In keeping with the emphasis on atmospheric physics in this book, the purpose of the present chapter is to illustrate the use of basic physical principles in the study of some aspects of atmospheric chemistry, rather than to provide a comprehensive treatment of atmospheric chemistry as a whole. We therefore focus on stratospheric chemistry, which provides some simple yet important applications of the basic principles and also some examples of interactions between chemistry and dynamics.

In Section 6.1 we outline some of the basic thermodynamics of chemical reactions, while in Section 6.2 we introduce some elementary aspects of chemical kinetics, including the concepts of reaction rates and chemical lifetimes. In Section 6.3 we focus on bimolecular reactions and show how physical reasoning can give an expression for the reaction rate. The process of photodissociation is introduced in Section 6.4. Once these basic ideas have been established, we apply them to stratospheric ozone in Section 6.5, first describing the Chapman theory (which involves oxygen compounds only) and then introducing the effects of catalytic cycles. The principles of chemical transport by atmospheric flows are discussed in Section 6.6, with a qualitative description of the main global-scale meridional transport structures in the middle atmosphere. Finally, in Section 6.7, we bring several of these ideas together in a general description of the processes implicated in the formation of the Antarctic ozone hole.