Carbonaceous aerosols affect the global radiative balance by absorbing and scattering radiation, which leads to warming or cooling of the atmosphere, respectively. Black carbon is the main light-absorbing component. A portion of the organic aerosol known as brown carbon also absorbs light. The climate sensitivity to absorbing aerosols rapidly increases with altitude, but brown carbon measurements are limited in the upper troposphere. Here we present aircraft observations of vertical aerosol distributions over the continental United States in May and June 2012 to show that light-absorbing brown carbon is prevalent in the troposphere, and absorbs more short-wavelength radiation than black carbon at altitudes between 5 and 12 km. We find that brown carbon is transported to these altitudes by deep convection, and that in-cloud heterogeneous processing may produce brown carbon. Radiative transfer calculations suggest that brown carbon accounts for about 24% of combined black and brown carbon warming effect at the tropopause. Roughly two-thirds of the estimated brown carbon forcing occurs above 5 km, although most brown carbon is found below 5 km. The highest radiative absorption occurred during an event that ingested a wildfire plume. We conclude that high-altitude brown carbon from biomass burning is an unappreciated component of climate forcing.
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All data used in this paper were collected as part of the NASA DC3 and SEAC4RS missions and became available to the general public on 15 October 2014 through the NASA data archive. Georgia Tech researchers were funded through NASA Radiation Sciences Program grant NNX14AP74G. NASA Tropospheric Composition Program supported J.D. through grant NNX12AB80G, and P.C.-J. and J.L.J. through grants NNX12AC03G and NNX15AT96G. H.F. was supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1650044. The authors thank G. Chen and C. Corr for discussion with Y.Z.
The authors declare no competing financial interests.
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Zhang, Y., Forrister, H., Liu, J. et al. Top-of-atmosphere radiative forcing affected by brown carbon in the upper troposphere. Nature Geosci 10, 486–489 (2017). https://doi.org/10.1038/ngeo2960
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