Water in clouds

Condensed phase interactions can be divided roughly into two further categories chemical and physical. The latter involves all purely physical processes such as condensation of species of low volatility onto the surfaces of aerosol particles, adsorption, and absorption into liquid cloud and rainwater. Here, the interactions may be quite complex. For example, cloud droplets require a CCN, which in many instances is a particle of sulfate produced from SO2 and gas-particle conversion. If this particle is strongly acidic (as is often the case) HNO3 will not deposit on the aerosol particle rather, it will be dissolved in liquid water in clouds and rain. Thus, even though HNO3 is not very soluble in... 

It is much more difficult to describe the relationship of the bulk field gradients, easily recognised in the flow of water in clouds and of oxygen in the ozone layer described in Section 3.4, to that of the gradients controlling the chemical flow in cell liquids. The effects of electric fields due to charge distribution in various parts of the cell is an obvious possibility. 

The major terrestrial water reservoirs, the oceans, are well mixed and of rather uniform composition. Upon evaporation, an isotopic separation occurs because the light molecules are more readily evaporated. Thus water in clouds is isotopically light compared to ocean water. Upon condensation, heavier water molecules condense more readily, causing a reversed fractionation. The degree of isotopic fractionation depends on the ambient temperature and other factors which are discussed in Chapter 9. 

Collection of supercooled liquid water in clouds is simple, using only a plate or screen exposed to RAM air the water is later melted and stored prior to analysis (6 ). Collection of frozen cloud particles is a little more problematical since the liquid water content can be low, and individual particles are more subject to bounce-off during impactive collection. Collection of snow particles aboard the aircraft is most difficult of all due to the low aerodynamic diameter exhibited by these particles in RAM air streams. Successful methods for the collection of snow and ice clouds are still in an active stage of development. 

J. Tyndall, The Forms of Water in Clouds and Rivers, Ice and Glaciers, Henry... 

The presence of strontium and radioactive strontium compounds in the atmosphere results from both natural and anthropogenetic activities (see Section 6.2.1). Strontium is emitted into the atmosphere as strontium oxide (i. e., SrO) from emission during thermal processes. As is the case of other metallic compounds, SrO cannot be destroyed however, it can be transformed from one form to another. When SrO comes in contact with water in clouds or during washout by rain, it ionizes to form Sr+. There is no evidence for interaction of SrO with C02 (g)or other compounds in the atmosphere. 

The constituents of cloud water derive from two sources one is material incorporated with the condensation nuclei, the other is the dissolution of gases from the sm-rounding air. As the numbers of particles serving as cloud condensation nuclei are most numerous in the size range of the accumulation mode, cloud water generally represents a dilute solution of this fraction of the aerosol. But the components of the aerosol fraction are already fully oxidized and, therefore, not very reactive. On the other hand, many of the gases that dissolve in cloud water have a potential for further oxidation. The aqueous concentration of such substances depends on their abundance in the gas phase before cloud condensation sets in and on the individual gas-liquid (Henry s law) partition coefficients, which causes a redistribution of the substances between the two phases. The amoimt of liquid water in clouds is in the range 0.1-0.5 g/m , so that the volume of liquid... 

There is a sharp separation between adsorbed water and liquid water, in which an adsorbent may be suspended and dissolved. In adsorbed water the structure is imposed by the adsorption field, while in liquid water the (dis)order is the one characteristic of the bulk phase. This state of affairs suggests that the adsorbed-to-liquid transition is a phase transition. On another side, water on many surfaces continuously undergoes adsorbed-to-liquid transition and vice versa. Familiar examples of adsorbents where the state of water frequently cycles between the adsorbed and liquid phases are soils and the skin of terrestrial mammalians. It is also noted that the formation of liquid water in clouds may occur via heterogeneous nucleation (i.e., via multilayer adsorption), either spontaneously on dust particles or artificially on Agl crystals formed by condensation of sublimated Agl,... 

Sulfuric acid. Sulfur dioxide (SO2), formed mostly by the burning of high-sulfur coal, forms sulfurous acid (H2SO3) in contact with water. The atmospheric pollutants hydrogen peroxide (H2O2) and ozone (O3) dissolve in the water in clouds and oxidize the sulfurous acid to sulfuric acid ... 

We saw in Chapter 5 that there is a driving force tending to make dispersions of precipitates in alloys coarsen and we would expect a dispersion of droplets in water vapour to do the same. Water droplets in clouds, however, carry electrostatic charges and this gives a different result for the driving force. 

Fig. 9.1. Rain falls when the water droplets in clouds turn to ice. This con only happen if the clouds are below 0°C to begin with. If the droplets are clean, ice can form only in the unlikely event that the clouds cool down to the homogeneous nucleation temperature of -40°C. When dust particles are present they can catalyse nucleation at temperatures quite close to 0°C. This is why there is often heavy rainfall downwind of factory chimneys.

Water is constantly evaporated from rivers, lakes, and oceans, and released from vegetation through evapo-transpiration. Water vapor travels through the atmosphere, eventually forming small droplets or ice crystals in clouds. Some particles grow sufficiently... 

Fig. 6-2 Comparison of infrared absorbance of a vertical column of atmospheric CO2 and H2O vapor. The nearly total absorbance by H2O between 5 and 7 /

We will illustrate the necessity of including solute from CCN by a simple calculation, recalling that pH = 5.6 is the supposed equilibrium value for water in contact with 300 ppm of CO2. (That calculation will appear later.) In clean, marine air, the concentration of submicrometer aerosol particles (by far the most numerous) is small, say 0.25 pg m . It is known from measurements that the molecular form is often NH4HSO4, and we assume it is all dissolved in 0.125 g/m of liquid water in a cloud - which is typical for fair-weather marine clouds. Thus the average concentration of sulfate ion [SO4 ], mol/L, is... 

Anthropogenic Modifications of the Acid-Base Balance of Rainwater Alkalinity in Cloud Water "Acid Rain"... 

Adsorption of water on salt crystals plays a key role in many atmospheric and environmental processes. Alkah halides in particular play an important role in the first stages of drop growth in clouds. To understand the atomistic details of the wetting and dissolution processes that take place in these crystals, we apphed SPFM to the smdy of the adsorption of water vapor on single crystal surfaces and the role of surface defects, such as steps. 

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