O-3 oxidation of SO2 in sea-salt aerosol water: Size distribution of non-sea-salt sulfate during the First Aerosol Characterization Experiment (ACE 1)

Non sea-salt sulfate (NSS) of 2.2-2.3 nmol m(-3) total magnitude in aerosol s observed during the First Aerosol Characterization Experiment (ACE-1) at Cape Grim, Tasmania, was trimodally distributed with similar to 1 nmol NSS m(-3) in > 0.7 mu m ambient diameter (diam) coarse seasalt mode aerosols; d espite this low NSS concentration, [H2SO4(g)] was so low that < 1% of this NSS could have been due to H2SO4 scavenging. Ammonium was not associated wi th these coarse seasalt aerosols, suggesting that cloud processing was a mi nor contributor to the NSS found in > 0.7 mu m diam aerosols. The mechanism of O-3 oxidation of SO2 in sea-salt aerosol water (SSAW) is assessed for i ts capability to explain this coarse aerosol NSS. Limitation of this mechan ism's NSS contribution is largely due to SSAW's buffering capacity since it s reaction rate is reduced by 2 orders of magnitude at pH 6 versus the pH g reater than or equal to 8 of unreacted SSAW. However, the buffering capacit y of sea-salt aerosols may have been significantly enhanced over that of bu lk 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 estimat ed actual buffering capacity of sea-salt aerosols observed during ACE 1 was 50%, or more, enhanced over that due to bulk seawater alkalinity. Consider ing this enhanced buffering capacity, O-3 oxidation of SO2 in SSAW can prod uce sufficient NSS to explain 70-90% of the similar to 1 nmol m(-3) found i n > 0.7 mu m diam coarse sea-salt aerosols with cloud processing and furthe r oxidation of SO2 in SSAW (i.e., pH < 6) by other sea-salt conversion mech anisms contributing the remainder. The amount of NSS produced by sea-salt c onversion mechanisms during the ACE 1 remote Southern Ocean experiment vied with homogeneous and cloud processing in their contribution to the total o bserved NSS of 2.2-2.3 nmol m(-3).