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Volume 14, issue 20
Atmos. Chem. Phys., 14, 10989–11010, 2014
https://doi.org/10.5194/acp-14-10989-2014
© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 14, 10989–11010, 2014
https://doi.org/10.5194/acp-14-10989-2014
© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 20 Oct 2014

Research article | 20 Oct 2014

Exploiting simultaneous observational constraints on mass and absorption to estimate the global direct radiative forcing of black carbon and brown carbon

X. Wang1, C. L. Heald1,2, D. A. Ridley1, J. P. Schwarz3,4, J. R. Spackman5,6, A. E. Perring3,4, H. Coe7, D. Liu7, and A. D. Clarke8 X. 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
  • 3Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA
  • 4Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA
  • 5Physical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA
  • 6Science and Technology Corporation, Boulder, Colorado, USA
  • 7Centre for Atmospheric Science, University of Manchester, Manchester, UK
  • 8Department of Oceanography, University of Hawaii, Honolulu, HI, USA

Abstract. Atmospheric black carbon (BC) is a leading climate warming agent, yet uncertainties on the global direct radiative forcing (DRF) remain large. Here we expand a global model simulation (GEOS-Chem) of BC to include the absorption enhancement associated with BC coating and separately treat both the aging and physical properties of fossil-fuel and biomass-burning BC. In addition we develop a global simulation of brown carbon (BrC) from both secondary (aromatic) and primary (biomass burning and biofuel) sources. The global mean lifetime of BC in this simulation (4.4 days) is substantially lower compared to the AeroCom I model means (7.3 days), and as a result, this model captures both the mass concentrations measured in near-source airborne field campaigns (ARCTAS, EUCAARI) and surface sites within 30%, and in remote regions (HIPPO) within a factor of 2. We show that the new BC optical properties together with the inclusion of BrC reduces the model bias in absorption aerosol optical depth (AAOD) at multiple wavelengths by more than 50% at AERONET sites worldwide. However our improved model still underestimates AAOD by a factor of 1.4 to 2.8 regionally, with the largest underestimates in regions influenced by fire. Using the RRTMG model integrated with GEOS-Chem we estimate that the all-sky top-of-atmosphere DRF of BC is +0.13 Wm−2 (0.08 Wm−2 from anthropogenic sources and 0.05 Wm−2 from biomass burning). If we scale our model to match AERONET AAOD observations we estimate the DRF of BC is +0.21 Wm−2, with an additional +0.11 Wm−2 of warming from BrC. Uncertainties in size, optical properties, observations, and emissions suggest an overall uncertainty in BC DRF of −80%/+140%. Our estimates are at the lower end of the 0.2–1.0 Wm−2 range from previous studies, and substantially less than the +0.6 Wm−2 DRF estimated in the IPCC 5th Assessment Report. We suggest that the DRF of BC has previously been overestimated due to the overestimation of the BC lifetime (including the effect on the vertical profile) and the incorrect attribution of BrC absorption to BC.

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