Abstract. Organic aerosols (OA) that strongly absorbs solar radiation in the near-UV are referred to as brown carbon (BrC). However the sources, evolution, and optical properties of BrC remain highly uncertain, and contribute significantly to uncertainty in the estimate of the global direct radiative effect (DRE) of aerosols. Previous modeling studies of BrC optical properties and DRE have been unable to fully evaluate model performance due to the lack of direct measurements of BrC absorption. In this study, we develop a global model simulation (GEOS-Chem) of BrC and test it against BrC absorption measurements from two aircraft campaigns in the continental U.S. (SEAC4RS and DC3). To our knowledge, this is the first study to compare simulated BrC absorption with direct ambient measurements. We show that the BrC absorption properties from biomass burning estimated based on previous laboratory measurements overestimate the aircraft measurements of ambient BrC absorption. In addition, applying a photochemical scheme to simulate bleaching/degradation of BrC improves model skill. The airborne observations are consistent with a mass absorption coefficient (MAC) of freshly emitted biomass burning OA of 0.57 m2 g−1 at 365 nm with an absorption Ångström exponents (AAE) = 3.1 for 300 & 600 wavelengths pair, coupled with a 1 day whitening e-folding time. Using the GEOS-Chem chemical transport model integrated with the RRTMG radiative transfer model, we estimate that the top-of-atmosphere all-sky direct radiative effect (DRE) of OA is −0.350 Wm−2, 10 % higher than that without consideration of BrC absorption. Therefore, our best estimate of the absorption DRE of BrC is +0.042 Wm−2. We suggest that the DRE of BrC has been overestimated previously due to the lack of observational constraints from direct measurements and omission of the effects of photochemical whitening.;
