Comparison between ozone column depths and methane lifetimes computed by one- and three-dimensional models at different atmospheric O 2 levels.

Recently, Cooke et al . (Cooke et al . 2022 R. Soc. Open Sci. 9 , 211165. (doi:10.1098/rsos.211165)) used a three-dimensional coupled chemistry-climate model (WACCM6) to calculate ozone column depths at varied atmospheric O 2 levels. They argued that previous one-dimensional (1-D) photochemical model studies, e.g. Segura et al . (Segura et al . 2003 Astrobiology 3 , 689-708. (doi:10.1089/153110703322736024)), may have overestimated the ozone column depth at low pO 2 , and hence also overestimated the lifetime of methane. We have compared new simulations from an updated version of the Segura et al . model with those from WACCM6, together with some results from a second three-dimensional model. The discrepancy in ozone column depths is probably due to multiple interacting parameters, including H 2 O in the upper troposphere, lower boundary conditions, vertical and meridional transport rates, and different chemical mechanisms, especially the treatment of O 2 photolysis in the Schumann-Runge (SR) bands (175-205 nm). The discrepancy in tropospheric OH concentrations and methane lifetime between WACCM6 and the 1-D model at low pO 2 is reduced when absorption from CO 2 and H 2 O in this wavelength region is included in WACCM6. Including scattering in the SR bands may further reduce this difference. Resolving these issues can be accomplished by developing an accurate parametrization for O 2 photolysis in the SR bands and then repeating these calculations in the various models.