= Disequilibrium chemistry in an exoplanet atmosphere
<Disequilibrium chemistry in an exoplanet atmosphere> occurs when transport or externally driven reactions prevent the composition from reaching local <thermochemical equilibrium>. Deep gas brought upward can retain its earlier composition if <chemical reactions> become too slow in cooler layers. At higher altitudes, <atmospheric photochemistry> can instead destroy or create species using stellar <photons>.
Models coupling <chemical kinetics> with vertical diffusion for <HD 189733 b> and <HD 209458 b> demonstrate that <chemical quench levels> and photochemical destruction can affect the same molecule at different heights.<Disequilibrium Carbon, Oxygen, and Nitrogen Chemistry in the Atmospheres of HD 189733b and HD 209458b by Julianne I. Moses and others>[Abstract: transport-enhanced methane/ammonia followed by high-altitude photochemical removal]{ref} An <atmospheric retrieval> that assumes equilibrium everywhere can consequently infer a different composition or <atmospheric pressure-temperature profile> from one that allows transport.
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