Aerosol fine fraction in the Venice Lagoon: particle composition and sources.

The work presents a characterisation of aerosol sampled during three campaigns conducted in the Venice Lagoon from 30 June to 21 July 2004, from 15 February to 10 March 2005 and from 8 May to 25 May 2006. The results yield information about the physical-chemical characteristics of fine aerosol, the possible sources and its fate. Sulphate (SO42-), nitrate (NO3-) and ammonium (NH4+) are identified as the main water soluble 4 3 components. The sum of these ions in the spring campaign 2006 varies from 51% to nearly 100% of PM2.5 fraction aerosol. NH4+ is found to be significantly correlated to non-sea-salt sulphate (nSS-SO42-) and NO3-, thus indicating the prevalent presence of ammonium nitrate and sulphate. The overall lack of a clear diurnal and seasonal cycle of sulphate suggests a transport from the Po Valley, while the diurnal and seasonal profile of nitrate concentrations suggests a prevalence of local generation. Sulphates from sea water through oxidation of dimethylsulfide (DMS) are not negligible (about 10% of the anthropogenic sulphate). The marine aerosol contribution to PM1 and PM2.5 fractions, calculated using Na+ as a tracer of sea-salts. is low (range 1-6%). In some cases chlorine depletion is observed. The content of chlorine in the aerosol particle is mostly higher than expected for marine environments (considered on the basis of Cl-/Na+ ratio). Therefore, specific sources for the element have been suggested. Oxalic acid anion, which accounts for 55% of the organic compounds examined, presents mean values 180 ng m(-3) and 161 ng m(-3) in the winter and spring campaigns, comparable with reported values in low polluted urban areas. In the winter campaign there is a high correlation between formate and acetate (R-2 = 0.93). suggesting that a common source makes a high contribution to the measured concentrations.