Numerical simulations of homogeneous freezing processes in the aerosol chamber AIDA

[摘要] The homogeneous freezing of supercooledH₂SO₄/H₂O aerosols in an aerosol chamber is investigated with amicrophysical box model using the activity parameterization of the nucleation rate byKoop et al. (2000). The simulations are constrained by measurements of pressure, temperature, total water mixingratio, and the initial aerosol size distribution, described in a companion paperMöhler et al. (2003). Model results are compared to measurements conducted in the temperature range between194 and 235 K, with cooling rates in the range between 0.5 and 2.6 K min^-1, and at air pressures between170 and 1000 hPa. The simulations focus on the time history of relative humidity with respect to ice, aerosol size distribution,partitioning of water between gas and particle phase, onset times of freezing, freezing threshold relativehumidities, aerosol chemical composition at the onset of freezing, and the number ofnucleated ice crystals. The latter four parameters can be inferred from the experiments, the former three aid in interpreting themeasurements. Sensitivity studies are carried out to address the relative importance of uncertainties of basic quantities such astemperature, total H₂Omixing ratio, aerosol size spectrum, and deposition coefficient of H₂Omolecules on ice. The ability of the numerical simulations to provide detailedexplanations of the observations greatly increases confidence in attempts to model this process under real atmospheric conditions,for instance with regard to the formation of cirrus clouds or polar stratospheric ice clouds, provided that accurate temperatureand humidity measurements are available.

[学科分类] 大气科学