Products and mechanism of the gas-phase reaction of radicalsNO3with a-pinene

Torsten Berndt* and Olaf Bo� geInstitut e.V ., Permoserstr. 15, 04303 L eipzig, Germanyf u� r T ropospha� renforschung

The gas-phase reaction of radicals with a-pinene has been studied under Ñow conditions in the pressure range 20 \ p/mbarNO3\200 at 298 K using gas chromatographyÈmass spectrometry/Ñame ionisation detection (GCÈMS/FID), MS and long-pathFTIR spectroscopy as detection techniques. radicals were generated by thermal decomposition of He, orNO3 N2O5 . N2 O2ÈN2mixtures served as carrier gas. In the absence of in the carrier gas, a-pinene oxide (ca. 60%) and an organic nitrate (ca. 40%)O2were found to be the main products with minor amounts of a-campholene aldehyde. The yields were slightly pressure dependent.In the presence of pinonaldehyde was the predominant product. When NO was added for conversion of the formed peroxylO2 ,radicals via the following product yields were obtained at p \ 200 mbar and moleculeRO2 ] NO] RO] NO2 , [O2][ 1017cm~3, pinonaldehyde 75 ^ 6%, a-pinene oxide 15 ^ 3%, organic nitrates (total) 14^ 3%, a-campholene aldehyde 3^ 1%. In theabsence of NO, nitroperoxy-group-containing substances were observed, arising from the reaction ARO2 ] NO2] RO2NO2 .reaction mechanism is proposed and a tropospheric application of the results is discussed.

Terpenes, in particular a-pinene, are amongst the most abun-dant naturally emitted hydrocarbons in the troposphere, witha globally averaged emission of 550 Tg a~1. On a global scale,the emission of biogenic hydrocarbons exceeds the emission ofnon-methane hydrocarbons with anthropogenic origin, 90 Tga~1.1 Thus, biogenic hydrocarbons play an important role inthe chemistry of the troposphere. Their degradation processcan be initiated by reaction with OH radicals, andO3 NO3radicals. Rate constants for most of these reactions at roomtemperature are available with high accuracy.2h4 Tropo-spheric lifetimes calculated with the assumption [OH]\ 106molecule cm~3, molecule cm~3 and[O3]\ 7.5 ] 1011

molecule cm~3 5 show that the night-time[NO3]\ 2.5 ] 108reaction of radicals with terpenes is an important sinkNO3for such substances, for example a-pinene : h,qOH \ 5.2 qO3

\4.3 h, min. Note that the commonly used night-qNO3

\ 10.8time concentration for radicals of 2.5 ] 108 moleculeNO3cm~3 seems to be inappropriate for situations with relativelyhigh terpene concentration levels because the terpenes maysuppress the radical concentration.6,7 In general, in spiteNO3of the importance of terpenes in the troposphere, knowledgeof the product distributions and mechanisms of the radical- orozone-initiated degradation is very limited. This fact is validespecially for radical reactions.NO3Jay and Stieglitz8 investigated product distributions fromthe reaction of radicals with a-pinene, b-pinene andNO3sabinene in synthetic air in a 10 l glass Ñask. As well as a largenumber of unidentiÐed products (nitrates, hydroxynitrates andketonitrates), pinonaldehyde from a-pinene and nopinonefrom b-pinene were detected, but no yields were obtained.From an FTIR chamber study,9 the formation of nitro-oxynitroperoxy compounds in the initial stage of the reactionof radicals with a-pinene, b-pinene, *3-carene and D-NO3limonene was observed. Because of rapid particle formation,Ðnal products of the gas-phase reaction could not be identi-Ðed. Recently, product yields from the reaction of rad-NO3icals with a-pinene obtained under atmospheric conditions ina 200 m3 chamber have been published :10 pinonaldehyde58%, organic nitrates (total) 12%, a-pinene oxide \3%.

