Model studies of the oxidation of light hydrocarbons in the troposphere and stratosphere

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Abstract
Our model studies of light non‐methane hydrocarbons (NMHCs) in the troposphere and lower stratosphere for seasonally varying insolation and transport conditions indicate that NMHC mixing ratios (MRs) are generally highest in the winter. These results reflect our assumptions of a constant surface source for each season. Comparisons of these calculated NMHC MRs with measurements suggest that the sources of the light NMHC may vary seasonally. There is a serious discrepancy between calculated and observed values of formic and acetic acids. It appears that the direct emissions or the aqueous phase chemistry of these acids, or their precursors, are of importance. An injection of NMHCs into the upper troposphere to stimulate transport of boundary‐layer air by rapid vertical convection in clouds, has a significant impact on the local chemistry on a time‐scale of a few days. The most important effects besides the increase in NMHCs and their products are the generation of peroxyacetyl nitrate precursors and the local depletion of NOx. The magnitude of the NOx depletion would be enhanced if the rate coefficients for the loss of the peroxyacetyl radical, which are currently not well determined, were slower.