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Unlabelled: Air quality impacts of volatile organic compound (VOC) and nitrogen oxide (NO(x)) emissions from major sources over the northwestern United States are simulated. The comprehensive nested modeling system comprises three models: Community Multiscale Air Quality (CMAQ), Weather Research and Forecasting (WRF), and Sparse Matrix Operator Kernel Emissions (SMOKE). In addition, the decoupled direct method in three dimensions (DDM-3D) is used to determine the sensitivities of pollutant concentrations to changes in precursor emissions during a severe smog episode in July of 2006. The average simulated 8-hr daily maximum O3 concentration is 48.9 ppb, with 1-hr O3 maxima up to 106 ppb (40 km southeast of Seattle). The average simulated PM2.5 (particulate matter with an aerodynamic diameter < 2.5 microm) concentration at the measurement sites is 9.06 microg m(-3), which is in good agreement with the observed concentration (8.06 microg m(-3)). In urban areas (i.e., Seattle, Vancouver, etc.), the model predicts that, on average, a reduction of NO(x) emissions is simulated to lead to an increase in average 8-hr daily maximum O3 concentrations, and will be most prominent in Seattle (where the greatest sensitivity is -O.2 ppb per % change of mobile sources). On the other hand, decreasing NO(x) emissions is simulated to decrease the 8-hr maximum O3 concentrations in remote and forested areas. Decreased NO(x) emissions are simulated to slightly increase PM2.5 in major urban areas. In urban areas, a decrease in VOC emissions will result in a decrease of 8-hr maximum O3 concentrations. The impact of decreased VOC emissions from biogenic, mobile, nonroad, and area sources on average 8-hr daily maximum O3 concentrations is up to 0.05 ppb decrease per % of emission change, each. Decreased emissions of VOCs decrease average PM2.5 concentrations in the entire modeling domain. In major cities, PM2.5 concentrations are more sensitive to emissions of VOCs from biogenic sources than other sources of VOCs. These results can be used to interpret the effectiveness of VOC or NO(x) controls over pollutant concentrations, especially for localities that may exceed National Ambient Air Quality Standards (NAAQS)., Implications: The effect of NO(x) and VOC controls on ozone and PM2.5 concentrations in the northwestern United States is examined using the decoupled direct method in three dimensions (DDM-3D) in a state-of-the-art three-dimensional chemical transport model (CMAQ). NO(x) controls are predicted to increase PM2.5 and ozone in major urban areas and decrease ozone in more remote and forested areas. VOC reductions are helpful in reducing ozone and PM2.5 concentrations in urban areas. Biogenic VOC sources have the largest impact on O3 and PM2.5 concentrations.