O3 oxidation of SO2 in sea-salt aerosol water - Size distribution of non-sea-salt sulfate during the First Aerosol Characterization Experiment (ACE 1)

• Published in: Journal of Geophysical Research 1999-09, Vol.104 (D17), p.21
• Main Authors: Sievering, H, Lerner, B, Slavich, J, Anderson, J, Posfai, M, Cainey, J
• Format: Article
• Language: English
• ISSN: 0148-0227

Non-sea-salt sulfate (NSS) of 2.2-2.3 nmol/cu m total magnitude in aerosols observed during the First Aerosol Characterization Experiment (ACE-1) at Cape Grim, Tasmania, was trimodally distributed with about 1 nmol NSS/cu m in more than 0.7-micron ambient diameter (diam) coarse sea-salt mode aerosols; despite this low NSS concentration, the H2SO4(g) concentration was so low that less than 1 percent of this NSS could have been due to H2SO4 scavenging. Ammonium was not associated with these coarse seasalt aerosols, suggesting that cloud processing was a minor contributor to the NSS found in more than 0.7-micron diam aerosols. The mechanism of O3 oxidation of SO2 in sea-salt aerosol water (SSAW) is assessed for its capability to explain this coarse aerosol NSS. The limitation of this mechanism's NSS contribution is largely due to SSAW's buffering capacity since its reaction rate is reduced by two orders of magnitude at pH 6 versus the pH at least 8 of unreacted SSAW. However, the buffering capacity of sea-salt aerosols may have been significantly enhanced over that of bulk seawater alkalinity. This appears to be due to carbonate resulting from small fragments of biogenic CaCO3 in the ocean's surface microlayer. Given the observed nonsoil calcium excess over that in bulk seawater, the estimated actual buffering capacity of sea-salt aerosols observed during ACE 1 was 50 percent or more, enhanced over that due to bulk seawater alkalinity. Considering this enhanced buffering capacity, O3 oxidation of SO2 in SSAW can produce sufficient NSS to explain 70-90 percent of the about 1 nmol/cu m found in more than 0.7-micron diam coarse sea-salt aerosols with cloud processing and further oxidation of SO2 in SSAW (i.e., pH less than 6) by other sea-salt conversion mechanisms contributing the remainder. (Author)