Case studies of new particle formation and evaporation processes in the western Mediterranean regional background

Case studies of new particle formation (NPF), subsequent growth and possible particle shrinkage occurring in the western Mediterranean regional background are presented in this work. Owing to the mid-altitude height of the station (720 m.a.s.l.), aerosol processes were highly influenced by mixing layer height and development, and mountain breezes. Nucleation processes were observed to occur both under cold and warm conditions, when solar radiation and sulphuric acid concentrations were sufficiently high. Intense bursts of NPF were recorded when the station resided above the polluted mixing layer with little influence of upslope transport of pollution. NPF and growth was also observed to occur within polluted air masses residing over the site, owing to the probable abundance of anthropogenic and natural organic vapours promoting rapid particle growth after nucleation. Measurable growth rates for the NPF episodes ranged from 1.3 to 6.9 nm h−1. Reductions in modal diameters, indicating particle shrinkage, were also observed and were attributed to the evaporation of semi-volatile species from the particulate phase to the gas phase. Particle shrinkage was observed both for pre-existing particles and for freshly formed particles. In the latter case, an “arch” formation was observed in the size distribution contour plot, as the nucleating particles grew and subsequently evaporated until the mode disappeared completely. In agreement with the few articles on particle shrinkage published to date, evaporation appears to be favoured under warm temperatures, high solar radiation, low relative humidity and atmospheric dilution, causing particle-to-gas transformations.