Abstract
Abstract. 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 (NOx = NO + NO2) and volatile organic compounds (VOCs) in the cities of Houston and Dallas-Fort Worth. We compared the model results with satellite retrievals of tropospheric nitrogen dioxide (NO2) columns and airborne in-situ observations of several trace gases including NOx and a number of VOCs. The model and satellite NO2 columns agree well for regions with large power plants and for urban areas that are dominated by mobile sources, such as Dallas. However, in Houston, where significant mobile, industrial, and in-port marine vessel sources contribute to NOx emissions, the model NO2 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 NO2 showed good agreement with the aircraft observations. In contrast, the model-simulated NO2 is ~60% higher than the aircraft observations in the Houston plumes. Further analysis indicates that the NEI-2005 NOx emissions over the Houston Ship Channel area are overestimated while the urban Houston NOx 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 NOx 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 (O3) plumes observed by the NOAA WP-3D aircraft, although the deficiencies in the model O3 simulations indicate that many challenges remain for a full understanding of the O3 formation mechanisms in Houston.
Highlights
Texas is the second most populous state in the US, according to 2000 and 2010 Census data
Ozone, which is strongly enhanced during photochemical smog events, is a regulated pollutant, and US Environment Protection Agency (EPA) ozone standards have consistently been violated in the Houston-Galveston area for decades
4.1.1 NOx emission sources in Texas In Fig. 1, boxes representing 9 regions with large NOx emission sources in Texas and one large power plant in Mexico are overlaid on maps of the NEI05-REF emissions and of the National Aeronautics and Space Administration (NASA) tropospheric satellite NO2 columns averaged over the Texas Air Quality Study (TexAQS) 2006 time period
Summary
Texas is the second most populous state in the US, according to 2000 and 2010 Census data (http://factfinder2.census. gov). A primary objective of the measurements made during the Texas Air Quality Studies in 2000 and 2006 (TexAQS 2000 and 2006) was to identify NOx and VOC emission sources and understand their roles in ozone pollution in Texas (Parrish et al, 2009). We evaluate NOx and VOC emissions in the EPA NEI2005 using regional model simulation results together with satellite and aircraft observations during TexAQS 2006. Because the satellite-retrieved NO2 columns have uncertainties caused by the application of an air mass factor (Boersma et al, 2004; Kim et al, 2009; Lamsal et al, 2010; Heckel et al, 2011), more definitive conclusions regarding the emission inventory are obtained using other independent observational data sets (e.g., aircraft measurements). The model simulations of ozone plumes with the default NEI-2005 and with updated emissions based on the findings in this study are compared with the aircraft observations, and the importance of the updated emissions in the ozone plume simulations is discussed
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