Better constraints on sources of carbonaceous aerosols using a combined 14C-macro tracer analysis in a European rural background site
The source contributions to carbonaceous PM2.5 aerosol were investigated\nat a European background site at the edge of the Po Valley, in Northern\nItaly, during the period January--December 2007. Carbonaceous aerosol\nwas described as the sum of 8 source components: primary (1) and\nsecondary (2) biomass burning organic carbon, biomass burning elemental\ncarbon (3), primary (4) and secondary (5) fossil organic carbon,\nfossil fuel burning elemental carbon (6), primary (7) and secondary\n(8) biogenic organic carbon. The mass concentration of each component\nwas quantified using a set of macro tracers (organic carbon OC, elemental\ncarbon EC, and levoglucosan), micro tracers (arabitol and mannitol),\nand 14C measurements. This was the first time that 14C measurements\ncovered a full annual cycle with daily resolution. This set of 6\ntracers, together with assumed uncertainty ranges of the ratios of\nOC-to-EC, and the reference fraction of modern carbon in the 8 source\ncategories, provides strong constraints to the source contributions\nto carbonaceous aerosol. The uncertainty of contributions was assessed\nwith a Quasi-Monte Carlo (QMC) method accounting for the variability\nof OC and EC emission factors, the uncertainty of reference fractions\nof modern carbon, and the measurement uncertainty.\n\nDuring winter, biomass burning composed 64 % (�<82>±15 %) of the total\ncarbon (TC) concentration, while in summer secondary biogenic OC\naccounted for 50 % (�<82>±16 %) of TC. The contribution of primary biogenic\naerosol particles was negligible during the entire year. Moreover,\naerosol associated with fossil sources represented 27 % (�<82>±16 %)\nand 41 % (�<82>±26 %) of TC in winter and summer, respectively. The contribution\nof secondary organic aerosol (SOA) to the organic mass (OM) was significant\nduring the entire year. SOA accounted for 30 % (�<82>±16 %) and 85 %\n(�<82>±12 %) of OM during winter and summer, respectively. While the\nsummer SOA was dominated by biogenic sources, winter SOA was mainly\ndue to biomass burning and fossil sources. This indicates that the\noxidation of semi-volatile and intermediate volatility organic compounds\nco-emitted with primary organics is a significant source of SOA,\nas suggested by recent model results and Aerosol Mass Spectrometer\nmeasurements. Comparison with previous global model simulations,\nindicates a strong underestimate of wintertime primary aerosol emissions\nin this region. The comparison of source apportionment results in\ndifferent urban and rural areas showed that the sampling site was\nmainly affected by local aerosol sources during winter and regional\nair masses from the nearby Po Valley in summer. This observation\nwas further confirmed by back-trajectory analysis applying the Potential\nSource Contribution Function method to identify potential source\nregions.