Impact of wildfires on regional ozone and PM2.5: Considering the light absorption of Brown carbon

The influence of wildfire brown carbon (BrC) in moderating the formation of O-3 and fine particulate matter (PM2.5) due to its absorption of ultraviolet (UV) radiation is, for the first time, investigated using a modified community multiscale air quality (CMAQ) model. The wavelength-dependent imaginary refractive index of the organic carbon from wildfires, which are needed by the CMAQ model for its inline photolysis rate calculation, are generated experimentally from wood burning aerosols from a combustion chamber. On high emission days of the Bastrop County Complex fire in Texas in early September 2011, BrC UV absorption reduces the daytime average NO2 photolysis rate by up to 38% in the core region of the wildfire plume compared to the base scenario that does not consider BrC absorption. Consequently, O-3 production in the smoke plume is suppressed compared to the base scenario 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 without considering BrC absorption, the predicted increase of O-3 is 5-15% lower when BrC absorption is considered. Similarly, considering the BrC UV absorption leads to approximately 1% (or similar to 2-3 mu g m(-3)) lower estimation of the wildfire emissions' impact on total PM2.5. This change is small because secondary aerosols, which are the components affected by BrC absorption, only account for a small fraction of the total PM2.5 in wildfire impacted regions in this study. In addition, our study shows that assumptions about aerosol mixing state (core-shell vs. homogeneous) in the inline photolysis rate calculation would not significantly affect out assessment of the impact of wildfire BrC light absorption on O-3 and PM2.5.

Automated summary

This study investigates the influence of wildfire brown carbon (BrC) on the formation of O-3 and fine particulate matter (PM2.5) through its absorption of ultraviolet (UV) radiation. The results show that BrC UV absorption reduces the formation of O-3 and PM2.5 in the wildfire plume. Additionally, the impact of BrC absorption on O-3 and PM2.5 is relatively small compared to other factors.