Using a small aircraft, we conducted nine flights from 27 May through 5 August 2003 to collect vertical profiles of ozone, carbon monoxide, and total aerosol scattering in the 0--6 km column off the coast of Washington State. During 27 May, 2 June, and 5 August 2003 we observed air masses containing highly correlated and well-defined layers of O3, CO, and aerosol enhancements within the 0--6 km column. The largest of these pollution episodes, or events, occurred on 2 June 2003 when O3 and CO mixing ratios exceeded 100 and 220 ppbv, respectively. A suite of meteorological data indicated that these polluted layers were not due to local sources. NOAA Hysplit backward trajectories indicate that the polluted air masses originated from a vast region in Asia bounded by 35¿--70¿N and 70¿--170¿E and transited the Pacific Ocean at middle and upper latitudes to North America in 7--10 days. Satellite data and global aerosol transport models suggest that Siberian boreal fire emissions were the primary source of the three events. Our observations indicate that ozone and aerosol scattering enhancement ratios, O3/CO and σsp (550 nm)/CO ERs, during these episodes were relatively large and varied from 0.15 to 0.84 ppbv ppbv-1 and 0.24 to 1.24 Mm-1 ppbv-1, respectively. These observations coupled with ground-based measurements suggest that under certain circumstances, Siberian smoke was transported to the surface and had a significant influence on regional air quality in western Washington State. Notably, the 2 June event coincides with a period when O3 concentrations exceeded the U.S. Environmental Protection Agency 8-hour standard (>84 ppbv) in Enumclaw, Washington, and the 5 August event corresponded with air quality degradation in Seattle, Washington, when 24-hour average fine particle (PM2.5) concentrations exceeded 18 ¿g m-3 during the first week of August.