As part of the NW-AIRQUEST's continuing evaluation of CMAQ for the Pacific Northwest, this project compares CMAQ version 4.3 at 4 km grid resolution against detailed observations of aerosol properties and gaseous pollutant concentrations obtained in August 2001. The coordinated Pacific Northwest 2001 (PNW2001) and Pacific 2001 field campaigns provide extensive airborne and ground-based measurements of air pollutants and meteorological variables. Together they are the most comprehensive and current air quality campaign for this region. Routine observations in Washington and Oregon add spatial and temporal coverage to the intensive field campaigns.

Evaluation 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). These results are despite a good representation of sulfur dioxide and despite general agreement with other CMAQ simulations of PM_2.5 and ozone. Other simulations may also have difficulty with number distributions, but few studies have examined the issue. This is especially significant given the recent implication of ultrafine particles and number concentrations on human health. An examination of the aerosol number and mass concentrations and gaseous chemical environment will be presented. The analysis will be brought into perspective with other current CMAQ analyses in the region.