During the spring of 2002, vertical profiles of O3, CO, nonmethane volatile organic compounds (VOCs), and total aerosol scattering were collected over the northwestern coast of Washington State as part of the University of Washington's Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) research campaign. These observations coincided with NOAA's Intercontinental Transport and Chemical Transformation 2002 (NOAA-ITCT 2K2) project. Thirteen research flights were conducted from 29 March through 23 May and several well-defined polluted layers of varying thickness (~0.2 to >3 km) were observed at altitudes between 0 and 6 km. These layers were characterized by correlated enhancements of O3, CO, VOCs, and particles. We observed rapid transpacific transport of polluted air masses on 15 April and 14, 17, and 23 May 2002, with ΔO3 and ΔCO (where Δ refers to the enhancement over background) exceeding 30 and 60 ppbv, respectively, and total aerosol scattering of green light (σsp (550 nm)) exceeding 65 Mm-1. These episodes were efficient in transporting O3 to the northeast (NE) Pacific troposphere, with ΔO3/ΔCO ratios in the pollution layers varying from 0.22 to 0.42 mol mol-1. In contrast, the average Δσsp (550 nm)/ΔCO ratio of the mid-May events (0.66 ¿ 0.21 (1σ)) was more than twice that of the 15 April event (0.32 ¿ 0.05). The correlation between O3, CO, aerosols, and VOCs coupled with back-trajectory analyses, satellite data, and the GEOS-CHEM global chemical transport model indicate that the primary source of pollution observed on 15 April originated from a mixture of Asian anthropogenic and biomass-burning emissions. For the May events, our analyses indicate that the early onset of the 2002 Siberian fire season was a significant source of the pollution episodes observed in May.