Atmospheric Aerosol Water-Soluble Organic Carbon Measurement: A Theoretical Analysis

The measurement of Water-Soluble Organic Carbon (WSOC) in atmospheric aerosol is usually carried out by sample collection on filters, extraction in ultrapure water, filtration, and measurement of the total organic carbon. This paper investigates the role of different conditions of sampling and extraction as well as the range of solubilities of the organic compounds that contribute to the WSOC. The sampling and extraction of WSOC can be described by a single parameter, <i>P</i>, expressing the ratio of water used per volume of air sampled on the analyzed filter. Two cases are examined in order to bound the range of interactions of the various organic aerosol components with each other. In the first we assume that the organic species form an ideal solution in the particle and in the second that the extraction of a single compound is independent of the presence of the other organics. The ideal organic solution model predicts that species with water solubility as low as 10^–4 g L^–1 contribute to the measured WSOC. In the other end, the independent compounds model predicts that low-solubility (as low as 10^–7 g L^–1) compounds are part of the WSOC. Studies of the WSOC composition are consistent with the predictions of the ideal organic solution model. A value of <i>P</i> = 0.1 cm³ m^–3 is proposed for the extraction of WSOC for typical organic aerosol concentrations (1–10 μg m^–3). WSOC measurements under high concentration conditions often used during source sampling will tend to give low WSOC values unless higher <i>P</i> values are used