Aqueous‐phase chemical processes in deliquescent sea‐salt aerosols: A mechanism that couples the atmospheric cycles of S and sea salt

Abstract

The aqueous‐phase chemistry of deliquescent sea‐salt 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 sea‐salt aerosols, the chemical reactions of these species and sea‐salt ions in the deliquescent solution, and changes in the aerosol composition that occur as a result of these processes. The calculations indicate that deliquescent sea‐salt aerosols are strongly buffered with a pH that remains close to 8 until the amount of acid added to the aerosol solution exceeds the alkalinity of sea salt. The oxidation of chloride by O3 and by free radicals is found to proceed at extremely slow rates, and thus these reactions cannot explain the high‐chloride deficits recently observed over the North Atlantic Ocean. On the other hand, the oxidation of dissolved SIV by O3 in sea‐salt aerosols is found to proceed at rates approaching 0.1 eq L−1 hr−1 and appears to be sufficiently rapid to qualitatively explain the observations of nss‐SO4= in sea‐salt aerosols over the North Atlantic Ocean. The high rate of SIV oxidation is found to proceed until the amount of nss‐SO4= generated in the aerosol is sufficiently large to overwhelm the buffering capacity of the deliquescent solution and lower the pH below 6. As a result, the calculations suggest the existence of a removal mechanism for atmospheric S that is largely controlled by the alkalinity of seawater and the flux of this alkalinity into the atmosphere in sea salt. It is estimated that this process will generate about 0.75 neq m−3 of nss‐SO4= associated with sea salt in the marine boundary layer and ultimately remove about (1–4) × 1011 moles of SO2 from the atmosphere annually. Comparison of this loss rate with other elements of the atmospheric S cycle suggests that sea salt may remove a significant amount of S from the marine atmosphere and thereby depress the SO2 concentration in the marine boundary layer and limit the number of cloud condensation nuclei generated from the oxidation of SO2.