Sulfuric acid clouds

An aspect of life in clouds that is beyond the scope of this book (cold environments) is potential life in Venusian clouds. The surface of Venus is too hot (464°C) for liquid water or carbon-based life (Cockell 1999). Atmospheric constraints include sulfuric acid clouds and high doses of ultraviolet radiation in principle, these atmospheric constraints can be overcome (Cockell 1999 Schulze-Makuch et al. 2004), which means that Venus could be close to possessing a habitable environment. However, it still remains to be demonstrated that the residence time in Venusian clouds is sufficiently long to create a self-sustaining ecosystem. 

H2O, CO, OCS, and SO have spatially and temporally variable abundances. Variations in the abundances of water vapor, CO, and sulfur gases are of particular interest because they result from the solar UV-diiven photochemistry that maintains the global sulfuric acid cloud cover. 

The sulfur cycle is closed as follows. Sulfuric acid cloud droplets vaporize to a gas mixture of H2SO4, SO3, and H2O at the cloud base. Reduction of SO3 to SO2 occurs via the reaction... 

The SO2 abundance in Venus atmosphere decreases at high altitudes due to the formation of the global sulfuric acid cloud layer (45-70 km), which is produced through SO2 photolysis via the net photochemical reaction ... 

Working in his father s business, which produced niter (saltpeter, potassium nitrate) from seaweed by treating it with strong acids, Courtois noticed that when he added excess sulfuric acid, clouds of violet vapor rose over the solution, then condensed into dark, shiny crystals. Courtois investigated the chemical properties of the new material during the next few months and prepared some of its compounds. However Courtois was busy with the war effort (niter is a component of gunpowder), and he may have been feeling the financial strain of his research, so he told two other chemists about his crystals and asked them to continue the work. 

Sulfuric acid is produced in the upper atmosphere of Venus by the Sun s photochemical action on carbon dioxide, sulfur dioxide, and water vapor. Ultraviolet photons of wavelengths less than 169 nm can photodissociate carbon dioxide into carbon monoxide and atomic oxygen. Atomic oxygen is highly reactive. When it reacts with sulfur dioxide, a trace component of the Venusian atmosphere, the result is sulfur trioxide, which can combine with water vapor, another trace component of Venus s atmosphere, to yield sulfuric acid. In the upper, cooler portions of Venus s atmosphere, sulfuric acid exists as a liquid, and thick sulfuric acid clouds completely obscure the planet s surface when viewed from above. The main cloud layer extends from 45-70 km above the planet s surface, with thinner hazes extending as low as 30 km and as high as 90 km above the surface. The permanent Venusian clouds produce a concentrated acid rain, as the clouds in the atmosphere of Earth produce water rain. 

Because of the mixture of VOCs in the atmosphere, the composition of smog reaction products and intermediates is extremely complex. formed via reaction 16, is important because when dissolved in cloud droplets it is an important oxidant, responsible for oxidising SO2 to sulfuric acid [7664-93-9] H2SO4, the primary cause of acid precipitation. The oxidation of many VOCs produces acetyl radicals, CH CO, which can react with O2 to produce peroxyacetyl radicals, CH2(C0)02, which react with NO2... 

Similar heterogeneous reactions also can occur, but somewhat less efticientiy, in the lower stratosphere on global sulfate clouds (ie, aerosols of sulfuric acid), which are formed by oxidation of SO2 and COS from volcanic and biological activity, respectively (80). The effect is most pronounced in the colder regions of the stratosphere at high latitudes. Indeed, the sulfate aerosols resulting from emptions of El Chicon in 1982 and Mt. Pinatubo in 1991 have been impHcated in subsequent reduced ozone concentrations (85). 

Oxidation of sulfur dioxide in aqueous solution, as in clouds, can be catalyzed synergistically by iron and manganese (225). Ammonia can be used to scmb sulfur dioxide from gas streams in the presence of air. The product is largely ammonium sulfate formed by oxidation in the absence of any catalyst (226). The oxidation of SO2 catalyzed by nitrogen oxides was important in the eady processes for manufacture of sulfuric acid (qv). Sulfur dioxide reacts with chlorine or bromine forming sulfuryl chloride or bromide [507-16 ]. 

A solution of sulfur trioxide [7446-11-9] dissolved in chlorosulfonic acid [7990-94-5] CISO H, has been used as a smoke (U.S. designation FS) but it is not a U.S. standard agent (see Chlorosulfuric acid Sulfuric acid and sulfur trioxide). When FS is atomized in air, the sulfur trioxide evaporates from the small droplets and reacts with atmospheric moisture to form sulfuric acid vapor. This vapor condenses into minute droplets that form a dense white cloud. FS produces its effect almost instantaneously upon mechanical atomization into the atmosphere, except at very low temperatures. At such temperatures, the small amount of moisture normally present in the atmosphere, requires that FS be thermally generated with the addition of steam to be effective. FS can be used as a fill for artillery and mortar shells and bombs and can be effectively dispersed from low performance aircraft spray tanks. FS is both corrosive and toxic in the presence of moisture, which imposes limitations on its storage, handling, and use. 

Figure 4-13 shows an example from a three-dimensional model simulation of the global atmospheric sulfur balance (Feichter et al, 1996). The model had a grid resolution of about 500 km in the horizontal and on average 1 km in the vertical. The chemical scheme of the model included emissions of dimethyl sulfide (DMS) from the oceans and SO2 from industrial processes and volcanoes. Atmospheric DMS is oxidized by the hydroxyl radical to form SO2, which, in turn, is further oxidized to sulfuric acid and sulfates by reaction with either hydroxyl radical in the gas phase or with hydrogen peroxide or ozone in cloud droplets. Both SO2 and aerosol sulfate are removed from the atmosphere by dry and wet deposition processes. The reasonable agreement between the simulated and observed wet deposition of sulfate indicates that the most important processes affecting the atmospheric sulfur balance have been adequately treated in the model. 

