, Matthew Jordon, , ,
Simultaneous transmissivity and absorptivity measurements were carried out in the visible at a laser wavelength of 532 nm on drop-cast, carbon-black-laden filters under ambient (laboratory) conditions. The focus of this investigation was to establish the feasibility of this approach to estimate the absorption coefficient (i.e., of the isolated particles) and particle mass- specific absorption cross section, and compare results to earlier work with the same carbon-black source. Transmissivity measurements were carried out with a laser probe beam positioned normal to the particle-laden filter surface. Absorptivity measurements were carried out using a laser- heating approach to record the sample temperature rise and decay with time from the ambient temperature. The sample temperature was recorded using a fine-wire thermocouple that was integrated into the transmission arrangement by placing the thermocouple flush with the filter back surface. The advantage of this approach is that one can estimate directly the sample absorptivity instead of resolving the difference between reflectivity (filter surface scattering) and transmissivity. The current approach also provides the filter optical characteristics, as well as an estimate of filter effects on the absorption coefficient due to particle absorption enhancement or shadowing. For this investigation, measurements were carried out with several blank uncoated quartz filters. A range of solution concentrations were prepared with well-characterized carbon black in deionized water (i.e., a water-soluble carbonaceous material referred to as a surrogate black carbon or carbon black). The solution was then drop cast using a calibrated syringe onto blank filters to vary particle loading. After evaporation of the water, the measurements were repeated with the coated filters.
Aerosol Science and Technology
absorption coefficient, carbon-black particles, mass-specific absorption cross section, particle- laden filters, simultaneous transmission/absorption photometry