Thermodynamic properties and nucleation processes of upper tropospheric and lower stratospheric aerosol particles

Atmospheric aerosol particles are subject to thermodynamic and kinetic processes through interaction with their environment. Such processes involve, among others, composition changes due to uptake of gas phase molecules and liquid-to-solid phase transitions induced by crystal nucleation. An atmospheric region of particular interest is the upper troposphere/lower stratosphere (UT/LS), where aerosol particles are involved in cloud formation and heterogeneous reactions, which are important for ozone chemistry, radiation, and the dehydration of air crossing the tropopause. In this work, laboratory experiments, employing optical microscopy combined with Raman spectroscopy, have been performed to study aerosol processes relevant to the UT /LS. The dissociation reaction of the bisulfate ion, HSOi ~ SO~-+ H+, is investigated in aqueous H2S04 solutions with concentrations of 0.54-15.23 mol kg-1 in the temperature range of 180-326 K using Raman spectroscopy. All investigated H2S04 solutions show a continuous increase in the degree of dissociation of HSOi with decreasing temperature, in contrast to predictions from thermodynamic models of aqueous H2S04 solutions. A Pitzer ion interaction model is used to derive a thermodynamically consistent formulation of the thermodynamic dissociation constant of the bisulfate ion, Kn(T), that is in agreement with the experimental data. The new formulation of Kn(T) is valid from 180 K to 473 K. Calculations with the ion interaction model reveal considerable differences in ion activity coefficients, water activities, water vapor pressure, and HCI solubilities, when compared to existing thermodynamic models of H2S04/H20 solutions, in particular at temperatures typical for the UT /LS. Cirrus ice cloud formation is studied by measuring upper limits of the homogeneous and pseudoheterogeneous ice nucleation rate coefficients in aqueous (NH4hS04 droplets. At temperatures of about 215 K, the experiments reveal values of these rate coefficients that are smaller than '"106 cm-3 s-1 and ",3.2.102 cm-2 s-l, respectively. These values are smaller than the nucleation rate coefficients obtained by AFT-IR (Aerosol Flowtube-Infrared) spectroscopy studies, but are in agreement with emulsion studies and other optical microscope experiments. The reasons for the discrepancies between the different data sets cannot be resolved but are suspected to have their origin in the evaluation procedures of the experimental data. Polar Stratospheric Cloud formation is studied by investigating, both theoretically and experimentally, the homogeneous nucleation and pseudo-heterogeneous nucleation of NAD and NAT in HN03/H20 and HN03/H2S04/H20 solution droplets. For polar stratospheric conditions, the upper limits of the homogeneous nucleation rate coefficients of NAD and NAT in liquid aerosols derived from the experiments are 2.10-5 cm-3 s-1 and 8.10-2 cm-3 s-l, respectively. The upper limits of the pseudo-heterogeneous nucleation rate coefficients of NAD and NAT are 1.5. 10-6 cm-2 S-1 and 9.10-4 cm-2 s-1, respectively. These experimentally derived nucleation rate coefficients are lower by up to 8 orders of magnitude than values in recently published nucleation parameterizations used in denitrification studies. From the nucleation rate coefficients presented here, maximum hourly production rates of nitric acid hydrate particles at stratospheric conditions are calculated which yield about 3.10-10 cm-3 (air) h-1 and 6.10-6 cm-3 (air) h-1 for homogeneous and pseudo-heterogenous nucleation, respectively. These production rates are too low to explain the number densities of large nitric acid containing particles recently observed in the Arctic stratosphere and, therefore, are insufficient to account for the subsequent