1 Development of a versatile source apportionment analysis based on positive matrix factorization: a case study of the seasonal variation of organic aerosol sources in Estonia. Athanasia Vlachou 1 , Anna Tobler 1 , Houssni Lamkaddam 1 , Francesco Canonaco 1 , Kaspar R. 5 Daellenbach 1, † , Jean-Luc Jaffrezo 2 , María Cruz Minguillόn 3 , Marek Maasikmets 4 , Erik Teinemaa 4 , Urs Baltensperger 1 , Imad El Haddad 1 and André S.H. Prévôt 1 1 Department of General Energy Research, Paul Scherrer Institute, Villigen PSI, CH-5232, Switzerland 2 Université Grenoble Alpes, CNRS, IRD, G-INP, IGE, 38000 Grenoble, France 10 3 Institute of Environmental Assessment and Water Research (IDAEA), CSIC, 08034 Barcelona, Spain 4 Estonian Environmental Research Centre, 10617, Tallinn, Estonia † Now at: Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, P. O. Box 64, 00014, Helsinki, Finland 15 Correspondence to: André S. H. Prévôt (andre.prevot@psi.ch) and Imad El Haddad (imad.el-haddad@psi.ch) Abstract. Bootstrap analysis is commonly used to capture the uncertainties of a bilinear receptor model such as the positive matrix factorisation (PMF) model. This approach can estimate the factor related uncertainties and partially assess the rotational ambiguity of the model. The selection of the environmentally plausible solutions though can be challenging and a systematic approach to identify and sort the factors is needed. For this, 20 comparison of the factors between each bootstrap run and the initial PMF output, as well as with externally determined markers, is crucial. As a result, certain solutions that exhibit sub-optimal factor separation should be discarded. The retained solutions would then be used to test the robustness of the PMF output. Meanwhile, analysis of filter sample with the Aerodyne aerosol mass spectrometer and the application of PMF and bootstrap analysis on the bulk water soluble organic aerosol mass spectra has provided insight into the source 25 identification and their uncertainties. Here, we investigated a full yearly cycle of the sources of organic aerosol (OA) at three sites in Estonia, Tallinn (urban), Tartu (suburban) and Kohtla-Järve (KJ, industrial). We identified six OA sources and an inorganic dust factor. The primary OA types included biomass burning, dominant in winter in Tartu accounting for 73%±21% of the total OA, primary biological OA which was abundant in Tartu and Tallinn in spring (21%±8% and 11%±5%, respectively) and two other primary OA types lower in mass. A 30 sulphur containing OA was related to road dust and tire abrasion which exhibited a rather stable yearly cycle and an oil OA was connected to the oil shale industries in KJ prevailing at this site comprising 36%±14% of the total OA in spring. The secondary OA sources were separated based on their seasonal behaviour: a winter oxygenated OA dominated in winter (36%±14% for KJ, 25%±9% for Tallinn and 13%±5% for Tartu) and was correlated with benzoic and phthalic acid implying an anthropogenic origin. A summer oxygenated OA was the 35 main source of OA in summer at all sites (26%±5% in KJ, 41%±7% in Tallinn and 35%±7% in Tartu) and exhibited high correlations with oxidation products of a-pinene like pinic acid and 3-methyl-1, 2, 3- butanetricarboxylic acid (MBTCA) suggesting a biogenic origin. 1. Introduction Particulate matter of aerodynamic diameter smaller than 10 μm (PM 10 ) has been extensively explored in many 40 sites around the globe due to their various adverse effects upon human health and climate. In Europe, several monitoring networks have been measuring PM 10 for long time periods and an increasing trend in concentrations from north to south was noticed (Fuzzi et al., 2015, Putaud et al., 2010). Despite of this, some North European Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1099 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 7 November 2018 c  Author(s) 2018. CC BY 4.0 License.