A field, study was, undertaken during the summer of 2000,to assess the impact of the presence of organic compounds, on the liquid water concentrations of PM2.5 samples. The selected site, located in Research Triangle Park,North Carolina, was in a semi-rural environment with expected impacts from both biogenic and anthropogenic emissions. Eight samples, collected on Teflon. filters over 48-h periods, were analyzed for their liquid water concentrations by using a beta gauge to measure the change in PM2.5 mass on the filter as a function of relative humidity in a humidity-controlled environmental chamber. The filter samples were also analyzed for mass, elemental and organic carbon, and anionic concentrations. For six of the eight samples, the measured liquid water concentrations exceeded those predicted from. the presence of the inorganic constituents. A relationship between the excess liquid water and the measured organic carbon mass was found, although the standard deviations associated with the measurements were large. Through modeling studies it was determined that, on average, about 80% of the liquid water in the PM2.5 could be accounted for by inorganic ions present, with the remaining 20% associated with organic compounds. The liquid water data were consistent with the presence of aqueous, solutions supersaturated with respect to the inorganic ions in the. aerosol. (C) 2002 Elsevier Science Ltd. All rights reserved.