= Solution
Four mechanisms that can produce <disequilibrium chemistry in an exoplanet atmosphere> are:
* \b[<Atmospheric photochemistry>.] Stellar ultraviolet photons initiate reactions whose products need not follow local <thermochemical equilibrium>. The <ozone layer> on <Earth> is a solar-system example. Calculations for <HD 189733 b> predict enhanced <hydrogen cyanide> and <acetylene> from the processing of <methane> and <ammonia>; these are model examples rather than assertions of an unambiguous detection.
* \b[Vertical transport and chemical quenching.] When the <eddy mixing time> is shorter than the <chemical relaxation time>, gas retains a deeper abundance above its <chemical quench level>. The excess <carbon monoxide> in <Jupiter>'s cool atmosphere exemplifies <carbon monoxide–methane quenching>. Models of <HD 189733 b> predict quenched <methane> and <ammonia> abundances differing from their local <chemical equilibrium> values. The enhancement or depletion depends on the underlying <atmospheric pressure-temperature profile>.
* \b[<Horizontal chemical quenching>.] If the <atmospheric advection time> is short, winds move chemically processed gas into regions with different irradiation or <temperature> faster than it can re-equilibrate. Transport of gas within <Earth>'s <ozone layer> moves material away from its local photochemical production regions. Models of <HD 209458 b> show that dayside <carbon monoxide>-rich composition can persist into the cooler nightside instead of forming the local <chemical equilibrium> amount of <methane>.
* \b[Condensation with sedimentation or rainout.] Finite-rate cloud formation can depart from phase equilibrium, while <atmospheric condensate rainout> removes elements from a layer and changes its gas composition. <Earth>'s <water> <atmospheric cold trap> limits the supply of <water> to the stratosphere. In <HD 209458 b> models, titanium-bearing condensates can settle and suppress upper-atmospheric <titanium monoxide>. The remaining gas can still be in local <chemical equilibrium> with its depleted inventory: rainout is an open-column effect, not necessarily a failure of equilibrium among all gas reactions.
The exoplanet transport and photochemical examples follow https://arxiv.org/abs/1102.0063[kinetic atmosphere calculations] and https://doi.org/10.1051/0004-6361/201322895[models including horizontal transport]; the condensate example is examined in https://arxiv.org/abs/0902.3995[cold-trap calculations].
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