In this study, NPF events at three sites of differing characteristics (rural, urban, background, urban roadside) but in close proximity are studied. The different conditions in each area not only have an effect on the frequency of the events, but also affect their development. A general pattern is found in which the condensation sink increases with the degree of pollution of the site, but this is counteracted by increased particle growth rates at the more polluted location.
The frequency of NPF events is similar at the rural and urban background locations, with a high proportion of events occurring at both sites on the same day. The frequency of NPF events at the urban roadside site is slightly less, and higher particle growth rates must result from rapid gas to particle conversion of traffic-generated pollutants. The role of the urban environment leads to an increment of 20% in N16-20nm in the urban background compared to that of the rural area in NPF events occurring at both sites. The role of the origin of incoming air masses is also considered and an association of regional events with cleaner air masses is found. Due to lower availability of condensable species, NPF events that are associated with cleaner atmospheric conditions have lower growth rates of the newly formed particles. The decisive effect of the condensation sink in the development of NPF events and the survival probability of the newly formed particles is underlined. This influences the overall contribution of NPF events to the number of ultrafine particles in an area.