= Solution
Three routes away from local <thermochemical equilibrium> are:
* Vertical transport and quenching. When the <eddy mixing time> becomes shorter than the <chemical relaxation time>, gas retains a deeper composition. In cool, directly imaged giant <exoplanets>, <carbon monoxide–methane quenching> can preserve CO and suppress the methane expected in cool equilibrium layers. In <Jupiter>, CO carried from deeper hot layers, and <phosphine> maintained against upper-atmosphere chemical loss, are examples of transported disequilibrium species.
* Horizontal transport and quenching. <Atmospheric advection time> shorter than the local <chemical relaxation time> carries abundances between regions of different <temperature>. A <hot Jupiter> can carry CO-rich dayside gas into cooler nightside gas that would otherwise favour <methane>. In the Solar System, upper-atmosphere CO produced on the illuminated side of <Venus> can be redistributed to its dark side by circulation; this is an example of nonlocal production and transport, rather than local dark-side thermochemical equilibrium.
* <Atmospheric photochemistry>. Ultraviolet photons dissociate or ionize molecules and initiate reaction networks. Models of irradiated hydrogen-rich <exoplanet atmospheres> can produce enhanced <hydrogen cyanide> and hydrocarbon precursors from methane/nitrogen chemistry; whether these products accumulate depends on ultraviolet flux and transport. The <terrestrial atmosphere> gives a clear Solar-System example: oxygen photodissociation and subsequent reactions maintain the <ozone layer>, which is not a purely thermochemical-equilibrium abundance.
\b[Vertical mixing, horizontal advection, and photochemistry supply three mechanisms and examples in both exoplanets and Solar-System atmospheres.] A photochemical steady state balances production and loss; it is different from a <Gibbs free energy> minimum. These examples describe mechanisms and model expectations rather than asserting unique observational attribution for every planet.
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