Atmospheric photochemistry

Atmospheric photochemistry is the network of chemical reactions initiated when absorbed photons dissociate or ionize atmospheric species. The initial fragments can react with other molecules, so a small abundance of short-lived products can control the chemistry of much more abundant gases. The reaction rates depend on the stellar spectrum, absorption and the amount of shielding above a layer.

The Chapman ozone equilibrium illustrates coupling between photodissociation, molecular formation and destruction. In an exoplanet atmosphere, such reactions can change spectral absorbers or supply material for atmospheric haze. A photochemical steady state balances production and loss while reactions continue; it need not be thermochemical equilibrium, because stellar radiation continuously drives the system.

A photochemical steady state has approximately constant local abundances because production and loss balance for each species. If a molecule is produced at a rate $P$ per unit volume and removed with a first-order rate coefficient $k$, then $dn/dt=P-kn$ and the stationary number density is $n=P/k$. The relaxation time is $1/k$ when the coefficients stay fixed.

This state is useful for short-lived intermediates in atmospheric photochemistry. It does not mean that molecular reactions stop or that thermodynamic equilibrium has been reached. Transport adds another term to the abundance balance; if it is important, the local chemical production and loss rates need not balance each other by themselves.

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.

An ozone layer is an atmospheric altitude range with enhanced ozone abundance. Oxygen-dissociating ultraviolet radiation is strongest aloft while three-body formation is fastest at greater density, so their competition can concentrate ozone at intermediate altitude.