Evaluations of NOx and highly reactive VOC emission inventories in Texas and their implications for ozone plume simulations during the Texas Air Quality Study 2006
Satellite and aircraft observations made during the 2006 Texas Air Quality Study (TexAQS) detected strong urban, industrial and power plant plumes in Texas. We simulated these plumes using the Weather Research and Forecasting-Chemistry (WRF-Chem) model with input from the US EPA's 2005 National Emission Inventory (NEI-2005), in order to evaluate emissions of nitrogen oxides (NO(x) = NO + NO(2)) and volatile organic compounds (VOCs) in the cities of Houston and Dallas-FortWorth. We compared the model results with satellite retrievals of tropospheric nitrogen dioxide (NO(2)) columns and airborne in-situ observations of several trace gases including NOx and a number of VOCs. The model and satellite NO(2) columns agree well for regions with large power plants and for urban areas that are dominated by mobile sources, such as Dallas. How-ever, in Houston, where significant mobile, industrial, and inport marine vessel sources contribute to NO(x) emissions, the model NO(2) columns are approximately 50 %-70 % higher than the satellite columns. Similar conclusions are drawn from comparisons of the model results with the TexAQS 2006 aircraft observations in Dallas and Houston. For Dallas plumes, the model-simulated NO(2) showed good agreement with the aircraft observations. In contrast, the model-simulated NO(2) is similar to 60 % higher than the aircraft observations in the Houston plumes. Further analysis indicates that the NEI-2005 NO(x) emissions over the Houston Ship Channel area are overestimated while the urban Houston NO(x) emissions are reasonably represented. The comparisons of model and aircraft observations confirm that highly reactive VOC emissions originating from industrial sources in Houston are underestimated in NEI-2005. The update of VOC emissions based on Solar Occultation Flux measurements during the field campaign leads to improved model simulations of ethylene, propylene, and formaldehyde. Reducing NO(x) emissions in the Houston Ship Channel and increasing highly reactive VOC emissions from the point sources in Houston improve the model's capability of simulating ozone (O(3)) plumes observed by the NOAA WP-3D aircraft, although the deficiencies in the model O(3) simulations indicate that many challenges remain for a full understanding of the O(3) formation mechanisms in Houston.