Abstract
Abstract. Organic aerosols (OA) that strongly absorb solar radiation in the near-UV are referred to as brown carbon (BrC). 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 US (SEAC4RS and DC3). To the best of our knowledge, this is the first study to compare simulated BrC absorption with direct aircraft measurements. We show that BrC absorption properties estimated based on previous laboratory measurements agree with the aircraft measurements of freshly emitted BrC absorption but overestimate aged BrC absorption. In addition, applying a photochemical scheme to simulate bleaching/degradation of BrC improves model skill. The airborne observations are therefore consistent with a mass absorption coefficient (MAC) of freshly emitted biomass burning OA of 1.33 m2 g−1 at 365 nm 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-the-atmosphere all-sky direct radiative effect (DRE) of OA is −0.344 Wm−2, 10 % higher than that without consideration of BrC absorption. Therefore, our best estimate of the absorption DRE of BrC is +0.048 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.
Highlights
Carbonaceous aerosols, including both black carbon (BC) and organic aerosols (OA), are among the largest sources of uncertainty in the estimate of the global direct radiative effect (DRE) and forcing (DRF) of aerosols
We explore how assumptions for brown carbon (BrC) sources, processing, and properties impact the comparisons with these observational constraints and estimate the resulting global direct radiative effect of BrC under these conditions
We use the Goddard Earth Observing System (GEOS)-Chem model coupled with the RRTMG model to investigate the mass optical properties and direct radiative effect of brown carbon (BrC)
Summary
Carbonaceous aerosols, including both black carbon (BC) and organic aerosols (OA), are among the largest sources of uncertainty in the estimate of the global direct radiative effect (DRE) and forcing (DRF) of aerosols. Wang et al.: Observational constraints on brown carbon simulation manathan et al, 2007; Washenfelder et al, 2015) These BrC emissions are typically mixed with co-emitted BC and nonabsorbing OA, challenging the measurement community’s ability to evaluate the optical properties of ambient BrC. Previous modeling studies have typically used either the lower or higher bound from laboratory studies to estimate the minimum or maximum absorption properties of BrC (Feng et al, 2013; Lin et al, 2014) It is unclear what fraction of the OA is BrC and how this differs with source and ambient combustion conditions (Pokhrel et al, 2017). We develop a model simulation of BrC, test it against BrC absorption measurements from two aircraft campaigns in the United States (SEAC4RS and DC3), and optimize it to match these observational constraints. We explore how assumptions for BrC sources, processing, and properties impact the comparisons with these observational constraints and estimate the resulting global direct radiative effect of BrC under these conditions
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