A radiative transfer scheme that considers absorption, scattering, and distribution of light-absorbing elemental carbon (EC) particles collected on a quartz-fiber filter was developed to explain simultaneous filter reflectance and transmittance observations prior to and during thermal/optical analysis for carbonaceous aerosol. The model is applied to study ambient filter samples from the United States and Hong Kong, China, and how they differ from each other and from reference carbon black samples. Most particles in ambient samples deposit into the top half of the filter, resuspended carbon black particles are only found close to the filter surface. Pyrolized/charred organic carbon (POC) generated during thermal analysis reduces filter reflectance and transmittance in a fashion that suggests a uniform distribution of POC throughout the filter. When heated in oxygen, most EC evolves earlier than the within-filter charring for certain ambient samples. This shows the different natures of EC, and also results in an inexact optical correction to separate organic and elemental carbon by thermal analysis, especially when the POC/EC ratio is large. Particle absorption in the filter is estimated for comparisons with the EC/POC measurements, suggesting a mass absorption efficiency of similar to2.7 +/- 0.2 m(2) (gC)(-1) for reference carbon black and > 15 m(2) (gC)(-1) for ambient EC, similar to previous estimates in the literature. (C) 2004 Elsevier Ltd. All rights reserved.