Organic source apportionment by NMR and GC/MS techniques at two Po Valley sites in the cold season during the SUPERSITO campaign in 2013

Understanding secondary organic aerosol (SOA) formation processes is a major challenge for the ambient aerosol science, due to the complexity of chemical matrices and the processes involved. Several analytical techniques have been explored for aerosol organic source apportionment. Proton-nuclear magnetic resonance (H-NMR) spectroscopy is an emerging one, suitable especially for water-soluble organic carbon (WSOC), and that can be used for organic characterization in three different approaches: a) analysis of molecular tracers, b) analysis of functional groups, c) factor analysis with spectral deconvolution. The latter offers a quantitative and simplified description of the thousands of individual species. NMR “factors” extracted by factor analysis typically correspond to a large group of WSOC constituents with similar chemical composition and time trend behaviour that are characteristic of different sources and/or atmospheric processes (Zhang et al. 2011). Two intensive field campaigns were conducted in the Po Valley, Italy, in the cold season during 2013, in February and October, in the framework of the Supersito project funded by Region Emilia-Romagna (http://www.arpa.emr.it/index.asp?idlivello=1459). Samples of fine aerosol particles (PM2.5 and PM1) were collected at an urban site (Bologna, BO) and a rural one (San Pietro Capofiume, SPC). ISAC-CNR in Bologna analysed PM1 by H-NMR spectroscopy, while Ferrara University analysed several polar organic tracers such as primary saccharides, sugar alcohols, anhydrosugars and carboxylic acids by GC/MS in PM2.5 (Pietrogrande et all. 2014). Multivariate factor analysis (positive matrix factorization, PMF) provided four H-NMR factors at both the urban and the rural sites. Dominance of biomass burning is evident at both sites. Specifically, primary organic aerosol from biomass burning (BB-POA) accounted for 30% of WSOC in BO, and 40% in SPC. Two biomass burning secondary organic aerosol factors (BB-SOA) represented 60% of WSOC in BO and are 50% in SPC. Only 10% of WSOC could be attributed to sources other than biomass burning, at both sites. The correlations between PMF H-NMR factors and the polar organic tracers measured by GC/MS were also evaluated. The molecular tracers included anhydrosugars, which are known biomass burning tracers, and carboxylic acids, which can be formed by traffic exhaust, photo oxidation of unsaturated carboxylic acids, oxidation of polyciclic aromatic hydrocarbons or phtalates, products of secondary photo-oxidation reactions, wood combustion. The results indicate that the anhydrosugars (levoglucosan and its isomers, galactosan and mannosan) showed good correlations with BB-POA factor at both sites. Glycolic, malic, phtalic, pimelic, 2-cheto glutaric, succinic, glutaric, adipic, azelaic acids, erythritol and arabinose showed instead high correlations with the BB-SOA factors. In conclusion, the comparison of datasets from distinct analytical methodology proved to be particularly useful to constrain the source apportionment of the polar fraction of organic particulate matter.