Removal of trace substances from the atmosphere

The concentration of trace constituents in the atmosphere would rise quickly if sink mechanisms did not assure thecleansing of the atmosphere. Thus, Hales (1978) calculated from the sulfur source strength and the tropospheric volume that the average sulfur concentration would increase annually about 70/rgm-3 without the action of removal processes. This means that within 1 year remote tropospheric air would become at least as polluted as dirty urban locations are today. 

We have seen in Chapter 3 that in the case of several trace gases (H2, CH4, CO, 03, N20) these sink mechanisms are provided by chemical reactions producing components that are utilized by the biosphere. Another possibility for the removal of some gaseous species (sulfur, nitrogen and organic compounds) is their transformation into particulate matter (see Chapters 3 and 4). 

The aim of this chapter is to present briefly our ideas on the removal of aerosol particles and water soluble gases (e.g. S02, NH3, N02), both during dry weather conditions (dry removal) and during periods with cloud and precipitation formation (wet removal). Because of wet removal, precipitation water contains many soluble (and insoluble) materials, as we will see at the end of this chapter. 

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Fig. 7-13 Physical transformations of trace substances in the atmosphere. Each box represents a physically and chemically definable entity. The transformations are given in F, (from the ith to the /th box). Q, represents sources contributing to the mass or burden, M,> in the ith box. Rd, and Rw, are dry and wet removals from M,. The dashed box represents what may be called the fine-particle aerosol and could be a single box instead of the set of four sub-boxes (i = 1,2,3,4). The physical transformations are as follows ...

Atmospheric chemistry deals with chemical compounds in the atmosphere, their distribution, origin, chemical transformation into other compounds, and finally, their removal from the atmospheric domain. These substances may occur as gases, liquids, or solids. The composition of the atmosphere is dominated by the gases nitrogen and oxygen in proportions that have been found invariable in time and space at altitudes up to 100 km. All other components are minor ones with many of them occurring only in traces. Atmospheric chemistry thus deals primarily with trace substances. 

Preparation of the intermediate for Allobarbitone (diethyl diallyl malonate). Diethyl malonate is dissolved in anhydrous alcohol and treated with one mole of clean sodium meted per every one mole of the ester. To this solution add one mole of allyl chloride and reflux for about 4 hours. Another equimolar ratio (1 mole of sodium per mole of ester) of sodium is added, followed by the same ratio of allyl chloride (1 mole per 1 mole), and this mixture is boiled for 2 hours. The alcohol is removed by distillation and the ester is extracted with benzene and distilled or evaporated in vacuo, recrystallized with a suitable "dry" solvent, and filtered. Evaporate again to remove traces of solvent. Keep this product, and any other substances that require dry reagents or solvents, stored away from contact with the atmosphere. When evaporating, filter the air coming into the evaporating vessel with a suitable drying agent. Use a little common sense. 

Thus the lifetime of a constituent with a first order removal process is equal to the inverse of the first order rate constant for its removal. Taking an example from atmospheric chemistry, the major removal mechanism for many trace gases is reaction with hydroxyl radical, OH. Considering two substances with very different rate constants for this reaction, methane and nitrogen dioxide... 

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