Our results have shown that CMAQ model performance in terms of surface species and column CO and NO2 predictions is reasonably good; larger biases exist in its column predictions for tropospheric O3 residuals and AOD. Vertical transport, chemistry, aqueous processes, and dry deposition are the major processes contributing to the formation and fate of total odd oxygen (Ox), and those for PM2.5 include emissions, vertical transport, PM processes, and aqueous processes. Export of Ox from the continental U.S. occurs only in summer and at a rate of 0.66 Gmol day-1. In contrast, export of PM2.5 occurs for all seasons and at rates of 29.32, 34.18, 26.56, and 25.68 Gg day-1 for winter, spring, summer, and fall, respectively. These results indicate a need to use a different control strategy for O3 and PM2.5, namely, monitor and control PM2.5 throughout the year, rather than for high O3 seasons (May-September). Simulations have also been conducted to examine the sensitivity of CMAQ predictions to model treatments (e.g., cloud chemistry) and inputs (e.g., volatile organic compound emissions). Under future emission scenarios, surface O3 mixing ratios will increase in summer, surface PM2.5 concentrations may increase or decrease, depending on the emission scenarios and the geographical regions. Under future climate conditions with 0.7 ˚C increase in surface temperatures, surface O3 mixing ratios will increase in summer and surface PM2.5 concentrations will decrease in the eastern U.S. but increase in the western U.S.