The impact of satellite-adjusted NO<sub>x</sub> emissions on simulated NO<sub>x</sub> and O<sub>3</sub> discrepancies in the urban and outflow areas of the Pacific and Lower Middle US

Abstract

Abstract. We analyze the simulation results from a CMAQ model and GOME-2 NO2 retrievals over the United States for August 2009 to estimate the model-simulated biases of NOx concentrations over six geological regions (Pacific Coast = PC, Rocky Mountains = RM, Lower Middle = LM, Upper Middle = UM, Southeast = SE, Northeast = NE). By comparing GOME-2 NO2 columns to corresponding CMAQ NO2 columns, we produced satellite-adjusted NOx emission ("GOME2009") and compared baseline emission ("BASE2009") CMAQ simulations with GOME2009 CMAQ runs. We found that the latter exhibited decreases of −5.6%, −12.3%, −21.3%, and −15.9 % over the PC, RM, LM, and SE regions, respectively, and increases of +2.3% and +10.0% over the UM and NE regions. In addition, we found that changes in NOx emissions generally mitigate discrepancies between the surface NOx concentrations of baseline CMAQ and those of AQS at EPA AQS stations (mean bias of +19.8% to −13.7% over PC, −13.8% to −36.7% over RM, +149.7% to −1.8% over LM, +22.5% to −7.8% over UM, +31.3% to −7.9% over SE, and +11.6% to +0.7% over NE). The relatively high simulated NOx biases from baseline CMAQ over LM (+149.7%) are likely the results of over-predictions of simulated NOx emissions, which could shed light on those from global/regional Chemical Transport Models. We also perform more detailed investigations on surface NOx and O3 concentrations in two urban and outflow areas, PC (e.g., Los Angeles, South Pasadena, Anaheim, La Habra and Riverside) and LM (e.g., Houston, Beaumont and Sulphur). From two case studies, we found that the GOME2009 emissions decreased surface NOx concentrations significantly in the urban areas of PC (up to 30 ppbv) and in those of LM (up to 10 ppbv) during the daytime and that simulated NOx concentrations from CMAQ with GOME2009 compare well to those of in-situ AQS observations. A significant reduction in NOx concentrations resulted in a comparable increase in surface O3 concentrations in the urban areas of PC (up to 30 ppbv) and the resulting simulated O3 concentrations compare well with in-situ surface O3 observations over South Pasadena, Anaheim, and Riverside. Over Houston, Beaumont, and Sulphur, large reductions in NOx emissions from CMAQ with GOME2009 coincides with large reduced concentrations of simulated NOx. These concentrations are similar to those of the EPA AQS NOx observations. However, the resulting simulated increase in surface O3 at the urban stations in Houston and Sulphur exacerbated preexisting high O3 over-predictions of the baseline CMAQ. This study implies that simulated low O3 biases in the urban areas of PC are likely caused by simulated high NOx biases, but high O3 biases in the urban areas of LM cannot be explained by simulated high NOx biases over the region. This study also suggests that both in-situ surface NOx and O3 observations should be used simultaneously to resolve issues pertaining to simulated high/low O3 bias and that remote-sensing data could be used as a constraint for bottom-up emissions. In addition, we also found that daytime O3 reductions over the outflow regions of LM following large reductions in NOx emissions in the urban areas are significantly larger than they are over outflow regions of PC. These findings provide policymakers in the two regions with information critical to establishing strategies for mitigating air pollution.