A top-down analysis of emissions from selected Texas power plants during TexAQS 2000 and 2006

Airborne measurements were taken downwind of eleven Texas power generation facilities in 2000 and 2006 as part of the two Texas Air Quality Study (TexAQS) campaigns. From these measurements, we determine emission ratios of NOx (= NO + NO2), SO2, and CO to coemitted CO2 for each facility. These measurements provide an independent external assessment of reported emission ratios from continuous emission monitoring systems (CEMS). During the TexAQS study years, we find the SO2/CO2 and NOx/CO2 emission ratios derived from measurements aboard the aircraft agree quantitatively with inventory values from CEMS, with standard deviations of less than +/- 14%. We document significant decreases in atmospheric mixing ratios of NOx as a result of emission reductions due to controls implemented at the W. A. Parish plant after TexAQS 2000. For several of the facilities, CO emissions appear relatively constant in time. Derived CO/CO2 emission ratios agree substantially better with Texas Commission on Environmental Quality inventories in 2006 than in 2000, which we attribute to better inventory data from three facilities that installed CO CEMS between the two study years and not because of any significant change in CO emissions. Other plants appear to have varying CO emissions over time, complicating comparison to annual inventory values. Finally, we use two independent NO2 measurements, along with measurements of O-3, NO3, and N2O5, to quantify the fraction of NOx directly emitted as NO2 from the Oklaunion Power Plant, providing the first quantitative estimate of NO2 emissions from a power generation facility using ambient data.