Here, we investigate the gas-phase reaction of radicalsNO3with a-pinene under Ñow conditions, with special attention tothe product distribution. By variation of the experimentalconditions (system pressure, composition of the carrier gas), itwas expected to get information concerning the individual

pathways leading to the observed products. The sum of allexperimental Ðndings should allow us to postulate a reactionmechanism applicable to the troposphere. For an extrapo-lation of experimental results from laboratory studies to thetroposphere, a more detailed knowledge of the elementarysteps of the considered chemical process seems to be neces-sary. Flow-tube experiments represent an e†ective way of elu-cidating these processes.

ExperimentalThe experimental set-up has been described previously.11,12Reactions were conducted in a 2.13 cm id quartz Ñow-tube inthe pressure range 20 \ p/mbar \ 200 at 298 K. radicalsNO3were generated in a thermal pre-reactor, located in a side tube,by thermal decomposition of at ca. 450 K.N2O5

N2O5] M ] NO3] NO2 ] M (1)

For GC analysis, a GCÈMS/FID system (HP 5890 withHP-MSD 5971) was connected to the tube ca. 85 cm down-stream of the mixing point for the radicals by means of aNO3vacuum system. IdentiÐcation of the products, a-pinene oxide,a-campholene aldehyde and pinonaldehyde, was achievedusing mass spectra and retention times of authentic samples.Absolute calibrations have been performed for a-pinene anda-pinene oxide by means of the FID. According to the methodof “e†ective carbon numberÏ,13 the response factor of a-campholene aldehyde was assumed to be equal to that froma-pinene oxide. Because of observed loss processes, especiallyin the transfer unit to the GC-system, and problems with themetering unit (low vapour pressure), a quantitative determi-nation of pinonaldehyde by this method was found to bedefective.

For on-line FTIR detection, a 2050 cm3 cell with a Whitemirror system (optical path length 10 m) was attached to theÑow tube downstream of the port for the GC sampling. In thespectral range 1000È4000 cm~1, by co-adding of 200 scansand an instrumental resolution of 8 cm~1, FTIR analysis wasperformed by a Nicolet spectrometer (Magna 750) with aMCT detector. Spectra were recorded when either only the

Ñow was switched on and the thermal pre-reactor wasN2O5cold or all Ñows and a-pinene) were set and the pre-(N2O5reactor was hot. In the resulting di†erence spectrum, con-sumed appeared as negative bands and all products, asN2O5well as residual a-pinene, as positive bands. Calibration curves

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were recorded for a-pinene and a-pinene oxide ; absorptioncross-sections for and were taken from pre-NO211 N2O514vious observations. Because of difficulties with the handling ofthe reference substance, an acceptable determination of anabsorption cross-section for pinonaldehyde was impossible.According to the approach of Hatakeyama et al.,15 for thecross-section of the CwH stretching of the aldehyde group at2714 cm~1 an averaged value, p \ (1.28^ 0.05)] 10~19 cm2molecule~1, arising from a series of monoaldehydes was used ;pentanal 1.25 ^ 0.02, hexanal 1.34^ 0.05, heptanal1.29^ 0.01, octanal 1.24^ 0.03 (unit : 10~19 cm2 molecule~1 ,base 10). From a comparison of various cycloalkyl nitrates,14an averaged absorption cross-section for a nitrate group ofp \ (1.2^ 0.1)] 10~18 cm2 molecule~1 (base 10) has beenestimated for the asymmetric stretching near 1660 cm~1.

About 50 cm downstream of the mixing point for the NO3radicals, a mass-selective detector (HP-MSD 5971) connectedto the Ñow tube by means of an uncoated deactivated columnserved as an on-line MS monitor. Spectra were recorded inthe 70 eV electron impact mode.

