As part of the Pacific 2001 Air Quality Study in August 2001, aerosol samples were collected at three sites in the Lower Fraser Valley, BC twice daily over a 2-week period. In this paper, the results for two compounds in the samples, levoglucosan and dehydroabietic acid (DHAA), both tracers for biomass burning plumes, are presented. Concentrations of the compounds were generally low throughout the study. Average values for levoglucosan were 14.4, 14.7, and 26.0 ng m(-3) for the urban, mixed urban/forest, and rural sites respectively. Elevations in the concentrations during two periods indicate evidence of biomass burning. Satellite images from 15 to 18 August show the transport of smoke plumes from forest fires in northern Washington and southern British Columbia into the Lower Fraser Valley. The residues of the smoke plumes in the river valleys probably contributed to the elevated ground-level levoglucosan levels. Emission ratios from reported source studies, 4.5-10% for levoglucosan/organic carbon and 4% for DHAA/organic carbon, were used to estimate the contributions of biomass burning to aerosol organic carbon. Average contributions of biomass burning to organic carbon varied significantly throughout the region. Contributions were between 5 and 10%, 12 and 27% and 5 and 12% for the urban, rural and mixed urban/forest sites respectively, but contributions up to 75% were found. Crown Copyright (c) 2006 Published by Elsevier Ltd. All rights reserved.