Characterization of ultrafine particles and the occurrence of new particle formation events in an urban and coastal site of the Mediterranean area

. In this work, new particle formation events (NPF) occurred at two locations in Southern Italy, the urban background site of Lecce (ECO station) and the coastal site of Lamezia Terme (LMT station), are identified and analysed. The study aims to compare the properties of NPF events at the two sites located 225 km away from each other and characterized by marked differences in terms of emission sources and local weather dynamics. Continuous measurements of 15 particle number size distributions, in the size range from 10 nm to 800 nm, were performed at both sites by a Mobility Particle Size Spectrometer (MPSS). The occurrence of NPF events, observed throughout the study period that lasted five years, produced different results in terms of frequency of occurrence, 25 % of the days at ECO and 9 % at LMT. NPF events showed seasonal patterns, higher frequency during spring and summer at the urban background site, while at the coastal site during the autumn-winter period. Some of these events happened simultaneously at both sites, indicating the occurrence of 20 the nucleation process on a large spatial scale (regional event). Cluster analysis of 72 h back-trajectories showed that during the NPF events the two stations were influenced by similar air masses, most of which originated from the North-Western directions. Local meteorological conditions characterized by high pressure, with a prevalence of clear skies, low level of relative humidity (RH < 52 %), and moderate winds (3-4 m s -1 ) dominated the NPF events at both sites. Notable differences were observed in SO 2 and PM 2.5 concentrations, resulting in ~65 % and ~80 % lower at LMT compared to ECO, 25 respectively. It is likely that the lower level of SO 2 , recognized as one of the main gas precursors involved in the nucleation process, can be responsible for the smaller NPF frequency of occurrence (~60 % less than ECO) observed in LMT.