Published in:Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Berichte des Forschungszentrums Jülich 4146, VI, 149 S. (2004). = Wuppertal, Univ., Diss., 2004
Language:
English
Origination:
Footnote:
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Description:
The objective of the doctoral thesis presented here is to contribute to an improved understanding of the formation of ice clouds and their micro-physical characteristics. Homogeneous and heterogeneous freezing experiments were carried out with different aerosol types at temperatures between 238 and 185 K and cooling rates between −0.3 and −3.0Kmin$^{−1}$ in the aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere). Dynamic cloud processes were simulated in the AIDA by controlled decreasing of pressure and temperature. Homogeneous ice nucleation was examined by means o f freezing processes of fully dissolved sulphuric acid (SA) and ammonium sulphate (AS) droplets. Heterogeneous ice nucleation was triggered by pure soot particles (SOOT), soot particles coated with sulphuric acid or ammonium sulphate (SOOT+SA, SOOT+AS) and two mineral dust types (Arizona Test Dust, ATD and Sahara dust, SD). The sulphuric acid droplets nucleated ice at relative humidities with respect to ice (RH$_{ice,nuc}$) of 139 - 166% (236 - 196 K). This is in accordance with both previous results of AIDA experiments (Möhler et al., 2003) and literature data (Koop et al., 2000). The AS-aerosols generated ice crystals at relative humidities with respect to ice that were significantly below the homogeneous freezing threshold (115 - 136%). This may be explained by the presence of (micro-) crystalline ammonium sulphate and therefore heterogeneous effects. The number of ice crystals formed in the homogeneous freezing experiments increased with decreasing temperature or increasing cooling rate, independently of the starting concentration of aerosol particles. This result is in accordance with the parameterisation of Kärcher and Lohmann (2002a) and confirms that an additional insertion of homogeneously freezing aerosols has no important impact on the microphysics of ice clouds. For heterogeneous freezing processes with pure soot and mineral dust particles (238 - 190 K), RH$_{ice,nuc}$ is clearly below the homogeneous freezing ...