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

Luka Drinovec,Emily Anne Bruns,Jean-Eudes Petit,Rajan K Chakrabarty,Robert Wolf,Peter Zotter,Agnes Filep,Griša Močnik,Hans Moosmüller,Asta Gregorič,André S H Prévôt,Ian J Arnold,Olivier Favez,Jean Sciare

doi:10.5194/amt-10-1043-2017

Abstract

Abstract. 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 Ångström 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 Ångström 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.

Highlights

Black carbon is a primary aerosol tracer for high-temperature combustion emissions and can be used to characterize the time evolution of its sources (Lack et al, 2014)
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
We have tested the hypothesis that the filter-loading effect present in filter-based photometers used for BC determination depends on the optical properties of particles present in the filter matrix, especially on coatings of black carbon particles

Summary

Introduction

Black carbon is a primary aerosol tracer for high-temperature combustion emissions and can be used to characterize the time evolution of its sources (Lack et al, 2014). Investigations performed on ambient black carbon (BC) mass equivalent concentrations have shown a stronger correlation with adverse public health effects than the ones observed for the total mass of particulate matter (Janssen et al, 2011, 2012; Grahame et al, 2014; Olstrup et al, 2016). Black carbon absorbs sunlight very efficiently and heats the atmosphere with the top-of-the-atmosphere forcing exceeding 1 W m−2, and even though its lifetime is short compared to CO2, it is still the second-most-important warming agent (Bond et al, 2013). While health effects are of interest in the context of local air pollution, climate change is investigated on regional and global scales, and co-benefits from black carbon emission reductions are of great interest (Smith et al, 2009). Measurements are conducted at locations suitable for all three different scales, with absorption filter photometers being the most common instrumentation employed to determine ambient black carbon mass concentrations

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Results

Conclusion