Liquid Water in the Atmosphere

Water is abundant on our planet, distinguishing Earth from all other planets in the solar system. More than 97% of Earth s water is in the oceans, with 2.1% in the polar ice caps and 0.6% in aquifers. The atmosphere contains only about one part in a hundred thousand (0.001%) of Earth s available water. However, the transport and phase distribution of this relatively small amount of water (estimated total liquid equivalent volume of 13,000 km3) are some of the most important features of Earth s climate. The existence of varying pressures and temperatures in the atmosphere and at the Earth s surface causes water to constantly transfer among its gaseous, liquid, and solid states. Clouds, fogs, rain, dew, and wet aerosol particles represent different forms of that water. Aqueous atmospheric particles play a major role in atmospheric chemistry, atmospheric radiation, and atmospheric dynamics. 

Clouds cover approximately 60% of the Earth s surface. Average global cloud coverage over the oceans is estimated at 65% and over land at 52% (Warren et al. 1986, 1988). 

Atmospheric Chemistry and Physics From Air Pollution to Climate Change, Second Edition, by John H. Seinfeld and Spyros N. Pandis. Copyright 2006 John Wiley Sons, Inc. 

FIGURE 7.1 Atmospheric water vapor concentration as a function of temperature and relative humidity. 

The occurrence of clouds shows dramatic geographical variation and is generally restricted to the lowest 4-6 km of the troposphere. Clouds form and evaporate repeatedly. Only a small fraction (around 10%) of the clouds that form actually generate precipitation. Thus nine out of ten clouds evaporate without ever generating rain droplets. Even in cases where precipitation develops, the raindrops often evaporate on their way falling through the cloud-free air, so the drops never reach the surface. 

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Bowden, D.J., Clegg. S.L., and Brimblecombe, P. The Henry s law constant of trifluoroacetic acid and its partitioning into liquid water in the atmosphere. Chemosphere, 32(2) 405-420, 1996. 

Hydrogen peroxide (H202) and 03 are the natural strong oxidants present in rainwater. These oxidants can potentially oxidize nearly all the S02 in a parcel of air. Box 3.8 shows that under such conditions rainfall may well have pH values lower than 3. This illustrates the high acid concentrations possible in the atmosphere as trace pollutants are transferred from the gas phase to droplets. Liquid water in the atmosphere has a volume about a million times smaller than the gas phase thus a substantial increase in concentration results from dissolution. 

Chemical reactions occur in the gas phase and in the aqueous phase (cloud droplets) that both oxidize elemental mercury to divalent mercury and reduce the divalent mercury to elemental mercury. The most important gas phase oxidation pathways are the reactions with ozone (Hall 1995) and OH radicals (Sommar etal. 2001). Small amounts of Hg , which are dissolved in liquid water in the atmosphere (fog or clouds), can also be oxidized by ozone (Munthe 1992) or by OH radicals (Garfeldt et al. 2001). The oxidation in the aqueous phase occurs at a significantly higher rate than in the gas phase but, due to the low solubility of Hg in water and the low liquid water content in the atmosphere, the overall rate of oxidation is comparable to the gas phase oxidation rate (Pirrone etal. 2001). Reduction of divalent mercury back to Hg may also occur by sulfite (S03 ) ions or HO2 radicals furthermore, complexation of divalent mercury with soot may occur to form particulate divalent mercury (figure 17.2). 

Acids are common constituents of the atmosphere. The acid that is always present in the atmosphere is CO2, a very weak acid, which tends to make liquid water in the atmosphere slightly acidic because of the following reaction ... 

Figure 14.49 shows the absorption of light from an overhead sun by liquid cloud droplets, water vapor inside the cloud, and water vapor in a column in the atmosphere for a 1-km stratus cloud whose top is 2 km above the ground (Davies et al., 1984 see also Goldstein and Penner, 1964). There are small amounts of absorption in the tail end of the red region of the visible attributed to water vapor in and outside the cloud. The absorption increases into the near-IR (the region from 780 to 2500 nm or 12,800-4000 cm-1) and mid-IR (2.5-50 fim or 4000-200 cm-1) where liquid water in the cloud absorbs (e.g., see Evans and Puckrin, 1996). 

