Global Modeling and Source Attribution of Atmospheric Brown Carbon and Its Associated AAOD Using an Innovative Absorptivity Basis Set Approach

Brown carbon (BrC), a subset of light-absorbing organic aerosols, contributes significantly to global warming not only by absorbing atmospheric radiation but also by accelerating snowmelt in the cryosphere through reducing surface albedo. However, accurately quantifying its global distribution and absorption remains challenging for numerical models, stemming from its complex composition, variable optical properties, and dynamic atmospheric transformations. This study developed an absorptivity basis set module for BrC (ABS-BrC) simulation, which categorizes BrC into four classes based on light-absorbing ability and accounts for chemical aging processes to better represent the variability in BrC absorption. Global simulations using ABS-BrC reveal notable disparities between source contributions to BrC concentrations and their associated absorption aerosol optical depth (AAOD). While secondary formation dominates global BrC concentrations (45.8%), its contribution to AAOD is disproportionately small (16.6%). In contrast, wildfire-sourced BrC, despite contributing less than a third of the total concentration, is responsible for the majority (54.2%) of global BrC AAOD. Notably, dark BrC (d-BrC), a strongly absorbing and water-insoluble subset, contributes 30-64% to global BrC AAOD. Its contribution is particularly critical in the Arctic during JJA and SON, where it accounts for 67% of BrC absorption, highlighting its critical role in polar warming. Implementing the ABS-BrC module in Earth system models is recommended for a more accurate assessment of BrC's radiative impact on the atmosphere and cryosphere.