Initial a-pinene concentrations were (3.4È5.5)] 1012 mol-ecule cm~3 and product yields were determined with respectto reacted a-pinene. If a-pinene was used in excess(3.65] 1014 molecule cm~3), product yields were obtainedwith respect to consumed radicals derived from con-NO3sumed Flow velocities varied from 1.03 to 1.40 m s~1.N2O5 .Under the experimental conditions used, at the point for theGC-sampling the disappearance of the radicals was com-NO3plete to [99%.

was prepared according to the method of DavidsonN2O5et al.16 a-Campholene aldehyde was synthesized from a-pinene oxide (Aldrich) by zinc bromide catalysed rearrange-ment.17 Pinonaldehyde was prepared by ozonolysis ofa-pinene (99]%, Aldrich) followed by reduction with zinc inacetic acid. All organic products were puriÐed by distillationand the purity was conÐrmed by GCÈMS/FID. The gasesused had stated purities : NO (99.5%), (99.9995%), MesserO2Griesheim; (99.999%) and He (99.999%), Linde. ForN2further puriÐcation, the carrier gases were passed through anactive carbon trap maintained at ca. 200 K and then throughtwo oxygen gas-clean cartridges.

Results and DiscussionAs a result of the sum of all experimental Ðndings, Fig. 1shows the proposed reaction mechanism taking into consider-ation the formation of the observed main products. For sim-pliÐcation, only the attack of radicals in the 3-position isNO3considered, forming a tertiary alkyl radical. It is well estab-lished, that attack proceeds very selectively forming theNO3most stable radical.18 An attack in the 2-position results in theformation of a secondary alkyl radical and, therefore, thispathway should be of minor importance. The addition of NO3radicals to alkenes (pathway 2) represents an exothermicprocess and the adduct radicals are generated with excessenergy,19 the asterisk in Fig. 1 stands for an excited state. Forthese excited radicals, there is a competition between col-lisional stabilization with the carrier gas molecules (M) viapathway 7 and unimolecular decomposition via pathways 3, 4and probably 8. Oxirane formation analogous to pathway 3has been detected for a series of alkenes20h22 to date and rep-resents the only decomposition pathway for such radicals.Additionally, in this system, the formation of a-campholenealdehyde (pathway 4) was detected, probably initiated bybreaking of a CwC bond in the b-position or by an NO2splitting o† from the nitrate group. Furthermore, a structur-ally unidentiÐed organic nitrate was observed, probably alsoarising from the excited adduct radical via pathway 8. Arearrangement step can initiate the formation of this sub-stance. The collisionally stabilized adduct radical from

Fig. 1 Proposed reaction mechanism. The asterisk denotes a specieswith excess energy.

pathway 7 can also produce the above-mentioned species, a-pinene oxide (oxirane), a-campholene aldehyde and theunidentiÐed organic nitrate, in a thermal reaction via path-ways 5, 6 and 9, respectively. In competition to that, there isthe addition of via pathway 10 representing the predomi-O2nant fate of alkyl radicals under tropospheric conditions. Theperoxyl radical formed reacts either in a self reaction or withNO, when NO is present, resulting in the corresponding oxylradical (pathway 13). In the reaction with the peroxylNO2 ,radical can be converted to a peroxy nitrate. Such substancesare thermally unstable23 and an equilibrium is established(pathway 11/12). According to pathway 14, the main productin the presence of pinonaldehyde, is formed.O2 ,

Using the given reaction pattern, the experimental Ðndingsare discussed in detail in the following section.

Reactions in the absence of O2

First, investigations were performed using direct MS as detec-tion technique in the 70 eV electron impact mode. Fig. 2shows a di†erence MS spectrum between the two states, the

radical generation was switched on or switched o†. AlsoNO3shown in Fig. 2 is a reference MS spectrum of a-pinene oxide.The di†erence spectrum was recorded in the range 75È200 uusing He as carrier gas at p \ 200 mbar, [a-pinene]reacted\2.6] 1012 molecule cm~3. In the di†erence MS spectrum,negative peaks characterize reacted a-pinene and positivepeaks are signals of the products. Taking into account super-positions, e.g. m/z\ 93 and m/z\ 137 from the M ] 1 peakof a-pinene, the detected product spectrum is in reasonableagreement with the reference spectrum of a-pinene oxide. Inaccordance

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Fig. 2 (a) Obtained di†erence MS spectrum (75È200 u, p \ 200mbar, He, [a-pinene] molecule cm~3) and (b) refer-reacted \ 2.6 ] 1012ence MS spectrum of a-pinene oxide using direct MS technique

with former investigations,22,24 a-pinene oxide can be formedby an splitting o† from the excited, as well as from theNO2collisionally stabilized, adduct radical via pathways 3 and 5,respectively.

