Advanced source apportionment of carbonaceous aerosols by coupling offline AMS and radiocarbon size-segregated measurements over a nearly 2-year period
Authors: Vlachou Athanasia , Daellenbach Kaspar R. , Bozzetti Carlo , Chazeau Benjamin , Salazar Gary A. , Szidat Soenke , Jaffrezo Jean-Luc , Hueglin Christoph , Baltensperger Urs , Haddad Imad El , Prévôt André S. H. 
Published: May 2018

Abstract. Carbonaceous aerosols are related to adverse human health effects. Therefore,
identification of their sources and analysis of their chemical composition is
important. The offline AMS (aerosol mass spectrometer) technique offers quantitative separation of organic
aerosol (OA) factors which can be related to major OA sources, either primary
or secondary. While primary OA can be more clearly separated into sources,
secondary (SOA) source apportionment is more challenging because different
sources – anthropogenic or natural, fossil or non-fossil – can yield
similar highly oxygenated mass spectra. Radiocarbon measurements provide
unequivocal separation between fossil and non-fossil sources of carbon. Here
we coupled these two offline methods and analysed the OA and organic carbon
(OC) of different size fractions (particulate matter below 10 and
2.5 µm – PM10 and PM2.5, respectively) from the Alpine
valley of Magadino (Switzerland) during the years 2013 and 2014
(219 samples). The combination of the techniques gave further insight into
the characteristics of secondary OC (SOC) which was rather based on the type
of SOC precursor and not on the volatility or the oxidation state of OC, as
typically considered. Out of the primary sources separated in this study,
biomass burning OC was the dominant one in winter, with average
concentrations of 5.36 ± 2.64 µg m−3 for PM10 and
3.83 ± 1.81 µg m−3 for PM2.5, indicating that wood
combustion particles were predominantly generated in the fine mode. The
additional information from the size-segregated measurements revealed a
primary sulfur-containing factor, mainly fossil, detected in the coarse size
fraction and related to non-exhaust traffic emissions with a yearly average
PM10 (PM2.5) concentration of
0.20 ± 0.24 µg m−3
(0.05 ± 0.04 µg m−3). A primary biological OC (PBOC) was also
detected in the coarse mode peaking in spring and summer with a yearly average
PM10 (PM2.5) concentration of
0.79 ± 0.31 µg m−3
(0.24 ± 0.20 µg m−3). The secondary OC was separated
into two oxygenated, non-fossil OC factors which were identified based on
their seasonal variability (i.e. summer and winter oxygenated organic carbon, OOC) and a third anthropogenic OOC factor which correlated
with fossil OC mainly peaking in winter and spring, contributing on average 13 % ± 7 %
(10 % ± 9 %) to the total OC in PM10 (PM2.5). The winter OOC was also connected
to anthropogenic sources, contributing on average
13 % ± 13 % (6 % ± 6 %) to the total OC in PM10 (PM2.5). The
summer OOC (SOOC), stemming from
oxidation of biogenic emissions, was more pronounced in the fine mode, contributing on average 43 % ± 12 %
(75 % ± 44 %) to the total OC in PM10 (PM2.5). In total the non-fossil OC
significantly dominated the fossil OC throughout all seasons, by contributing
on average 75 % ± 24 % to the total OC. The results also
suggested that during the cold period the prevailing source was residential
biomass burning while during the warm period primary biological sources and
secondary organic aerosol from the oxidation of biogenic emissions became
important. However, SOC was also formed by aged fossil fuel combustion
emissions not only in summer but also during the rest of the year.