The formation of new atmospheric particles by gas–to–particle conversion leads to enhanced concentrations of nanoparticles. We have studied the formation and growth of new particles in urban Atlanta and in the remote atmosphere in locations ranging from the North Pole to Mauna Loa, Tasmania and the South Pole. Key to this work was our development of new measurement techniques for freshly formed nucleation mode particles between 3 and 10 nm. In this paper we show that measured aerosol size distributions in the 3–10 nm diameter range often increase with decreasing size down to our minimum detectable size of 3 nm, presumably because nucleation was occurring during the measurement. Furthermore, we show that the Atlanta nucleation mode size distributions are consistent with a collision–controlled nucleation process in which accommodation coefficients for all collisions between condensing molecules and molecular clusters and between molecular clusters are assumed to be equal to one, and in which evaporation from molecular clusters is neglected, as would be expected for a highly supersaturated vapour.