Effects of Water-soluble Organic Carbon on Aerosol pH

Abstract. Water soluble organic carbon (WSOC) is a ubiquitous and significant fraction of fine particulate matter. Despite advances in aerosol thermodynamic equilibrium models, there is limited understanding on the comprehensive impacts of WSOC on aerosol acidity (pH). We address this limitation by studying submicron aerosol that represent the two extremes in acidity levels found in the atmosphere: strongly acidic aerosol from Baltimore, MD, and weakly acidic conditions characteristic of Beijing, China. These cases are then used to construct mixed inorganic/organic single-phase aqueous particles, and thermodynamically analyzed by the E-AIM and ISORROPIA models (in combination with activity coefficient model AIOMFAC) to evaluate the effects of WSOC on the H + ion activity coefficients ( γ H+ ) and activity (pH). We find that addition of organic acids and non-acid organic species concurrently increases γ H+ and aerosol liquid water. When allowed to modulate pH, these effects mostly offset each other, giving pH changes of γ H+ . The model simulations were run at 70 %, 80 %, and 90 % relative humidity (RH) levels and the effect of WSOC was inversely related to RH. At 90 % RH, WSOC altered aerosol pH by up to ~ 0.2 pH units, though the effect was up to ~ 0.6 pH units at 70 % RH. The offsetting nature of these effects suggests that aerosol pH is sufficiently constrained by the inorganic constituents alone and under conditions where liquid-liquid phase separation is not anticipated to occur.