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Recent factor analysis studies of data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network in which the separate carbon fractions have been used have found factors that have been assigned to emissions from diesel and spark-ignition vehicles. In this study, positive matrix factorization (PMF) was applied to the chemical composition of PM2.5 collected at the San Gorgonio IMPROVE site, downwind of the Los Angeles area. Therefore, in addition to the general identification of the possible emission sources of the ambient aerosols at this site, this study explicitly examines the feasibility of separating diesel emission from gasoline emission using the organic and elemental carbon (OC/EC) thermal fractions. The results suggest the feasibility of separating diesel/gasoline emission profiles based on the species concentration data including the OC/EC fractions. Also in the analysis of these data, two crustal factors were identified with one being associated with local suspended soil and the other being associated with transported Asian desert dust. The other sources include secondary sulfate/OC, secondary nitrate, and reacted sea salt. (C) 2004 Elsevier Ltd. All rights reserved.