The Houston-Galveston-Brazoria urban area contains industrial petrochemical sources that emit volatile organic compounds and nitrogen oxides, resulting in rapid and efficient ozone production downwind. During September to October 2006, the NOAA WP-3D aircraft conducted research flights as part of the second Texas Air Quality Study (TexAQS II). We use measurements of NOx, SO2, and speciated hydrocarbons from industrial sources in Houston to derive source emission ratios and compare these to emission inventories and the first Texas Air Quality Study (TexAQS) in 2000. Between 2000 and 2006, NOx/CO2 emission ratios changed by an average of −29% ± 20%, while a significant trend in SO2/CO2 emission ratios was not observed. We find that high hydrocarbon emissions are routine for the isolated petrochemical facilities. Ethene (C2H4) and propene (C3H6) are the major contributors to ozone formation based on calculations of OH reactivity for organic species including C2–C10 alkanes, C2–C5 alkenes, ethyne, and C2–C5 aldehydes and ketones. Measured ratios of C2H4/NOx and C3H6/NOx exceed emission inventory values by factors of 1.4–20 and 1–24, respectively. We examine trends in C2H4/NOx and C3H6/NOx ratios between 2000 and 2006 for the isolated petrochemical sources and estimate a change of −30% ± 30%, with significant day-to-day and within-plume variability. Median ambient mixing ratios of ethene and propene in Houston show decreases of −52% and −48%, respectively, between 2000 and 2006. The formaldehyde, acetaldehyde, and peroxyacetyl nitrate products produced by alkene oxidation are observed downwind, and their time evolution is consistent with the rapid photochemistry that also produces ozone.