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A chemical quench level is an atmospheric region where the chemical relaxation time becomes comparable to a transport time. Below it, sufficiently fast chemical reactions maintain near-local thermochemical equilibrium. Above it, slower reactions cannot keep up with transported gas, allowing the composition to retain information about deeper conditions.

For vertical mixing over length $L$, a useful estimate is ${\tau }_{\mathrm{mix}}\sim L^2/K_{zz}$ with vertical eddy diffusion coefficient $K_{zz}$. Comparing this with a reaction-network relaxation time locates an approximate quench region. The quenched atmospheric mixing ratio can remain nearly constant even though number density falls upward. Different species can quench at different depths, and atmospheric photochemistry or atmospheric condensate rainout can subsequently alter their abundances.

The chemical relaxation time estimates how rapidly a perturbed abundance returns toward chemical equilibrium. Comparing it with the eddy mixing time identifies a chemical quench level.

For the A-to-B model in chemical equilibrium, deviations decay by exponential decay with ${\tau }_{\mathrm{chem}}=1/(k_f+k_r)=2.5\mathrm{s}$. After one such interval, $1/e$ of the initial deviation remains; reaching one percent takes $\ln (100){\tau }_{\mathrm{chem}}\simeq 11.51\mathrm{s}$. Multiplying both rate constants by two preserves their ratio and the equilibrium composition, but halves ${\tau }_{\mathrm{chem}}$ to $1.25\mathrm{s}$. The equilibrium exchange rates double to $0.15M/s$. Faster exchange therefore coexists with the same final composition and the same free energy minimum.

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.^[ref] An atmospheric retrieval that assumes equilibrium everywhere can consequently infer a different composition or atmospheric pressure-temperature profile from one that allows transport.

Nitrogen–ammonia quenching occurs when transport preserves a nonequilibrium partition between molecular nitrogen and Ammonia. Their net conversion may be writtenbut its actual relaxation time depends on a multistep reaction network. Near a chemical quench level, that time exceeds the eddy mixing time, so the abundance can retain a deeper value instead of following local thermochemical equilibrium.

Coupled diffusion and chemistry models illustrate ammonia enhancement by transport and destruction at high altitudes by atmospheric photochemistry.^[ref] Interpreting an ammonia feature therefore requires an atmospheric pressure-temperature profile and a transport model, not only a molecular identification.

For a representative hydrogen-rich hot Jupiter, increasing altitude lowers pressure, slows collision-driven chemistry, and increases exposure to stellar UV. Three principal regimes follow.

At high pressures, illustratively $P\gtrsim 1$--$10$ bar, collisions and sufficiently high temperature make thermochemical equilibrium a useful approximation. Molecular abundances minimize the free energy subject to elemental conservation. Condensation and atmospheric condensate rainout can remove selected elements from the gas.

At intermediate pressures, illustratively $10^{-3}\lesssim P\lesssim 1$ bar, vertical mixing and horizontal winds can outrun chemical relaxation time. The chemical quench level is defined by ${\tau }_{\mathrm{chem}}\sim {\tau }_{\mathrm{mix}}$; above it, some abundances retain values from a deeper or hotter region. Horizontal chemical quenching similarly occurs when chemical adjustment is slower than advection between the day and night hemispheres.

At low pressures, illustratively $P\lesssim 10^{-3}$ bar, atmospheric photochemistry becomes important: photodissociation and radical reactions change equilibrium abundances and can produce hydrocarbons and atmospheric hazes. At still lower pressures, roughly $10^{-6}$ bar and above in altitude, photoionization and an escaping thermosphere can become important.

These pressures label a representative sketch, not universal interfaces. UV optical depth, stellar spectrum, gravity, metallicity, reaction rates and eddy diffusivity determine the transitions; quenching is species-dependent.

Convection mixes material vertically where the Schwarzschild criterion or Ledoux criterion allows it. Its chemical effect is controlled by competition between transport and reaction, rather than by a direct alteration of the equilibrium constant. Parametrize vertical transport by a vertical eddy diffusion coefficient $K_{zz}$; the eddy mixing time over a scale height isLet ${\tau }_{\mathrm{chem}}$ be the chemical relaxation time of the relevant interconversion. If reactions are faster, the gas follows local thermochemical equilibrium. If transport becomes faster, composition can be carried upward faster than it re-equilibrates. A chemical quench level is approximately where the two times match:For example, hot deep gas may contain abundant carbon monoxide. Strong mixing can maintain it in cooler layers where equilibrium would favour methane; this is carbon monoxide–methane quenching. Similar reasoning applies to nitrogen–ammonia quenching. The cooler nightside commonly has slower reaction rates and can preserve such quenched composition more readily, whereas sufficiently rapid dayside chemistry can restore local equilibrium. Neither statement is universal without the actual reaction network and temperature.

The observable atmosphere of a strongly irradiated hot Jupiter may be radiative and stable even if the deeper interior is convective. Deep convection influences the photosphere only if some mixing crosses the intervening radiative region, for example through overshoot or other dynamical transport. Moreover, transport from day to night is horizontal circulation, not vertical convection; its relevant competition is atmospheric advection time versus chemical relaxation time, giving horizontal chemical quenching. Convection can affect either hemisphere indirectly through connected vertical transport; strong deep convection alone does not guarantee a quenched observable composition.

Four mechanisms that can produce disequilibrium chemistry in an exoplanet atmosphere are:

The exoplanet transport and photochemical examples follow kinetic atmosphere calculations and models including horizontal transport; the condensate example is examined in cold-trap calculations.