There was no experimental evidence of further productsusing this method.

GCÈMS/FID analysis conÐrmed the occurrence of a-pineneoxide as a main product under these experimental conditions.Additionally, the formation of a-campholene aldehyde wasobserved. An unambiguous assignment was possible from themass spectrum and retention time, respectively, of authenticsamples. The formation of a-campholene aldehyde can beexplained in the following way (for demonstration the break-ing bonds are displayed with a wavy line) :

In the position b to the radical, the CwC bond breaks and anew Ðve-membered ring can be established. An thenNO2

splits o† from the nitrate group and breaking of the CwCbond of the former double bond results in the formation of anew double bond and a carbonyl group. The sequence of theelementary processes given here is highly uncertain. The for-mation of a-campholene aldehyde can be also initiated by thesplitting o† of As pointed out for a-pinene oxide, a-NO2 .campholene aldehyde can arise from the excited adductradical as well as from the collisionally stabilized adductradical via pathways 4 and 6, respectively.

By means of FTIR spectroscopy, the following results havebeen achieved. In the upper part of Fig. 3 a typical di†erencespectrum recorded with as the carrier gas is given, p \ 200N2mbar, [a-pinene] molecule cm~3, consumed0 \ 5.4] 1012a-pinene : 53%. Residual a-pinene has been subtracted andnegative bands originating from removed have beenN2O5compensated using spectra of authentic samples. Besides thespectral features of (l\ 1601 and 1629 cm~1), absorp-NO2tions at l\ 1278 and 1651 cm~1 typical for organic nitrateswere detected. Furthermore, in the CwH vibration range near3000 cm~1, absorptions attributed to a-pinene oxide(l\ 2927 and 2987 cm~1) were visible, in accordance with theexperimental result found with the other detectors. For com-parison, Fig. 4 shows the reference spectra of a-pinene oxideas well as pinonaldehyde. There was no sign of a signiÐcantcontribution to the absorptions in the CwH vibration rangenear 3000 cm~1 from substances other than a-pinene and a-pinene oxide. After subsequent subtraction of the spectrum ofa-pinene oxide, no further noticeable absorptions in the con-sidered range were present.

The inÑuence of total pressure on the product distributionwas studied in the pressure range 20 \ p/mbar \ 200 usingFTIR spectroscopy. Fig. 5 presents the yields of a-pineneoxide and the organic nitrate (described as nitrate) obtainedwith respect to consumed a-pinene as well as the ratio ofremoved a-pinene and The product yields were foundN2O5 .to be only slightly pressure dependent. With decreasing total

Fig. 3 Typical FTIR spectra recorded at p \ 200 mbar using pureas carrier gas (i.e. without and with mol-N2 O2) [O2]\ 4.58] 1018

ecule cm~3. Absorptions arising from reacted and residual a-N2O5pinene are compensated. Spectral features : organic nitrate, l\ 1278and 1651 cm~1 ; l\ 1601 and 1629 cm~1 ; a-pinene oxide,NO2 ,l\ 2927 and 2987 cm~1 ; nitrooxynitroperoxy compound(peroxynitrate), l\ 1293, 1666 and 1720 cm~1 ; and pinonaldehyde,l\ 1741, 2714 and 2963 cm~1.

