Chemical quench level

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.

For a vertical eddy diffusivity $K_{zz}$, mixing over a length $\ell$ takes ${\tau }_{\mathrm{mix}}\sim {\ell }^2/K_{zz}$. The condition that transport outrun a chemical relaxation time therefore gives $K_{zz}\gtrsim {\ell }^2/{\tau }_{\mathrm{chem}}$. Taking $\ell =0.1H$ instead of one atmospheric scale height $H$ changes this estimate by a factor of one hundred. The atmospheric scale height itself depends on both temperature and gravity.

Atmospheric advection time shorter than the chemical relaxation time allows winds to carry a composition into regions where it differs from local thermochemical equilibrium. In a hot Jupiter, this can transport dayside carbon chemistry into the cooler nightside.

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.

Carbon monoxide–methane quenching preserves a transported carbon partition after local reactions become too slow to restore thermochemical equilibrium. A useful net reaction isThis is a net stoichiometric balance, rather than a single elementary reaction whose rate can simply be read from the equation. Temperature, pressure and the kinetic pathway determine the preferred composition and conversion time.

In models of hot Jupiters, vertical transport can maintain methane away from its local equilibrium abundance, while atmospheric photochemistry removes it at higher levels.^[ref] A methane band therefore probes both chemistry and circulation, rather than acting as a unique local temperature measurement.

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.