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. Carbonaceous aerosols are related to adverse human health effects. Therefore,identification of their sources and analysis of their chemical composition isimportant. The offline AMS (aerosol mass spectrometer) technique offers quantitative separation of organicaerosol (OA) factors which can be related to major OA sources, either primaryor secondary. While primary OA can be more clearly separated into sources,secondary (SOA) source apportionment is more challenging because differentsources – anthropogenic or natural, fossil or non-fossil – can yieldsimilar highly oxygenated mass spectra. Radiocarbon measurements provideunequivocal separation between fossil and non-fossil sources of carbon. Herewe coupled these two offline methods and analysed the OA and organic carbon(OC) of different size fractions (particulate matter below 10 and2.5  µ m – PM 10 and PM 2.5 , respectively) from the Alpinevalley of Magadino (Switzerland) during the years 2013 and 2014(219 samples). The combination of the techniques gave further insight intothe characteristics of secondary OC (SOC) which was rather based on the typeof SOC precursor and not on the volatility or the oxidation state of OC, astypically considered. Out of the primary sources separated in this study,biomass burning OC was the dominant one in winter, with averageconcentrations of 5.36  ±  2.64  µ g m −3 for PM 10 and3.83  ±  1.81  µ g m −3 for PM 2.5 , indicating that woodcombustion particles were predominantly generated in the fine mode. Theadditional information from the size-segregated measurements revealed aprimary sulfur-containing factor, mainly fossil, detected in the coarse sizefraction and related to non-exhaust traffic emissions with a yearly averagePM 10 (PM 2.5) concentration of0.20  ±  0.24  µ g m −3 (0.05  ±  0.04  µ g m −3) . A primary biological OC (PBOC) was alsodetected in the coarse mode peaking in spring and summer with a yearly averagePM 10 (PM 2.5) concentration of0.79  ±  0.31  µ g m −3 (0.24  ±  0.20  µ g m −3) . The secondary OC was separatedinto two oxygenated, non-fossil OC factors which were identified based ontheir seasonal variability (i.e. summer and winter oxygenated organic carbon, OOC) and a third anthropogenic OOC factor which correlatedwith fossil OC mainly peaking in winter and spring, contributing on average 13 %  ±  7 %(10 %  ±  9 %) to the total OC in PM 10 (PM 2.5) . The winter OOC was also connectedto anthropogenic sources, contributing on average13 %  ±  13 % (6 %  ±  6 %) to the total OC in PM 10 (PM 2.5) . Thesummer OOC (SOOC), stemming fromoxidation of biogenic emissions, was more pronounced in the fine mode, contributing on average 43 %  ±  12 %(75 %  ±  44 %) to the total OC in PM 10 (PM 2.5) . In total the non-fossil OCsignificantly dominated the fossil OC throughout all seasons, by contributingon average 75 %  ±  24 % to the total OC. The results alsosuggested that during the cold period the prevailing source was residentialbiomass burning while during the warm period primary biological sources andsecondary organic aerosol from the oxidation of biogenic emissions becameimportant. However, SOC was also formed by aged fossil fuel combustionemissions not only in summer but also during the rest of the year.

atmospheric chemistry and physics

Advanced source apportionment of carbonaceous aerosols by coupling offline AMS and radiocarbon size-segregated measurements over a nearly 2-year period