Hydrogen-induced methane lifetime response

In a fixed-production hydroxyl radical box, adding hydrogen by $\Delta [{\mathrm{H}}_2]$ changes $K$ to $K+k_{{\mathrm{H}}_2+OH}\Delta [{\mathrm{H}}_2]$. Methane lifetime scales as $K/P$ when its hydroxyl radical reaction coefficient is held fixed. The pre-existing hydrogen contribution is already in $K$ and must not be counted again.