Abstract. A measurement flight was performed by a motorglider equipped with an aerosol size spectrometer and nano-particle counter over a large area in the rural vicinity of Budapest, Hungary. The flight was carried out in the early afternoon under unstable air dynamical conditions in August 2010. These conditions allowed flying in glider mode, with the engine switched off, using thermal lifts for altitude gain. A significant part of the flight was spent in thermals that allow studying how the atmospheric dynamics acts on the particle formation. It was found that hygroscopic growth affects the particle size distribution in the 30–500 nm interval. In the 30–280 nm interval, the growth rate was found to be similar to that of ammonium sulphate particles. Indirect signs of cloud droplet formation were found during cloud cross, as a concentration drop in size bins fallen in the 280–400 nm size range. In three thermal lifts significantly higher ultrafine particle concentration (30 < d < 280 nm) was measured, with decreased average diameter. The results support the supposition that convective eddies in the boundary layer affect the aerosol composition via hygroscopic growth and secondary particle formation.