To evaluate the impact of regional wildfires in east Texas on fine particulate matter concentration and composition, source apportionment calculations were performed on a set of samples collected at three sites in Houston during a wood smoke episode. Separately, samples collected at the same sites on days not influenced by wood smoke, were analyzed for comparison. The analysis of the data collected on non--wood smoke episode days indicated that the major contributors to PM2.5 mass were secondary sulfate, diesel and gasoline powered vehicles, other organics (difference between the measured organic mass by carbon (OMC) and the sum of the primary OC source contributions), wood combustion and meat cooking. Secondary sulfate (not apportioned to a primary source) represented almost 100% of the sulfate measured whereas primary sources were found to account for the majority of the measured organic carbon at three sites (64--69%). On the wood smoke episode days, major sources of PM2.5 were found to be the same as on the days without wood smoke, except for the contribution of the meat cooking source, which became insignificant on the wood smoke episode days. The contribution of the wood combustion source increased by an average of 200% at all sites on wood smoke episode days, whereas the contributions of other primary sources did not increase significantly during the wood smoke episode. PM2.5 mass almost doubled during the wood smoke episode, largely because of the contributions of the secondary sources. The mass concentrations of secondary sulfate and organics not attributed to primary sources increased 68% and 228% at each site, respectively, during the wood smoke episode. The increase in the contributions of secondary sulfate aerosols during the wood smoke episode was examined using a 3-D photochemical grid model. The simulations, together with ambient data, indicated that the increases in sulfate concentrations observed during this wood smoke episode were consistent with heterogeneous/surface reactions on wood smoke particles.