Evaluation of Multiangle Absorption Photometry for Measuring Aerosol Light Absorption

Abstract

A new multi-angle absorption photometer for the measurement of aerosol light absorption was recently introduced which builds on the simultaneous measurement of radiation transmitted through and scattered back from a particle-loaded fibre filter at multiple detection angles. The absorption coefficient of the filter-deposited aerosol is calculated from the optical properties of the entire filter system by a radiative transfer scheme. In the course of the reno aerosol optics study, the response characteristics of multi-angle absorption photometry (MAAP) for white aerosol, pure black carbon aerosol from different sources, external mixtures of black and white aerosol, and internally mixed ambient aerosol was investigated. The MAAP response characteristics were compared to basic filter transmittance and filter reflectance approaches. The MAAP method showed close agreement to a reference absorption measurement by extinction minus scattering with the slopes of regression lines varying between 0.99 #+-# 0.01 and 1.07 #+-# 0.02 for pure black carbon particles and external mixtures to 1.03 #+-# 0.05 for internally mixed 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 non-absorbing aerosol, the MAAP approach reduced the magnitude of the apparently measured absorption coefficient by one order of magnitude compared to a basic transmittance measurement. (orig.)