Environ. Scl. Technol. 1993, 27, 979-981
A Biogenic Precursor of Peroxyproplonyl Nitrate: Atmospheric Oxidation of cie3-Hexen-1-01 Daniel Grosjean,' Edwin L. Williams 11, and Eric Grosjean DGA, Incorporated, 4526 Telephone Road, Suite 205, Ventura, California 93003
Introduction The contribution of biogenic emissions to ozone and aerosol formation in the atmosphere has received attention for a number of years (1-3). While recent studies have focused on isoprene and terpenes (4-6),biogenic emissions also include oxygenated species such as alcohols and esters (7-12).Among these, the unsaturated alcohol cis-3-hexen1-01, CH3CH2=CH=CHCH2CH20H, long known as leaf alcohol, has been identified as a major component in air emissions from a number of plant species including grass, trees, and agricultural plants (7-11). As an olefin, cis3-hexen-1-01 is expected to be rapidly oxidized in the atmosphere in pathways initiated by its reactions with ozone, the hydroxyl radical, and the nitrate radical (10, 13,14), thus yielding carbonyl and carboxylic acid reaction products (13-15). In the presence of oxides of nitrogen, these reactions will also lead to nitrogen-containing products including peroxyacylnitrates, RC(O)OON02 (13, 14). The most abundant peroxyacyl nitrate, peroxyacetyl nitrate (R = CH3, PAN), has been extensively studied for its role as reservoir in the long-range transport of odd reactive nitrogen (16). Precursors of PAN include both anthropogenic and biogenic hydrocarbons such as isoprene (17,18).The oxidation of isoprene also leads to another peroxyacyl nitrate, peroxymethacryloyl nitrate, R = CH2=C(CH3)-, MPAN (17-19). The second most abundant peroxyacyl nitrate, peroxypropionyl nitrate (R = CzHs, PPN) has been measured in ambient air downwind of urban areas (20,211and has been assumed to originate solely from the oxidation of anthropogenic hydrocarbons. In this study, we have investigated the oxidation of the unsaturated alcohol cis-3-hexen-1-01 under laboratory conditions that closely simulate those relevant to the atmosphere. PPN has been identified as a major reaction product, thus demonstrating the possibility of a biogenic contribution to PPN by atmospheric oxidation of plantemitted species including unsaturated alcohols such as cis-3-hexen-1-01. Experimental Methods
Sunlight Irradiations of 3-Hexen-l-ol-NO Mixtures. Sunlight irradiations of 1-2 ppm cis-3-hexen-1-01 (purity >95%,Aldrich ChemicalCo., Milwaukee, WI) and 0.22-0.46 ppm nitric oxide in purified air were carried out at ambient temperature in a 3.5-m3all-Teflon collapsible chamber constructed from transparent 200A FEP Teflon film (17,22,23).The particle-free matrix air was purified by passing ambient air through large cartridges containing activated carbon, silica gel, molecular sieves, and permanganate-coated alumina and contained less than 1ppb of reactive hydrocarbons, ozone, and oxides of nitrogen; less than 0.1-0.5 ppb of formaldehyde, acetaldehyde, and propanal; and less than 0.05-0.1 ppb of peroxyacylnitrates including PAN and PPN. Ozone was measured by
* Corresponding author. 0013-936X/93/0927-0979$04.00/0
@ 1993 American Chemical Soclety
ultraviolet photometry using a calibrated Dasibi 1108 continuous analyzer. Oxides of nitrogen were measured by chemiluminescence using a Monitor Labs 8840 continuous analyzer calibrated using the diluted output of a certified NO2 permeation tube maintained at 30.0 f 0.1 "C in a thermostated water bath. Carbonyl Reaction Products. Carbonyl products were isolated as their 2,4-dinitrophenylhydrazonesby sampling the reaction mixture through small (218 cartridges coated with twice recrystallized 2,4-dinitrophenylhydrazine (DNPH) as described previously (24,25).In order to minimize sampling artifact, the cartridge samples were collected downstream of annular denuders coated with potassium iodide (17). The sampling flow rate was 0.9 Llmin. Following collection, the cartridges were eluted with HPLC-grade acetonitrile, and aliquots of the acetonitrile extracts were analyzed by liquid chromatography with ultraviolet detection. The DNPH derivatives were separated on a Whatman Partisphere CIScolumn, 110 X 4.7 mm, with 55:45 by volume CHBCN-HPOeluent at a flow rate of 1mL/min. The detection wavelength was 360 nm. Quantitative analysis involved the use of external hydrazone standards, from which calibration