Atmospheric gases photochemical processes

In cloud droplets in remote regions the metal concentrations are likely to be low. Here more typically the reaction proceeds with oxidants such as dissolved hydrogen peroxide (or other atmospheric peroxides) and ozone. Hydrogen peroxide is an especially important droplet phase oxidant, because the gas is very soluble in water so can dissolve from the atmosphere. Additionally, it is readily produced within droplets in the atmosphere via photochemical processes. Oxidation by hydrogen peroxide is also significant, because the reaction is faster in acidic solutions, which means that the oxidation process does not become much slower as droplets become more acidic with the production of sulfuric acid. This oxidation can be represented as... 

Fig. 7-11 Compilation of the most important photochemical processes in the atmosphere, including estimates of flux rates expressed in moles per year between the earth s surface and the atmosphere and within the atmosphere. (Modified with permission from P. J. Crutzen, Atmospheric interactions - homogeneous gas reactions of C, N, and S containing compounds. In B. Bolin and R. Cook (1983). "The Major Biogeochemical Cycles and Their Interactions," pp. 67-112, John Wiley, Chichester.)...

The definition of environmental chemistry given above is illustrated for a typical environmental pollutant in Figure 2.3. Pollutant sulfur dioxide is generated in the anthrosphere by combustion of sulfur in coal, which has been extracted from the geosphere. The S02 is transported to the atmosphere with flue gas and oxidized by chemical and photochemical processes in the atmosphere to sulfuric acid. The sulfuric acid, in turn, falls as acidic precipitation, where it may have detrimental effects, such as toxic effects, on trees and other plants in the biosphere. Eventually the sulfuric acid is carried by stream runoff in the hydrosphere to a lake or ocean, where its ultimate fate is to be stored in solution in the water or precipitated as solid sulfates and returned to the geosphere. 

The experiments and model calculations regarding photochemical processes in surface snow clearly demonstrate that photochemical transformations in the snow are very diverse. As in the atmospheric gas and liquid phase, the OH radical plays a critical role in these transformations. However, the sinks of this radical are not well defined. The reactions with organic compounds are probably the most important OH destruction reactions. However, due to the limited information of the concentrations of single organic compounds in snow it is currently impossible to assemble a detailed mechanism for snow chemistry. Therefore, we decided to introduce a class of compounds, which represents organic material. Additional investigations of organic components in snow can be used to further refine the mechanism. 

Several studies have investigated empirically the flux of chemicals within snow or between snow and the atmosphere (Guimbaud et al., 2002 Albert and Shultz, 2002 Herbert et al., 2006). In particular, measured concentration gradients within the atmospheric boundary layer or within the snow pack have been used to calculate a chemical s flux into or out of the snow pack. This approach has resulted in miscellaneous parameterizations to calculate fluxes of, for example, carbonyl compounds and NO c species from the snow pack as a result of photochemical processes in snow (Domind and Shepson, 2002 Hutterli et al., 1999 Guimbaud et al., 2002 Grannas et al., 2002). However, flux measurements can only be used to derive kinetic transport parameters, such as diffusivities and mass transport coefficients, if the chemicals involved are reasonably persistent and do not undergo rapid conversions within the snow pack. For example, measurements of the flux of carbonyl compounds out of snow are more likely to reflect the kinetics of formation in the snow pack than the kinetics of snow-air gas exchange. As a result, there is a very limited number of experimental studies that provide quantitative information on the rate of chemical transport in snow. 

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

Combustion processes are the most important source of air pollutants. Normal products of complete combustion of fossil fuel, e.g. coal, oil or natural gas, are carbon dioxide, water vapour and nitrogen. However, traces of sulphur and incomplete combustion result in emissions of carbon monoxide, sulphur oxides, oxides of nitrogen, unburned hydrocarbons and particulates. These are primary pollutants . Some may take part in reactions in the atmosphere producing secondary pollutants , e.g. photochemical smogs and acid mists. Escaping gas, or vapour, may... 

Oxidation-reduction reactions in water are dominated by the biological processes of photosynthesis and organic matter oxidation. A very different set of oxidation reactions occurs within the gas phase of the atmosphere, often a consequence of photochemical production and destruction of ozone (O3). While such reactions are of great importance to chemistry of the atmosphere - e.g., they limit the lifetime in the atmosphere of species like CO and CH4 - the global amount of these reactions is trivial compared to the global O2 production and consumption by photosynthesis and respiration. 

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