Alkyl nitrate formation from the nitrogen oxide (NOx)-air photooxidations of C2-C8 n-alkanes

Abstract

The yields of alkyl nitrates formed in the NO/sub x/-air photooxidations of the homologous series of n-alkanes from ethane through n-octane have been determined at 299 +/- 2K and 735 torr total pressure for two different chemical systems. Alkyl peroxy radicals were generated by reaction of the n-alkanes with OH radicals (generated from the photolysis of methyl nitrite in air) or Cl atoms (from photolysis of Cl/sub 2/ in air). The alkyl nitrate yields obtained from the two systems, corrected for secondary reactions, were in agreement within the experimental errors and increased monotonically with the carbon number of the n-alkane, from less than or equal to1% for ethane to approx. 33% for n-octane, with the yields apparently approaching a limit of approx. 35% for large n-alkanes. The relative yields of the various secondary alkyl nitrate isomers in the n-pentane through n-octane systems were in good agreement with those expected from OH radical or Cl atom reaction with the corresponding secondary C-H bonds. However, the relative yields of the primary alkyl nitrates in the propane and butane systems were a factor of approx. 2 lower than expected. The data are consistent with the alkyl nitrates being formed almost entirely from themore » reaction of peroxy radicals with NO, and the ratios of the corrected alkyl nitrate yields thus reflect the fraction of RO/sub 2/ radicals which react with NO to form alkyl nitrates. These nitrate yields from the reaction of RO/sub 2/ radicals with NO are important inputs into chemical computer models of the atmospheric NO/sub x/-air photooxidations of the large n-alkanes.« less