The filter-loading effect by ambient aerosols in filter absorption photometers depends on the coating of the sampled particles

摘要

Black carbon is a primary aerosol tracer for high-temperaturecombustion emissions and can be used to characterize the time evolution ofits sources. It is correlated with a decrease in public health andcontributes to atmospheric warming. Black carbon measurements are usuallyconducted with absorption filter photometers, which are prone to severalartifacts, including the filter-loading effect – a saturation of theinstrumental response due to the accumulation of the sample in the filtermatrix. In this paper, we investigate the hypothesis that thisfilter-loading effect depends on the optical properties of particles presentin the filter matrix, especially on the black carbon particle coating. Weconducted field campaigns in contrasting environments to determine theinfluence of source characteristics, particle age and coating on themagnitude of the filter-loading effect. High-time-resolution measurements ofthe filter-loading parameter in filter absorption photometers show daily andseasonal variations of the effect. The variation is most pronounced in thenear-infrared region, where the black carbon mass concentration isdetermined. During winter, the filter-loading parameter value increases withthe absorption Ångström exponent. It is suggested that this effectis related to the size of the black carbon particle core as the wood burning(with higher values of the absorption Ångström exponent) producessoot particles with larger diameters. A reduction of the filter-loadingeffect is correlated with the availability of the coating material. As thecoating of ambient aerosols is reduced or removed, the filter-loadingparameter increases. Coatings composed of ammonium sulfate and secondaryorganics seem to be responsible for the variation of the loading effect. Thepotential source contribution function analysis shows that high values ofthe filter-loading parameter in the infrared are indicative of localpollution, whereas low values of the filter-loading parameter result fromageing and coating during long-range transport. Our results show that thefilter-loading parameter can be used as a proxy for determination of theparticle coating, thus allowing for differentiation between local/fresh andtransported/aged particles.