Figure 1.1 The thick clouds surrounding the planet Venus are made up of carbon dioxide, nitrogen, and sulfuric acid. Living things cannot survive in such harsh conditions.

The clouds around Venus contain relatively large droplets of sulfuric acid, which occasionally rain down on the surface of the planet, or at least they try to, because the temperature is so high that the droplets evaporate before they actually reach the surface. (This almost rain is called virga, the term for any kind of precipitation that evaporates before it reaches the ground.) On Earth, however, the sulfuric acid does not evaporate but falls to the ground as acid rain, an environmental pollutant that can destroy buildings and harm plants and animals. 

Acid rain. Natural (unpolluted) precipitation is naturally acidic with a pH often in the range of 5 to 6 caused by carbonic acid from dissolved carbon dioxide and sulfurous and sulfuric acids from natural emissions of SO and H2S. Human activity can reduce the pH very significantly down to the range 2 to 4 in extreme cases, mainly caused by emissions of oxides of sulfur. Because atmospheric pollution and clouds travel over long distances, acid rain is not a local problem. The problem may manifest itself a long way from the source. Problems associated with acid rain include ... 

Similar to the history of many other elements, iodine s discovery was serendipitous in the sense that no one was looking for it specifically. In 1811 Bernard Courtois (1777—1838), a French chemist, attempted to remove sodium and potassium compounds from the ash of burned seaweed in order to make gunpowder. After removing these chemicals from the ash, he added sulfuric acid (H SO j) to the remaining ash. However, he mistakenly added too much acid, which produced a violet-colored vapor cloud that erupted from the mixture. This violet vapor condensed on all the metallic objects in the room, leaving a layer of sohd black iodine crystals. Sir Humphry Davy (1778—1829) confirmed this discovery of a new element and named it iodine after the Greek word iodes, which means violet, but it was Courtois who was given credit for the discovery of iodine. 

If one follows the solution viscosity in concentrated sulfuric acid with increasing polymer concentration, then one observes first a rise, afterwards, however, an abrupt decrease (about 5 to 15%, depending on the type of polymers and the experimental conditions). This transition is identical with the transformation of an optical isotropic to an optical anisotropic liquid crystalline solution with nematic behavior. Such solutions in the state of rest are weakly clouded and become opalescent when they are stirred they show birefringence, i.e., they depolarize linear polarized light. The two phases, formed at the critical concentration, can be separated by centrifugation to an isotropic and an anisotropic phase. A high amount of anisotropic phase is desirable for the fiber properties. This can be obtained by variation of the molecular weight, the solvent, the temperature, and the polymer concentration. 

A second pathway to the formation of sulfuric acid depends on the presence of hydrogen peroxide (H2O2) in clouds, fog, rain, and other forms of water in the atmosphere. Hydrogen peroxide is now known to form in such locations when hydroperoxyl radicals react with each other ... 

To destroy these sulfur compounds Courtois added sulfuric acid, and on one eventful day in 1811 he must have added it in excess (54). To his astonishment lovely clouds of violet vapor arose, and an irritating odor like that of chlorine permeated the room. When the vapors condensed on cold objects, no liquid was formed, but there appeared instead a quantity of dark crystals with a luster surprisingly like that of a metal (45). 

We have seen in Chapter 8 that reactions in the aqueous phase present in the atmosphere in the form of clouds and fogs play a central role in the formation of sulfuric acid. Thus, an additional mechanism of particle formation and growth involves the oxidation of SOz (and other species as well) in such airborne aqueous media, followed by evaporation of the water to leave a suspended particle. 

Borrmann, S S. Solomon, J. E. Dye, D. Baumgardner, K. K. Kelly, and K. R. Chan, Heterogeneous Reactions on Stratospheric Background Aerosols, Volcanic Sulfuric Acid Droplets, and Type I Polar Stratospheric Clouds Effects of Temperature Fluctuations and Differences in Particle Phase, J. Geophys. Res., 102, 3639-3648 (1997b). 

M. Loewenstein, G. V. Ferry, K. R. Chan, and B. L. Gary, Particle Size Distributions in Arctic Polar Stratospheric Clouds, Growth, and Freezing of Sulfuric Acid Droplets, and Implications for Cloud Formation, J. Geophys. Res., 97, 8015-8034 (1992). 

Iraci, L. T., A. M. Middlebrook, and M. A. Tolbert, "Laboratory Studies of the Formation of Polar Stratospheric Clouds Nitric Acid Condensation on Thin Sulfuric Acid Films, J. Geophys. Res., 100, 20969-20977 (1995). 

To this point we have dealt only with spheres, which have the advantage that their extinction properties are easily calculated while still giving substantial guidance into extinction by small particles in general. And there are many particles that are indeed spherical cloud droplets sulfuric acid droplets... 

However, more than one reaction pathway may exist, in which case the rate equation will contain sums of terms representing the competing reaction pathways. For example, one of the oxidation reactions that convert the atmospheric pollutant sulfur dioxide to sulfuric acid (a component of acid rain) in water droplets in clouds involves dissolved ozone, O3 (see Sections 8.3 and 8.5) ... 

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