Biomass Burning Dominates Brown Carbon Absorption in the Rural Southeastern U.S.

Wednesday, 17 December 2014: 10:50 AM
Rebecca A Washenfelder1, Alexis R Attwood2, Charles A Brock3, Steven S Brown4, Hongyu Guo5, Rodney J J Weber6, Lu Xu7, Nga Lee Ng8, Elizabeth A. Stone9, Eric S Edgerton10, Karsten Baumann11, Weiwei Hu12, Brett B Palm12, Jose L Jimenez12, Juliane Fry13, Benjamin R Ayres13, Danielle Draper13 and Hannah Allen13, (1)NOAA Boulder, Boulder, CO, United States, (2)NOAA Boulder, Denver, CO, United States, (3)NOAA Earth System Research Lab, Boulder, CO, United States, (4)NOAA Earth System Research Lab, Chemical Sciences Division, Boulder, CO, United States, (5)Georgia Institute of Technology Main Campus, Earth and Atmospheric Sciences, Atlanta, GA, United States, (6)Georgia Inst Technology, Atlanta, GA, United States, (7)Georgia Institute of Technology Main Campus, Atlanta, GA, United States, (8)Georgia Institute of Technology, Atlanta, GA, United States, (9)University of Iowa, Iowa City, IA, United States, (10)Atmospheric Research & Analysis, Inc., Cary, NC, United States, (11)Atmospheric Research & Anal., Morrisville, NC, United States, (12)University of Colorado at Boulder, Boulder, CO, United States, (13)Reed College, Portland, OR, United States
Aerosol scattering and absorption are still among the largest uncertainties in quantifying radiative forcing. Brown carbon has a wavelength-dependent absorption that increases in the UV spectral region, and its major atmospheric sources include biomass burning, anthropogenic combustion of fossil fuels, and secondary organic aerosol. The rural Southeastern U.S. is influenced by high isoprene concentrations and varying concentrations of biomass burning aerosol, making it an ideal place to compare the relative contributions of these two sources to the brown carbon absorption budget.

During the Southern Oxidant and Aerosol Study in summer 2013, we deployed a new field instrument that uses cavity enhanced spectroscopy with a broadband light source to measure aerosol optical extinction as a function of wavelength. The instrument consists of two broadband channels which span the 360–390 and 385–420 nm spectral regions using two light emitting diodes (LED) and a grating spectrometer with charge-coupled device (CCD) detector. We combine these data with direct absorption measurements of water-soluble organic carbon obtained from a novel UV/VIS-WSOC instrument, and with aerosol composition measurements.

We examine these data sets to determine: 1) the optical closure between measured dry aerosol extinction and values calculated from aerosol composition and size distribution; 2) the magnitude of brown and black carbon absorption; 3) the relative contributions of biomass burning, anthropogenic, and secondary organic aerosol contributions to brown carbon absorption in the Southeast U.S. during the summer. We conclude that biomass burning is a major contributor to optical absorption by organic aerosol in the rural southeastern U.S.