate (10). Atmospheric Implications. Pinonaldehyde has been reported to be an important product from reactions of R-pinene with NO3, OH, and O3 (14-17). However, this is to our knowledge the first time that the yield of pinonaldehyde from the reaction between NO3 and R-pinene has been quantified. The observation of high yields of pinonaldehyde from the NO3/R-pinene reaction indicates that, although NO3 is adding to the double bond, a large fraction of the nitrogen is being re-released during the course of the oxidation in the form of NO2 as indicated in the mechanism in Figure 5. The yields of hydroxy- and oxoalkylnitrates from R-pinene are low in comparison to cycloalkenes such as cyclohexene and 1-methylcyclohexene and are likely to be due to the bicyclic structure of R-pinene. Other monocyclic and bicyclic terpenes that contain a similar structure to R-pinene (for example, limonene, terpinene, and ∆3-carene) are likely to show the same behavior and will therefore not act as efficient nighttime reservoirs of NOx. This is in stark contrast to the other biogenically important compound, isoprene, where laboratory experiments show that a very large fraction of the reacting NO3 will probably be bound up as bifunctional nitrates (1, 18). An interesting aspect of the work is the observation of a peroxy acylnitrate from secondary reactions of pinonaldehyde in the system that has been assigned to 3-acetyl-2,2dimethylcyclobutane acetylperoxynitrate. In the atmosphere, pinonaldehyde formed during the night can form the acylperoxnitrate during the following day via reaction with OH or possibly photolysis. As pinonaldehyde is also a product from the reactions of R-pinene with OH and ozone, the peroxynitrate will also be formed during the day from these reactions. Recent studies show that the reaction of pinonaldehyde with OH radicals is fast (8, 19); therefore, significant levels of 3-acetyl-2,2-dimethylcyclobutane acetylperoxynitrate could be formed during the day on a short time scale in regions emitting terpenes and influenced by high NOx levels. Since

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Received for review November 13, 1996. Revised manuscript received February 19, 1997. Accepted February 24, 1997.X ES960958N X

Abstract published in Advance ACS Abstracts, May 1, 1997.

VOL. 31, NO. 7, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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