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Fig. 4 Obtained FTIR reference spectra of a-pinene oxide and pin-onaldehyde

pressure, a minor rise in the a-pinene oxide yield with con-comitant decrease in the yield of the organic nitrate wasdetected. At low pressure, the product formation is governedby pathways from the excited adduct radical. With increasingtotal pressure, pathway 7 becomes more important togetherwith the pathways from the stabilized adduct radical. Usingthis model, the ratio of the rate constants, andk3 : k4 : k8must not be equal and the product yields becomek5 : k6 : k9 ,pressure dependent. Dlugokencky and Howard25 observed an

yield of 67% at 1.33 mbar (1 Torr) in He, from the reac-NO2tion of radicals with a-pinene, using LIF for detec-NO3 NO2tion. Taking into account that a-pinene oxide and areNO2formed in equimolar amounts and the yield of a-campholenealdehyde is of minor importance, an extrapolation of ourexperimental Ðndings to p \ 1.33 mbar results in a yield ofa-pinene oxide of ca. 65%, in excellent agreement. Note thatsuch an extrapolation can be highly uncertain and misinter-pretations are possible. The ratio *[a-pinene]/*[N2O5]

Fig. 5 Product yields of a-pinene oxide and organic nitrate andthe ratio of removed a-pinene and as a function of total pres-N2O5sure of N2

increased with increasing total pressure, resulting in a limitingvalue of ca. 1 (0.94^ 0.08) at 200 mbar. This behaviour wasalready found in former studies using this equipment11,12 andcan be explained by a more e†ective wall loss of radicalsNO3with decreasing total pressure. Similar observations have beenmade for Cl atoms under comparable conditions.26

From the 1 : 1 relation between consumed a-pinene andradicals derived from consumed it was concludedNO3 N2O5that addition of a second radical to the adduct radicalNO3forming a dinitrate was of minor importance and the detected

organic nitrate should be present as mononitrate. Further-more, the N balance was found to be fulÐlled, at 200 mbar

indicat-([NO2] ] [organic nitrate])/*[N2O5]\ 2.08^ 0.23,ing also that did not react with the adduct radical. ByNO2means of the other detection techniques, direct MS and GCÈMS/FID, no further information concerning the structure ofthe organic nitrate was obtained. Most likely, nitrate forma-tion via pathways 8 and 9 is initiated by a rearrangement, in asimilar way to the formation of a-campholene aldehyde.Rearrangements in the reactions of terpenes are well known.

At p \ 200 mbar, using as the carrier gas, 58 ^ 2% a-N2pinene oxide and 45^ 3% organic nitrate analysed withFTIR and 6 ^ 1% a-campholene aldehyde detected withGC/FID were found. Assuming that the structurally unknownorganic nitrate consists of 10 C atoms (there was no indicationof other small hydrocarbons in the GC analysis) a C balanceof 109^ 6% follows. Obviously, the use of the averaged cross-section for the organic nitrate gave rise to an overestimationof the yield of this substance causing the C balance to be sig-niÐcantly higher than 100%. For each of the detected pro-ducts, formation via the excited adduct radical (pathways 3, 4and 8) and via the collisionally stabilized adduct radical(pathways 5, 6 and 9) have to be taken into consideration. Incompetition with pathway 10, pathways 5, 6 and 9 should besuppressed by addition of sufficient O2 .

Reactions in the presence of O2

Experiments have been performed in mixtures withO2ÈN2 O2concentrations up to 4.58 ] 1018 molecule cm~3, i.e. close toatmospheric conditions. GCÈMS analysis of reactions withthe maximum concentration at 200 mbar showed aO2decrease in the product yields of a-pinene oxide and a-campholene aldehyde compared with those obtained in pure

a-pinene oxide : a-N2 , yield(N2)/yield(O2ÈN2) \ 4.67,campholene aldehyde : This isyield(N2)/yield(O2ÈN2) \ 2.13.attributed to the suppression of pathways 5 and 6 in the pres-ence of sufficient in the carrier gas. In the total ion chro-O2matogram, an additional peak was observed, attributed topinonaldehyde by means of the mass spectrum and the reten-tion time of an authentic sample. In principle, pinonaldehydeshould be formed according to the reaction sequence : path-ways 10] 13 ] 14. A critical point is pathway 13. NO wasabsent in these experiments and from the ratio *[a-pinene]/

it must be concluded that rad-*[N2O5]\ 0.99 ^ 0.14, NO3icals did not contribute signiÐcantly to the formation of oxylradicals via The selfRO2 ] NO3 ] RO] NO2] O2 .27,28reaction of the peroxyl radicals should govern the formationof the oxyl radicals :

