Surface-active organic molecules are common constituents of atmospheric aerosol particles, raindrops, and snowflakes. If these compounds are present as surface films, transfer of gases into the atmospheric water systems could be impeded, evaporation could be slowed, and the aqueous chemical reactions could be influenced. To investigate these possibilities, we have reviewed the chemical literature pertaining to organic films on aqueous surfaces: their composition, structure, properties, and effects. We then review the surface-active organic compounds in atmospheric water. We report the results of new measurements of surface tension of aqueous solutions of common atmospheric organic compounds (β-pinene, n-hexanol, eugenol, and anethole) and demonstrate that the compounds produce films with properties similar to those of the more well known surfactants. We conclude that organic films are probably common on atmospheric aerosol particles and that they may occur under certain circumstances on fog droplets, cloud droplets, and snowflakes. If present, they will increase the lifetimes of aerosol particles, fog droplets, and cloud droplets, both by inhibiting water vapor evaporation and by reducing the efficiency with which these atmospheric components are scavenged. The presence of the films will not cause a significant reduction of solar radiation within the aqueous solution. It appears likely, however, that the transport of gaseous molecules into and out of the aqueous solution will be impeded by factors of several hundred or more when organic films are present. Since incorporated gas molecules provide much of the oxidizing potential of atmospheric water droplets, the organic films will play a major role in droplet chemistry by strongly inhibiting solution oxidation.

Organic films on atmospheric aerosol particles, fog droplets, cloud droplets, raindrops, and snowflakes

Laboratory studies have shown that thunderstorm charging caused by the interactions of ice crystals and graupel pellets is affected in sign and magnitude by temperature and cloud liquid water content; the presence of water droplets is a requirement for substantial charge transfer. Relationships showing the dependence of charge transfer on ice crystal size and velocity have previously been reported and now, in a continuation of the laboratory studies, the effect of liquid water content on the charge transfer has been investigated. The experiments show that positive graupel charging occurs at temperatures above a “charge sign reversal temperature” with negative charging at lower temperatures. The reversal temperature moves to lower temperatures when the liquid water content is increased. However, at low values of liquid water content, the sign of the graupel charging is inverted being positive at low temperatures and negative at higher temperatures. A discussion is presented of the various charge transfer theories. The results are consistent with the idea of two competing mechanisms whose relative success depends on the temperature and liquid water content. Positive graupel charging occurs when the graupel surface grows from the vapor and the crystals interact with a negative surface charge caused by a temperature gradient across the rime ice surface layer. Negative graupel charging occurs when the surface growth effect is swamped by freezing droplets which create either a pseudo contact potential with which the crystals interact, or a positive surface charge, due to dislocations in the rime ice, which is removed during glancing crystal interactions. Relationships between charge transfer, liquid water content, temperature, ice crystal size, and velocity have been determined and the equations may be used in numerical models of the development of thunderstorm electric fields. A one-dimensional model indicates that the charge separation rates noted here are adequate to account for thunderstorm electrification. Use of the equations with cloud parameter values obtained in a thunderstorm research flight, leads to a predicted charge reversal level around −13°C, which is in agreement with the analysis of the electric field in the thunderstorm studied.

The effect of liquid water on thunderstorm charging

Current theories of snow metamorphism indicate that sublimating snow crystals have rounded shapes, while growing crystals have shapes that depend on growth rates. At slow growth rates, crystals are rounded. At moderate rates, they have flat faces with rounded edges. At fast growth rates, crystals have flat faces with sharp edges, and they have hollow faces at very fast growth rates. The main growth/sublimation mechanism is thought to be by the homogeneous nucleation of new layers at or near crystal edges. It was also suggested that the equilibrium shape of snow crystals would be temperature dependent: rounded above −10.5°C, and faceted below. To test these paradigms, we have performed SEM investigations of snow samples having undergone metamorphism under natural conditions, and of snow samples subjected to isothermal metamorphism at −4° and −15°C in the laboratory. In general, current theories predicting crystal shapes as a function of growth rates, and of whether crystals are growing or sublimating, are verified. However, the transition in equilibrium shapes from rounded to faceted at −10.5°C is not observed in our isothermal experiments that reveal a predominance of rounded shapes after more than a month of metamorphism at −4 and −15°C. Some small crystals with flat faces that also have sharp angles at −15°C, are observed in our isothermal experiments. These faces are newly formed, and contradict current theory. Several hypotheses are proposed to explain their occurrence. One is that they are due to sublimation at emerging dislocations. Microsc. Res. Tech. 62:33–48, 2003. © 2003 Wiley-Liss, Inc.

Snow metamorphism as revealed by scanning electron microscopy.

Laboratory measurements show that the friction of rubber sliding on ice can be drastically reduced by ionic contaminants, such as common salt. Interface observations help to explain this in terms of the properties of ice and salt. The results are relevant to the grip of road tyres on icy roads.

http://iopscience.iop.org/article/10.1088/0022-3727/16/3/011/pdf

Friction of rubber on ice in the presence of salt

Small water ice crystals are the main component of cold tropospheric clouds such as cirrus. Because these clouds cover large areas of our planet, their role in the radiation budget of incoming and outgoing radiation to the planet’s surface is important. At present, the representation of these clouds in climate and weather models is subject to improvements: a large part of the uncertainty error stems from the lack of precise micro-physical and radiation model schemes for ice crystal clouds. To improve the cloud representations, a better understanding of the life time dynamics of the clouds and their composition is necessary, comprising a detailed understanding of the ice particle genesis, and development over their lifetime. It is especially important to understand how the development of ice crystals over time is linked to the changes in observable variables such as water vapour content and temperature and how they change the light scattering properties of the crystals. Recent remote and aircraft based in-situ measurements have shown that many ice particles show a light scattering behaviour typical for crystals having rough surfaces or being of complex geometrical shapes. The aim of this thesis was to develop the experimental setup and experiments to investigate this further by studying the surface morphology of small water ice crystals using scanning electron microscopy (SEM). The experiments I developed study the growth of water ice crystals inside an SEM chamber under controlled environmental conditions. The influence of water vapour supersaturation, pressure and temperature is investigated. I demonstrate how to retrieve the surface topology from observed crystals for use as input to computational light scattering codes to derive light scattering phase functions and asymmetry parameters, which can be used as input into atmospheric models. Difficulties with the method for studying the growth of water ice crystals, such as the effect of the electron beam-gas ionization and charging effects, the problem of facilitating repeated and localized ice growth, and the effect of radiative influences on the crystal growth are discussed. A broad set of nucleation target materials is studied.

The Growth and Morphology of Small Ice Crystals in a Diffusion Chamber

The Effect of Impurities on the Surface Structure of Evaporating Ice

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The Effect of Impurities on the Surface Structure of Evaporating Ice