Study of new particle formation events in southern Italy

New particle formation (NPF) events were studied in the urban background of Lecce (Italy) at the ECO Environmental-Climate Observatory, GAW/ACTRIS regional station. The objective of this study was to obtain long-term information on nucleation phenomenon in the Salentum peninsula, a coastal area of the central Mediterranean basin for which no extensive data exists. Particle number size distributions of the atmospheric aerosol were continuously collected by a Mobility Particle Size Spectrometer (MPSS) in the size range of 10-800 nm. For this investigation, data was analyzed for the period from January 2015 to July 2016 together with simultaneous measurements of gaseous pollutants (SO2, NO, NO2, and O-3) and meteorological parameters. Throughout the observation period, nucleation events have been identified in approximately 21% of days. The classification of the events showed a seasonal trend with higher frequencies in warm seasons with a maximum in September (38%) while the highest non-event days fraction was observed in cold seasons, similar to 90% in November and December. The fraction of undefined days peaked in August and September, 15 and 19% respectively. The analysis of reactive gas showed that NO, NO2, and O-3 did not play an evident role in the formation process, as observed by the comparison between event and non-event monthly median concentrations. Nucleation phe-nomenon seemed instead to be very sensitive to SO2 that was present with higher (about 40%) monthly median concentrations during event days compared to non-event days. The condensation sink (CS), calculated for the complete observational period, showed similar values comparing event and non-event days. It is noteworthy that all the events occurred during the central hours of the day, high pressure, dry air (RH<55%) and without significant cloud cover. The event days were characterized by winds originating predominately from the north and northwest directions, as shown by the analysis of the back-trajectories.