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Sources of secondary organic carbon at 15 field sites across the United States (U.S.) during the years 2003–2010 have been examined. Filter samples have been taken for 24-h at a site in Research Triangle Park, NC; at SEARCH sites in southeastern U.S. during May and August 2005; at LADCO sites from Mar 2004–Feb 2005; Riverside, CA during SOAR in 2005; Cleveland, OH during CMAPS; and Pasadena and Bakersfield, CA during CalNex (see text for acronyms.) Samples were extracted, derivatised, and analysed for organic tracers by GC-MS. The mass fraction method described by Kleindienstet al. was used to determine the contributions of the tracers to secondary organic carbon mass. Secondary organic aerosol masses were determined using laboratory-derived values for the organic mass–organic carbon (OM/OC) ratio. Results from the analysis show secondary organic carbon in the eastern and midwestern U.S. to be consistently dominated by SOA from biogenic emissions during the spring–summer period. SOA from biogenic emissions are far less important in the western U.S. during the same period with isoprene emissions being particularly weak. These sites in the western U.S. are in more densely populated, polluted regions of California and are probably not representative of sites in the rural western U.S. The ratio of tracers from monoterpenes can also provide information regarding presumed sources. Similarly, the ratio of isoprene tracers can provide information on reaction pathways (NOXvs. non-NOX) leading to the formation of SOA in the atmosphere. Updated tables for the identity and fragmentation of SOA molecular tracers and for mass fractions of four biogenic class types (isoprene, monoterpenes, sesquiterpenes, 2-methyl-3-buten-2-ol) and two anthropogenic class types (aromatic hydrocarbons and 2-ring PAHs) are given. [ABSTRACT FROM AUTHOR]