Aerosol pH in the marine boundary layer: A review and model evaluation

Impacts of sea-salt-aerosol pH on oxidation processes, sulfur cycling, and surface-ocean fertilization are uncertain; estimates vary from pH<1 to >9 and the pH-dependence of some transformations is poorly characterized. We modeled these processes under clean and polluted conditions. At pH 8, S(IV)+O3 in sea salt is the principal S-oxidation pathway. At pH 5.5, S(IV) oxidation by HOCl dominates. Decreased SO2 solubility at pH 3 slows S(VI) production. The relative contribution of H2SO4(g) scavenging to S(VI) in sea salt increases with decreasing pH. Significant sea-salt dehalogenation is limited to acidified aerosol. Volatilization rates of BrCl and Br2 do not vary significantly between pH 5.5 and 3, whereas HCl production via acid displacement increases by a factor of 20. At pH 5.5 and 8, virtually all HNO3 is scavenged by sea salt. Modeled HNO3 increases at pH 3 but remains substantially lower than particulate NO-3. Discrepancies between measurements and modeled results are assessed based on measurement artifacts, uncertainties in rate and equilibrium constants, organic reactants and surface films, and dynamics.