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A study of carbonaceous particulate matter (PM) was conducted in the Middle East at sites in Israel, Jordan, and Palestine. The sources and seasonal variation of organic carbon, as well as the contribution to fine aerosol (PM2.5) mass, were determined. Of the 11 sites studied, Nablus had the highest contribution of organic carbon (OC), 29%, and elemental carbon (EC), 19%, to total PM2.5 mass. The lowest concentrations of PM2.5 mass, OC, and EC were measured at southern desert sites, located in Aqaba, Eilat, and Rachma. The OC contribution to PM2.5 mass at these sites ranged between 9.4% and 16%, with mean annual PM2.5 mass concentrations ranging from 21 to 25 ug m(-3). These sites were also observed to have the highest OC to EC ratios (4.1-5.0), indicative of smaller contributions from primary combustion sources and/or a higher contribution of secondary organic aerosol. Biomass burning and vehicular emissions were found to be important sources of carbonaceous PM in this region at the non-southern desert sites, which together accounted for 30%-55% of the fine particle organic carbon at these sites. The fraction of measured OC unapportioned to primary sources (1.4 mu gC m(-3) to 4.9 mu gC m(-3); 30%-74%), which has been shown to be largely from secondary organic aerosol, is relatively constant at the sites examined in this study. This suggests that secondary organic aerosol is important in the Middle East during all seasons of the year. (C) 2010 Elsevier Ltd. All rights reserved.