The photochemistry of acetone in the upper troposphere A source of oddhydrogen radicals

This paper summarizes measured photodissociation quantum yields for acetone in the 290-320 nm wavelength region for pressures and temperatures 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 (STRAT) field campaign, in which measurements of OH, HO2, odd-nitrogen, and other compounds were collected over Hawaii, andwest of California during fall and winter of 1995/1996. OH and HO 2 concentrations within 2 to 5 km layers just below the tropopause are -50% larger than expected from 03, CH 4, and H20 chemistry alone. Although not measured during STRAT, acetone is inferred from CO measurements and acetone-CO correlations from a previous field study. These inferred acetone levels are a significant source of odd-hydrogen radicals that can explain a large part of the discrepancy in the upper troposphere. For lower altitudes, the inferred acetone makes a negligible contribution to HOx (HO+HO2), but influences NOy partitioning. A majorfractionofHOx production by acetone is through CH20 formation, and the HOx discrepancy can also be explained by CH 20 levels in the 20 to 50 pptv range, regardless of the source.