Take the logarithm to base ten and write pressure in units of $1\mathrm{bar}$, so its argument is dimensionless. Continuity of the atmospheric pressure-temperature profile givesThusIf the logarithm means $\ln$, the equivalent constant is $C=900/\ln 100\simeq 195.4\mathrm{K}$. A plot of temperature against logarithmic pressure is vertical in each isothermal region and straight between the two endpoints.

The processes can be organized by the supplied pressure ranges, although the exact boundaries require reaction rates, irradiation and mixing information:

The profile identifies plausible chemical regimes, but does not fix their transition pressures by itself. In particular, cloud formation depends on the species-specific condensation curve, and ultraviolet processing depends on shielding.

Use the standard local chemical quench level approximation: carbon monoxide-rich gas from the hot deep region is transported upward, and conversion becomes slower as the gas enters the cooler layers. Assume that neither a faster loss process nor strong compositional fractionation removes carbon monoxide above the quench level. For an effective mixing length $L$, the eddy mixing time is ${\tau }_{\mathrm{mix}}\simeq L^2/K_{zz}$, so outrunning conversion near $1\mathrm{bar}$ requiresThe usual order-of-magnitude choice is $L\simeq h_b$, the local atmospheric scale height at $T_b=1400\mathrm{K}$. Equal planetary mass and radius give the same gravity as Jupiter; with the same mean molecular weight, the atmospheric scale height scales linearly with temperature. To use the supplied $30\mathrm{km}$ reference, additionally adopt a representative Jovian reference temperature $T_J=150\mathrm{K}$. ThenUsing $k_{B}T/(\mu m_{H}g)$ directly with $\mu \simeq 2.3$ and Jovian $g\simeq 25\mathrm{m}{\mathrm{s}}^{-2}$ gives $h_b\simeq 200\mathrm{km}$ and $K_{zz}\gtrsim 4\times 10^5{\mathrm{m}}^2{\mathrm{s}}^{-1}$, the same order of magnitude. The supplied Jovian height is approximate and does not specify its reference temperature. Inserting $30\mathrm{km}$ unchanged for the hot gas would instead give $9\times 10^3{\mathrm{m}}^2{\mathrm{s}}^{-1}$ and neglect this temperature scaling.

The pressure separation is relevant to a stronger, whole-column transport estimate. With constant gravity and mean molecular weight, the profile givesIf one additionally requires diffusion through the entire column within $10^5\mathrm{s}$, a sufficient conservative condition is $K_{zz}\gtrsim \Delta z^2/{\tau }_{\mathrm{chem}}\sim 10^7{\mathrm{m}}^2{\mathrm{s}}^{-1}$. It is not a necessary local quench condition: the chemical relaxation time is expected to become much longer in the cooler gas, so a longer total transit time can still preserve carbon monoxide.

The usual quench estimate is $K_{zz}$ of order $10^6{\mathrm{m}}^2{\mathrm{s}}^{-1}$ under the stated scale-height assumptions. A unique bound for survival to $10^{-3}\mathrm{bar}$ cannot be inferred from one reaction time without assumptions about its variation, the mixing length and upper-atmospheric losses.

Assume a hydrogen-helium perfect gas with mean particle mass $2.3m_H$, Jupiter gravity $g\simeq 24.8\mathrm{m}{\mathrm{s}}^{-2}$, and mixing over one atmospheric scale height. At $T=1000\mathrm{K}$,The eddy mixing time is ${\tau }_{\mathrm{mix}}\simeq H^2/K_{zz}$. A chemical quench level at one bar requires ${\tau }_{\mathrm{mix}}\lesssim {\tau }_{\mathrm{chem}}=10^5\mathrm{s}$, hence the vertical eddy diffusion coefficient must be of orderThe number depends quadratically on the assumed mixing length; using a fraction of $H$ reduces it accordingly.

Above the chemical quench level, neglect photochemistry, condensation, and molecular diffusion. The quenched atmospheric mixing ratio $f_A=n_A/n$ is approximately constant, while the number density isThus the abundance fraction is frozen, but the absolute number density falls with pressure and altitude. Examples are carbon monoxide–methane quenching, through $\mathrm{CO}+3{\mathrm{H}}_2\rightleftharpoons {\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}$, and nitrogen–ammonia quenching, through ${\mathrm{N}}_2+3{\mathrm{H}}_2\rightleftharpoons 2{\mathrm{NH}}_3$.

Approximate the atmosphere above $R_p$ as isothermal with constant gravity and atmospheric scale height $H$, soVertical transport over one scale height has eddy mixing timeThe chemical quench level satisfies ${\tau }_{\mathrm{chem}}(z_q)={\tau }_{\mathrm{mix}}$. Since ${\tau }_{\mathrm{chem}}=\eta z$,Above this level, mixing is faster than reaction and freezes the deeper abundance of A. Larger $K_{zz}$ moves the quench level deeper and raises $P_q$. Important examples are carbon monoxide–methane quenching and nitrogen–ammonia quenching; phosphine destruction is another tracer of vertical quenching.

Above a chemical quench level, rapid eddy mixing can keep a species' number mixing ratio $f_A$ approximately fixed at its deeper value, provided condensation, photochemical loss, and molecular diffusion remain negligible. The species' number density is then $n_A=f_{A}p/(k_{B}T)$; constant mixing ratio does not imply constant number density. In an isothermal atmosphere, $n_A$ decreases exponentially with altitude on the atmospheric scale height.