Sources of high PM2.5 concentrations in Milan, Northern Italy: molecular marker data and CMB modelling.

In Milan (MI), the largest city in Northern Italy, the annually average PM2.5 concentration is above 25 μg m(-3), the value that the EU established as a target for 2010, and the upper limit from 2015 onwards (2008/30/CE). Over a three-year period (2006-2009) PM concentrations and chemical compositions were measured in an urban site (MI), a rural site (OB) and a remote site (ASC) in Northern Italy. Chemical characterization (EC/OC, inorganic ions, elements, C20-C32 n-alkanes, C2-C5 mono and dicarboxylic acids, levoglucosan and PAHs) was carried out on PM2.5 samples from the three sites, and PM10 from MI. Molecular markers were used in Chemical Mass Balance (CMB) modelling to estimate the contributions of primary sources to OC, and then PM mass from each source was reconstructed in MI, OB and ASC for different seasons. Estimates of the traffic (TR) source contribution to PM2.5 mass ranged from 4.1 (± 2.0) μg m(-3) during the summer, to 13.3 (± 6.7) μg m(-3) during the winter in MI. TR was the main primary source for PM2.5 concentrations in MI (17-24%). Its contribution was lower at the OB site (7-9%) and at the remote ASC site (3-4%). TR is a local source, while biomass burning (BB) is a diffuse regional source in Northern Italy: during fall and winter, BB was 25-30% and 27-31% of PM2.5 at MI and OB respectively. Other primary sources accounted for a small amount of the PM2.5, i.e. natural gas combustion (0-1%), plant debris (0-4%), road dust (RD=0-4%; but 15% at ASC during winter and 10% of PM10 at MI during summer) and sea salt (0-1%). Secondary inorganic+organic aerosol constituted the major part of the PM2.5 mass during spring and summer (50-65%) at the three sites.