TY  - THES
AU  - Mangold, Alexandor
TI  - Untersuchungen zur Mikrophysik von Eiswolken: Simulationsexperimente in der Aerosolkammer AIDA
VL  - 4146
IS  - Juel-4146
SN  - 0944-2952
PB  - Univ. Wuppertal
VL  - Dr. (Univ.)
CY  - Jülich
M1  - PreJuSER-40727
M1  - Juel-4146
T2  - Berichte des Forschungszentrums Jülich
SP  - VI, 149 S.
PY  - 2004
N1  - Record converted from VDB: 12.11.2012
N1  - Wuppertal, Univ., Diss., 2004
AB  - 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 threshold. Mineral dust freezes at lower values of RHice (100 - 120%) than pure soot (111 - 134%). A sulphuric acid or ammonium sulphate coating of the soot particles raises the respective values close to the homogeneous freezing threshold (120 - 160% at 230 - 185K). For heterogeneous freezing experiments, no clear increase in the number of ice crystals can be observed with decreasing temperature. With increasing cooling rate, the number of ice crystals only increases for SOOT- and SD-particles. For ATD-particles, there is limited evidence that the starting concentration of the aerosol has an influence on the number of ice crystals formed. Therefore, heterogeneously freezing aerosol particles (especially mineral dust particles) may influence the microphysics of ice clouds and thus have the potential to influence the climate. This confirms results of modelling studies (Kärcher and Lohmann, 2003), which consider freezing processes of externally mixed homogeneous and heterogeneous aerosols. RH$_{ice,nuc}$ and the share of ice-forming particles of an aerosol are parameters for its potential impact on the climate. In conclusion, the aerosol types examined here can, according to these two parameters, be put in the following order of increasing freezing efficiency: SA (RH$_{ice}$ $\thickapprox$ 155%; Nice $\thickapprox$1.4%), SOOT+SA and SOOT+AS (both 145%; 1.7%), AS (130%; 10%), SOOT (120%; 16%), SD (110%; 37%) and ATD (110%; 70%). This means that the freezing efficiency increases across the scale of fully dissolved, homogeneously freezing aerosols to coated to pure, heterogeneously freezing aerosol particles.
LB  - PUB:(DE-HGF)11 ; PUB:(DE-HGF)3
UR  - https://juser.fz-juelich.de/record/40727
ER  -