Assessing the wintertime contribution of biomass smoke to organic aerosol at 15 sites in Switzerland by analysing filter samples using aerosol mass spectrometry

Burning biomass and other human activities lead to the emission of particle matter (PM) consisting of ions, elemental (EC) and organic carbon (OA). In winter, high proportions of OA are related to biomass burning in the alpine region as well as in urban regions as Grenoble and Zurich (Lanz et al. 2010; Richard et al. 2011). Besides traffic and cooking, biomass burning contributes even in the megacity Paris considerable amounts of OA (Crippa et al. 2013). Due to the carcinogenic potential of biogenic smoke, it is crucial to examine its contribution in different regions in order to allow an effective mitigation process. The Aerosol mass spectrometer (AMS, Aerodyne) provides measurements of OA for which positive matrix factorization (PMF) is able to separate the proportion of biomass burning (BBOA) from other primary sources such as traffic (hydrocarbon-like OA, HOA) , or from secondary oxygenated OA (OOA), formed in-situ in the atmosphere via the oxidation of volatile organic compound precursors (e.g. Lanz et al. 2010). While the information accessible through analysis of AMS mass spectra is highly useful, widespread or long-term AMS data collection is greatly restricted by the high instrument cost and complex maintenance. On the other hand, the Aerosol Chemical Speciation Monitor (ACSM) is designed for low costs and maintenance, but it also operates only at unit mass resolution, preventing the assessment of oxidation state. In order to overcome these limitations and to assess the contribution of BBOA compared to other sources, we explored the application of laboratory AMS measurements on aerosol filter samples. Such samples are relatively easy and inexpensive to collect and store, and are already routinely collected at many air quality stations over the world. The approach consists of water extraction of the particulate material from quartz filters and subsequent atomization of the resulting solutions into the AMS. The extraction efficiency is estimated as ~80% and the mass spectra obtained by this methodology are very similar to the corresponding on-line measurements and that for different settings (e.g. different sites and different seasons). We present here the first application of this technique to filter samples collected during 2 consecutive winters (2008 and 2009) at 15 stations in Switzerland with different exposure characteristics (including a complete yearly cycle for one of the stations). Data are analysed by PMF and combined with other measurements, including organic and elemental carbon (OC/EC), ions, levoglucosan (marker for biomass burning), and radiocarbon content (14C), to provide an improved estimation of the biomass smoke contribution to OA (Figure 1). BBOA contribution and emission profiles at different stations will be discussed and related to the prevailing topographical, meteorological and combustion conditions. This work was supported by the Swiss Federal office for the Environment and the Swiss National Science Foundation. Lanz, V. et al. (2010) Atmos. Chem. Phys., 10, 10453-10471. Richard, A. et al. (2011) Atmos. Chem. Phys., 11, 8945-8963. Crippa, M. et al. (2013) Atmos. Chem. Phys., 13, 961 981.