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Journal article

Better constraints on sources of carbonaceous aerosols using a combined 14C-macro tracer analysis in a European rural background site

Gilardoni S, Vignati E, Cavalli F, Putaud J, Larsen B, Karl M, Stenstr??m K, Genberg J, Henne S, Dentener F ...see all

Atmospheric Chemistry and Physics, vol. 11, issue 12 (2011) pp. 5685-5700

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Abstract

The source contributions to carbonaceous PM2.5 aerosol were investigated
at a European background site at the edge of the Po Valley, in Northern
Italy, during the period January--December 2007. Carbonaceous aerosol
was described as the sum of 8 source components: primary (1) and
secondary (2) biomass burning organic carbon, biomass burning elemental
carbon (3), primary (4) and secondary (5) fossil organic carbon,
fossil fuel burning elemental carbon (6), primary (7) and secondary
(8) biogenic organic carbon. The mass concentration of each component
was quantified using a set of macro tracers (organic carbon OC, elemental
carbon EC, and levoglucosan), micro tracers (arabitol and mannitol),
and 14C measurements. This was the first time that 14C measurements
covered a full annual cycle with daily resolution. This set of 6
tracers, together with assumed uncertainty ranges of the ratios of
OC-to-EC, and the reference fraction of modern carbon in the 8 source
categories, provides strong constraints to the source contributions
to carbonaceous aerosol. The uncertainty of contributions was assessed
with a Quasi-Monte Carlo (QMC) method accounting for the variability
of OC and EC emission factors, the uncertainty of reference fractions
of modern carbon, and the measurement uncertainty.

During winter, biomass burning composed 64 % (�±15 %) of the total
carbon (TC) concentration, while in summer secondary biogenic OC
accounted for 50 % (�±16 %) of TC. The contribution of primary biogenic
aerosol particles was negligible during the entire year. Moreover,
aerosol associated with fossil sources represented 27 % (�±16 %)
and 41 % (�±26 %) of TC in winter and summer, respectively. The contribution
of secondary organic aerosol (SOA) to the organic mass (OM) was significant
during the entire year. SOA accounted for 30 % (�±16 %) and 85 %
(�±12 %) of OM during winter and summer, respectively. While the
summer SOA was dominated by biogenic sources, winter SOA was mainly
due to biomass burning and fossil sources. This indicates that the
oxidation of semi-volatile and intermediate volatility organic compounds
co-emitted with primary organics is a significant source of SOA,
as suggested by recent model results and Aerosol Mass Spectrometer
measurements. Comparison with previous global model simulations,
indicates a strong underestimate of wintertime primary aerosol emissions
in this region. The comparison of source apportionment results in
different urban and rural areas showed that the sampling site was
mainly affected by local aerosol sources during winter and regional
air masses from the nearby Po Valley in summer. This observation
was further confirmed by back-trajectory analysis applying the Potential
Source Contribution Function method to identify potential source
regions.

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