The given peroxyl radical represents a tertiary peroxyl radical.In the literature,29 the rate constant for the self reaction of

radicals is 3] 10~17 cm3 molecule~1 s~1.tert-C4H9O2Taking this value for with an assumed initial concentra-k13a ,tion for the peroxyl radicals of 2] 1012 molecule cm~3, only0.01% of the peroxyl radicals have been converted at t \ 1 s

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(the residence time of our experiments is in this range). Inorder to play a signiÐcant role under our experimental condi-tions, must be fourÈÐve orders of magnitude higher thank13athe value for radicals. To date, rate constants fortert-C4H9O2the self reaction of nitrooxy-peroxyl radicals are not availablein the literature. The oxyl radicals produced from pathway 13decompose with elimination of forming the ÐnalNO2 ,product pinonaldehyde :

Pathway 14 represents the predominant fate of these oxyl rad-icals. Under all conditions used, there was no experimentalevidence for a competitive reaction such as isomerisation.

FTIR analysis has been performed at a total pressure of 200mbar with concentrations of 2.67 ] 1013È4.58] 1018 mol-O2ecule cm~3. A typical di†erence FTIR spectrum recordedfrom a reaction with the maximum concentration is givenO2in the lower part of Fig. 3, residual a-pinene is subtracted andbands originating from consumed are compensated, [a-N2O5molecule cm~3, consumed a-pinene :pinene]0 \ 5.4 ] 101266%. Instead of the absorptions at l\ 1278 and 1651 cm~1from the unidentiÐed organic nitrate found in pure in theN2 ,presence of absorptions at l\ 1293, 1666 and 1720 cm~1O2 ,were detected typically for substances of the nitrooxy-nitroperoxy type.9 The attack of radicals at the 3-NO3position gives rise to the formation of a nitroperoxy group inthe 2-position, as shown below:

As expected from the GCÈMS analysis, spectral features orig-inating from pinonaldehyde were present : carbonyl stretchingat l\ 1741 cm~1, CwH stretching from the aldehyde groupat l\ 2714 cm~1 and further absorptions in the CwH vibra-tion range. For comparison, the reference spectrum of pinon-aldehyde is given in Fig. 4. Furthermore, absorptions arisingfrom residual a-pinene oxide at 2927 and 2987 cm~1 werevisible.

Fig. 6 shows the product yields with respect to reacted a-pinene as a function of the concentration. Values givenO2with molecule cm~3 are those found in pure[O2]\ 1012 N2 .Nitrate stands for the unidentiÐed organic nitrate and per-oxynitrate stands for the nitrooxynitroperoxy compoundformed via pathway 11. For quantitative determination of thelatter species, the asymmetric stretching at l\ 1666 cm~1 wasused. According to the competition between addition of O2via pathway 10 and pathways 5, 6 and 9, for mol-[O2][ 1014ecule cm~3 a noticeable e†ect was observed and the additionpathway becomes predominant for molecule[O2]P 1017cm~3.

As known from a series of acyclic monoalkenes,22 the yieldof a-pinene oxide decreases with increasing content,O2resulting in an apparently constant value attributed to the for-mation from pathway 3. For demonstration of the compara-bility of results arising from FTIR as well as GC analysis, theyield of a-pinene oxide obtained with GC/FID at one con-O2centration is also given. For a clear representation, only theerror bar from the GC analysis is displayed.

The yield of the unidentiÐed organic nitrate decreased inthe same way as observed for a-pinene oxide. Because of thelimited sensitivity of the FTIR analysis and an overlap withabsorptions from the nitrooxynitroperoxy compound,yields\ 20% (ca. 5] 1011 molecule cm~3) could not be

Fig. 6 Product yields of a-pinene oxide, the organic nitrate and thenitrooxynitroperoxy compound (peroxynitrate) as a function of O2concentration at a total pressure of 200 mbar. Values given at

molecule cm~3 were found in pure[O2]\ 1012 N2 .

determined. It was therefore impossible to decide whether theunidentiÐed organic nitrate was present in signiÐcant amountswith molecule cm~3 or not (i.e. was pathway 8[O2]P 1017signiÐcant or not).

