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Volume 18, issue 2
Atmos. Chem. Phys., 18, 635–653, 2018
https://doi.org/10.5194/acp-18-635-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Atmos. Chem. Phys., 18, 635–653, 2018
https://doi.org/10.5194/acp-18-635-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Jan 2018

Research article | 19 Jan 2018

Exploring the observational constraints on the simulation of brown carbon

Xuan Wang1,a, Colette L. Heald1,2, Jiumeng Liu3, Rodney J. Weber4, Pedro Campuzano-Jost5,6, Jose L. Jimenez5,6, Joshua P. Schwarz7, and Anne E. Perring6,7 Xuan Wang et al.
  • 1Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA
  • 2Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
  • 3Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA, USA
  • 4School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
  • 5Department of Chemistry, University of Colorado Boulder, Boulder, CO, USA
  • 6Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA
  • 7Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA
  • anow at: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA

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.

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Brown carbon (BrC) contributes significantly to uncertainty in estimating the global direct radiative effect (DRE) of aerosols. We develop a global model simulation of BrC and test it against BrC absorption measurements from two aircraft campaigns in the continental United States. We suggest that BrC DRE has been overestimated previously due to the lack of observational constraints from direct measurements and omission of the effects of photochemical whitening.
Brown carbon (BrC) contributes significantly to uncertainty in estimating the global direct...
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