Impacts of a Large Wildfire on Regional Ozone and PM2.5 Considering the Light Absorption of Brown Carbon
MINSU CHOI, Jie Zhang, Qi Ying,
Texas A&M University Abstract Number: 372
Working Group: Carbonaceous Aerosol
AbstractThe enhanced UV absorption of wildfire brown carbon (BrC) on the formation of O
3 and fine particulate matter (PM
2.5) is, for the first time, evaluated using a modified community multiscale air quality (CMAQ) model with wavelength-dependent imaginary refractive indexes for primary and secondary organic aerosols. On high emission days of the Bastrop County Complex fire in Texas in early September 2011, BrC UV absorption reduces the daytime average NO
2 photolysis rate by up to 38% in the core region of the wildfire plume compared to the base case that does not consider BrC absorption. Consequently, the predicted daytime increase of O
3 due to wildfire emissions is uniformly lower throughout the mixing layer than the base case due to reduced HOx radical concentrations. In the core region, the predicted O
3 increase due to wildfire reaches as high as 47-123 ppb in the base case, but the increase is reduced by 4-19 ppb when BrC absorption is considered. While the fire emission leads to increases of PM2.5 up to 200-300 µg m
-3, considering BrC light absorption only leads to a small reduction of PM
2.5 by ~1% because secondary aerosols only account for a small fraction of the total PM
2.5 in wildfire impacted regions. Our study demonstrates that UV absorption by BrC must be considered in evaluating the impact of wildfires on O
3 air quality, and the assumption of the aerosol mixing state (core-shell vs. homogeneous) in the online photolysis rate calculation would not significantly affect the evaluation results.