Laboratory studies on the optical, physical, and chemical properties of fresh and aged biomass burning aerosols

Abstract. Atmospheric brown carbon (BrC) plays a significant role in global warming, yet the evolution of its optical properties during aging remains poorly understood, leading to substantial uncertainties in its climate effects. In this study, we investigate the aging process of BrC and its driving factors using laboratory-generated biomass burning emissions, including four types of straw and one type of wood. Upon OH oxidation, there exists a large increase in OA fraction after 2 d aging, followed by a minor increase during aging to 7 d. The particle growth is dominated by the change in OA content and thus shows a similar trend during aging. The mass absorption efficiency (MAE) of fresh BrC measured at 370 nm is 2.1–5.7 m2 g−1. A sharp decline in MAE is observed after 2 d aging, equally attributed to photobleaching and secondary organic aerosol formation. Although a negative correlation is observed between particle size and MAE, the reduction in MAE is mainly driven by the decline in the imaginary part (k) of BrC, with particle size playing a minor role. Combined with positive matrix factorization (PMF) analysis, the study reveals that oxygenated OA, characterized by higher O / C ratios but lower MAE, increases significantly with aging. In contrast, two hydrocarbon-like OA factors with lower O / C ratios and higher MAE decrease over time. These results emphasize the importance of categorizing BrC based on its MAE and atmospheric behavior in climate models.