Aging Lifetime of Atmospheric Brown Carbon Determined From Measurements of Wildfire Carbonaceous Aerosol in Western Canada

Abstract The atmospheric abundance of black carbon (BC) and brown carbon (BrC) aerosol particles has been measured using aethalometry at four locations (Kamloops, BC; Prince George, BC; Joussard, AB; East Trout Lake, SK) in Western Canada through the 2023 and 2024 wildfire seasons (June to November). Intense wildfires led to higher median (Interquartile = Q 3‐ Q1) BC mass concentrations in 2023 (0.45 (0.80–0.19), 0.55 (1.22–0.20), 0.28 (0.69–0.00), 0.16 (0.37–0.00) μg m −3 ) than in 2024 (0.31 (0.60–0.00), 0.25 (0.51–0.00), 0.18 (0.53–0.00) μg m −3 ) for Kamloops, Prince George, Joussard, and East Trout Lake, respectively (with no data for East Trout Lake in 2024). The season‐ and site‐wide means for the aerosol Absorption Angstrom Exponent for 2023 and 2024 are 1.7 and 1.5, respectively, indicating the presence of BrC in both years. The Trajectory Fire Interception Method (TFIM), which determines the spatial and temporal overlap between atmospheric back trajectories and satellite fire counts was used to calculate the average atmospheric transport time of fire plumes for the heavily wildfire‐affected sites in 2023 (Prince George, Kamloops, Joussard). Consistent with laboratory studies of multiphase oxidative aging of smoke particles, the TFIM approach demonstrates that the BrC absorption (at 375 nm, normalized to BC) decreases by about a factor of two as the average atmospheric transport time increases from 15 hr to greater than 46 hr. This corresponds to a BrC whitening e ‐folding timescale of 27 hr averaged across the three sites. Contrary to past studies that have analyzed individual fire plumes, an innovative aspect of this work is that the BrC aging timescale is derived from data accumulated continuously during the entire 2023 wildfire season at multiple locations. In addition, the aging timescale includes smoke with origins in the boreal forest, whereas most prior studies have been conducted at lower latitudes.