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Chapman ozone equilibrium balances production and destruction of atomic oxygen and ozone in a minimal oxygen-only atmospheric photochemistry model. Write square brackets for number densities. Define photolysis rate coefficients $J_1,J_3$ and collisional rate constants $K_2,K_4$ byThe collision partner $\mathrm{M}$ removes excess formation energy without being consumed. The third-body requirement reflects conservation of energy and momentum conservation during ozone formation.^[ref]

Adding the reaction rate equations for atomic oxygen and ozone cancels their rapid interconversion. A local steady state givesIf ozone loss is predominantly photolytic, $J_3\gg K_4[\mathrm{O}]$, its separate balance isCombining the two balances yieldsThe two factors of two in oxygen-atom production and destruction cancel in the first balance. This is a sunlit local steady state, with specified temperature, radiation and background number densities; transport and additional catalytic chemistry need not obey this four-reaction estimate.

Sydney Chapman developed a quantitative explanation of atmospheric ozone photochemistry. In the basic mechanism, ultraviolet radiation dissociates molecular oxygen, an oxygen atom combines with another molecular oxygen and a third collision partner to form ozone, and further reactions remove it. His 1930 treatment connected the production and destruction reaction rates with the distribution and seasonal variation of ozone, while explicitly setting aside uncertain atmospheric transport. It was a testable physical model with stated omissions, rather than a complete account of all observed ozone changes.^[ref]