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We present a systematic study of the spectral consequences of departures fromchemical equilibrium in the atmospheres of L and T dwarfs, and for even coolerdwarfs. The temperature/pressure profiles of the non-equilibrium models arefully consistent with the non-equilibrium chemistry. Our grid ofnon-equilibrium models includes spectra for effective temperatures from 200 Kto 1800 K, three surface gravities, four possible values of the coefficient ofeddy diffusion in the radiative zone, and three different CO/CH$_4$ chemicalreaction prescriptions. We find that the non-equilibrium overabundance of COtranslates into flux suppressions in the M ($\sim4-$5 $\mu$m) band of at most$\sim$40% between effective temperatures of 600 and 1800 K. The effect islargest around $T_{\rm eff} \approx 1100$ K. The underabundance of ammonia dueto non-equilibrium chemistry translates into flux enhancements of no more than$\sim$20% for the $T_{\rm eff}$ range from 300 to 1800 K, with the largesteffects at the lowest values of $T_{\rm eff}$. The magnitude of the departurefrom chemical equilibrium increases with decreasing gravity, with increasingeddy diffusion coefficient, and with decreasing speed of the CO/CH$_4$reaction. Though these effects are modest, they lead to better fits with themeasured T dwarf spectra. Furthermore, the suppression in the M band due tonon-equilibrium enhancements in the CO abundance disappears below $\sim$500 K,and is only partial above $\sim$500 K, preserving the M band flux as a usefuldiagnostic of cool atmospheres and maintaining its importance for searches forbrown dwarfs cooler than T dwarfs.

A Systematic Study of Departures from Chemical Equilibrium in the Atmospheres of Substellar Mass Objects