A Wind Tunnel Experimental Study of the Wet Deposition of Atmospheric Pollutants

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 particl…

New particle-dependent parameterizations of heterogeneous freezing processes: sensitivity studies of convective clouds with an air parcel model

Abstract. Based on the outcome of laboratory results, new particle-dependent parameterizations of heterogeneous freezing were derived and used to improve and extend a two-dimensional spectral microphysics scheme. They include (1) a particle-type-dependent parameterization of immersion freezing using the numbers of active sites per mass, (2) a particle-type and size-resolved parameterization of contact freezing, and (3) a particle-type-dependent description of deposition freezing. The modified microphysical scheme was embedded in an adiabatic air parcel model with entrainment. Sensitivity studies were performed to simulate convective situations and to investigate the impact of ice nuclei con…

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 …

Drop Shapes and Axis Ratio Distributions: Comparison between 2D Video Disdrometer and Wind-Tunnel Measurements

Abstract Comparisons of drop shapes between measurements made using 2D video disdrometer (2DVD) and wind-tunnel experiments are presented. Comparisons are made in terms of the mean drop shapes and the axis ratio distributions. Very close agreement of the mean shapes is seen between the two sets of measurements; the same applies to the mean axis ratio versus drop diameter. Also, in both sets of measurements, an increase in the oscillation amplitudes with increasing drop diameter is observed. In the case of the 2DVD, a small increase in the skewness was also detected. Given that the two sets of measurements were conducted in very different conditions, the agreement between the two sets of dat…

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 u…

Homogeneous freezing of single sulfuric and nitric acid solution drops levitated in an acoustic trap

Abstract The freezing temperatures of single supercooled drops of binary and ternary sulfuric and nitric acid solutions were measured while varying the acid concentration. An acoustic levitator was used which allows to freely suspend single solution drops in air without electrical charges thereby avoiding any electrical influences which may affect the freezing process. The drops of typically 500 µm in radius were monitored by a video camera during cooling cycles down to − 85 °C to simulate the upper tropospheric and stratospheric temperature range. The present data confirm that liquid solution droplets can be supercooled far below the equilibrium melting point by approximately 35 °C. They f…

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 becom…

On the scavenging of SO2 by large and small rain drops: V. A wind tunnel and theoretical study of the desorption of SO2 from water drops containing S(IV)

An experimental and theoretical study has been carried out to investigate the rate of desorption of SO2 from water drops falling at terminal velocity in air. The experiments were carried out in the Mainz vertical wind tunnel in which water drops of various sizes containing S(IV) in various concentrations were freely suspended in the vertical airstream of the tunnel. The results of these experiments were compared with the predictions of three theoretical models, and with the experiments of Walceket al. This comparison shows that the predictions of the diffusion model of Kronig and Brink in the formulation given by Walcek and Pruppacher agree well with the experimental results for all relevan…

Laboratory Studies of Scattering Properties of Polluted Cloud Droplets: Implications for FSSP Measurements

Abstract Laboratory experiments were conducted in the Mainz vertical wind tunnel to study the effects of pollutants dissolved or suspended in cloud droplets on the droplet size measurements of a Forward Scattering Spectrometer Probe (FSSP). The FSSP is a widely used instrument to derive microphysical properties of atmospheric clouds. Individual droplets of different well-defined sizes were freely falling at their terminal velocities in the wind tunnel while the intensity of radiation emitted by the He–Ne laser of the FSSP and scattered by the droplets was measured. For this purpose, the FSSP was adapted and mounted to the wind tunnel. The intensity of radiation scattered by the droplets in …

Shapes and oscillations of raindrops with reduced surface tensions: Measurements at the Mainz vertical wind tunnel

Abstract Important characteristics of raindrops pertinent to fields in atmospheric sciences such as weather radar or pollution scavenging are drop shape, oscillation frequency and amplitude, as well as the internal circulation. Atmospheric raindrops are never pure water drops but contain additional components like aerosol particles and dissolved species. Surface active substances, when present, reduce the surface tension of raindrops and, thus, increase the drop deformation which in turn affects breakup and coalescence, pollutant scavenging, and, finally, the formation of precipitation. Experiments were performed at the Mainz vertical wind tunnel with raindrops freely suspended at their ter…

A Wind Tunnel and Theoretical Study of the Melting Behavior of Atmospheric Ice Particles. IV: Experiment and Theory for Snow Flakes

Abstract An experiment in the Mainz vertical Cloud Tunnel is described in which natural and laboratory-made aggregates of snow crystals (snow flakes) were melted under free fall conditions in the vertical air stream of the tunnel, which was allowed to warm up at the rates experienced by falling snow flakes in the atmosphere. The variation of the fall mode, the fall velocity, and the percentage of ice melted, as a function of percentage of distance travelled for 99% melting was recorded by cinematography. The laboratory results were confirmed by the results of a theoretical heat transfer model which we developed for the melting of a snow flake. In this model a snow flake was idealized by an …

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 p…

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 ma…

A Wind Tunnel Study of the Effects of Turbulence on the Growth of Cloud Drops by Collision and Coalescence

A set of wind tunnel experiments was carried out to investigate the growth of single drops by collision coalescence with small droplets in laminar and turbulent flow. Analysis of the experiments shows that under otherwise similar conditions, there exists a tendency toward a faster drop growth under turbulence. The observed growth under laminar conditions agrees well with computed continuous growth of a collector drop using collision efficiencies reported in the literature.

