New aerosol production has been observed at about 50 days per year at Hyytiala in Finland during the past few years. Selected events are analysed on the hypothesis that they are initiated by a vapour produced by solar radiation on a precursor. The vapour is removed by condensation on existing aerosol, and it is shown that the vapour concentration is determined from the ratio of radiation intensity to removal rate. Values of the ratio found for initiation and cut-off of nucleation are consistent, supporting the hypothesis that nucleation bursts occur when the vapour concentration exceeds a certain value. Comparisons are made to days when bursts did not occur and the variability of the data is examined. Precursor concentrations obviously vary strongly, but it is found that both the radiation intensity and its ratio to removal rate must exceed certain minimum values for bursts to be likely. (C) 2001 Elsevier Science Ltd. All rights reserved.