Cloud water droplets

In the atmosphere DMS is oxidised mainly in the gas phase. Oxidation in cloud-water droplets is insignificant as the low solubility of DMS mitigates the effects of its rapid aqueous oxidation by ozone ( , McElroy, W.J., Central Electricity Research Laboratories, personal communication). Gas-phase oxidation is initiated principally by reaction with OH radicals ( ) and methanesulphonic acid (MSA) is one of the products (2). MSA has a very low vapour pressure and will be rapidly scavenged by aqueous aerosols and cloud droplets wherein further oxidation to sulphate by OH may occur. Although the kinetics and mechanism of this process have yet to be unambiguously determined, it is possible that emissions of DMS could be both a significant source of "background" sulphur and, upon oxidation, of non sea-salt sulphate. 

We stated that not all clouds precipitate. Indeed, from only a very small proportion of clouds does precipitation actually reach the ground surface below. The basic problem is that cloud water droplets or ice particles are frequently too small to fall from the cloud base or to survive on the way to the ground because they evaporate. Whereas a cloud droplet is on average 8 pm in diameter, a rain drop is between 500 and 5000 pm (0.5-5 mm) this means that a small rain drop is as large in volume as 240 000 cloud drops. Assuming 240 cloud droplets cm (cf Table 2.25), there is only one rain drop in 1 L of air. Several microphysical processes occur in clouds depending on temperature, vertical resolution, dynamic and other parameters that result in growth of a particle (Fig. 2.39) and different precipitation forms (Table 2.26). 

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. 

The gas-liquid equUihiium constants determine the solubility of gaseous molecule into the liquid phase so that they are important parameters for the kinetic analysis of multiphase reactions. Liquid reactions are important in the atmosphere for fog and cloud water droplets with the particle diameter of 1-100 pm. When those water droplets coexist in the atmosphere, water soluble molecules X in the gas phase is absorbed by the droplet, and the gas-liquid equilibrium,... 

Most of our previous studies have been devoted to reactive oxygen species (ROS) at the air-water interface because such species are ubiquitous and play a crucial role in atmospheric chemistry, in environmental processes, water treatment technologies and biochemical reactions. The complex chemistries associated to these species, and their interconnection across different reaction media, have recently been reviewed [53]. The stability of ozone, molecular oxygen, hydrogen peroxide, hydroxyl and hydroperoxyl radicals, and other related compounds at the air-water interface had been established through classical molecular dynamics simulations [54—56]. Those studies, in particular, suggested that many of the compounds could accumulate at the surface of cloud water droplets, influencing in this way the overall chemistry of the troposphere. Recenfly, combined QM/MM MD simulations have confirmed the marked aflSnity of ROS species such as HO2 [27] and ozone [30] for the air-water interface. 

Atmosphere—Water Interaction. Although water is a very minor component of the atmosphere, less than 10 vol % of the atmosphere consisting of water, many important reactions occur ki the water droplets of cloud, fog, and rain. The atmosphere is an oxic environment ki its water phase, gigantic quantities of reductants, such as organic substances, Fe(II), SO2, CH SCH (dimethyl sulfide), and nitrogen oxides, are oxidized by oxidants such as oxygen, OH radicals, H2O2, and Fe(III). 

A cloud is cloudy because it is a suspension of vast numbers of minute, spherical water droplets. The droplets are too small and light to fall under gravity and they are stable, that is to say they do not coarsen and become large enough to fall." ... 

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.

Pollution can cause opposite effects in relahon to precipitation. Addition of a few particles that act as ice nuclei can cause ice particles to grow at the expense of supercooled water droplets, producing particles large enough to fall as precipitation. An example of this is commercial cloud seeding with silver iodide particles released from aircraft to induce rain. If too many particles are added, none of them grow sufficiently to cause precipitation. Therefore, the effects of pollution on precipitation are complex. 

Galloway, J.N. Likens, G.E. Hawley, M.E. Science, 1984,226, pp 829-831. Castillo, R. An Investigation of the Acidity of Stratus Cloud Water and Its Relationship to Droplet Distribution, pH of Rain and Weather Parameters, Ph.D. Thesis, Dept. Atmos. Sci., State University of New York, Albaity, NY, 1979. 

The total wet deposition flux consists of 2 contributory factors. The first derives from the continuous transfer of Hg to cloud water, described by chemistry models. There are 2 limiting factors 1) the uptake of gas phase Hg(0), which is regulated by the Hemy s corrstant and 2) the subsequent oxidation of Hg(0) to Hg(ll), which is governed by reaction rate constants and the irritial concentratiorrs of the oxidant species. The total flirx depends on the hquid water content of the cloud and the percentage of the droplets in the cloud that reach the Earth s surface. 

Light is scattered all around us— the fact that the sky above us appears blue, the clouds white, and the sunset shades of reds and oranges is a consequence of preferential scattering of light from air molecules, water droplets, and dust particles. This scattered light carries messages about the scattering objects. 

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