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Three research flights were conducted during August 2001 as part of the PNW2001 field study in the Puget Sound region. Each flight presented a case study of unique sources, meteorological conditions, and spatial distributions of pollutants. On 10 August, an urban smog episode was sampled downwind of the Puget Sound Seattle–Vancouver corridor containing high mixing ratios of O3 (∼80 ppbv) and high aerosol scattering coefficients (σsp) (∼35 M m−1). The ozone and σsp measured during this flight were typical of summer pollution events in the Puget Sound region. The AIRPACT forecast model (MM5–CALMET-CALGRID) showed variable results when compared to surface measurements. In addition, predicted surface O3 maxima were several hours too early and fell off sharply relative to observations. AIRPACT generally under-predicted upper-level O3 concentrations and misplaced the observed O3 maxima. On 16 August, a second pollution region was encountered in a broad layer between 2 and 4.5 km above a well-defined temperature inversion. Meteorological analysis and satellite imagery suggest the high O3 mixing ratios (60 ppbv) and very high σsp (80 M m−1) were from fires burning in Washington, Oregon, and Northern California. IMPROVE aerosol chemistry data from sites in the Cascade Mountains showed evidence of biomass burning emissions reaching the surface 2 days prior to this flight. The last flight, on 20 August, sampled slightly polluted background conditions in the region. Ozone levels reached only 30 ppbv and σsp reached 20 M m−1 at the surface, but was much lower aloft. Therefore, this flight was representative of background conditions during the summer in this region. The results from these research flights provide evidence of enhanced O3 and σsp, extending to 4 km altitude over Puget Sound, during the summer months from varying sources. They further demonstrate the current capability of the AIRPACT O3 forecast system and, in one case, illustrate the need to include biomass-burning emissions in the model forecasts. [Copyright &y& Elsevier]