Effects of Water-soluble Organic Carbon on Aerosol pH
<p>Water soluble organic carbon (WSOC) is a ubiquitous and significant fraction of fine particulate matter.&#160; Despite advances in aerosol thermodynamic equilibrium models, there is limited understanding on the comprehensive impacts of WSOC on aerosol acidity (pH).&#160; 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<sup>+</sup> ion activity coefficients (&#947;<sub>H+</sub>) and activity (pH).&#160; We find that addition of organic acids and non-acid organic species concurrently increases &#947;<sub>H+</sub> and aerosol liquid water.&#160; Under the highly acidic conditions typical of the eastern U.S. (inorganic-only pH ~1), these effects mostly offset each other, giving pH changes of < 0.5 pH units even at organic aerosol dry mass fractions in excess of 60%.&#160; Under conditions with weaker acidity typical of Beijing (inorganic-only pH ~4.5), the non-acidic WSOC compounds had similarly minor effects on aerosol pH, but organic acids imparted the largest changes in pH compared to the inorganic-only simulations.&#160; Organic acids affect pH in the order of their pKa values (oxalic acid > malonic acid > glutaric acid).&#160; Although the inorganic-only pH was above the pK<sub>a</sub> value of all three organic acids investigated, pH changes in excess of 1 pH unit were only observed at unrealistic organic acid levels (aerosol organic acid concentrations > 35 &#181;g m<sup>-3</sup>) in Beijing.&#160; The model simulations were run at 70%, 80%, and 90% relative humidity (RH) levels and the effect of WSOC was inversely related to RH.&#160; 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.&#160; The somewhat offsetting nature of these effects suggests that aerosol pH is sufficiently constrained by the inorganic constituents alone under conditions where liquid-liquid phase separation is not anticipated to occur.</p>