NOx Emissions Uncertainty of the EPA National Emissions Inventory 2005 over the Southern US

Simulation results from the Community Multiscale Air Quality (CMAQ) model version 4.7.1 over the Conterminous United States (CONUS) for August 2009 are analyzed to investigate the uncertainty of the EPA National Emissions Inventory 2005 over the six geographical regions (i.e., Pacific Coast=PC, Rocky Mountain=RM, Lower Middle=LM, Upper Middle=UM, Southeast=SE, and Northeast=NE). We found that NOx emissions from NEI 2005, simply scaled by comparing the Global Monitoring Experiment 2 (GOME-2) and CMAQ NO2 columns, decreased significantly over the LM and the SE US when compared to those of other regions and the resulting NOx concentrations were as much as those from corresponding AQS stations (mean biases become -1.8% from +149.7% and -7.9% from +31.3%). Moreover, CMAQ with the scaled NOx emissions better captures in-situ observed daytime (1-5 PM, local hour) O3 concentrations from the measurements over the LM and the SE US. The model predicted that monthly averaged daytime surface O3 concentrations with the modified emissions would decrease significantly by as much as 10 ppbv, particularly over the neighboring areas of Houston, New Orleans, Tampa, and Jacksonville. These results suggest that anthropogenic NOx emissions over the southern US reported in the EPA National Emissions Inventory 2005 are too high.