As pointed out above in the explanations of the GCÈMSanalysis, for a-campholene aldehyde the same behaviour wasfound as described for a-pinene oxide. The concentration ofthis compound (ca. 1011 molecule cm~3 ) was close to thedetection limit of the GC detectors. Thus, a quantitative deter-mination in all cases was difficult.

Instead of the three above-mentioned species, thermallyunstable compounds from the nitrooxynitroperoxy species23and the Ðnal main product, pinonaldehyde, were found.Product yields at p \ 200 mbar with molecule[O2][ 1017cm~3 are : pinonaldehyde 46 ^ 2%, peroxynitrate(nitrooxynitroperoxy compound) 22 ^ 2%, a-pinene oxide16 ^ 3% (15 ^ 3% GC/FID ) and a-campholene aldehyde3 ^ 1% (GCÈMS). This results in a C balance of 87 ^ 8%(86 ^ 8%). The N balance was found to be fulÐlled, ([NO2]] 2[peroxynitrate])/*[N2O5]\ 2.10 ^ 0.16.

Reactions with addition of NO

All experiments were performed at a total pressure of 200mbar with molecule cm~3 ; a-pinene was used in[O2][ 1017excess (3.65] 1014 molecule cm~3) and product yields wereobtained with respect to consumed radicals derived fromNO3removed Wall-loss processes were assumed to be negli-N2O5 .gible. The ratio *[a-pinene] con-/*[N2O5]\ 0.99^ 0.14Ðrmed this. NO was added together with a-pinene to thecarrier gas. For radicals, in competition with the reac-NO3tion with a-pinene via pathway 2 the reaction with NO alsohas to be taken into consideration.

NO ] NO3] 2NO2 (15)

With cm3 molecule~1 s~1 4 andk2 \ 6.16] 10~12 k15\ 2.6] 10~11 cm3 molecule~1 s~1 30 and an initial NO concentra-tion of 8.7 ] 1012 molecule cm~3, a ratio of the initial reac-tion rates follows, indicating that more than 90%r2/r15 \ 9.9of the radicals reacted via pathway 2. In the presence ofNO3NO, peroxyl radicals react with NO forming the correspond-ing oxyl radicals :

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By means of FTIR spectroscopy, pinonaldehyde with a yieldof 75^ 6% was detected. According to the suppression ofpathway 11, absorptions arising from nitrooxynitroperoxycompounds were completely absent. Weak absorptions atl\ 1664 and ca. 1290 cm~1 indicated the occurrence ofnitrate-group-containing substances with a yield of 14 ^ 3%,assuming a mononitrate. There was no information concern-ing the structure of these compounds and pathways for theirformation seem to be highly speculative. If the primary attackof radicals at the 2-position is considered, a secondaryNO3peroxyl radical is formed and the occurrence of a compoundof the ketonitrate type is possible. Furthermore, the formationof a nitrate group is also possible in the reaction of peroxylradicals with NO.29

If the observed absorptions are predominantly caused by adinitrate, a yield of 7^ 2% for this substance follows.

Because, in these experiments, a-pinene was used in excess,determination of a-pinene oxide from absorptions in theCwH vibration range was impossible. The presence of NO inthe carrier gas should not a†ect the formation of a-pineneoxide and a-campholene aldehyde via pathways 3 and 4,respectively. Under comparable conditions, experimental evi-dence for this fact was found in the reaction of radicalsNO3with isoprene in a former study.31 Therefore, yields of bothspecies obtained in the absence of NO should be also valid forthese experiments.

