Enhanced particle formation and growth due to mixing processes in the tropopause region

Enhancement of the binary homogeneous nucleation rate of H2SO4 and H2O and condensation of H2O and HNO3 on liquid sulphate aerosol particles due to mixing processes in the tropopause region were investigated by conducting theoretical studies and by using a section aerosol box model. If two air parcels with a large initial temperature and humidity difference are mixed with each other the nucleation rate within the resulting air parcel will be enhanced. This is due to the curvature of the saturation vapour pressure curve. The theoretical studies show that in the tropopause region enhancements of the nucleation rate of up to five orders of magnitude can occur. Since the mixing causes strong supersaturations also the condensation rate will be enhanced, especially for HNO3. The simulation with the sectional aerosol box model shows that after the mixing of two air parcels up to 40 wt% of HNO3 is taken up by the smallest aerosol particles. Since the initial air parcels are initialized with 0 wt% HNO3 this corresponds to a very strong enhancement. Measurements during the STREAM 1998 campaign show,, an entrainment of stratospheric air into the troposphere during the flight on 15 July leading to a production of new particles. Box model studies reproduce fairly well the measured number of newly formed particles. (C) 2003 Elsevier Science Ltd. All rights reserved.