CMAQ simulation of aerosol size distributions with ternary nucleation

Several investigators have tested CMAQ's prediction of PM2.5, but little attention has been paid to its representation of the entire size distribution. Evaluation of CMAQ version 4.4 has shown an under-prediction of aerosol number by a factor of 10 to 100, especially in the Aitken mode (i.e., ultrafine or less than 100 nm diameter). This error was consistent in space and time and not obviously related to an error in gaseous concentrations, meteorological input, or aerosol mass. While many factors could be responsible for this error, the binary H2SO4-H2O nucleation parameterization in the model is not appropriate for the sulfate-poor Pacific Northwest environment. Therefore, a ternary H2SO4-H2O-NH3 parameterization was implemented in CMAQ. Performance will be presented by comparison to the binary parameterization and to number and size distribution measurements from the Pacific Northwest 2001 (PNW2001) and Pacific 2001 field campaigns.