Analytical Chemistry Seminar: Rebecca Washenfelder
Jointly sponsored by the Department of Chemistry and Biochemistry, CIRES, and the Environmental Program
Optical properties of brown carbon aerosol in the near-ultraviolet spectral region
by Rebecca Washenfelder - NOAA Chemical Sciences Division
Aerosol scattering and absorption are among the largest uncertainties in radiative forcing. Black carbon is a strong radiative forcing agent, and absorbs strongly throughout the ultraviolet and visible spectral regions. In contrast, brown carbon has a wavelength-dependent absorption that increases sharply in the ultraviolet spectral region and its importance in radiative forcing is more unknown. Part of this uncertainty arises from the need to characterize potential sources of brown carbon aerosol, which include fossil fuel combustion, biomass burning, and secondary organic aerosol aging processes.
We have developed a new method to measure aerosol optical extinction as a function of wavelength for in situ aerosol, using cavity enhanced spectroscopy. We have demonstrated this method over the 360-420 nm spectral region, and plan to extend it to 300 nm. We retrieve complex refractive indices as a function of wavelength from the measured extinction cross sections. In the laboratory, this technique has allowed us to examine a proposed mechanism to produce brown carbon aerosol from the reaction of ammonia or amino acids with carbonyl products in secondary organic aerosol.
During the Southern Oxidant and Aerosol Study in summer 2013, we acquired field measurements of aerosol optical extinction at 360-420 nm. We combined these data with direct absorption measurements of water-soluble organic carbon obtained from a UV/VIS-WSOC instrument, and with aerosol composition measurements. I will present the magnitude of brown and black carbon absorption and the relative contributions of biomass burning, anthropogenic, and secondary organic aerosol contributions to brown carbon absorption in the Southeast U.S. during the summer.