Evaluation of equivalent black carbon source apportionment using observations from Switzerland between 2008 and 2018

Abstract

Black carbon (BC) or soot is a constituent of particulate matter (PM) which is relevant for negative human health and climate effects, and despite the lack of direct legal limits, it is recognised as an important atmospheric pollutant to monitor, understand, and control. Aethalometers are instruments which continuously monitor BC by measuring absorption at a number of distinct wavelengths. If collocated elemental carbon (EC) observations are used to transform these values into BC mass, by convention, the result is named equivalent black carbon (EBC). BC emitted by different combustion processes has different optical absorption characteristics, and this can be used to apportion EBC mass into traffic (EBCTR) and woodburning (EBCWB) components with a data processing technique known as the aethalometer model. The aethalometer model was applied to six EBC monitoring sites across Switzerland (using data between 2008 and 2018) and was evaluated by investigating diurnal cycles, model coefficients, and ambient temperature dependence of the two EBC components. For one monitoring site, San Vittore, the aethalometer model failed to produce plausible outputs. The reason for this failure was likely due to a high load of freshly emitted wood smoke during the winter which should be thought of as a third distinct emission source. After model evaluation, the trend analysis indicated that EBCTR concentrations at the remaining five locations significantly decreased between 2008 and 2018. EBCWB also demonstrated significant decreases in most monitoring locations but not at a monitoring site south of the Alps with a high PM load sourced from biomass burning. Ratios of EBC and particulate matter with a diameter of less than 2.5 µm (PM2.5) suggested that EBC contributes 6 %–14 % of the PM2.5 mass in Switzerland. The aethalometer model is a useful data analysis procedure but can fail under certain conditions; thus, careful evaluation is required to ensure the method is robust and suitable in other locations.