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
hsieveri@carbon.cudenver.edu

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.