SENSITIVITY OF SULFUR-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 investig ated using a steady state box model. The model simulates, for certain species, the condensation onto, and chemical reactions within, delique scent sea-salt particles, The chemical reactions are controlled by the pH of the solution, and results show that the presence of gas-phase n itric acid, HNO3, and ammonia, NH3, can have a significant effect on c alculated 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 o bserved for 500 pptv NH3, 50 pptv HNO3. Thus nitric acid is more effec tive than ammonia. In conditions appropriate for the marine boundary l ayer (50 pptv of SO2, and HNO3, and 200 pptv of NH3), nitric acid is s till the governing factor, and the [nss-SO42-], after 2, days, is decr eased by approximately 25% compared to the concentration formed when c onditions show no HNO, or NH3. Results also show that comparisons with nss-SO42- observations are nearly 1 order of magnitude lower, with ob servations giving a [nss-SO42-]:Na+ ratio of 0.01-0.04 compared to pre dicted values of 0.005 or lower. Other mechanisms of nss-sulphate prod uction may have to be used to explain all the nss-sulphate found id se a-salt aerosol. Although in cases where [SO2(g)] is 10-100 pptv the de crease 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 decr easing SO2 emissions the role of HNO3 on the global sulphur budget cou ld be very significant.