How Does Thermal-Optical Analysis for Atmospheric Elemental Carbon Behave Optically? Investigations of the Apparent Specific Absorption Cross Section
Joseph M. Conny
Thermal-optical analysis (TOA) is a principal method for measuring elemental carbon (EC) associated with atmospheric soot. It relies on changes in the optical behavior of carbon in particulate matter (PM) to indicate when carbon measured as EC separates thermally from organic carbon (OC) in the sample. We cannot assume that char produced by the instrument during pyrolysis of OC separates physically from native EC during analysis because, as both are products of incomplete combustion, they are chemically and physically similar substances. For the thermal-optical transmission method (TOT), an emphasis on optical behavior presents it as a method for the accurate measurement of light-absorbing particulate carbon and thus allows EC to be defined as black carbon (BC) as in the aethelometer. To achieve accuracy in EC as BC, three assumptions concerning the TOT s optical behavior are necessary. First, the absorptivity of carbonaceous PM remains constant prior to OC pyrolysis, i.e., no substantive change to particle refractive index occurs that would affect absorptivity. Second, the absorptivity of pyrolyzed OC remains constant after its formation within a high-temperature step in the non-oxidizing (He) phase. Third, pyrolyzed OC has the same absorptivity as native EC. Tests of these assumptions using three types of samples (indoor air particles, urban (outdoor) PM, and smoldering forest fire emissions) showed that all three assumptions are invalid. Absorptivity during the course of TOT was determined as the apparent specific absorption cross section, C, for the carbonaceous component. Tandem TOT runs were used to factor out scattering by non-carbonaceous material and scattering by the sample substrate quartz fibers, both of which are exacerbated at pyrolysis temperatures. In comparison with the aethelometer s specific attenuation cross section, the C value for the urban air sample at the point of maximum TOT charring was moderately larger (15 m2 g-1) than the value for the aethelometer s specific attenuation cross section for urban PM reported by others (12 m2 g-1). The C value for the smoldering fire emissions sample at maximum charring was also larger (31 m2 g-1) than the aethelometer s specific attenuation cross section reported by others for biomass burning influenced PM (20 m2 g-1 to 26 m2 g-1 ). C was further separated into separate components for OC char and native EC. Results showed that changes in the duration of the high temperature step in helium produced better agreement between the C values for char and EC. However, the two sample types required different step durations. While it is possible to validate the third assumption concerning the equivalence of the absorptivities of char and EC by modifying the TOT thermal desorption protocol, to achieve TOT accuracy different sample types will require different thermal desorption protocols.
To Be Determined
absorption coefficient, absorptivity, atmospheric aerosol, black carbon, elemental carbon, specific absorption cross section, thermal-optical analysis, TOT
How Does Thermal-Optical Analysis for Atmospheric Elemental Carbon Behave Optically? Investigations of the Apparent Specific Absorption Cross Section, To Be Determined
(Accessed June 1, 2023)