INVESTIGATION OF ORGANIC AEROSOL SAMPLING ARTIFACTS IN THE LOS-ANGELES BASIN

Filter collection of particulate organic carbon is complicated by adsorption and volatilization sampling artifacts which inhibit attempts to accurately assess aerosol concentrations. Insight into the adsorption artifact can be gained by considering that adsorption on sampling filters is conceptually similar to gas-particle partitioning with the filter acting as a model aerosol. We found that significant adsorption of gas-phase organic compounds occurs on quartz fiber filters, and the presence of typical loadings of particulate material on the sampling filter does not significantly affect the magnitude of the adsorption artifact. Also placing a Teflon filter upstream of a pair of quartz fiber filters has no effect on total organic carbon loadings on the final quartz fiber filter, but adsorbed organic carbon loadings are much higher on quartz fiber filters behind Teflon filters than on quartz fiber filters behind quartz fiber filters. Organic carbon sampled from the atmosphere is unlikely to attain equilibrium between that in the gas phase and that adsorbed on the quartz fiber sampling filter. As a result, a quartz fiber filter behind a quartz fiber filter is exposed to a lower gas-phase concentration than a quartz fiber filter behind a Teflon filter. This explains the difference between Teflon-quartz and quartz-quartz backup filters and suggests that Teflon-quartz backup filters provide a better estimate of adsorbed vapor on quartz fiber front filters than do quartz-quartz backup filters. Under typical sampling conditions adsorption is the dominant artifact in the sampling of particulate organic carbon, and longer sampling periods reduce the percentage of collected material that is adsorbed vapor.