Methane consumes hydroxyl radicals during its initial oxidation and therefore influences atmospheric oxidative capacity and the removal of other trace gases. Subsequent oxidation generates peroxy radicals. In the presence of sufficient nitrogen monoxide, these convert nitrogen monoxide to nitrogen dioxide without consuming ozone, allowing net photochemical ozone production. At low nitrogen monoxide, peroxide formation and ozone-destroying radical reactions compete. Methane oxidation ultimately produces carbon monoxide and carbon dioxide; the sign and rate of the associated ozone response depend on the radical/nitrogen oxides regime, not methane alone.
Methane reaching the stratosphere is oxidized and is an important source of stratospheric water vapor. That water can feed HOx radical family chemistry and alter radiative cooling and polar-cloud conditions. Methane also converts chlorine atoms into hydrogen chloride, opposing some chlorine-driven ozone loss. These effects compete: increasing methane does not have one universal ozone sign at every altitude and latitude.
Methane is an infrared-active greenhouse gas with absorption in the terrestrial thermal spectrum. Its direct forcing is supplemented by chemically induced changes in tropospheric ozone and stratospheric water vapor. Its finite atmospheric lifetime makes sustained emissions and emission pulses different experiments: stopping emissions permits the methane burden to decline much faster than the long tail of a carbon-dioxide perturbation. Quantitative comparisons require a specified emission mass, time horizon and treatment of indirect effects, as formalized by global warming potential.
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