The photochemistry of acetone in the upper troposphere: A source of odd‐hydrogen 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, HO 2 , odd‐nitrogen, and other compounds were collected over Hawaii, and west 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 O 3 , CH 4 , and H 2 O 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 HO x (HO+HO 2 ), but influences NO y partitioning. A major fraction of HO x production by acetone is through CH 2 O formation, and the HO x discrepancy can also be explained by CH 2 O levels in the 20 to 50 pptv range, regardless of the source.