curves, Le., absorbance (peak height) vs concentration, were constructed. More details regarding the sampling and analytical protocols have been given elsewhere (24,25) including collection efficiency, cartridge elution recovery, precision, accuracy, and interlaboratory comparison studies. Confirmation of the structure of the carbonyl DNPH derivatives was obtained by measuring their 3601430-nm absorbance ratio as a test for dicarbonyls (24,25) and, when necessary, by comparingthe chemicalionization mass spectra of both hydrazone standards and carbonyl-DNPH samples (26). Peroxyacyl Nitrates. Peroxyacyl nitrates were measured by electron capture gas chromatography (20,21,27) using SRI 8610 gas chromatographs equipped with Valco 140 BN detectors. The columns used were 70 X 0.3 cm Tefon-lined stainless steel columns packed with 10% Carbowax on Chromosorb P, acid-washed and DMCStreated. The column and detector temperatures were 36 and 60 "C, respectively, and the carrier gas was ultrahighpurity nitrogen. The column flow rate was 58 mL/min. Air from the Teflon chamber was continuously pumped through a short section of 6-mm diameter Teflon tubing connected to a 6.7-mLstainless steel sampling loop housed in the GC oven and was injected every 30 min using a timer-activated 10-port sampling valve. To calibrate the EC-GC instrument, PAN and PPN were synthesized in the liquid phase as described before (21)using commercially available anhydrides as starting materials. Parts per billion levels of PAN and PPN in the gas phase were obtained by dilution, with purified air, of the output of a diffusion vial containing solutions of PAN or PPN in n-dodecane and maintained at 2 OC in the freezer compartment of a small refrigerator (20,213.A silica gel trap was inserted upstream of the diffusion vial to minimize water condensation. Calibration involved side-by-side Envlron. Sci. Technol., Vol. 27, No. 5, 1993 878
readings with the EC-GC instrument and with a calibrated chemiluminescent NO, analyzer, which employs a surface converter to reduce oxides of nitrogen to NO and, therefore, responds quantitativelyto organic nitrates and peroxyacyl nitrates (28) including PAN and PPN (20, 21). Results and Discussion
Sunlight irradiation of mixtures of cis-3-hexen-1-01and NO in purified air resulted in the rapid conversion of NO to NO2 and in the formation of ozone, propanal, and PPN as major reaction products (Table I). Up to 40-65 ppb of PPN were formed, thus accounting for 14-20% of the initial NO concentration. The presence of PPN was verified using the following tests. Its retention time, on two EC-GC instruments, matched those of P P N synthesized in the liquid phase and of PPN prepared in situ by sunlight irradiation of propanal-NO mixtures in pure air. The compound decomposed when passing the reaction mixture through a tube heated to 170 OC at which peroxyacyl nitrates decompose but alkyl nitrates (RONO,) do not (17,20,21). The compound also decomposed upon addition of excess NO, and this a t a rate that matched well that of pure P P N as measured in separateexperiments (29).
Under the conditions of our study, the oxidation of cis3-hexen-1-01is initiated by its reaction with the hydroxyl radical (reaction with ozone becomes important after the NO-NO2 crossover). By analogy with other alkenes (10, 13, 14), the OH-cis-3-hexen-1-01 reaction is expected to involve addition on the unsaturated carbon-carbon bond (H-atom abstraction from the CH2 and CHzOH groups is also possible as a minor reaction pathway):
CH3CH2CH=CHCH2CHzOH
+
OH
-
OH
I
(la)
(1b)
-
CH3CHpCH0 + EHOHCH2CH20H
CH3CH2tHOH
+
-
+ CHzOHCHpCHO (2b)
CHzOHCHfiHO
Oz -Hop
-
Pa)
H02
+ CH&H&HOH (2d
+
CH3CH2CH0
(24
While propanal was indeed identified and was a major reaction product, hydroxypropanal could not be positively identified due to lack of a reference standard. In the above reaction sequence, we have omitted the RO2 + NO RON02 reaction (30),which yields two c6 dihydroxynitrates, and the reaction of alkoxy radicals with oxygen, which may compete with their unimolecular decomposition
-
980
Environ. Sci. Technol., Vol. 27, No. 5, 1993
run2
1.o
2.0
0.22 4.5
0.46 4.3
55 110
330 240 43.0 0.20
15.0 61 29
65 214 325 670 63.7 0.14 20.9 96 86
Samples of 42-min (run 1) and 60-min (run 2) duration, with midpoints at 150 (run 1)and 106min (run 2) after the onset of sunlight irradiation.
