Aqueous-Phase Chemical Processes in Deliquescent Seasalt Aerosols

Abstract

The aqueous-phase chemistry of deliquescent seasalt aerosols in the remote marine boundary layer is investigated with a steady-state box model. The model simulates the scavenging of soluble and reactive gaseous species by the seasalt aerosols, the chemical reactions of these species and seasalt ions in the deliquescent solution, and changes in the aerosol composition that occur as a result of these processes. Because of the alkalinity of seasalt, the calculations indicate that deliquescent seasalt aerosols are strongly buffered and as a result their pH remains close to 8 until the amount of acid added to the aerosol solution exceeds the alkalinity. The oxidation of chloride by O3 as well as by free radicals are found to proceed at extremely slow rates and thus these reactions can not explain the high chloride deficits recently observed over the North Atlantic Ocean. On the other hand, the oxidation of dissolved SIV by O3 in seasalt aerosols is found to proceed at an extremely high rate and appears to explain the observations of nss-SO42- in seasalt aerosols over the Noth Atlantic Ocean. The high rate of SIV oxidation is found to proceed until the amount of nss-SO42- generated in the aerosol is sufficiently large to overwhelm the buffering capacity and lower the pH below 5.5. As a result, the calculations suggest a strong coupling between the cycles of S and alkalinity in the marine boundary layer that may act to limit the generation of cloud condensation nuclei from the oxidation of biogenic S emissions.