THE PHOTOCHEMISTRY OF ACETONE IN THE UPPER TROPOSPHERE - A SOURCE OF ODD-HYDROGEN RADICALS

This paper summarizes measured photodissociation quantum yields for ac etone in the 290-320 nm wavelength region for pressures and temperatur es characteristic of the upper troposphere. Calculations combine this laboratory data with trace gas concentrations obtained during the NASA and NOAA sponsored Stratospheric Tracers of Atmospheric Transport (ST RAT) field campaign, in which measurements of OH, HO2, odd-nitrogen, a nd other compounds were collected over Hawaii, and west of California during fall and winter of 1995/1996. OH and HO2 concentrations within 2 to 5 km layers just below the tropopause are similar to 50% larger t han expected from O-3, CH4, and H2O chemistry alone. Although not meas ured during STRAT, acetone is inferred from CO measurements and aceton e-CO correlations from a previous field study. These inferred acetone levels are a significant source of odd-hydrogen radicals that can expl ain a large part of the discrepancy in the upper troposphere. For lowe r altitudes, the inferred acetone makes a negligible contribution to H Ox (HO+HO2), but influences NOy partitioning. A major fraction of HOx production by acetone Is through CH2O formation, and the HOx discrepan cy can also be explained by CH2O levels in the 20 to 50 pptv range, re gardless of the source.