The size of the interface between atmosphere and hydrosphere is immense (see Appendix E) 71% of the earth s surface (361 x 106 km2) is covered by water. In addition, the atmosphere contains about 13 x 1015 kg of water vapor. Expressed as liquid volume, this amounts to 13 x 1012 m3 or 2.5 cm per m2 of earth surface. This is a small volume compared to the total ocean volume of 1.37 x 1018 m3, but it is important in terms of the additional interfacial area between water and air. Although most of the water in the atmosphere is present as water vapor, roughly 50% of the earth s surface is covered by clouds which contain between 0.1 and 1 g of liquid water per cubic meter of air. The water is present in droplets with a typical diameter of 20 pm. Thus, clouds supply an air-water interface area of the order of 0.1 m2 per cubic meter of air (Seinfeld, 1986). For a cloud cover 500 m thick this would yield an air-water contact zone of 50 m2 per m2 of earth surface. 

The gases in the air are held in an envelope around the Earth by its gravity. The atmosphere is approximately 100 km thick (Figure 11.1), and about 75% of the mass of the atmosphere is found in the layer nearest the Earth called the troposphere (Figure 11.2). Beyond this layer, the atmosphere reaches into space but becomes extremely thin. Nearly all atmospheric water vapour (or moisture) is found in the troposphere, which also contains the liquid water in the oceans, rivers and lakes. 

As the steam lost energy, it was collected in a separate chamber where it cooled and condensed back into liquid water. In the process of condensing in a sealed chamber, the steam created a vacuum, a space empty of matter. This vacuum allowed atmospheric pressure to drive the piston down again, preparing it for another round of work. 

Relative humidity %RH, lOOP/Po, IOOjc The vapor pressure of water in the atmosphere (P) usually expressed as a percentage of the saturation vapor pressure of pure liquid water (R,) at the same temperature. 

Relative to the levels of the species we have been considering, water vapor is at a high concentration in the atmosphere. Liquid water, in the form of clouds and fog, is frequently present. Small water droplets can themselves be viewed as microscopic chemical reactors where gaseous species are absorbed, reactions take place, and species evaporate back to the gas phase. Droplets themselves do not always leave the atmosphere as precipitation more often than not, in fact, cloud droplets evaporate before coalescing to a point where precipitation can occur. In terms of atmospheric chemistry, droplets can both alter the course of gas-phase chemistry through the uptake of vapor species and act as a medium for production of species that otherwise would not be produced in the gas phase or would be produced by different paths at a lower rate in the gas phase (Fig. 10). Concentrations of dissolved species in cloud, fog, and rain droplets are in the micromolar range, and therefore one usually assumes that the atmospheric aqueous phase behaves as an ideal solution. 

Almost all of the fresh liquid water is groundwater, with at least half of the total present at depths of greater than 1000 m. (Much less than 1% of the total is in the root zone of the soil.) Lakes and rivers, despite their extreme importance for humans, are a quantitatively insignificant component of the world s water supply. There is about as much water in the atmosphere as there is in all the world s rivers. 

Only a very small percentage of all the water in the climate system is actually present in the atmosphere (Table 2.12). Of the atmospheric water, most is in the vapor phase (Fig. 2.37) the liquid water content (LWC) of clouds is only in the order 1 g the cloud ice water content (IWC) still less, down to 0.0001 g m . But clouds play a huge role in the climate system, whereas precipitation closes the cycle for water and also for substances dissolved in it (wet deposition). Some of the processes (droplet formation, transfer processes, deposition, and chemistry) will be described later. The aim of this chapter is to describe the phenomenology of water in the atmosphere so far as we need it for an understanding of chemical processes. 

Bnijsman E., H. F. M. Mass and W. A. H. Asman (1986) Anthropogenic ammonia emissions in Enrope. Atmospheric Environment 21, 1009-1022 Bnrford, I R. and I M. Bremner (1972) Is phosphate reduced to phosphine in waterlogged sods Soil Biology and Biochemistry 4, 489-495 Buxton, G. V. (1982) Basic radiation chemistry of liquid water. In The study of fast processes and transient species by electron pulse radiolysis (Eds. J. H. Baxendale and F. Busi), D. Reidel Publ. Comp., pp. 241-266... 

AU the water on Earth is connected in a global water cj cle ( FIGURE 18.15). Most of the processes depicted here rely on the phase changes of water. For in.stance, warmed by the Sun, liquid water in the oceans evaporates into the atmosphere as water vapor and condenses into liquid water droplets that we see as clouds. Water droplets in the clouds can crystallize to ice, which can precipitate as hail or snow. Once on the ground, the hail or snow melts to liquid water, which soaks into the ground. If conditions are right, it is also possible for ice on the ground to sublime to water vapor in the atmosphere. 

