Exploring the observational constraints on the simulation of brown carbon

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 (SEAC 4 RS 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 m 2 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 allsky 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.

Direct Radiative Effect of Absorbing Aerosols: Sensitivity to Mixing State, Brown Carbon, and Soil Dust Refractive Index and Shape

“…Knowing the contribution of BrC to OA, the MAC OA may be translated to MAC BrC using equation 2 of Wang et al (): where f is the fraction of OA mass that is BrC. In our case, BrC contributed to 50% and 25% of the BF and BB OA, respectively.…”

Radiative Effect and Climate Impacts of Brown Carbon with the Community Atmosphere Model (CAM5)

“…The BrC bleaching parameterization is based on that applied by Wang et al (2018). The half-life of BrC is assumed to be about 1 day with the aging of BrC dependent on the atmospheric concentration of OH ([OH], molec cm -3 ).…”

“…The radiative effect of BrC has been estimated in a number of modeling studies using global chemical transport models combined with radiative transfer models (Feng et al, 2013;Lin et al, 2014;Saleh et al, 2015; 25 Jo et al, 2016;Wang X. et al, 2018). In Feng et al (2013), global mean absorption aerosol optical depth (AAOD) increases with two different BrC absorption assumptions: moderately (Chen and Bond, 2010) and strongly (Kirchstetter et al, 2004) absorbing, with 66% of BB/BF organic carbon emissions assumed to be BrC.…”

“…The BrC REari calculated in Saleh et al (2015) varied between +0.12 W m -2 and +0.22 W m -2 , with the variation attributed 10 to core-shell and externally mixed assumptions, respectively. Recent work by Wang et al (2018) included the Saleh et al (2015) parameterization in addition to a BrC bleaching parameterization that ages BrC to 0.25% of its original absorption over ~1 day. This lower threshold reflects the observation that BrC doesn't fully bleach (Wang et al, 2016;Forrister et al, 2015).…”

“…We also implement a BrC bleaching parameter in the model similar to Wang et al (2018) to analyze its effect on the BrC direct and semi-direct effects. It is important to mention that this study does not include absorption by BB SOA (Lin et al, 2014;Saleh et al, 2015) 25 or absorbing aromatic SOA Jo et al, 2016;Wang X. et al, 2018). This is neglected due to the lack of aromatic SOA speciation in the model.…”