Sensitivity of sulphur dioxide oxidation in sea salt to nitric acid and ammonia gas phase concentrations

The role of sea-salt aerosol in the sulphur dioxide budget is investigated using a steady state box model. The model simulates, for certain species, the condensation onto, and chemical reactions within, deliquescent sea-salt particles, The chemical reactions are controlled by the pH of the solution, and results show that the presence of gas-phase nitric acid, HNO3, and ammonia, NH3, can have a significant effect on calculated concentrations of non-sea-salt (nss) sulphate, [nss-SO42-], formed within the solution. For example, in the case of high [HNO3(g)] (100 pptv) the calculated [nss-SO42-] can be decreased by similar to 40% after 2 days (with a 75% decrease at HNO3 concentrations of 1 ppbv). When NH3 is added, a decrease in calculated [nss-SO42-] of 20% is observed for 500 pptv NH3, 50 pptv HNO3. Thus nitric acid is more effective than ammonia. In conditions appropriate for the marine boundary layer (50 pptv of SO2, and HNO3, and 200 pptv of NH3), nitric acid is still the governing factor, and the [nss-SO42-], after 2, days, is decreased by approximately 25% compared to the concentration formed when conditions show no HNO, or NH3. Results also show that comparisons with nss-SO42- observations are nearly 1 order of magnitude lower, with observations giving a [nss-SO42-]:Na+ ratio of 0.01-0.04 compared to predicted values of 0.005 or lower. Other mechanisms of nss-sulphate production may have to be used to explain all the nss-sulphate found id sea-salt aerosol. Although in cases where [SO2(g)] is 10-100 pptv the decrease in [nss-SO42-] is 15-30%, at values of [SO2(g)] > 100 pptv the effects of both nitric acid and ammonia on calculated [nss-SO42-] are small. In a future scenario of increasing NOx(HNO3) emissions and decreasing SO2 emissions the role of HNO3 on the global sulphur budget could be very significant.