The ratios of observed organic carbon (OC) to elemental carbon (EC) from the rural sites of the IMPROVE network are analyzed for the 5-year period from 2000 to 2004. Among these years, nationwide OC/EC peaks are observed most consistently in the summer of 2002. Several potential factors are analyzed, including biomass burning, secondary organic aerosols (SOA) formation from biogenic sources and in-cloud processing, long-range transport from East Asia, and meteorological conditions over the U.S. We find that biomass burning and SOA formation make the most significant contributions using the global GEOS-Chem model simulations. The effect of model estimated in-cloud SOA formation is significant compared to the estimate of (non-cloud) biogenic SOA. The impacts of Canadian and western U.S. fires are larger than fires in Russia or Mexico in summer. The dry meteorological condition of the summer of 2002 tends to promote higher OC/EC ratios by inducing larger fire emissions, SOA formation, and a longer OC lifetime. Published by Elsevier Ltd.