An initial evaluation of the Models-3 Community Multiscale Air Quality (CMAQ) model aerosol component reveals CMAQ's varying ability to simulate observed visibility indices and aerosol species concentrations. The visibility evaluation, using National Weather Service observations from 139 airports for 11--15 July 1995, shows that CMAQ reasonably captured the general spatial and temporal patterns of visibility degradation, including major gradients, maxima and minima. However, CMAQ's two visibility prediction methods, Mie theory approximation and mass reconstruction, both underpredict visibility degradation (i.e., overpredict visibility). The mean bias, normalized mean bias (NMB), mean error and normalized mean error (NME) for the Mie calculations are -5.9 dv, -21.7%, 7.0 dv and 25.4%, respectively. For the reconstruction simulations, these statistics are -9.8 dv, -35.5%, 10.0 dv and 36.2%, respectively. Most simulated values (~90% Mie and ~85% reconstruction) fall within a factor of two of the observations, although r2 = 0.25 (Mie) and r2 = 0.24 (reconstruction). The speciated aerosol evaluation uses observations of sulfate, nitrate, PM2.5, PM10 and organic carbon obtained from 18 stations of the Interagency Monitoring of Protected Visual Environments (IMPROVE) network in June 1995. This evaluation reveals that, with the exception of sulfate (mean bias: 0.15 ¿g/m3, NMB: 3.1%), the model consistently underpredicts aerosol concentrations of nitrate (-0.10 ¿g/m3, -33.1%), PM2.5 (-3.9 ¿g/m3, -30.1%), PM10 (-5.66 ¿g/m3, -29.2%) and organic carbon (-0.78 ¿g/m3, -33.7%). Sulfate was simulated best by the model (r2 = 0.63, mean error = 1.75 ¿g/m3, NME = 36.2%), followed by PM2.5 (0.55, 5.00 ¿g/m3, 38.5%), organic carbon (0.25, 0.94 ¿g/m3, 40.6%), PM10 (0.13, 9.85 ¿g/m3, 50.8%) and nitrate (0.01, 0.33 ¿g/m3, 104.3%). Except for nitrate, 75--80% of simulated concentrations fall within a factor of two of the IMPROVE observations.