Atmospheric variability and binary homogeneous nucleation: A parametrisation and conditions required for a significant effect

The binary water–sulphuric acid nucleation has long been considered an important path for new particle formation in the atmosphere. The formation rate depends nonlinearly on temperature and vapour concentrations. We aim to find out if atmospheric small scale and mesoscale spatial and temporal variability can increase the nucleation rate significantly. Simultaneous and separate effects of variation in temperature, humidity and sulphuric acid concentration on the mean nucleation rate have been modelled. We have assumed that the distributions of variables are Gaussian and compare the mean of simulated nucleation rates with the nucleation rate in mean conditions without variation. According to our studies the atmospheric variability can increase the mean nucleation rate by orders of magnitude but the significant effect is limited to specific conditions. The effect seems to be more important in the lower troposphere than in the upper troposphere, where temperature and sulphuric acid concentrations are lower. In addition, our model simulations indicate that the atmospheric variability can only explain part of the difference between measured and theoretically required sulphuric acid concentrations in conditions where particle formation has been observed. We have parametrised the effect of atmospheric variability as a simple correction factor for the effect of temperature and humidity variation.