(31)and which yields the two c6 dihydroxyketones, CH3C H ~ C H O H C ( O ) C H Z C H ~ Oand H CH&HzC(O)CHOHCHzCHzOH. These c6 dihydroxynitrates and dihydroxyketones may indeed form but were not observed by electron capture gas chromatography and by liquid chromatography, respectively. As we have verified in separate experiments involving sunlight irradiation of propanal-NO mixtures at ppb levels in air (13,141,PPN forms readily and in high yields from propanal in a sequence of reactions initiated by the reaction of propanal with OH (H-atom abstraction from the carbonyl carbon):
-
+ OH CH,CH,CO + H,O (3a) CH,CH,CO + 0, CH,CH,C(O)OO (3b) CH,CH,C(O)OO + NO, CH,CH,C(O)OONO, (PPN) CH,CH,CHO
(343
I
EHOHCH~CH~OH + o, CH3CH2CHOHCH(6)CH&HzOH
initial cis-3-hexen-1-01,ppm initial NO, ppm initial hydrocarboniN0 ratio, ppmippm NO-NO2 crossover time, from start of run, min concentration, ppb reaction products ozone max, ppb propanal, ppbn PPN max, ppb PPN yield, percent of initial NO PAN max, ppb acetaldehyde, ppb" formaldehyde, ppbn
OH
followed by reaction of the alkyl radicals formed in eqs l a and l b with oxygen (R 0 2 ROz), by reaction of the corresponding peroxy radicals with NO (ROz + NO RO + NOz), and by unimolecular decomposition of the alkoxy radicals, both of which yield propanal and hydroxypropanal: CH3CH2CH(6)CHOHCH&H,0H
run1
+
CH3CHztH --CHCH&HzOH
CH3CH2CH -tHCH2CHzOH
+
Table I. Summary of Results
PPN was not observed to form until after the NO-NO, crossover: In the presence of excess NO, PPN rapidly decomposes via the reverse reaction of equilibrium 3c followed by the reaction of NO with the peroxypropionyl radical, leading to products including acetaldehyde (29): CH,CH,C(O)OO
+ NO
-
CH,CH,C(O)O
-
CH,CH,C(O)O
CH3CH, + 0,
+ NO CH,CH,O + 0,
CH,CH,02
-
-
CO,
+ NO,
+ CH,CH,
CH3CH,0,
+ CH,CH,O CH3CH0 + HO,
-+
NO,
(4a) (4b) (4c) (4d) (4e)
In turn, acetaldehyde leads to peroxyacetyl nitrate via a sequence identical to that shown above for propanal (reaction 3). Indeed, acetaldehyde and PAN were identified as reaction products (see Table I), with PANIPPN concentration ratios of 0.34 f 0.1 and acetaldehyde1 propanal concentration ratios of 0.20 f 0.05. Formaldehyde was also identified and is a product of acetaldehyde oxidation, PAN thermal decomposition, and other oxidation pathways. After the NO-NO2 crossover, ozone accumulates as a reaction product and reacts with cis-3-hexen-1-01, thus also yielding propanal (and, subsequently, PPN via reaction 3):
CH3CH2CH=CHCHpCH20H + O3
-
-
CH3CH2CH0
+ CH,OHCH~~HO~
(54
other products
(*)
Rate constants for the reactions of cis-3-hexen-1-01with OH and with ozone have not been measured. As a disubstituted alkene, cis-3-hexen-1-01is expected to react rapidly with both electrophiles (10). From considerations of substituent effects (32)and from available kinetic data for structural homologues (13),we expect cis-3-hexen-l01 to be slightly more reactive than cis-2-pentene (13)and about as reactive as cis-3-hexene (32),Le., we estimate room temperature rate constants to be about 7.5 x and 2.5 x cm3 molecule-l s-l for the OH and 0 3 reactions, respectively. Thus, for typical atmospheric - ~[os] conditions, e.g., [OH] = 1.0 X lo6 molecule ~ m and = 50 ppb, the half-life of cis-3-hexen-1-01against removal by either OH or ozone is only 1-3 h. Therefore, cis-3hexen-1-01emitted by biogenic sources is expected to be rapidly oxidized to propanal, PPN, and other products. While PAN is formed in the oxidation of important biogenichydrocarbons including isoprene (17') and several abundant terpenes such as d-limonene and a-pinene (33), it has been assumed until now that PPN is solely of anthropogenic origin and that its presence in air may be indicative of long-range transport of urban pollution. Our results indicate that, in fact, the production of PPN may have an important biogenic component. Atmospheric removal processes for PPN include its thermal decomposition (13)and presumably its reaction with OH. In the presence of NO,, both processes lead to acetaldehyde and subsequently to PAN as major products. Both acetaldehyde and PAN are significant sources of free radicals, thus "fueling" oxidant production downwind of areas where biogenic emissions of cis-3-hexen-1-01 contribute to PPN formation. Acknowledgments Dr. John B. Palmer of Southern California Edison Co. provided technical advice. Mrs. Denise M. Velez prepared the draft and final versions of the manuscript. This work was supported by the Southern California Edison Co., Rosemead, CA, and by internal R&D funds, DGA, Inc., Ventura, CA. Literature Cited (1) Rasmussen, R. A. J. Air. Pollut. Control Assoc. 1972,22, 537-543. (2) Altshuller, A. P. Atmos. Enuiron. 1983, 17, 2131-2165. (3) Chameides, W. L.; Lindsay, R. W.; Richardson, J.; Kiang, C. S. Science 1988,241, 1473-1475.
