Modeling Study on the spatiotemporal distribution of global atmospheric brown carbon

    Brown carbon (BrC), a certain type of organic carbon (OC) with light-absorbing ability at visible and near-ultraviolet spectrum, pays an important role in the Earth's radiation budget and atmospheric warming. It is primarily produced through the incomplete combustion of biomass and fossil fuels, as well as the formation of secondary organic aerosols. Recent laboratory and field studies have identified a class of BrC known as dark brown carbon which exhibits black carbon (BC)-like properties and strong absorbing ability, with k value between 0.2 to 0.4 in the visible spectrum. However, few modeling studies have taken dark brown carbon into account, leading to  an underestimation of its direct radiative forcing on the climate system.

    In this study, we utilized the global-regional nested transport model, the Aerosol and Atmospheric Chemistry Model of the Institute of Atmospheric Physics (IAP-AACM), to simulate global brown carbon distribution. Following previous studies, we classified brown carbon into four categories based on its absorptivity: very weak brown carbon, weak brown carbon, moderate brown carbon, and dark brown carbon. Using the mass ratio of BrC/OC in the published paper, we constructed emission inventories for anthropogenic and biomass burning from EDGAR_v8.1 and GFED_v4.1, respectively, and assigned each emission source to one of the four BrC categories based on its absorptive properties. The model simulations were performed at a 1° × 1° resolution for both winter and summer in 2022 by coupling physical and chemical modules including dry deposition, wet scavenging, secondary formation, and aging. Model results were compared with AERONET and satellite-based aerosol absorption optical depth (AAOD), showing reasonable agreement.

    This research includes dark brown carbon from biomass burning and secondary BrC from aromatic secondary organic aerosol (SOA). The Volatility Basis Set (VBS) scheme was employed to simulate SOA formation. The results indicate that secondary BrC contributes approximately 2% ~ 8% to global BrC absorption. Dark brown carbon accounts for 2% ~ 14% BrC concentrations, but contributes 47% ~ 88% to global BrC absorption. These findings highlight the significant role of dark brown carbon in solar absorption and its potential impact on the Earth's radiative forcing.