The role of primary and secondary brown carbon in carbonaceous aerosol absorption: a global modelling study

Brown carbon (BrC) remains one of the most uncertain components in the aerosol–radiation interactions due to the uncertainties in its different sources, secondary formation pathways, chemical aging, and optical properties. In this study, we investigate the global contribution of the different primary and secondary BrC sources to the aerosol absorption for the year 2018 with the Multiscale Online Nonhydrostatic AtmospheRe CHemistry (MONARCH) chemistry-transport model at global scale. All significant primary and secondary BrC formation pathways have been implemented in MONARCH. Primary BrC emissions include particulate organic aerosol from biomass burning (BB), biofuel (BF), fossil fuel (FF), and shipping (SH) sources. BB emissions are generated online following Liu et al. (2013) and Saleh et al. (2014), while BF, FF, and SH emissions are sourced from the Global Emission Modeling System (GEMS) inventory (https://gems.pku.edu.cn). Primary BrC undergoes photochemical bleaching through oxidation by OH radicals. Secondary BrC formation is represented through the oxidation of aromatic and terpene volatile organic compounds by OH and NO₃ radicals, allowing for both daytime and nighttime darkening processes, with yields dependent on simulated NOₓ conditions. Secondary BrC is further aged through ozonolysis and OH oxidation. For the BrC absorption aerosol optical depth (AAOD) calculation, the species-specific imaginary refractive indices are assigned to account for the differences in absorptivity. Model results are evaluated against the GHOST dataset, which provides harmonized global observations of aerosol optical properties derived from AERONET. Observed AAOD at 440 nm is partitioned into BrC and black carbon contributions following Bahadur et al. (2012). Our results confirm the dominant role of BB in BrC absorption near source regions, while highlighting the significance of secondary BrC formation in urban and polluted environments. The simulations also demonstrate the importance of the hygroscopicity in the BrC absorption calculations, emphasizing its relevance for accurately representing the aerosol radiative effects in modelling studies.