(4) Lamb, B.; Guenther, A.; Gay, D.; Westberg, H. Atmos. Enuiron. 1987,21, 1695-1705. (5) Jacob, D. J.; Wofsy, S. C. J. Geophys. Res. 1988,93,14771486. (6) Hatakeyama, S.; Isumi, K.; Fukuyama, T.; Akimoto, H. J. Geophys. Res. 1989, 94, 13013-13024. (7) Graedel, T. E. Chemical compounds in the atmosphere; Academic Press: New York, 1978. (8) Ohta, K. Geochem. J. 1984, 18, 135-141. (9) Isidorov, V. A.; Zenkevich, I. G.;Ioffe, B. V. Atmos. Enuiron. 1985, 19, 1-8. (10) Arey, J.; Winer, A. M.; Atkinson, R.; Aschmann, S. M.; Long, W. D.; Morrison, C. L. Atmos. Enuiron. 1991,25A, 10631075. (11) Winer,A.M.;Arey, J.;Atkinson,R.;Aschmann,S. M.;Long, W. D.; Morrison, C. L.; Olszyk, D. M. Atmos. Environ. 1992, 26A, 2647-2659. (12) Arey, J.; Corchnoy, S. B.; Atkinson, R. Atmos. Enuiron. 1991,25A, 1377-1381. (13) Atkinson, R. Atmos. Enuiron. 1990,24A, 1-41. (14) Carter, W. P. L. Atmos. Enuiron. 1990,24A, 481-518. (15) Grosjean, D. Enuiron. Sci. Technol. 1989, 23, 1506-1514. (16) Ridley, B. A.; et al. J. Geophys. Res. 1990,95,13949-13961. (17) Grosjean, D.; Williams, E. L., 11;Grosjean, E. Atmospheric chemistry of isoprene and of its carbonyl products. Environ. Sci. Technol. 1993, in press. (18) Tuazon, E. C.; Atkinson, R. Int. J. Chem. Kinet. 1990,22, 1221-1236. (19) Bertman, S. B.;Roberts, J. M. Geophys. Res.Lett. 1991,18, 1461-1464. (20) Williams, E. L., 11;Grosjean, D. Environ. Sci. Technol. 1991, 25, 653-659. (21) Grosjean, D.; Williams, E. L., 11; Grosjean, E. Enuiron. Sci. Technol. 1993, 27, 110-120. (22) Grosjean, D. Environ. Sci. Technol. 1990,24, 1428-1432. (23) Grosjean, D. Environ. Sci. Technol. 1985, 19, 1059-1065. (24) Grosjean, D. Enuiron. Sci. Technol. 1991, 25, 710-715. (25) Druzik, C. M.; Grosjean, D.; Van Neste, A.; Parmar, S. S. Int. J. Enuiron. Anal. Chem. 1990,38,495-512. (26) Grosjean, D. Anal. Chem. 1983,55, 2436-2439. (27) Williams, E. L., II; Grosjean, D. Atmos. Enuiron. 1990,24A, 2369-2377. (28) Grosjean, D.; Harrison, J. Enuiron. Sci. Technol. 1985,19, 749-752. (29) Grosjean, D.; Williams, E. L., 11; Grosjean, E. Thermal decomposition of Cz-Cd peroxyacyl nitrates. Atmos. E n uiron., submitted. (30) Carter, W. P. L.; Atkinson, R. J. Atmos. Chem. 1989, 8, 165-173. (31) Atkinson, R.; Carter, W. P. L. J. Atmos. Chem. 1991, 13, 195-210. (32) Grosjean, D. Atmos. Enuiron. 1992, 26A, 1395-1405. (33) Grosjean, D.; Williams, E. L., 11; Seinfeld, J. H. Enuiron. Sci. Technol. 1992,26, 1526-1532.
Received for review November 10, 1992. Revised manuscript received J a n u a r y 8, 1993. Accepted January 14, 1993.
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