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 NO x (= NO + NO 2 ), SO 2 , and CO to coemitted CO 2 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 SO 2 /CO 2 and NO x /CO 2 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 NO x 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/CO 2 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 NO 2 measurements, along with measurements of O 3 , NO 3 , and N 2 O 5 , to quantify the fraction of NO x directly emitted as NO 2 from the Oklaunion Power Plant, providing the first quantitative estimate of NO 2 emissions from a power generation facility using ambient data.