Chemical characterization of the ambient organic aerosol soluble in water: 2. Isolation of acid, neutral, and basic fractions by modified size‐exclusion chromatography

Abstract

A method employing size‐exclusion chromatography (SEC) with Total Organic Carbon detection is developed to isolate and quantify the water‐soluble organic carbon (WSOC) component of ambient aerosol particles by acid, neutral, and basic functional groups. The method provides unique quantitative insights into the characteristics and possible sources of a large fraction of the organic aerosol. The SEC is combined with a XAD‐8 method that separates WSOC into hydrophilic and hydrophobic fractions. Calibrations show the hydrophilic fraction separates into short‐chain aliphatic acids (WSOCxp_a), neutrals (WSOCxp_n, e.g., saccharides, polyols, and short‐chain carbonyls), and organic bases. Recovered hydrophobic fractions are separated into acids (WSOCxrr_a, e.g., aromatic) and neutrals (WSOCxrr_n, e.g., phenols). 13Carbon–nuclear magnetic resonance on ambient samples support the calibration‐based conclusions; however, calibrations only provide a guide to the type of ambient organic compounds expected in each group. Comparisons are made between XAD‐8/SEC results from urban Atlanta summer and winter and biomass burning samples. The largest isolated fraction of Atlanta summer WSOC is WSOCxp_a (29% μg C/μg C), suggesting aliphatic acids of less than C4 or C5 are the dominant secondary organic aerosol product. Combined with WSOCxrr_a, these acid groups are a higher fraction of summer organic carbon (20%) than winter (14%). They are correlated with each other (R2 = 0.74), with WSOCxp_n (R2 = 0.61 and 0.52), and with gaseous 24‐hour averaged volatile organic compounds linked to mobile sources. In biomass burning samples, neutrals (WSOCxp_n and WSOCxrr_n) dominate the WSOC. Atlanta winter samples have characteristics that appear to be a combination of summer and biomass samples.