Multi-angle absorption photometry—a new method for the measurement of aerosol light absorption and atmospheric black carbon

A new method is presented which determines the aerosol light absorption from the simultaneous measurement of radiation passing through and scattered back from a particle-loaded fibre filter. The measurement is performed at three detection angles to resolve the influence of light-scattering aerosol components on the angular distribution of the back scattered radiation. The fraction of light absorbed by the deposited aerosol (absorbance ABS) is obtained from a radiative transfer scheme. The method response normalised to the mass concentration of black carbon to test aerosols with mass fractions of the light-absorbing component between 1.2% and 100% (aerosol single scattering albedo 1.0–0.28) varied by >300% for a transmission measurement only, but by <50% for multi-angle absorption photometry. The compensation of light-scattering effects in filter-based aerosol absorption measurements is considerably improved. The detection limit is 3.5×10−4 absorbance units. The uncertainty of the absorbance is ±12%. The evaluation of the method using field data yielded an average mass specific absorption coefficient of 9.1–10.7 m2 g−1 at λ=0.55 μm for atmospheric black carbon in urban air.