Effects of water-soluble organic carbon on aerosol pH

Michael A Battaglia Jr,Rodney J Weber,Christopher J Hennigan,Athanasios Nenes

doi:10.5194/acp-19-14607-2019

Abstract

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 aerosols 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 Extended Aerosol Inorganics Model (E-AIM) and ISORROPIA models in combination with activity coefficient model AIOMFAC (Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficient) to evaluate the effects of WSOC on the H+ ion activity coefficients (γH+) and activity (pH). We find that addition of organic acids and nonacid organic species concurrently increases γH+ and aerosol liquid water. Under the highly acidic conditions typical of the eastern US (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 %. Under conditions with weaker acidity typical of Beijing (inorganic-only pH ∼4.5), the nonacidic WSOC compounds had similarly minor effects on aerosol pH, but organic acids imparted the largest changes in pH compared to the inorganic-only simulations. Organic acids affect pH in the order of their pKa values (oxalic acid > malonic acid > glutaric acid). Although the inorganic-only pH was above the pKa 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 µg m−3) in Beijing. 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 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.

Highlights

The acidity of atmospheric particles plays a critical role in many physicochemical processes
We explore the effects of Water-soluble organic carbon (WSOC) on aerosol pH in a systematic way by utilizing inorganic data to construct combinations of single, aqueous-phase particulate compositions and utilizing aerosol thermodynamic models to investigate the effects of different WSOC species and concentrations on γH+ and aH+
Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficient (AIOMFAC)-predicted aerosol pH and γH+ versus the organic dry mass fraction, along with aerosol liquid water used in the model evaluations for the nonacid species runs in Baltimore and Beijing at all relative humidity (RH) levels, are shown in Figs. 2 and 3

Summary

Introduction

The acidity of atmospheric particles plays a critical role in many physicochemical processes. Some of these processes include sulfur oxidation and halogen chemistry, with important implications for the formation of sulfates (Chameides, 1984); the oxidation of volatile organic compounds (VOCs) and ozone formation in marine environments (Keene et al, 1998); the gas–particle partitioning of many semivolatile species (Ahrens et al, 2012; Keene et al, 2004); and enhancements to secondary organic aerosol (SOA) formation (Hallquist et al, 2009). The inorganic salt constituents in atmospheric particles, such as ammonium sulfate ((NH4)2SO4) and ammonium bisulfate (NH4HSO4), contribute to particle acidity and water content, with effects on aerosol radiative forcing (Seinfeld and Pandis, 2016). Battaglia Jr. et al.: Effects of water-soluble organic carbon their bulk acidity can affect the health of both environmental ecosystems and the human populations therein either directly (Gwynn et al, 2000; Peters et al, 1996; Schindler, 1988; Spengler et al, 1996; Fang et al, 2017; Johnson et al, 2008) or by their effects on nutrient deposition (Myriokefalitakis et al, 2016, 2018; Kanakidou et al, 2016; Nenes et al, 2011)

Methods

Results

Conclusion