Numerical Simulations of Homogeneous Freezing Processes in the Aerosol Chamber AIDA

ATMOSPHERIC CHEMISTRY AND PHYSICS (Impact Factor: 5.51). 01/2002; 3(2003):195-210.
Source: DOAJ

ABSTRACT The homogeneous freezing of supercooled H2SO4/H2O aerosols in an aerosol chamber is investigated with a microphysical box model using the activity parameterization of the nucleation rate by Koop et al (2000). The simulations are constrained by measurements of pressure, temperature, total water mixing ratio, and the initial aerosol size distribution, described in a companion paper Möhler et al. (2002). Model results are compared to measurements conducted in the temperature range between 194 and 235 K, with cooling rates in the range between 0.5 and 2.6 K min-1, and at air pressures between 170 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 relative humidities, aerosol chemical composition at the onset of freezing, and the number of nucleated ice crystals. The latter three parameters can directly be inferred from the experiments, the former three aid in interpreting the measurements. Sensitivity studies are carried out to address the relative importance of uncertainties of basic quantities such as temperature, H2O mixing ratio, aerosol size spectrum, and deposition coefficient of H2O molecules on ice. The ability of the numerical simulations to provide detailed explanations 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 type-II polar stratospheric clouds, provided that accurate temperature and humidity measurements are available.

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    ABSTRACT: PAZI is a national research project supported by the German Secretary of Education and Research (BMBF) through the Helmholtz-Gesellschaft Deutscher Forschungszentren (HGF). Research in PAZI is performed in concert with the projects SiA, INCA, PartEmis, and PARTS funded by the European Commission. PAZI investigates the interaction of aerosols with cirrus clouds, with an emphasis on aviation -produced aerosols and contrails, and their impact on atmospheric composition, radiation, clouds, and climate. This overview summarizes important results obtained during the first phase and highlights the following issues. Measurements and models addressing the formation and evolution of bla ck carbon (BC) particles in burners and jet engines; physico-chemical characterization of aircraft-produced BC particles; measured freezing properties of liquid and BC particles; calculated global atmospheric distribution of BC from various sources; observed differences in cirrus properties between clean and polluted air masses; correlations between air traffic and cirrus cloud cover deduced from satellite observations; process
    Sausen, R.; Fichter, C.; Amanatidis, G.: Proceedings of the European Conference on Aviation, Atmosphere and Climate (AAC). Air pollution research report 83, European Commission, 197-206 (2004).
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    ABSTRACT: We apply a cloud-resolving model with explicit aerosol and ice microphysics and Lagrangian ice particle tracking to simulate the evolution of a cirrus cloud field observed during the US Atmospheric Radiation Measurement Program Intensive Operational Period in March 2000. This comprehensive data set includes remote sensing, radiosonde, and aircraft measurements of a midlatitude cirrus cloud system, supported by estimates of the dynamical cloud forcing. The dataset allows us to evaluate and study in great detail the process-oriented representation of the microphysical processes relevant to the formation and evolution of deep, stratiform cirrus (in particular ice crystal sedimentation and aggregation). The suite of explicitly resolved physical processes in our model enables us to better understand the sensitivity of the simulated cirrus properties on a large number ofmicrophysical and environmental parameters. The evolution of the domain-integrated cloud optical depth is largely dominated by homogeneous freezing processes.Wefind that the evolution of the observed cirrus cloud system is most dependent on updraught speed and ice supersaturation and that homogeneous freezing leads to a total, cloud-averaged ice crystal concentration of 0.1 cm−3 of air. It is not necessary to invoke heterogeneous ice nuclei to explain most of the data, but we cannot rule out that a small concentration (up to 0.002 cm−3) of such particles may have affected the cirrus cloud field in nature. Cloud-averaged ice particle size distributions are bimodal, separating two distinct growth regimes in the developed cloud. The small mode (ice particle sizes below a few 100μm) forms by homogeneous freezing of supercooled aerosol droplets and grows by deposition of water molecules from the gas phase. The large mode (sizes up to several 1000μm) forms and grows by aggregation. We demonstrate that the formation of the largest crystals by aggregation in deep cirrus is controlled in part by the nucleation of new ice crystals in dynamically active, highly supersaturated upper cloud regions. Furthermore, a pronounced increase in the number of aggregation events is predicted in sublimation zones. The combined effect of sublimation and sedimentation leads to the formation of a very thin (vertical extension ∼ 100 m) sublimation microlayer mainly composed of aggregated ice crystals, containing relatively high total ice crystal number concentrations (∼ 0.02 cm−3) comparable to those generated locally by homogeneous freezing in the upper cloud layers.
    Quarterly Journal of the Royal Meteorological Society 01/2011; 137(2011):374-393. · 3.33 Impact Factor
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    ABSTRACT: The low-temperature aerosol and cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe was used to investigate the effect of sulfuric acid coating on the ice nucleation efficiency of soot aerosol particles from a spark discharge generator. The uncoated (sulfuric acid-coated) soot aerosol showed a nearly lognormal size distribution with number concentrations of 300-5000 cm-3 (2500-56,000 cm-3), count median diameters of 70-140 nm (90-200 nm), and geometric standard deviation of 1.3-1.4 (1.5-1.6). The volume fraction of the sulfuric acid coating to the total aerosol volume concentration ranged from 21 to 81%. Ice activation was investigated in dynamic expansion experiments simulating cloud cooling rates between about -0.6 and -3.5 K min-1. At temperatures between 186 and ~235 K, uncoated soot particles acted as deposition nuclei at very low ice saturation ratios between 1.1 and 1.3. Above 235 K, ice nucleation only occurred after approaching liquid saturation. Coating with sulfuric acid significantly increased the ice nucleation thresholds of soot aerosol to saturation ratios increasing from ~1.3 at 230 K to ~1.5 at 185 K. This immersion mode of freezing nucleates ice well below the thresholds for homogeneous freezing of pure sulfuric acid solution droplets measured in previous AIDA experiments. A case study indicated that in contrast to the homogeneous freezing the nucleation rate of the immersion freezing mechanism depends only weakly on relative humidity and thereby the solute concentration. These results show that it is important to know the mixing state of soot and sulfuric acid aerosol particles in order to properly assess their role in cirrus formation.
    Journal of Geophysical Research 01/2005; 110. · 3.17 Impact Factor

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