K. Diehl

Sensitivity Studies of the Importance of Dust Ice Nuclei for the Indirect Aerosol Effect on Stratiform Mixed-Phase Clouds

Abstract New parameterizations of contact freezing and immersion freezing in stratiform mixed-phase clouds (with temperatures between 0° and −35°C) for black carbon and mineral dust assumed to be composed of either kaolinite (simulation KAO) or montmorillonite (simulation MON) are introduced into the ECHAM4 general circulation model. The effectiveness of black carbon and dust as ice nuclei as a function of temperature is parameterized from a compilation of laboratory studies. This is the first time that freezing parameterizations take the chemical composition of ice nuclei into account. The rather subtle differences between these sensitivity simulations in the present-day climate have significant implications for the anthropogenic indirect aerosol effect. The decrease in net radiation in these sensitivity simulations at the top of the atmosphere varies from 1 ± 0.3 to 2.1 ± 0.1 W m−2 depending on whether dust is assumed to be composed of kaolinite or montmorillonite. In simulation KAO, black carbon has a ...

A laboratory study of the effects of a kerosene-burner exhaust on ice nucleation and the evaporation rate of ice crystals

Abstract Laboratory experiments are described during which the influence of gases and particles from the exhaust of a kerosene burner on microphysical processes were studied. In one experimental investigation the evaporation rates of ice crystals polluted with the kerosene-burner exhaust were compared with the evaporation rates of pure ice crystals. During another experimental investigation the ice nucleating ability of the exhaust particles was studied in terms of the efficiency of the exhaust particles to act as deposition and condensation freezing nuclei, as immersion freezing nuclei, and as contact nuclei. The results of our experiments showed that the evaporation rate of ice crystals polluted with the kerosene-burner exhaust was significantly reduced compared to the evaporation rate of pure ice crystals, implying an increased lifetime of aircraft contrails in comparison to a cloud of pure ice crystals. We also found that the kerosene-burner exhaust particles act as ice nuclei in all studied modes of ice formation at temperatures as high as −20°C, particulary freezing between 20 and 70% of the drops at temperatures warmer than −28°C in the immersion mode. Since the temperature at the level of the contrails is typically below −30°C our result allows the speculation that drop formation at the cruising altitude of air planes is immediately followed by ice crystal formation via heterogeneous nucleation.

A laboratory study of the uptake of HNO3 and HCl vapor by snow crystals and ice spheres at temperatures between 0 and −40°C

Abstract A laboratory experiment is described during which the uptake of HNO3 and HCl vapor by dendritic snow crystals and by single crystalline and polycrystalline small ice spheres was studied at ppbv and ppmv gas levels and at temperatures between 0 and −40°C. In one experimental investigation the vapor was allowed to be adsorbed onto the surface of the ice particles. During another experimental investigation the ice particles were allowed to grow from water vapor on fine fibers in the presence of the HNO3 and HCl vapor. The results of our experiments show that under both conditions significant amounts of HNO3 and HCl became scavenged by the ice particles. Scavenging by adsorption was maximum for both vapors at temperatures near 0°C where a quasi-liquid layer exists on the surface of ice. With decreasing temperature the uptake of HNO3 and HCl vapor decreased and kept on decreasing for HCl with further decrease in temperature. In contrast, the uptake of HNO3 reached a minimum near −18°C to increase again strongly with further decrease in temperature. The temperature-dependent ,uptake of both vapors were explained on the basis of surface melting caused by the vapors. For the case that the ice crystals were growing in an atmosphere supersaturated with respect to ice while simultaneously being exposed to HNO3 or HCl vapor we noted that gas scavenging was less than during simple adsorption by a nongrowing crystal. Our experiments further showed that HNO3 once taken up by an ice particle would not desorb if the ice particle remained at ice saturation. Our experiments also indicated that some of the adsorbed HNO3 and HCl diffuses into the ice particle.

Laboratory Studies and Numerical Simulations of Cloud Droplet Formation under Realistic Supersaturation Conditions

Abstract In this paper, a new device is introduced to study the formation and growth of cloud droplets under near-atmospheric supersaturations. The new device, called the Leipzig Aerosol Cloud Interaction Simulator (LACIS), is based on a laminar flow tube. It has been designed to reproduce the thermodynamic conditions of atmospheric clouds as realistically as possible. A series of experiments have been conducted that prove the definition and stability of the flow field inside the LACIS as well as the stability and reproducibility of the generated droplet size distributions as a function of the applied thermodynamic conditions. Measured droplet size distributions are in good agreement with those determined by a newly developed Eulerian particle–droplet dynamical model. Further investigations will focus on the influences of latent heat release during vapor condensation on the tube walls and the development of a more suitable optical particle counter for droplet size determination.

A Wind Tunnel Study on the Shape, Oscillation, and Internal Circulation of Large Raindrops with Sizes between 2.5 and 7.5 mm

Abstract Precipitation prediction using weather radars requires detailed knowledge of the shape parameters of raindrops falling at their terminal velocities in air. Because the raindrops undergo oscillation, the most important shape parameters from the radar prediction point of view are the equilibrium drop shape, the time-averaged axis ratio, and the oscillation frequency. These parameters for individual water drops with equivalent diameter from 2.5 to 7.5 mm were investigated in a vertical wind tunnel using high-speed video imaging. A very good agreement was found between the measured and the theoretically determined raindrop shape calculated by a force balance model. A new method was developed to determine the equivalent drop diameter with the help of the oscillation frequency. The drop size determination by means of the frequency method was found to be three times more precise than by volumetric methods. The time-averaged axis ratio was found to be equal to the equilibrium axis ratio in the investigat...

