Seasonal Trends of Natural and Anthropogenic Sources Contributions to PM2.5 and PM10 in Southern Italy

A one-year (July 2013-July 2014) dataset of PM2.5 and PM10 was collected at the Environmental-Climate Observatory (regional station of the Global Atmosphere watch - GAW-WMO), recently built in an urban background area in Lecce (SE Italy, 40°20'8''N-18°07'28''E, 37 m asl) within the I-AMICA project (PON R&C 2007-2013). Roughly, one sample every three days was chemically analysed for a total of 226 simultaneous samples (113 for each size fraction). Elemental and organic carbon were determined via thermo-optical method (Sunset OC/EC analyser, NIOSH5040 protocol), major ions Cl-, NO3-, SO42-, C2O42-, Ca2+, Na+, K+, Mg2+ via IC and 23 metals via ICP-MS (Li, Al, Ti, V, Mn, Fe, Co, Cu, Zn, As, Se, Rb, Sr, Nb, Cd, Sb, Ba, La, Ce, Nd, Dy, Pb, Th). The dataset was analysed using mass closure stoichiometric calculations for sea-spray, secondary inorganic aerosol (SIA) and crustal matter and using Positive Matrix factorization model (PMF5) to investigate the seasonal trends of eight particle sources (sea-spray, nitrate, sulphate, biomass burning, crustal, crustal carbonates, traffic, and industrial). Several cases of sea-spray events were observed with an average contribution of 16% to the coarse fraction (PM10-2.5) and 3% to PM2.5. Larger contributions were observed in autumn and winter and in high winds periods. Sea-spray interacted with nitric acid with a consequent chloride depletion, 60% on average for both PM2.5 and PM10. The Cl- depletion was significantly larger at high temperature during spring and summer with a trend opposite to that of secondary nitrate that was lower during spring and summer due to its thermal instability. Secondary nitrate had larger concentration in the coarse fraction at high temperature, instead at lower temperature the fine fraction of nitrate dominated. Organic matter was evaluated as OM=1.6xOC and represented 31% (PM10) and 43% (PM2.5), EC represented 2.7% (PM10) and 3.1% (PM2.5). Carbonaceous species were higher during autumn and winter with OC well correlated with K+ supporting the relevant contribution of biomass burning found with PMF5. Secondary organic carbon, evaluated with the minimum OC/EC ratio, was entirely segregated in PM2.5 accounting for 80% of total OC. Two crustal contributions were found, one characterised by metal oxides representing long-range transport of dust including Saharan dust advection, and the other characterised by crustal carbonates (mainly calcium carbonates) compatible with the local soil composition (limestone). Secondary sulphate was mainly ammonium sulphate/bisulphate entirely segregated in PM2.5, however, during intense cases of Saharan dust advection, a coarse component of CaSO4 was observed.