HR: 0800h
AN: A41D-0733    [Abstracts]
TI: Optimizing Thermal-Optical Analysis for Atmospheric Black Carbon (BC): Determining the Beer-Lambert Mass Without a Fixed Mass Absorption Coefficient for BC
AU: * Conny, J M
EM: joseph.conny@nist.gov
AF: Surface and Microanalysis Science Division, National Institute of Standards and Technology, 100 Bureau Drive, Stop 8372, Gaithersburg, MD 20899, United States
AU: Norris, G
EM: Norris.Gary@epamail.epa.gov
AF: Human Exposure and Atmospheric Science Division, National Exposure Research Laboratory, U.S. EPA, MD E205-03, Research Triangle Pa, NC 27711, United States
AB: In thermal-optical transmission analysis (TOT), laser light passing through a particle-laden filter is monitored while carbonaceous material is removed in several heating steps and measured by flame ionization detection. In a helium atmosphere, the laser signal is attenuated by the pyrolysis of organic carbon (OC). Later, while carbon is removed in an oxidizing atmosphere, the laser signal returns to its value prior to pyrolysis (split point), whereupon the amount of carbon equivalent to the native BC is measured. Since pyrolyzed OC may actually evolve beyond the split point, the specific absorption cross sections of pyrolyzed OC and native BC must be equivalent. Moreover, OC pyrolysis must be sufficient so that unpyrolyzed OC is not measured as BC beyond the split point. Using response surfaces models of the apparent specific absorption cross sections for pyrolyzed OC and what the instrument measures as native BC, we determined the thermal conditions for establishing the equivalence of the apparent cross sections while insuring sufficient pyrolysis of OC. In this way, we have optimized TOT for BC mass based on the Beer-Lambert Law but without the need for an absolute mass absorption coefficient (or an absolute attenuation coefficient) for BC. Optimal thermal conditions for the equivalence of the cross sections were indicated by the intersection of the response surfaces. Concurrently, optimal conditions for sufficient pyrolysis of OC were indicated by a plateau in the response surface for the BC cross section. Modeling was based on extensive analyses of PM2.5 samples collected from Atlanta, Los Angeles, and Seattle. Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0345 Pollution: urban and regional (0305, 0478, 4251)
DE: 0365 Troposphere: composition and chemistry
DE: 0394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting