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Atmospheric black carbon (BC) is a leading climate warming agent, yetuncertainties on the global direct radiative forcing (DRF) remain large.Here we expand a global model simulation (GEOS-Chem) of BC to include theabsorption enhancement associated with BC coating and separately treat boththe aging and physical properties of fossil-fuel and biomass-burning BC. Inaddition we develop a global simulation of brown carbon (BrC) from bothsecondary (aromatic) and primary (biomass burning and biofuel) sources. Theglobal mean lifetime of BC in this simulation (4.4 days) is substantiallylower compared to the AeroCom I model means (7.3 days), and as a result,this model captures both the mass concentrations measured in near-sourceairborne field campaigns (ARCTAS, EUCAARI) and surface sites within 30%,and in remote regions (HIPPO) within a factor of 2. We show that the newBC optical properties together with the inclusion of BrC reduces the modelbias in absorption aerosol optical depth (AAOD) at multiple wavelengths bymore than 50% at AERONET sites worldwide. However our improved modelstill underestimates AAOD by a factor of 1.4 to 2.8 regionally, with the largestunderestimates in regions influenced by fire. Using the RRTMG modelintegrated with GEOS-Chem we estimate that the all-sky top-of-atmosphere DRFof BC is +0.13 Wm−2 (0.08 Wm−2 from anthropogenic sources and0.05 Wm−2 from biomass burning). If we scale our model to match AERONETAAOD observations we estimate the DRF of BC is +0.21 Wm−2, with anadditional +0.11 Wm−2 of warming from BrC. Uncertainties in size,optical properties, observations, and emissions suggest an overalluncertainty in BC DRF of −80%/+140%. Our estimates are at the lowerend of the 0.2–1.0 Wm−2 range from previous studies, and substantiallyless than the +0.6 Wm−2 DRF estimated in the IPCC 5th AssessmentReport. We suggest that the DRF of BC has previously been overestimated dueto the overestimation of the BC lifetime (including the effect on thevertical profile) and the incorrect attribution of BrC absorption to BC

atmospheric chemistry and physics

Exploiting simultaneous observational constraints on mass and absorption to estimate the global direct radiative forcing of black carbon and brown carbon