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

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

doi:10.5194/acp-17-4265-2017

Abstract

Abstract. With the present demand on fast and inexpensive aerosol source apportionment methods, the Aethalometer model was evaluated for a full seasonal cycle (June 2014–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 the AAE was elevated during winter (1.36 ± 0.07) compared to summer (1.12 ± 0.07). Quantified WB aerosol showed good agreement with levoglucosan concentrations, both in terms of correlation (R2 = 0.70) and in comparison to reference emission inventories. WB aerosol showed strong seasonal variation with high concentrations during winter (0.65 µg m−3, 56 % of total carbon) and low concentrations during summer (0.07 µg m−3, 6 % 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 µg m−3, 72 % of total carbon) and low during winter (0.13 µg m−3, 8 % of total carbon). Aethalometer model results were further compared to radiocarbon and levoglucosan source apportionment results. The comparison showed good agreement for 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. A performed sensitivity analysis suggests that this discrepancy can be explained by interference of non-light-absorbing biogenic carbon during winter. 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.

Highlights

Carbonaceous aerosol, i.e., the fraction of the aerosol containing carbon, contributes approximately 25 % to the mass of particulate matter with smaller diameter than 10 μm (PM10) in Europe (Fuzzi et al, 2015) and is presently estimated to be one of the most important climate forcers
By using Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) we find that the incoming air masses to Vavihill during the winter of this measurement campaign were influenced by approximately 45 % more precipitation than the average winter of 2000–2013 (p = 0.002, Fig. S5)
In this study we propose a small modification to the Aethalometer model which enables apportioned nonabsorbing carbon, here assumed to be biogenic carbon, to vary

Summary

Introduction

J. Martinsson et al.: Carbonaceous aerosol source apportionment. The magnitude of the carbonaceous aerosol impact on climate is still associated with significant uncertainty (IPCC, 2013). The carbonaceous aerosol originates mainly from three sources: wood burning, fossil fuel combustion and biogenic emissions. BC has a graphitic carbon structure and is known to efficiently absorb incoming solar radiation (Bond et al, 2013). This absorption leads to molecular vibration and rotation which causes emission of longwave radiation, heating the atmosphere. Organic aerosol (OA) is known to mainly scatter incoming sunlight, thereby cooling the climate. BrC is a much less effective light absorber than BC, deposition of BrC on bright surfaces such as snow or ice may cause significant changes in albedo (Doherty et al, 2010)

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Conclusion