A comprehensive performance evaluation of MM5-CMAQ for the Summer 1999 Southern Oxidants Study episode—Part Ⅱ: Gas and aerosol predictions

摘要

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.