Nitric acid, tropospheric sinks

Although the nitric acid molecule is subject to various reactions and to photodissociation, nevertheless it remains, and it becomes the most important of the molecules containing NO (HN04, N205, NO3,. . . ) in the lower stratosphere. However, it cannot accumulate because it crosses the tropopause into the troposphere, where it rapidly disappears because of its solubility in water. Thus, if N20 is the source of the nitrogen oxides in the stratosphere, nitric acid is the sink that prevents their accumulation beyond certain limits. But it is now known that the sequence of reactions (20), (21), and (22) results in a lower concentration of stratospheric ozone than would be possible in a pure oxygen atmosphere. 

The last step in the current manufacture of adipic acid involves oxidation by nitric acid, which results in the formation of nitrous oxide (N2O) that is released into the atmosphere. Given that N2O has no tropospheric sinks, it can rise to the stratosphere and be a factor in the destruction of the ozone layer. It also acts as a greenhouse gas (see Section 8.4.1). 

Thus it can be seen that nitric acid is a significant reservoir or sink for the oxides of nitrogen. In the lowest stratosphere, the nitric acid predominates over the NO2 and a major loss of NO from the stratosphere occurs by diffusion of the acid into the troposphere where it is rained out. 

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