Weekly cycles of observed and modeled NOx and O3 concentrations as a function of land use type and ozone production sensitivity of the US

Simulation results from the Community Multiscale Air Quality (CMAQ) model version 4.7 over the Conterminous United States (CONUS) for August 2009 are analyzed to evaluate the daily derivation of the U.S. EPA's Air Quality System (AQS) observed and modeled ground-level concentrations of nitrogen oxides (NOx) and ozone(O3) at stations classified according to a geographical-based land use designation or a O3-NOx-VOC chemical sensitivity regime. The physical land use designations are derived from the Advanced Very High Resolution Radiometer (AVHRR) global land cover characteristic data representing three features: urban, forest, and other. The O3 sensitivity regimes (NOx-saturated, mixed or NOx-sensitive) are inferred from low to high values of photochemical indicators based on the HCHO to NO2 ratio column data from the Global Ozone Monitoring Experiment 2 (GOME-2) and model. Interestingly, the AQS-observed weekly cycles of NOx over the AVHRR urban, other and forest region are similar to those over the GOME-2 NOx-saturated, mixed, and NOx-sensitive regimes, respectively. However, AQS-observed O3 weekly cycle over the AVHRR urban region is significantly different from that over the GOME-2 NOx-saturated regime. Whereas the weekend high O3 anomaly is clearly shown in both AQS and CMAQ over the NOx-saturated regime, the weekend effect is not captured over the geographically-based urban region, suggesting that the urban region includes other chemical regimes. This study suggests that chemical classification into NOx-saturated, mixed and NOx-sensitive regime stations gives a more accurate picture for the weekly O3 cycles than the geographically-based urban, forest, and other classification.