Cloud condensation nuclei formation from ultrafine aerosols

Atmospheric cloud condensation nuclei sources are a key uncertainty in the assessment of the effect of anthropogenic aerosols on clouds and climate. The ability of new ultrafine particles to grow to become CCN varies throughout the atmosphere and must be understood in order to understand the CCN budget. We have developed the Probability of Ultrafine aerosol Growth (PUG) model to answer questions regarding which processes compete to determine whether an ultrafine particle survives long enough to grow to CCN size, how the growth probability varies between different parts of the atmosphere and how uncertainties with respect to the magnitude and size distribution of ultrafine emissions translates into uncertainty in CCN generation. It was found in most cases that condensation is the dominant growth mechanism and coagulation with larger particles is the dominant sink mechanism for ultrafine particles. The probability of a new ultrafine particle generating a CCN varies from <0.1% to >90% in different parts of the atmosphere, though in the boundary layer a large fraction of the ultrafine particles have an probability between 1% and 60%. Some regions of the atmosphere, such as the tropical free troposphere, are areas with high growth efficiencies. However, variability within regions makes it difficult to determine which regions of the atmosphere are most efficient for generating CCN from ultrafine particles without more detailed simulation. It was found using bulk quantities to describe the amount of ultrafine aerosol, such as total mass, surface area, or number, cannot predict accurately and robustly the number of CCN generated without additional information about the ultrafine size distribution. An uncertainty of a factor of two in the diameter of new ultrafine aerosol may lead to uncertainties in the number of CCN generated as high as a factor of ten for a given mass of new ultrafine aerosol in certain atmospheric conditions.