The ice nucleating ability of pollen

Abstract Laboratory experiments are described where the water uptake by a variety of pollen was studied quantitatively, followed by the investigation of the ice nucleating ability of four kinds of pollen in the deposition and the condensation freezing modes. The diameters of the pollen selected for the freezing experiments were between 25 and 70 μm. The freezing experiments in the deposition mode including also pollen resuspended from decayed leaves, and crushed pollen grains were carried out at different temperatures down to −33 °C combined with various supersaturations with respect to ice up to 35%. The condensation freezing experiments were carried out at temperatures down to −18 °C at s…

The Effect of Turbulence on the Accretional Growth of Graupel

Abstract Wind tunnel experiments were carried out to investigate the influence of turbulence on the collection kernel of graupel. The collection kernel defines the growth rate of a graupel accreting supercooled droplets as it falls through a cloud. The ambient conditions were similar to those occurring typically in the mixed-phase zone of convective clouds, that is, at temperatures between −7° and −16°C and with liquid water contents from 0.5 to 1.3 g m−3. Tethered spherical collectors with radii between 220 and 340 μm were exposed in a flow carrying supercooled droplets with a mean volume radius of 10 μm. The vertical root-mean-square fluctuation velocity, the dissipation rate, and the Tay…

Comparative study on immersion freezing utilizing single-droplet levitation methods

Immersion freezing experiments were performed utilizing two distinct single-droplet levitation methods. In the Mainz vertical wind tunnel, supercooled droplets of 700 µm diameter were freely floated in a vertical airstream at constant temperatures ranging from −5 to −30 ∘C, where heterogeneous freezing takes place. These investigations under isothermal conditions allow the application of the stochastic approach to analyze and interpret the results in terms of the freezing or nucleation rate. In the Mainz acoustic levitator, 2 mm diameter drops were levitated while their temperature was continuously cooling from +20 to −28 ∘C by adapting to the ambient temperature. Therefore, in this case th…

Collision efficiencies empirically determined from laboratory investigations of collisional growth of small raindrops in a laminar flow field

In laboratory experiments at the vertical wind tunnel of the University of Mainz the collisional growth of drops with radii between 70 and 170 μm in radius were observed while the collector drop freely floated in a cloud of droplets with radii ranging from 1 to 7 μm. Previously existing tables with collision efficiency values were interpolated and completed in such a way that drop growth rates calculated with these collision efficiencies match with observed growth rates. These new tables provide collision efficiency values for a wide range of drop sizes and radius ratios p including those ranges where efficiency values missed so far. This is of high importance for small p-ratios where the c…

The ice nucleating ability of pollen:

Abstract Laboratory tests were conducted of the ice nucleating ability of four kinds of pollen in the immersion and the contact freezing modes. The diameters of the selected pollen were between 25 and 70 μm. The experiments were carried out at the Mainz vertical wind tunnel with freely suspended supercooled droplets at temperatures down to −28 °C. The immersion freezing experiments were conducted with drops of radii between 250 and 375 μm formed from distilled water with a defined amount of pollen added. The drops were freely floated in the wind tunnel while being supercooled. For the contact freezing experiments, a short burst of pollen was allowed to collide with freely suspended, superco…

Particle surface area dependence of mineral dust in immersion freezing mode: investigations with freely suspended drops in an acoustic levitator and a vertical wind tunnel

Abstract. The heterogeneous freezing temperatures of supercooled drops were measured using an acoustic levitator. This technique allows one to freely suspend single drops in the air without any wall contact. Heterogeneous nucleation by two types of illite (illite IMt1 and illite NX) and a montmorillonite sample was investigated in the immersion mode. Drops of 1 mm in radius were monitored by a video camera while cooled down to −28 °C to simulate freezing within the tropospheric temperature range. The surface temperature of the drops was contact-free, determined with an infrared thermometer; the onset of freezing was indicated by a sudden increase of the drop surface temperature. For compari…

A wind tunnel study of the effects of collision processes on the shape and oscillation for moderate-size raindrops

Abstract Drop–drop collision experiments were carried out at the Mainz vertical wind tunnel. Water drops of 2.5 mm diameter were freely floated at their terminal velocities in a vertical air stream and collided with 0.5 mm diameter droplets. The collisions were recorded with a high speed digital video camera at a frame rate of 1000 per second. Altogether 116 collision events were observed, 75 of which ended with coalescence, and the rest with filament type breakup. The coalescence efficiency and its dependence on the Weber number and on the eccentricity of the colliding drops showed good agreement with earlier numerical studies. Thirty-six recorded collisions were further analyzed in order …