Black carbon in the Lower Fraser Valley, British Columbia: Impact of 2017 wildfires on local air quality and aerosol optical properties

Exposure to wildfire smoke is a public health issue of increasing prominence in North America, particularly in western states and provinces. In this study, Aethalometer data collected at six sites in the Lower Fraser Valley (LFV), British Columbia, from September 2016 through August 2017 were analyzed to investigate the relative importance of fossil fuel and biomass burning contributions to black carbon (BC) in the region. Annual mean BC mass concentrations were in the range 0.3-0.8 mu g m(-3), and BC was apportioned to fossil fuel and biomass burning sources at hourly resolution using the dual-wavelength Aethalometer model, applying a typical a priori assumed biomass burning Absorption Angstrom Exponent (AAE) value close to 2. However, this approach was found to underestimate biomass contributions to BC mass by up to a factor of three during wildfire events that impacted air quality throughout the region in July and August 2017. Hourly resolution PM2.5 concentrations in excess of 100 mu g m(-3) were recorded at multiple sites and mean ambient PM2.5 concentrations for the wildfire smoke periods were up to nine times higher than mean concentrations for the remainder of the summer. A background extraction approach was applied to optimize BC source apportionment during these events and to enable the calculation of wildfire aerosol AAE, with similar values for the latter determined across all six sites (1.35 +/- 0.09). The relatively low AAE values observed suggest that evaporation and/or photo-bleaching of brown carbon occurred during transport of wildfire aerosol to the receptor sites. From an exposure perspective, for the sites investigated, up to 69 +/- 20% (+/- 1 sigma) of ambient BC is attributed to wildfires during these events. On an annual scale, however, fossil fuel combustion and residential/agricultural biomass burning are estimated to be more important contributors to ambient BC mass concentrations at every site.