Charge induced stability of water droplets in subsaturated environment

Atmospheric liquid and solid water particles are stabilized if they are coated with either negative or positive electric charge. The surface charge causes an increase of the partial pressure of water vapour close to the surface of each particle, effectively allowing the particles to remain in their condensed phase even if the environmental relative humidity drops below unity. The theory, briefly presented in this paper, predicts a zero parameter relation between surface charge density and water vapour pressure. This relation was tested in a series of Electrodynamic Balance experiments. The measurements were performed by stabilizing charged droplets of pure water near an ice-surface. We observed a divergence in radius as the temperature approached the freezing point from below. We find that the measurements confirm the theory within the experimental uncertainty. In some cases this generally overlooked effect may have impact on cloud processes and on results produced by Electrodynamic Balance experiments

Experimental quantification of contact freezing in an electrodynamic balance

Heterogeneous nucleation of ice in a supercooled water droplet induced by external contact with a dry aerosol particle has long been known to be more effective than freezing induced by the same nucleus immersed in the droplet. However, the experimental quantification of contact freezing is challenging. Here we report an experimental method to determine the temperature-dependent ice nucleation probability of size-selected aerosol particles. The method is based on the suspension of supercooled charged water droplets in a laminar flow of air containing aerosol particles as contact freezing nuclei. The rate of droplet–particle collisions is calculated numerically with account for Coulomb attraction, drag force and induced dipole interaction between charged droplet and aerosol particles. The calculation is verified by direct counting of aerosol particles collected by a levitated droplet. By repeating the experiment on individual droplets for a sufficient number of times, we are able to reproduce the statistical freezing behavior of a large ensemble of supercooled droplets and measure the average rate of freezing events. The freezing rate is equal to the product of the droplet–particle collision rate and the probability of freezing on a single contact, the latter being a function of temperature, size and composition of the contact ice nuclei. Based on these observations, we show that for the types of particles investigated so far, contact freezing is the dominating freezing mechanism on the timescale of our experiment

Experimental quantification of contact freezing in an electrodynamic balance

Heterogeneous nucleation of ice in a supercooled water droplet induced by external contact with a dry aerosol particle has long been known to be more effective than freezing induced by the same nucleus immersed in the droplet. However, the experimental quantification of contact freezing is challenging. Here we report an experimental method to determine the temperature-dependent ice nucleation probability of size-selected aerosol particles. The method is based on the suspension of supercooled charged water droplets in a laminar flow of air containing aerosol particles as contact freezing nuclei. The rate of droplet–particle collisions is calculated numerically with account for Coulomb attraction, drag force and induced dipole interaction between charged droplet and aerosol particles. The calculation is verified by direct counting of aerosol particles collected by a levitated droplet. By repeating the experiment on individual droplets for a sufficient number of times, we are able to reproduce the statistical freezing behavior of a large ensemble of supercooled droplets and measure the average rate of freezing events. The freezing rate is equal to the product of the droplet–particle collision rate and the probability of freezing on a single contact, the latter being a function of temperature, size and composition of the contact ice nuclei. Based on these observations, we show that for the types of particles investigated so far, contact freezing is the dominating freezing mechanism on the timescale of our experiment

Resurgence in Ice Nuclei Measurement Research

The accuracy and precision of current ice nuclei (IN) instruments is reported and progress made toward addressing apparent shortfalls in past ice nuclei measurements is studied. Most new IN instruments can measure at controlled conditions below water saturation and, in some cases, well below -40°C. The UF FRIDGE and the Tel Aviv University (TAU) FRIDGE have showed much lower concentrations of ice nuclei than any of the CFDC instruments. A postanalysis of the measurements has revealed that the petroleum jelly used to increase thermal contact between the collection filter and the cooling stage became mobile at the low pressure operational conditions and condensed on the ice nuclei, leading to a deactivation effect. The longer evaporation section of the ZINC instrument permits extension of measurements to a higher water supersaturation bound before water droplets begin to survive through to the optical detector