The filter loading effect by ambient aerosols in filter absorption photometers depends on the mixing state of the sampled particles
2016
Black carbon is a primary aerosol tracer for high-temperature
combustion emissions and can be used to characterize the time evolution of
its sources. It is correlated with a decrease in public health and
contributes to atmospheric warming. Black carbon measurements are usually
conducted with absorption filter photometers, which are prone to several
artifacts, including the filter-loading effect – a saturation of the
instrumental response due to the accumulation of the sample in the filter
matrix. In this paper, we investigate the hypothesis that this
filter-loading effect depends on the optical properties of particles present
in the filter matrix, especially on the black carbon particle coating. We
conducted field campaigns in contrasting environments to determine the
influence of source characteristics, particle age and coating on the
magnitude of the filter-loading effect. High-time-resolution measurements of
the filter-loading parameter in filter absorption photometers show daily and
seasonal variations of the effect. The variation is most pronounced in the
near-infrared region, where the black carbon mass concentration is
determined. During winter, the filter-loading parameter value increases with
the absorption Angstrom exponent. It is suggested that this effect
is related to the size of the black carbon particle core as the wood burning
(with higher values of the absorption Angstrom exponent) produces
soot particles with larger diameters. A reduction of the filter-loading
effect is correlated with the availability of the coating material. As the
coating of ambient aerosols is reduced or removed, the filter-loading
parameter increases. Coatings composed of ammonium sulfate and secondary
organics seem to be responsible for the variation of the loading effect. The
potential source contribution function analysis shows that high values of
the filter-loading parameter in the infrared are indicative of local
pollution, whereas low values of the filter-loading parameter result from
ageing and coating during long-range transport. Our results show that the
filter-loading parameter can be used as a proxy for determination of the
particle coating, thus allowing for differentiation between local/fresh and
transported/aged particles.
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