Sources of HOx and production of ozone in the upper troposphere over the United States

The sources of HO x (OH+peroxy radicals) and the associated production of ozone at 8–12 km over the United States are examined by modeling observations of OH, HO 2 , NO, and other species during the SUCCESS aircraft campaign in April–May 1996. The HO x concentrations measured in SUCCESS are up to a factor of 3 higher than can be calculated from oxidation of water vapor and photolysis of acetone. The highest discrepancy was seen in the outflow of a convective storm. We show that convective injection of peroxides (CH 3 OOH and H 2 O 2 ) and formaldehyde (CH 2 O) from the boundary layer to the upper troposphere could resolve this discrepancy. More generally, the data collected over the central United States during SUCCESS suggest that local convection was a major source of HO x and NO x to the upper troposphere. The OH and HO 2 observations together with the observations of NO allow us to directly calculate the ozone production in the upper troposphere and its dependence on NO x . We find an average net ozone production of 2 ppbv day −1 between 8 and 12 km over the continental United States in the spring. Ozone production was NO x ‐limited under essentially all the conditions encountered in SUCCESS. The high levels of HO x present in the upper troposphere stimulate ozone production and increase the sensitivity of ozone to NO x emissions from aircraft and other sources.