Evaluation of multiangle absorption photometry for measuring aerosol light absorption

A new multiangle absorption photometer for the measurement of aerosol light absorption was recently introduced that builds on the simultaneous measurement of radiation transmitted through and scattered back from a particle-loaded fiber filter at multiple detection angles. The absorption coefficient of the filter-deposited aerosol is calculated from the optical properties of the entire filter system, which are determined by a two-stream-approximation radiative transfer scheme. In the course of the Reno Aerosol Optics Study (RAOS), the response characteristics of multiangle absorption photometry ( MAAP) for white aerosol, pure black carbon aerosol from different sources, external mixtures of black and white aerosol, and ambient aerosol was investigated. The MAAP response characteristics were compared to basic filter transmittance and filter reflectance measurements. MAAP showed close agreement with a reference absorption measurement by extinction minus scattering. The slopes of regression lines vary between 0.99 +/- 0.01 and 1.07 +/- 0.02 for pure black carbon particles and external mixtures with ammonium sulphate to 1.03 +/- 0.05 for ambient aerosol. No effect of the filter aerosol loading or the single-scattering albedo omega(0) of the sampled aerosol on the MAAP response characteristics was observed. In contrast, transmittance and reflectance methods showed a clear impact of omega(0) and the filter loading on the response characteristics, which requires the application of a correction function for the reliable determination of the aerosol absorption coefficient. In the case of nonabsorbing aerosol, the MAAP approach reduced the magnitude of the apparently measured absorption coefficient by one order of magnitude compared to a basic transmittance measurement.