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Organic aerosols (OA) that strongly absorb solar radiation in the near-UV arereferred to as brown carbon (BrC). The sources, evolution, and opticalproperties of BrC remain highly uncertain and contribute significantly touncertainty in the estimate of the global direct radiative effect (DRE) ofaerosols. Previous modeling studies of BrC optical properties and DRE havebeen unable to fully evaluate model performance due to the lack of directmeasurements of BrC absorption. In this study, we develop a global modelsimulation (GEOS-Chem) of BrC and test it against BrC absorption measurementsfrom two aircraft campaigns in the continental US (SEAC4RS and DC3). Tothe best of our knowledge, this is the first study to compare simulated BrC absorptionwith direct aircraft measurements. We show that BrC absorption propertiesestimated based on previous laboratory measurements agree with the aircraftmeasurements of freshly emitted BrC absorption but overestimate aged BrCabsorption. In addition, applying a photochemical scheme to simulatebleaching/degradation of BrC improves model skill. The airborne observationsare therefore consistent with a mass absorption coefficient (MAC) of freshlyemitted biomass burning OA of 1.33 m2 g−1 at 365 nm coupled witha 1-day whitening e-folding time. Using the GEOS-Chem chemical transportmodel integrated with the RRTMG radiative transfer model, we estimate thatthe top-of-the-atmosphere all-sky direct radiative effect (DRE) of OA is−0.344 Wm−2, 10 % higher than that without consideration of BrCabsorption. Therefore, our best estimate of the absorption DRE of BrC is+0.048 Wm−2. We suggest that the DRE of BrC has been overestimatedpreviously due to the lack of observational constraints from directmeasurements and omission of the effects of photochemical whitening

Exploring the observational constraints on the simulation of brown carbon