© Author(s) 2008. This work is licensed under a Creative Commons License. Atmospheric Chemistry and Physics

Abstract. Organic carbon (OC) comprises a large fraction of fine particulate matter (PM2.5) in Mexico City. Daily and select 12-h PM2.5 samples were collected in urban and pe-ripheral sites in Mexico City from 17–30 March 2006. Sam-ples were analyzed for OC and elemental carbon (EC) us-ing thermal-optical filter-based methods. Real-time water-soluble organic carbon (WSOC) was collected at the pe-ripheral site. Organic compounds, particularly molecular markers, were quantified by soxhlet extraction with methanol and dichloromethane, derivitization, and gas chromatogra-phy with mass spectrometric detection (GCMS). A chemi-cal mass balance model (CMB) based on molecular marker species was used to determine the relative contribution of major sources to ambient OC. Motor vehicles, including diesel and gasoline, consistently accounted for 49 % of OC in the urban area and 32 % on the periphery. The daily con-tribution of biomass burning to OC was highly variable, and ranged from 5–26 % at the urban site and 7–39 % at the pe-ripheral site. The remaining OC unapportioned to primary sources showed a strong correlation with WSOC and was considered to be secondary in nature. Comparison of tem-porally resolved OC showed that contributions from primary aerosol sources during daylight hours were not significantly different from nighttime. This study provides quantitative understanding of the important sources of OC during the MI-LAGRO 2006 field campaign