HR: 15:10h
AN: A43E-07    [Abstracts]
TI: Toward Resolution on the Optics of Light-Absorbing Carbon
AU: * Bond, T C
EM: yark@uiuc.edu
AF: University of Illinois at Urbana-Champaign, Dept. of Civil Engineering, NCEL-MC250 205 N. Mathews Ave., Urbana, IL 61801 United States
AU: Bergstrom, R W
EM: bergstrom@baeri.org
AF: Bay Area Environmental Research Institute, 560 Third St. West, Sonoma, CA 95476 United States
AB: Compendia of the absorption cross-section of atmospheric particles yield a wide range of values for light-absorbing carbon; similar anthologies drawn from the combustion literature support a wide range of refractive indices. While these values have been tabulated in previous reviews, an acceptable resolution of the reported variability has been missing. Here, we present the results of an exhaustive review of reported optical properties of LAC: inferred refractive indices and measured absorption efficiencies. In all cases, we have returned to the original source of the data, and we have frequently re-examined the measurements that led to the reported values in light of current theoretical understanding. Instead of simply rejecting values measured with differing techniques, we attempt to use each study to bound the relevant properties of LAC and thereby achieve consensus between studies. Our revised tabulation shows that values of refractive index and absorptive properties of light-absorbing carbon are not as different as is commonly believed, and that many of the apparent discrepancies result from variations in interpreting measurements. We identify the origins of two common values in the atmospheric science community: the refractive-index values recommended by OPAC, and the 10 m2/g value widely cited for the mass absorption efficiency of pure LAC. Neither value is taken from material representative of atmospheric LAC, and we provide new recommendations and discuss the implications for aerosol models. Next, we discuss the parameters needed to represent absorbing aerosol in climate models. In particular, internal versus external mixing has been cited as a cause of large uncertainty in such modeling. While the number of mixing states is effectively infinite, we identify a limited number of boundaries that could be used to make such a representation manageable. Finally, we examine estimates of direct climate forcing by carbonaceous aerosols. We believe there is still high uncertainty in these estimates that is not bounded by presently-published model results. Paradoxically, we report that some of the variability in published estimates of climate forcing results from basic assumptions, and that model results agree much better when adjusted for these considerations.
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0365 Troposphere--composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting