Case Study of Particle Number Fluxes and Size Distributions during Nucleation Events in Southeastern Italy in the Summer

Concentrations, size distributions and particle number vertical turbulent fluxes were measured by the eddy-covariance method at an urban background site in southeastern Italy during the summer. CO2/H2O concentrations and fluxes were also determined together with meteorological parameters. Time series show that particles could be divided into two size classes with negatively-correlated temporal trends in diurnal hours: nanoparticles (diameter D-p < 50 nm) and larger particles (D-p > 50 nm). Larger particles include part of the Aitken mode and the accumulation mode. Nanoparticles peaked in diurnal hours due to the presence of several days with nucleation events when particles D-p > 50 nm were at minimum concentrations. Nucleation increased diurnal total particle concentration by a factor of 2.5, reducing mean and median diameters from D-mean = 62.3 +/- 1.2 nm and D-median = 29.1 +/- 1.3 nm on non-event days to D-mean = 35.4 +/- 0.6 nm and D-median = 15.5 +/- 0.3 nm on event days. During nucleation events, particle deposition increased markedly (i.e., downward fluxes), but no significant changes in CO2 concentrations and fluxes were observed. This is compatible with new particle formation above the measurement height and a consequent net transport towards the surface. Correlation with meteorology shows that the formation of new particles is correlated with solar radiation and favored at high wind velocity.