The kinetics of atmospheric reactions

Components of the atmosphere and emissions released into the atmosphere may react with each other with the possible production of further, so-called secondarily polluting components. A knowledge of the rate of these reactions, together with the associated chemical thermodynamics, make it possible to determine the probability of their course, which may help to explain the mechanism of atmospheric processes. 

The reaction rate depends on the concentration of the reacting substances, temperature, and the presence of other species (which may catalyse the process), and in photochemical reactions the rate depends also on the intensity and spectrum of the solar radiation. 

High dilution of polluting components in the atmosphere causes that only very rapid reactions will occur to a sufficient extent. For example, when the rate of a bimolecular reaction for a content of 10 vol.% of reacting components leads to a 50% conversion being achieved in 3.6 x 10 s, then in the case of a content decreased by a factor of 10, the resulting decrease of the reaction rate will give rise to the same 50% conversion only after 1 hour. This means that the rate of relevant reactions should be very high indeed [26] in order to be able to observe these reactions. 

There is a different situation in the case of reactions involving oxygen, carbon dioxide and water vapour. These stable components of the atmosphere are present in much higher concentrations. The oxygen content is frequently higher by a factor of 10 as compared to the content of polluting components, the concentration of carbon dioxide is higher by a factor of 10  

The kinetics of photochemical processes depend to a great extent on the solar radiation intensity. This also affects the production of photochemi-cally produced secondary polluting components, which varies according to the rhythm of the diurnal cycle of the solar intensity. 

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