Aerosol properties were studied during an intensive airborne measurement campaign that took place at Rotterdam in Netherlands in May 2008 within the framework of the European Aerosol Cloud Climate and Air Quality Interactions project (EUCAARI). The objective of this study is to illustrate seven events of new particle formation (NPF) observed with two Condensation Particle Counters (CPCs) operated on board the ATR-42 research aircraft in airsectors around Rotterdam, and to provide information on the spatial extent of the new particle formation phenomenon based on 1-s resolution measurements of ultra-fine particle (in the size range 3-10 nm diameter, denoted N3-10 hereafter) concentrations. The results show that particle production occurred under the influence of different air mass origins, at different day times and over the North Sea as well as over the continent. The number concentration of freshly nucleated particles (N3-10) varied between 5000 and 100 000 cm−3 within the boundary layer (BL). Furthermore the vertical extension for all nucleation events observed on the ATR-42 never exceeded the upper limit of the BL. The horizontal extent of N3-10 could not be delimited due to inflexible flight plans which could not be modified to accommodate real-time results. However, the NPF events were observed over geographically large areas; typically the horizontal extension was about 100 km and larger.