Heterogeneous Sulfur Conversion and the Size Distribution of nssSO4= in the MBL during ACE-1: The Role of O3-Oxidation in Sea-Salt Aerosol Water

H. Sievering, B. Lerner, and J. Slavich
Center for Environmental Sciences, Box 173364
University of Colorado at Denver
Denver, CO 80217

J Cainey
CSIRO Division of Atmospheric Research
PMB No.1
Aspendale, Victoria 3195, Australia
tel +61 3 9239 4564

Oxidation of SO2 by O3 as well as H2O2 in the aqueous phase is an important process in cloud droplets. The MBL contains sea-salt aerosols with a large amount of high pH water which supports O3-oxidation of SO2 to nssSO4=. During ACE-1, Cape Grim size distributed nssSO4 data from approximately 125m and 30m asl levels within the MBL show that 1 to 1.5 nmol nssSO4= m-3 out of a total 2 to 2.5 nmol nssSO4= m-3 was found associated with sea salt mode aerosols. Cloud production/processing appears to be a minor contributor to the 1-1.5 nmol nssSO4= m-3 found in the sea salt mode. The pH of water associated with sea salt aerosols probably remained near 8, since no sea salt mode chloride deficit was observed; alkalinity of sea salt aerosols was about 1 nmol m-3 . Thus, rapid oxidation of SO2 in sea salt aerosol water took place. Most, if not all, of the 1-1.5 nmol nssSO4= m-3 observed in the sea salt mode can be explained by O3-oxidation of SO2 in sea salt aerosol water.