Organic carbon (OC) and elemental carbon (EC) in fine particles (PM2.5) were measured at the Baltimore Supersite at Ponca Street for 9.5 months in 2002 using a Sunset Laboratory carbon analyzer with 1-hour time resolution. Monthly EC and CO diurnal averages were characterized by pronounced peaks in the early morning commute hours. OC concentration profiles were similar to CO and EC during the morning and evening rush hours, except when high ozone episodes occurred, i.e., generally in summer. Primary and secondary OC contributions were estimated during ozone episodes exceeding 100 ppb (1-hour average). The largest 1-hour ozone concentrations for the entire study period occurred between 11 and 13 August, during which secondary OC contributed, on average, ∼60% of the hourly OC concentrations and a maximum of 82%. Additionally, the annual EC emission rate is estimated for Maryland using published CO emission inventory data and a mean EC/CO ratio (0.0023 ± 0.0008) derived by regressing selected EC and CO measurements for each of the 9.5 months. The result, 2.31 ± 0.80 Gg EC yr−1, is similar to the estimate (2.95 Gg EC yr−1), determined on the basis of the PM2.5 emission inventory for Maryland and the mean EC abundance in PM2.5 measured with a U.S. Environmental Protection Agency speciation monitor in 2002. Finally, the Baltimore Supersite and much of the northeastern United States experienced severe smoke fumigation in early July 2002 owing to uncontrolled forest fires in Quebec, during which time, CO, EC, and OC were predominately associated with the Canadian smoke. During this episode, EC emissions are estimated to be 4.75 Gg EC from Canadian Quebec natural fires in July, and an annual release rate of 9.94 Gg EC yr−1 is estimated for all Canadian boreal forest fires.