Application to the troposphere

Table 1 shows a compilation of product yields obtained underdi†erent experimental conditions. At a total pressure of 200mbar with molecule cm~3, pinonaldehyde is the[O2][ 1017predominant reaction product. Taking into account that thenitrooxynitroperoxy compound only serves as a temporaryreservoir for peroxyl radicals forming, Ðnally, pinonaldehyde,the expected pinonaldehyde yield from the reactions withoutNO (68 ^ 4%) is in reasonable agreement with the valueobtained from reactions in the presence of NO (75 ^ 6%). Amodelling study28 provides a strong indication that at night-time, for all types of peroxyl radicals, the reaction with NO3

radicals via

RO2 ] NO3 ] RO] NO2] O2 (17)

is a main loss process. Under tropospheric conditions when (i)the removal of the peroxyl radicals, formed in the reaction of

radicals with a-pinene, via pathway 17 is dominant andNO3(ii) the peroxyl radicals react in a self reaction or in a cross-reaction with other tertiary peroxyl radicals, our resultsshould be applicable. In a cross-reaction with primary or sec-ondary peroxyl radicals, the formation of hydroxy-group-con-taining substances is also possible. In the reaction with HO2radicals, hydroperoxides can be formed.

Note that the given pinonaldehyde yield represents only alower limit. According to the competition between collisionalstabilization (pathway 7) and decomposition (pathways 3, 4and 8), with increasing total pressure up to 1000 mbar,pathway 7 becomes more important. Because of the uncer-tainty of the formation pathways for the organic nitratesdetected in the presence of NO, only a rough estimate for anextrapolation to tropospheric pressure conditions can begiven. Using the obtained product yield of a-pinene oxide atp \ 200 mbar with molecule cm~3 and the[O2][ 1017assumption that pathways 4 and 8 are negligible, derived fromthe Ñuxes over pathways 3 and 7 the following expression canbe given,19,22,24 cf. Fig. 1 :

yield (a-pinene oxide)\ 1/(1 ] k7[M]/k3) (18)

resulting in an a-pinene oxide yield of 3.5% at p \ 1000 mbar.The same tendency is expected for the yield of a-campholenealdehyde and at p \ 1000 mbar a yield should be valid.@3%With the increasing importance of pathway 7, according tothe reaction sequence pathways 7] 10 ] 13 ] 14, pinonal-dehyde should be formed at p \ 1000 mbar with a yield of80È85%. Because of the lack of mechanistic information forthe formation of the organic nitrates found in the experimentswith NO (yield : 14^ 3%/7 ^ 2%), no reasoned extrapolationfor these compounds can be given.

Our results are in reasonable agreement with those from astudy in a 200 m3 chamber under atmospheric conditions :10a-pinene oxide\ 3%, pinonaldehyde 58%, organic nitrates(total) 12%. In this study, an increase in the pinonaldehydeyield from 40% (0.5 m3 chamber) to ca. 60% (200 m3chamber) is reported, caused by a less e†ective wall loss in thelarger chamber. Further loss processes on the chamber wall aswell as on the surface of existing particles can explain the dif-ference from our pinonaldehyde yield. The atmospheric fate ofpinonaldehyde has already been discussed in the literature.32

Table 1 Product yields (%) at p \ 200 mbar

O2ÈN2 mixture

[O2][ 1017[O2][ 1017 molecule cm~3,

compound N2 molecule cm~3 presence of NO

a-pinene oxide 58^ 2 15^ 3a 15 ^ 3ba-campholene 6^ 1 3^ 1 3 ^ 1b

aldehydeorganic nitrate 45 ^ 3 \20 È

(l\ 1651 cm~1)pinonaldehyde È 46^ 2 75 ^ 6nitroxynitroperoxy È 22 ^ 2 È

compoundorganic nitrate È È 14 ^ 3c

(l\ 1664 cm~1) 7 ^ 2

C balanced 109 ^ 6 86^ 8 107 ^ 13/100 ^ 12

a GC/FID analysis ; b assumed to be equal to those from experiments without NO; c yield if a mononitrate or a dinitrate was assumed; d organicnitrates and nitrooxynitroperoxy compounds were assumed to consist of 10 C atoms.

3026 J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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Ðnancial support of the Deutsche Forschungsgemein-Theschaft is gratefully acknowledged.

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J. Chem. Soc., Faraday T rans., 1997, V ol. 93 3027

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