In the talk we discuss regional simulations of high particulate episodes in July 1995 and February 1998 with Models-3 and compare results with a very extensive set of measurements gathered over Eastern North America by AIRS, NAPS, IMPROVE, CAPMON, GAVM, CAAMP and CASTNET networks. This data include both hourly and daily measurements of concentrations of PM2.5 and PM10 as well as speciated aerosols. Simulations of two episodes characterized by very distinct meteorology allows for a comprehensive model evaluation. We present statistics of CMAQ performance, discuss model's strengths and weaknesses and look for causes of errors. In short, our results show that simulated PM2.5 and PM10 masses are highly accurate. However, model tends to underpredict SO4-- concentrations and overpredict NO3- concentrations.