Filter-based measurements of aerosol optical absorption are widely used to determine Black Carbon (BC) concentrations in real time. Measurements at multiple wavelengths permit the separation of contributions of BC from different combustion sources. Filterbased methods can suffer from non-linearity due increasing sample deposit on the filter. The dual-spot Aethalometer model AE33 provides a real-time determination of this loading effect, by determining the compensation parameter k in real time for each of the wavelengths. We present an interpretation of k in terms of aerosol composition. Optical properties of combustion aerosols were investigated under laboratory conditions. Optical properties of emitted aerosols varied greatly with the comb...