Carbonaceous aerosol source apportionment using the aethalometer model – Evaluation by radiocarbon and levoglucosan analysis at a rural background site in southern Sweden

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Martinsson, Johan ; Abdul Azeem, Hafiz ; Sporre, Moa K. ; Bergström, Robert ; Ahlberg, Erik ; Öström, Emilie ; Kristensson, Adam ; Swietlicki, Erik ; Eriksson Stenström, Kristina (2016)

With the present demand on fast and inexpensive aerosol source apportionment methods, the aethalometer model was evaluated for a full seasonal cycle (June 2014&ndash;June 2015) at a rural atmospheric measurement station in southern Sweden by using radiocarbon and levoglucosan measurements. By utilizing differences in absorption of UV and IR, the aethalometer model apportions carbon mass into wood burning (WB) and fossil fuel combustion (FF) aerosol. In this study, a small modification in the model in conjunction with carbon measurements from thermal-optical analysis allowed apportioned non-light absorbing biogenic aerosol to vary in time. The absorption differences between WB and FF can be quantified by the absorption Ångström exponent (AAE). In this study AAEWB was set to 1.81 and AAEFF to 1.0. Our observations show that AAE was elevated during winter (1.36&thinsp;±&thinsp;0.07) compared to summer (1.12&thinsp;±&thinsp;0.07). Quantified WB aerosol showed good agreement with levoglucosan concentrations, both in terms of correlation (<i>R</i><sup>2</sup>&thinsp;=&thinsp;0.70) and in comparison to reference emission inventories. WB aerosol showed strong seasonal variation with high concentrations during winter (0.65&thinsp;µg&thinsp;m<sup>&minus;3</sup>, 56&thinsp;% of total carbon) and low concentrations during summer (0.07&thinsp;µg&thinsp;m<sup>&minus;3</sup>, 6&thinsp;% of total carbon). FF aerosol showed less seasonal dependence, however black carbon (BC) FF showed clear diurnal patterns corresponding to traffic rush hour peaks. The presumed non-light absorbing biogenic carbonaceous aerosol concentration was high during summer (1.04&thinsp;µg&thinsp;m<sup>&minus;3</sup>, 72&thinsp;% of total carbon) and low during winter (0.13&thinsp;µg&thinsp;m<sup>&minus;3</sup>, 8&thinsp;% of total carbon). Aethalometer model results were further compared to radiocarbon and levoglucosan source apportionment results. The comparison displayed good agreement in apportioned mass of WB and biogenic carbonaceous aerosol but discrepancies were found for FF aerosol mass. The aethalometer model overestimated FF aerosol mass by a factor of 1.3 compared to radiocarbon and levoglucosan source apportionment. This discrepancy may possibly be explained by a relatively low <i>R</i><sup>2</sup> value in the fit of FF aerosol light absorption to carbon mass concentration. In summary, the aethalometer model offers a cost-effective, yet robust high-time resolution source apportionment at rural background stations compared to a radiocarbon and levoglucosan alternative.
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