Modeling the impacts of open biomass burning on regional O₃ and PM_2.5 in Southeast Asia considering light absorption and photochemical bleaching of Brown carbon

Open biomass burning in Southeast Asia has significant adverse impacts on air quality in the region and in downwind areas. These biomass burning events emit large amounts of light absorbing brown carbon (BrC). Once in the atmosphere, the light absorbing capacity of BrC is reduced by various oxidation processes. However, few modeling studies have been conducted to explicitly examine light absorption and bleaching on the prediction of ozone (O₃) and fine particulate matter (PM_2.5). In this study, a modified Community Multiscale Air Quality (CMAQ) model that explicitly tracks the concentrations of light absorbing and non-light absorbing organic aerosol components from different emission sources and the bleaching of BrC due to photooxidation and OH oxidation is applied to widespread open biomass burning events in March 2018 in Southeast Asia. Open biomass burning accounts for as much as 20–40 ppb (30–50%) of the maximum daily average 8-h ozone (MDA8 O₃) and 40–120 μg m⁻³ (60–90%) of the daily average PM_2.5 in the emission source regions. Compared to a simulation without BrC light absorption, the predicted MDA8 O₃ and PM_2.5 are as much as 16 ppb and 16 μg m⁻³ lower, respectively, than a simulation with light absorption. This confirms that neglecting the UV light absorption of BrC can lead to significant overpredictions of O₃ and PM_2.5 during the open biomass burning periods, which may lead to an overestimation of the adverse impacts of biomass burning on public health in Southeast Asia. The addition of BrC bleaching results in a 0.5–1% increase in MDA8 O₃ and 1–5% increase in PM_2.5 compared to the case without BrC bleaching. The results of this study indicate that light absorption by BrC needs to be considered in chemical transport modeling of large open biomass burning events. The BrC bleaching process is relatively slow and neglecting this process does not significantly change the predictions of MDA8 O₃ and PM_2.5 during open biomass burning.