Gas and aerosol predictions from CMAQ simulations with horizontal grid spacings of 8- and 32-km are evaluated against available observations from CASTNet, IMPROVE, AIRS-AQS, SOS99/SOS99NASH, SEARCH, and ARIES for the southeastern US for the period of 1–10 July 1999. The predictions evaluated in this work include mixing ratios of O 3 (hourly, maximum 1-h, and 8-h average), NO x , HNO 3 , NO y , and mass concentrations of PM 10 , PM 2.5 , and PM 2.5 components.Our evaluation has shown that CMAQ tends to underpredict maximum 1-h O 3 mixing ratios on high O 3 days at some sites. It overpredicts the maximum and minimum hourly O 3 mixing ratios for most low O 3 days, the daytime and nighttime hourly, and the maximum 8-h average O 3 mixing ratios on most days at all sites. The model performance for hourly O 3 mixing ratios generally meets EPA's criteria but deteriorates for maximum 1- and 8-h average O 3 mixing ratios. CMAQ underpredicts the mass concentrations of PM 10 , PM 2.5 , and PM 2.5 composition and fails to reproduce their temporal variations (except for sulfate). Largest underpredictions occur for organic matter (OM 2.5 ) and nitrate 2.5 among all PM components. These underpredictions and overpredictions may be caused by inaccurate meteorological predictions (e.g., the PBL height, wind speed/direction, vertical mixing, temperature, and relative humidity) and boundary conditions for chemical species (e.g., O 3 ), underestimation in emissions (e.g., NO x , NH 3 , and primary OM), as well as uncertainties in model assumptions and treatments in aerosol chemistry and microphysics.