Tropospheric sulfate distribution during SUCCESS: Contributions from jet exhaust and surface sources

Abstract

The distribution of SO 4 = aerosol over the central US during SUCCESS indicates that surface sources of SO 4 = and SO 2 in the western US caused SO 4 = enhancements up to 10 km altitude. The mean (median) SO 4 = mixing ratio in the mid‐ and upper‐troposphere increased from 24 (16) pptv over the Pacific ocean to 58 (29) pptv over the central plains. Above 10 km the SO 4 = mixing ratio was essentially the same in both regions, and also when the geographic classifications were further partitioned into upper tropospheric and lower stratospheric categories (mean near 40 pptv). No obvious enhancements of SO 4 = could be detected in jet exhaust plumes, but this may reflect the difficulty of keeping a large airborne sampling platform within a turbulent wake for time periods longer than a few seconds. Expected SO 4 = enhancements (based on observed CO 2 enhancements and emission factors for these two species) were generally much smaller than the variability of ambient SO 4 = mixing ratios, so our null result does not mean that aircraft do not emit H 2 SO 4 .