The size distribution of aerosol chemical compositions was studied over a 2-week period in August 2001 at three locations (one urban and two semi-rural) in the Lower Fraser Valley (LFV) of British Columbia, Canada. The size distributions varied diurnally and were bimodal with a pronounced peak at about 0.3-0.55 mu m (accumulation mode) and SO42- 2-7 mu m (coarse mode). Generally, on an equivalent basis, the aerosol was dominated by SO42- (balanced by NH4+) in the accumulation mode range; SO42- often exhibited a diurnal variation that was primarily the result of fog deposition overnight and photochemical formation during the day. SO42- in the 3.1-6.2 mu m particles was mostly of marine origin. Most of the smaller particle SO42- was from anthropogenic sources and it is estimated that about 75% of the SO42- in the smaller particles were due to secondary processes. Oxalate, C2O42-, although at low concentrations (generally < 0.1 SO42- displayed a bi-modal size distribution, the accumulation mode being similar to SO42- and the coarse mode peaking at 4 1-1.8 mu m. Frequently at night, there was also a significant increase in NO3- in the accumulation mode that is attributed to the co-condensation of gas phase NH3 + HNO3 to form aerosol NO3- and the reactive uptake of N2O5 onto aerosols. In the coarse mode, the sea salt Cl- was frequently deficient relative to Na+ yet accompanied by a corresponding increase in NO3-; this was attributed to reaction of NO3- precursors (e.g., HNO3 and N2O5) with sea salt aerosol Cl-. (c) 2006 Elsevier Ltd. All rights reserved.
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.