Evolution of Light Absorption Enhancement of Black Carbon Aerosols From Biomass Burning in Atmospheric Photochemical Aging

The light absorption enhancement (E_abs) of black carbon (BC) coated with non-BC materials is crucial in the assessment of radiative forcing, yet its evolution during photochemical aging of plumes from biomass burning, the globe's largest source of BC, remains poorly understood. In this study, plumes from open burning of corn straw were introduced into a smog chamber to explore the evolution of E_abs during photochemical aging. The light absorption of BC was measured with and without coating materials by using a thermodenuder, while the size distributions of aerosols and composition of BC coating materials were also monitored. E_abs was found to increase initially, and then decrease with an overall downward trend. The lensing effect dominated in E_abs at 520 nm, with an estimated contribution percentages of 47.5%–94.5%, which is far greater than light absorption of coated brown carbon (BrC). The effects of thickening and chemical composition changes of the coating materials on E_abs were evaluated through comparing measured E_abs with that calculated by the Mie theory. After OH exposure of 1 × 10¹⁰ molecules cm⁻³ s, the thickening of coating materials led to an E_abs increase by 3.2% ± 1.6%, while the chemical composition changes or photobleaching induced an E_abs decrease by 4.7% ± 0.6%. Simple forcing estimates indicate that coated BC aerosols exhibit warming effects that were reduced after aging. The oxidation of light-absorbing C_xH_y compounds, such as polycyclic aromatic hydrocarbons (PAHs), to C_xH_yO and C_xH_yO_>1 compounds in coating materials may be responsible for the photobleaching of coated BrC.