Evidence for Brown Carbon Whitening with Increasing Plume Age During Wildfire Seasons in Western Canada
MOHAMAD H. AL-JABIRI, Jason S. Olfert, Laura-Helena Rivellini, Stephanie R. Schneider, Jonathan Abbatt, University of Toronto
Abstract Number: 465
Working Group: Aerosol Processes and Properties in Changing Environments in the Anthropocene
Abstract
The atmospheric abundance of black carbon (BC) and brown carbon (BrC) aerosol particles has been measured using aethalometry at three locations (Kamloops, BC; Prince George, BC; Joussard, AB) in Western Canada through the 2023 and 2024 wildfire seasons (June to November). Intense wildfires in 2023 led to higher median BC levels (0.44, 0.55, 0.28 μg m-3) than in 2024 (0.31, 0.25, 0.18 μg m-3) for Kamloops, Prince George, and Joussard, respectively. The season-wide averages for the aerosol Absorption Angstrom Exponent for 2023 and 2024 are 2.05 and 2.1, respectively, indicating the presence of BrC. The average BrC absorption observed was 0.0285 Mm-1 and 0.010 Mm-1 at 375 and 470 nm, where the BrC is evaluated by subtracting the absorption due to BC at those wavelengths from the total measured absorption (using BC mass absorption coefficients of 24.1 and 19.1 m2 g-1 at 375 and 470 nm). The Trajectory Fire Interception Method (TFIM), where spatial and temporal overlap between atmospheric back trajectories and satellite fire counts is assessed, was used to calculate the average atmospheric transport time and age of a fire plume. The TFIM approach demonstrated that there is a marked decrease in the ratio of BrC to BC with increasing atmospheric transport time. Specifically, there is a decrease of between 44 to 56 % in BrC/BC at 375 nm as the average atmospheric transport time increases from < 10 hours to > 72 hours. This indicates that significant atmospheric BrC whitening occurs on a timescale of 2 to 3 days. This is the first evaluation of the BrC atmospheric whitening timescale using season-long data sets measured at multiple locations.