Aerosol nucleation inside and outside of cirrus

Recent observations have revealed elevated concentrations of ultrafine particles inside of cirrus clouds and in their vicinity. The formation mechanisms and the importance of these particles need yet to be elucidated. We have developed a Lagrangian aerosol model describing processes of neutral and charged water/sulfuric acid aerosol and their interaction with cirrus ice particles. The thermodynamic data for sulfuric acid uptake and loss by small neutral and charged clusters are based on laboratory measurements, allowing for a reliable description of both neutral and charged nucleation down to the very low temperatures occurring in the upper troposphere and lower stratosphere. The model implements a first order scheme for resolving the aerosol size distribution within the model bins which efficiently suppresses numerical diffusion. We operate the model offline on trajectories generated with a detailed 2D cirrus model which describes freezing, water uptake and loss, and vertical transport of cirrus ice particles. We discuss nucleation of sulfate aerosol particles from the gas phase on trajectories that lead to cirrus formation, and on trajectories which do not.