Air quality in the Pacific Northwest is strongly influenced by complex terrain and unique meteorological conditions within and near the Columbia River Gorge and the Cascade Mountain range. Topographical features interact closely with the regional flow, and therefore have an important impact on the dispersion of pollutants. In addition, unresolved subgrid variability in precursor emissions within the metropolitan areas also suggests the need for high resolution in photochemical modeling. In this study, the MM5/SMOKE/CMAQ modeling system was used for nested 4 km and 1 km domains encompassing the Portland metropolitan area and the Columbia River Gorge for an ozone episode that occurred at July, 1998 to investigate the impact of high resolution meteorological and photochemical modeling on ozone predictions for the region. The effects of high resolution meteorological input were compared with those of high resolution emissions by running CMAQ at 1 km with two scenarios: one with 1 km MM5 but 4 km emissions and the other with both MM5 and emissions at 1 km.

Using high resolution meteorological input alone appeared to have a larger impact on ozone predictions in terms of the position and peak levels of the ozone plume. Ozone concentrations were higher in the 1 km case compared to the 4 km case in areas along the Columbia River Gorge and the foothills of Cascades where the terrain effects are substantial. Using high resolution emissions appeared to affect small-scale features in ozone concentration patterns mainly within the urban area. In that case both ozone increases and decreases could be identified which were missing when using coarse resolution emissions.