Articles | Volume 15, issue 14
Atmos. Chem. Phys., 15, 7841–7858, 2015
Atmos. Chem. Phys., 15, 7841–7858, 2015

Research article 16 Jul 2015

Research article | 16 Jul 2015

Brown carbon aerosol in the North American continental troposphere: sources, abundance, and radiative forcing

J. Liu1,a, E. Scheuer2, J. Dibb2, G. S. Diskin3, L. D. Ziemba3, K. L. Thornhill3, B. E. Anderson3, A. Wisthaler4, T. Mikoviny5, J. J. Devi6, M. Bergin6, A. E. Perring7,8, M. Z. Markovic7,8, J. P. Schwarz7,8, P. Campuzano-Jost8,9, D. A. Day8,9, J. L. Jimenez8,9, and R. J. Weber1 J. Liu et al.
  • 1School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA
  • 2Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH 03824, USA
  • 3NASA Langley Research Center, Hampton, VA 23681, USA
  • 4Institute of Ion Physics and Applied Physics, University of Innsbruck, Innsbruck, Austria
  • 5Oak Ridge Associated Universities (ORAU), Oak Ridge, TN 37830, USA
  • 6School of Civil & Environmental Engineering, Georgia Institute of Technology, Atlanta, GA, USA
  • 7Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80305, USA
  • 8Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA
  • 9Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA
  • anow at: Atmospheric Sciences and Global Change Division, Pacific Northwest, National Laboratory, Richland, WA 99352, USA

Abstract. Chemical components of organic aerosol (OA) selectively absorb light at short wavelengths. In this study, the prevalence, sources, and optical importance of this so-called brown carbon (BrC) aerosol component are investigated throughout the North American continental tropospheric column during a summer of extensive biomass burning. Spectrophotometric absorption measurements on extracts of bulk aerosol samples collected from an aircraft over the central USA were analyzed to directly quantify BrC abundance. BrC was found to be prevalent throughout the 1 to 12 km altitude measurement range, with dramatic enhancements in biomass-burning plumes. BrC to black carbon (BC) ratios, under background tropospheric conditions, increased with altitude, consistent with a corresponding increase in the absorption Ångström exponent (AAE) determined from a three-wavelength particle soot absorption photometer (PSAP). The sum of inferred BC absorption and measured BrC absorption at 365 nm was within 3 % of the measured PSAP absorption for background conditions and 22 % for biomass burning. A radiative transfer model showed that BrC absorption reduced top-of-atmosphere (TOA) aerosol forcing by ~ 20 % in the background troposphere. Extensive radiative model simulations applying this study background tropospheric conditions provided a look-up chart for determining radiative forcing efficiencies of BrC as a function of a surface-measured BrC : BC ratio and single scattering albedo (SSA). The chart is a first attempt to provide a tool for better assessment of brown carbon's forcing effect when one is limited to only surface data. These results indicate that BrC is an important contributor to direct aerosol radiative forcing.

Short summary
Brown carbon (BrC) is found throughout the US continental troposphere during a summer of extensive biomass burning and its prevalence relative to black carbon (BC) increases with altitude. A radiative transfer model based on direct measurements of aerosol scattering and absorption by BC and BrC shows BrC reduces top-of-atmosphere forcing by 20%. A method to estimate BrC radiative forcing efficiencies from surface-based measurements is provided.
Final-revised paper