In general, the mechanical and thermophysical properties of a material depend on its phase. For example, as you know from your everyday experience, the density of ice is different from liquid water (ice cubes float in liquid water), and the density of liquid water is difierent from that of steam. Moreover, the properties of a material in a single phase could depend on its temperature and the surrounding pressure. For example, if you were to look up the density of liquid water in the temperature range of, say, 4° to 100°C, under standard atmospheric pressure, you would find that its density decreases with increasing temperamre in that range. Therefore, properties of materials depend not only on their phase bur also on their temperature and pressure. This is another important fret to keep in mind when selecting materials. 

Water dissolves all kinds of substances and reacts chemically with many of the atmospheric pollutants we ve encountered in earlier chapters. Several nitrogen oxides and sulfur oxides react readily with water in the atmosphere to produce acidic rainfall that can have a very negative impact on our environment and on many building materials. Dissolved substances in liquid water can include natural minerals and chemicals, manmade fertilizers and pesticides, gasoline and fuel additives, toxic chemicals of all types, and even unused antibiotics that people have flushed down the toilet. To minimize environmental damage and to ensure a safe and adequate water supply, we must limit all types of pollution. We must control both the amount of pollution and the types of pollution to which we subject our water supply because what we ve got is all there is. The water must be used, reused, and used s ain, over and over, in a never-ending cycle. If it becomes polluted, we will have to clean it up. 

Furthermore, plant growth is provided for by water flow and substance accumulation. The driving force for soil-plant-atmosphere water continuum is the difference in free energy between liquid water in the soil and water vap>our in the atmosphere. Cellular functions are ultimately linked to metabolic fluxes which direction is dictated by the change of the free energy, too. 

Atmospheric Water Water in the atmosphere it can be in a gaseous, liquid, or frozen form. 

It is often useful for the balance equation to show the nature of the phases in which the components are. In order to do this, a system of symbols as outlined in Table 1.1 is included in tiie balance equations. According to this system, the vaporization of pure liquid water in an atmosphere composed... 

Some compounds released to the atmosphere as air pollutants hydrolyze to form HCl. Such incidents have occurred as the result of leaks of liquid silicon tetrachloride, SiCl4. This compound reacts with water in the atmosphere to form a choking fog of... 

The detection of atmospheres on extrasolar planet is a very difficult task. 71% of the Earth is covered by oceans but up to now it is the only planet with water in liquid form on its surface. Venus might have had water on its early history, on Mars water may exist in a frozen state near the surface and climatic changes have occurred and formed river-like structures that are observed on its surface. There exists the possibility to find condensed water in the atmospheres of Jupiter and Saturn and in deeper layers of Uranus and Neptune. Subsurface oceans may exist on several satellites of the giant planets. But how can we detect water on extrasolar planets, how can we detect whether these objects have even an atmosphere ... 

Deliquescence and efflorescence. A substance is said to deliquesce (Latin to become liquid) when it forms a solution or liquid phase upon standing in the air. The essential condition is that the vapour pressure of the saturated solution of the highest hydrate at the ordinary temperature should be less than the partial pressure of the aqueous vapour in the atmosphere. Water will be absorbed by the substance, which gradually liquefies to a saturated solution water vapour will continue to be absorbed by the latter until an unsaturated solution, having the same vapour pressure as the partial pressure of water vapour in the air, is formed. In order that the vapour pressure of the saturated solution may be sufficiently low, the substance must be extremely soluble in water, and it is only such substances (e.g., calcium chloride, zinc chloride and potassium hydroxide) that deliquesce. 

When a liquid or solid substance is emitted to the air as particulate matter, its properties and effects may be changed. As a substance is broken up into smaller and smaller particles, more of its surface area is exposed to the air. Under these circumstances, the substance, whatever its chemical composition, tends to combine physically or chemically with other particles or gases in the atmosphere. The resulting combinations are frequently unpredictable. Very small aerosol particles (from 0.001 to 0.1 Im) can act as condensation nuclei to facilitate the condensation of water vapor, thus promoting the formation of fog and ground mist. Particles less than 2 or 3 [Lm in size (about half by weight of the particles suspended in urban air) can penetrate the mucous membrane and attract and convey harmful chemicals such as sulfur dioxide. In order to address the special concerns related to the effects of very fine, iuhalable particulates, EPA replaced its ambient air standards for total suspended particulates (TSP) with standards for particlute matter less than 10 [Lm in size (PM, ). 

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