Evaluating NOx Emission Inventories for Regulatory Air Quality Modeling Using Satellite and Model Data

Ozone modeling and meteorological analyses indicate that the southeastern U.S. sometimes is a source of ozone transport into Texas. Systematic errors in ozone modeling of southeastern U.S. ozone can confound efforts to quantify downwind effects of ozone transport. Model overestimates of southeastern U.S. ozone may result from biased NOx emissions and the purpose of this study was to assess the accuracy of the NOx emissions used in the Texas Commission on Environmental Quality's State Implementation Plan (SIP) modeling which are based on the 2005 National Emission Inventory. We used satellite NO2 column data from the Ozone Monitoring Instrument (OMI) together with NO2 columns from a regional air quality model to make top-down NOx emissions estimates for the southeastern U.S. Specifically, we used the KNMI DOMINO v2.0 and NASA SP2 NO2 column products and the Comprehensive Air quality Model with extensions (CAMx). Consistent with many models the CAMx NO2 columns were consistently lower than the retrieved NO2 columns across the modeling domain. We evaluated CAMx-predicted NOy in the free troposphere against aircraft data from the INTEX-A field experiment. This comparison showed that, like many other regional and global models, CAMx underestimated NO2, NOx, and NOy above 8 km in the troposphere. We addressed the CAMx low bias in upper tropospheric NO2 by evaluating the model's simulation of sources and sinks of NOx in the upper troposphere. We added transport of NOy from the stratosphere to the troposphere. We added lightning NOx emissions and an aircraft emission inventory with a detailed spatial distribution of aircraft cruise NOx emissions based on aircraft flight data. Finally, we revised the treatment of vertical transport of chemical species by deep convection. These changes improved model performance in simulating upper tropospheric NO2, provided better agreement with satellite NO2 column retrievals and the INTEX-A data, and allowed us to proceed with top-down emissions estimates. Two separate top-down NOx emissions estimates were developed from the DOMINO and NASA SP2 retrievals. Differences between the estimates based on DOMINO and SP2 were evaluated and the estimates were then compared with the NOx emission inventory used in TCEQ's SIP modeling episodes to assess the possibility of bias in the inventory.