Stabilization of the Mass Absorption Cross Section of Black Carbon for Filter-Based Absorption Photometry by the use of a Heated Inlet

In principle, mass concentrations of black carbon (BC) (M-BC) can be estimated by the measurement of the light absorption coefficient of BC. Filter-based methods, which quantify the absorption coefficient (b(abs)) from the change in transmission through a filter loaded with particles, have been widely used to measure M-BC. However, reliable determination of M-BC has been very difficult because of the large variability in the mass absorption cross section (C-abs), which is the conversion factor from b(abs) to M-BC. Coating of BC by volatile compounds and the co-existence of light-scattering particles contribute to the variability of C-abs. In order to overcome this difficulty, volatile aerosol components were removed before collection of BC particles on filters by heating a section of the inlet to 400 degrees C. We made simultaneous measurements of b(abs) by two types of photometers (Particle Soot Absorption Photometer (PSAP) and Continuous Soot Monitoring System (COSMOS)) together with M-BC by an EC-OC analyzer to determine C-abs at 6 locations in Asia. C-abs was stable at 10.5 +/- 0.7 m(2) g(-1) at a wavelength of 565 nm for BC strongly impacted by emissions from vehicles and biomass burning. The stable C-abs value provides a firm basis for its use in estimating M-BC by COSMOS and PSAP with an accuracy of about 10%. For the quantitative interpretation of the ratio of the C-abs to the model-calculated C-abs*, we measured Cabs for mono-disperse nigrosin particles in the laboratory. The C-abs/C-abs* ratio was 1.4-1.9 at the 100-200 nm diameters, explaining the ratio of 1.8 for ambient BC.