Comparison between ozone column depths and methane lifetimes computed by 1-D and 3-D models at different atmospheric O2 Levels

Recently, Cooke et al. (2022) used a 3-D coupled chemistry-climate model
(WACCM6) to calculate ozone column depths at varied atmospheric O2 levels.
They argued that previous 1-D photochemical model studies, e.g., Segura et
al. (2003), may have overestimated the ozone column depth at low pO2, 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 another 1-D and 3-D model.
The discrepancy in ozone column depths is likely due to multiple
interacting parameters, including lower boundary conditions, vertical and
meridional transport rates, and different chemical mechanisms, especially
the treatment of O2 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 pO2 is reduced when
absorption from CO2 and H2O 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 parameterization for O2 photolysis in the SR bands and then
repeating these calculations in the various models. Work is already
underway to this end.