Sea-salt aerosol chemistry in coastal areas: A model study

Abstract

A trajectory model of Coastal Chemistry and Deposition of Sea-Salt Particles (CCDSSP) has been developed. Release of reactive C1 and Br species takes place under a variety of conditions. The expulsion of HCl by strong acids was found to be the most important mechanism of C1 release. In a clean marine environment, Br is released mainly through a catalytic mechanism involving HOBr, BrC1, and Br2, as described in earlier model studies. This pathway is ineffective for chlorine release, and formation of HOCl and C12 leads to further activation of Br2. In environments rich in oxides of nitrogen, reactive uptake of N2O5, C1ONO2, and BrONO2 leads to release of chlorine and bromine from the sea-salt particles. The importance of these processes is enhanced in the winter half year, when reactive uptake of N2O5 causes approximately 40% of the total C1 release and BrONO2 becomes the dominant pathway of bromine release. Simulated peak winter concentrations of reactive halogen species were similar to those in summer simulations, leading to an increasing relative importance of halogens in VOC oxidation in wintertime, when C1 can account for 9% of the VOC oxidation. The model simulated sea-salt deposition along the trajectory quite satisfactorily when compared to measurements. With increasing time of transport from the coast, the sea-salt deposition becomes less important, while deposition of HCl keeps almost constant. The Cl- deficiency in the deposited, aged sea salt is thus fully compensated for, and an excess of C1- is found because of HCl deposition. The excess C1- is coupled with acidity which is not accounted for in deposition measurements.