Atmospheric Chemistry of 1-Octene, 1-Decene, and Cyclohexene: Gas-Phase Carbonyl and Peroxyacyl Nitrate Products

Abstract

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 carbonyl products of the alkene−ozone reaction (with sufficient cyclohexane added to scavenge OH) were heptanal from 1-octene, nonanal from 1-decene, formaldehyde from both, and pentanal from cyclohexene. For 1-octene and 1-decene, carbonyl formation yields were consistent with the simple mechanism:  RCH = CH_2 + 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 for ca. 0.16 of the cyclohexene−O_3 reaction. Sunlight irradiation of alkene−NO and aldehyde−NO mixtures leads to carbonyls, alkyl nitrates, and peroxyacyl nitrates. Major carbonyl products of the OH−alkene reaction were heptanal from 1-octene, nonanal from 1-decene, and formaldehyde from both. Experimental data indicated that addition is an important pathway of the overall OH−1-alkene reaction, decomposition is important for the β-hydroxyalkoxy radicals that form following OH addition, and reaction with O2 is of some importance for the alkoxy radicals that form in several pathways in the alkene−NO (sunlight), aldehyde−NO (sunlight), and peroxyacyl nitrate−NO (dark) systems. The extent of isomerization of alkoxy and β-hydroxyalkoxy radicals could not be assessed. Several peroxyacyl nitrates [RC(O)OONO_2] were formed including those with R = n-C_4H_9 from cyclohexene, R = n-C_5H_(11) from hexanal, and R = n-C_6H_(13) from heptanal and from 1-octene. The thermal decomposition rates of n-C_5H_(11)C(O)OONO_2 and n-C_6H_(13)C(O)OONO_2, which were synthesized in the liquid phase and were characterized in a number of tests using 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), respectively, at p = 1 atm of air. Comparison with data for lower molecular weight homologues including PAN (R = CH_3) suggests that the thermal stability of peroxyacyl nitrates may increase with the size of the n-alkyl substituent. The atmospheric persistence of the atmospheric oxidation products of 1-octene, 1-decene, and cyclohexene is briefly discussed.