A laboratory study on the uptake of HCl, HNO3, and SO2 gas by ice crystals and the effect of these gases on the evaporation rate of the crystals

The results of our new and earlier laboratory studies on the uptake of gases by ice crystals are summarized in terms of (1) the equilibrium phase diagram for a system gas/H2O, (2) the effect of these gases on the evaporation rate of ice crystals, and (3) in terms of the uptake of the gases by water drops. It is shown that the intrinsic quasi-liquid layer significantly affects the uptake of a gas by an ice surface in that, depending on the gas phase concentration, the layer thickness may be considerably increased by depressing the equilibrium freezing point causing additional surface melting. It is further shown that the evaporation rate of ice particles previously exposed to a gas may become significantly reduced in comparison to that of pure ice particles. Finally, it is shown that under atmospheric conditions the direct gas uptake by ice crystals may be neglected in comparison to the uptake of gases by water drops. In atmospheric clouds gases are therefore most likely taken up by ice crystals via the process of riming.

A Wind Tunnel Study of Turbulence Effects on the Scavenging of Aerosol Particles by Water Drops

Abstract Laboratory experiments are described where the effects of turbulence on the impaction scavenging of aerosol particles by water drops were investigated. During the experiments the drops were freely suspended at their terminal velocities in the Mainz vertical wind tunnel. Turbulence in the tunnel airstream was produced by placing a needle obstruction upstream of the floating drop. The energy dissipation rates e were between 0.03 and 0.5 m2 s−3. The power spectrum covered a range of k values between 102 and 3 × 103 m−1, agreeing with atmospheric observations within this range. Collector drops of 346-μm, 1.68-mm, and 2.88-mm radius were exposed to indium acetylacetonate aerosol particles (having mean radii in the range of 0.16–0.24 μm) in the turbulent core for exposure times between 70 and 100 s. The collection efficiency for each drop size was determined by analyzing the accrued aerosol mass and compared with efficiencies for laminar conditions found in the literature. The results showed no enhance...

Riming of Graupel: Wind Tunnel Investigations of Collection Kernels and Growth Regimes

Abstract Laboratory experiments were carried out in the vertical wind tunnel in Mainz, Germany, to study the collision coalescence growth of single spherical ice particles having initial radii between 290 and 380 μm while they were freely floated in a laminar flow containing a cloud of supercooled droplets with radii between 10 and 20 μm. The experiments were performed in a temperature range between −8 and −12°C, where riming proceeds in the atmosphere, and with cloud liquid water contents lying between 0.9 and 1.6 g m−3 (i.e., values typically found in mixed-phase clouds). The collection kernels were calculated from the mass increase of the rimed ice particles and the average liquid water content during the experiments. Surface temperature measurements of growing graupel indicated that a dry growth regime prevailed during the whole set of growth experiments. The collection kernels of rimed ice particles attained values between 0.9 and 2.3 cm3 s−1 depending on their collector momenta (mass × fall velocity...

A laboratory and theoretical study on the uptake of sulfur dioxide gas by small water drops containing hydrogen peroxide under laminar and turbulent conditions

Abstract Laboratory experiments are described where the uptake of SO2 gas by water drops containing H2O2 is investigated where the taken up S(IV) is quickly converted to S(VI). During the gas uptake the drops were freely suspended at their terminal velocity by means of the Mainz vertical wind tunnel. Two series of experiments were carried out, one with a laminar air flow in the wind tunnel, one with a turbulent air flow in the wind tunnel. Afterwards, the experimental results were compared against model computations using the so-called fully mixed convective diffusion model. The experimental results for laminar flow conditions showed that the fully mixed convective diffusion model for the uptake of SO2 by water drops is not only applicable to large water drops of millimeter size, as demonstrated in our previous papers, but also to small water drops of some hundreds micron size. Our experimental results for turbulent flow conditions allow, within the experimental error, to conclude that turbulence in the air surrounding falling drops has no measurable effect on the uptake of SO2 by freely falling water drops.

A Laboratory and Theoretical Study on the Uptake of SO2 Gas by Large and Small Water Drops Containing Heavy Metal Ions

Laboratory experiments have been carried out to investigate the uptake of sulfur dioxide by water drops containing heavy metal ions where the metal ions serve as catalysts to oxidise the taken up S(IV) into S(VI). During the gas uptake the drops were freely suspended at their terminal velocity in the airstream of the Mainz vertical wind tunnel. Two series of experiments were carried out, one with large millimeter size water drops containing manganese or iron ions, and the other with small water drops containing manganese ions and having radii in hundreds of micron size range. The experimental results were compared against model computations using the Kronig–Brink model and the fully mixed model, modified for the case that heavy metal ions present in the liquid phase act as catalysts for the oxidising process. The results of the model calculations show that there are only small differences between the predicted gas uptake according to the two models. In addition it was found that the experimental obtained results from the uptake of SO2 by water drops containing heavy metal ions for both, large and small water drops did agree with the model results.