Modeling Ozone Formation and Transport in the Cascadia Region of the Pacific Northwest

Abstract

The rapidly growing Cascadia region of the Pacific Northwest, consisting of western Washington, Oregon, and southwestern British Columbia, has experienced surface ozone concentrations that exceed federally mandated standards. A modeling system consisting of the prognostic meteorological model known as the Fifth-Generation Pennsylvania State University?National Center for Atmospheric Research Mesosale Model (MM5), the diagnostic meteorological model CALMET, and the photochemical air quality model CALGRID was developed to investigate ozone formation and transport in this region. To address both the complex topography within the model domain and the relatively sparse network of surface and upper-air meteorological observations, MM5 simulations were performed using 4D data assimilation and a relatively high-resolution inner domain (5-km grid). The MM5 results, however, failed to reproduce the observed wind patterns in some portions of the domain. As a result, it was necessary to employ the MM5 solution as the initial-guess wind field for CALMET (also with a 5-km grid). Objective analysis was applied within CALMET to interpolate the predicted winds with available surface observations. This method involved an iterative approach to find the optimal set of weighting factors within CALMET to merge the MM5 solution with the available meteorological observations. The predicted ozone concentration patterns for a July 1996 event were very complex but generally showed areas of maximum ozone (130 ppb) occurring to the south and east of Puget Sound and within and to the south of the Portland area (170 ppb). Widespread ozone buildup does not occur over the course of the episode; rather, the maximum ozone concentration occurs each day downwind of each urban center. There was no evidence for recirculation of pollutants from one day to the next within an urban area. It also does not appear that emissions from one urban center influence the neighboring downwind urban area. The predicted ozone concentrations showed good agreement with observations at the monitors located along the Interstate Highway No. 5 corridor. Model performance was less good at three sites located in regions of complex terrain.