Impact of organic acids on chloride depletion of inland transported sea spray aerosols

Abstract. Heterogeneous reactions on sea spray aerosols (SSA) are the main pathway to drive the circulation of chlorine, nitrogen, and sulfur in the atmosphere. The release of Cl will significantly affect the physicochemical properties of SSA. However, the impact of organic acids and mixing state on chloride depletion of SSA is still unclear. Hence, the size and chemical composition of individual SSA particles during the East Asian summer monsoon were investigated by a single particle aerosol mass spectrometer (SPAMS). According to the chemical composition, SSA particles were classified into SSA-Aged, SSA-Bio and SSA-Ca. In comparison to the aged Na-rich SSA particles (SSA-Aged), some additional organic species related to biological origin were observed in SSA-Bio, and each of two types accounts for approximately 50 % of total SSA particles. SSA-Ca may associated with organic shell of Na-rich SSA particles, which only accounts for ~ 3 %. Strongly positive correlations between Na and organic acids (including formate, acetate, propionate, pyruvate, oxalate, malonate, succinate, and glutarate) were observed for the SSA-Aged (r² = 0.52, p < 0.01) and SSA-Bio (r² = 0.61, p < 0.01), indicating the significance of organic acids in the chloride depletion during inland transport. The contribution of these organic acids to the chloride depletion is estimated to be up to 34 %. Interestingly, the degree of chloride depletion is distinctly different between SSA-Aged and SSA-Bio. It is most probably attributed to the associated organic coating in the SSA-Bio particles, which inhibit the displacement reactions between acids and chloride. As revealed from the mixing state of SSA-Bio, Cl / Na ratio increases with increasing phosphate and organic nitrogen, which is known to originate from biological activities. This finding provides some basis for the improvement of modeling simulations in chlorine circulation and a comprehensive understanding of the effects of organics on chloride depletion of SSA particles.