Concurrent observations of particle number densities and size distributions observed at two different heights (near ocean level and 475 m above sea level) in Ny-Alesund, Svalbard were studied with respect to the diurnal variation during a summer period in June 2004. The results show that observed variation in particle number density in the Arctic boundary layer may be strongly modulated by vertical mixing and dilution. The particles appeared to be formed in the early morning when solar intensity reached about 30% of the mid-day intensity. Based on differences in the observed number densities at the two heights it appears as if particles are formed in the lower part of the boundary layer. The formation rate of 10 nm diameter particles is estimated to be 0.11 cm(-3) s(-1) and the growth rate is in a range between 1 and 2 nm h(-1).