ATMOSPHERIC CHEMISTRY OF 1-OCTENE, 1-DECENE, AND CYCLOHEXENE - GAS-PHASE CARBONYL AND PEROXYACYL NITRATE PRODUCTS

Atmospheric reactions of 1-octene, 1-decene, and cyclohexene have been studied in laboratory experiments involving alkene-ozone mixtures in the dark and alkene-NO and aldehyde-NO mixtures in sunlight. Major car bonyl products of the alkene-ozone reaction (with sufficient cyclohexa ne added to scavenge OH) were heptanal from 1-octene, nonanal from 1-d ecene, formaldehyde from both, and pentanal from cyclohexene. For 1-oc tene and 1-decene, carbonyl formation yields were consistent with the simple mechanism: RCH = CH2 + O-3 --> 0.5(HCHO + RCHOO) + 0.5(RCHO + H 2COO). Other carbonyls were formed and accounted for ca. 0.10-0.20 of the subsequent reactions of the RCHOO biradical. Pentanal accounted fo r ca. 0.16 of the cyclohexene-O-3 reaction. Sunlight irradiation of al kene-NO and aldehyde-NO mixtures leads to carbonyls, alkyl nitrates, a nd peroxyacyl nitrates. Major carbonyl products of the OH-alkene react ion were heptanal from 1-octene, nonanal from 1-decene, and formaldehy de from both. Experimental data indicated that addition is an importan t pathway of the overall OH-1-alkene reaction, decomposition is import ant for the beta-hydroxyalkoxy radicals that form following OH additio n, and reaction with O-2 is Of some importance for the alkoxy radicals that form in several pathways in the alkene-NO (sunlight), aldehyde-N O (sunlight), and peroxyacyl nitrate-NO (dark) systems. The extent of isomerization of alkoxy and beta-hydroxyalkoxy radicals could not be a ssessed. Several peroxyacyl nitrates [RC(O)OONO2] were formed includin g those with R = n-C4H9 from cyclohexene, R = n-C5H11 from hexanal, an d R = n-C6H13 from heptanal and from 1-octene. The thermal decompositi on rates of n-C5H11C(O)OONO2 and n-C5H13C(O)OONO2, which were synthesi zed in the liquid phase and were characterized in a number of tests us ing electron capture gas chromatography, were (in units of 10(-4) s(-1 )) 0.72-2.02 (T = 291-299 K) and 0.61-1.19 (T = 291-295 K), respective ly, at p 1 = atm of air. Comparison with data for lower molecular weig ht homologues including PAN (R = CH3) suggests that the thermal stabil ity of peroxyacyl nitrates may increase with the size of the n-alkyl s ubstituent. The atmospheric persistence of the atmospheric oxidation p roducts of 1-octene, 1-decene, and cyclohexene is briefly discussed.