Results of the first balloon-borne profile measurements of particle number concentrations, together with fine-scale measurements of temperature and water vapor density under cloudless conditions are reported. The profiles showed regions near the inversion of a continental planetary boundary layer with high particle number concentrations of up to 600 cm-3 in the size range from 5 to 10 nm. These high concentrations of ultrafine particles correlated well with high fluctuations of the temperature and humidity caused by plumes penetrating the inversion. Such penetration caused intensive mixing, which in turn apparently significantly increased the nucleation rate. The observed particle number concentrations near the inversion layer were significantly higher than that in the mixed layer and at ground level. This corroborates the hypothesis that particles are formed near this inversion.