Absorption of light by soot particles: determination of the absorption coefficient by means of aethalometers

During a soot aerosol measurement campaign the response of two different aethalometers (AE10 with white light and AE30 with multiwavelength capability) to several types of soot was investigated. Diesel soot, spark-generated carbon particles, and mixtures of these soot particles with ammonium sulfate and oxidation products of α-pinene were used in this evaluation. The determination of the particles light absorption coefficient (babs) with the AE10 aethalometer is a difficult task because of an ill-defined spectral sensitivity of this instrument. Provided that the proper numerical corrections are performed, the AE30 instrument allows for the measurement of babs over a wide spectral range (λ=450–950 nm). During all experiments it was found that with increasing filter load the optical path in the aethalometer filter decreased. As a result, an increased underestimation of the measured aethalometer signals (babs or black carbon mass concentrations) occurs with increasing filter loads. This effect, which is attributed to a “shadowing” of the particles in the fiber matrix, is very pronounced for “pure” soot particles while almost negligible for aged atmospheric aerosols. An empirical correction for this bias is presented and requires information on the light scattering behavior (i.e. light scattering coefficient) of the sampled particles. Without this additional information, the applicability of the instruments is limited. Comparison with a reference method shows that multiple scattering in the nearly unloaded fiber filter is responsible for enhanced light absorption by a factor of about 2.14.