The sources of photochemical precursors for ozone, primarily NOX (NO + NO2) and reduced carbon (CO and hydrocarbons), were evaluated using principal component analysis (PCA) of concurrent measurements of [NOX], [NOY] (total reactive oxidized nitrogen species), [CO], [SO2], [C3H8], [C6H6], and [O3] collected at a rural Alabama field site during the summers of 1990 and 1992. Two separate data matrices were analyzed, including a low measurement frequency matrix that included all of the variables and a high measurement frequency matrix that included all of the variables except for the hydrocarbons. The results of the PCA indicated that the major sources of NOY in the region are (1) coal-fired power plants and (2) biomass burning and/or paper mills. The identifications are based on the [CO]/[NOY] and [SO2]/[NOY] emission ratios derived from the measurements in comparison with the expected emission ratios that are based upon the National Acid Precipitation Assessment Program (NAPAP) emission inventory and upon laboratory experiments. The results indicated that the NAPAP inventory, integrated over the southeastern region of the United States, may overestimate the current [SO2]/[NOY] emission ratio from coal-fired power plants and may underestimate the [CO]/[NOY] emission ratio from paper mills. The results also suggest that biomass burning in the southeastern United States may be an important component in the NOY inventory, both as a point source associated with paper mills and as a seasonally dependent area source. The variation of [O3] observed at the site was primarily related to the variation of [NOY].