We present a new 12-km nested resolution capability in the GEOS-Chem global model of atmospheric composition. This capability can be applied to simulations for any user-selected domain worldwide from March 2021 onward by accessing a new hourly cubed-sphere C720 (≈0.125°×0.15625° or 12×12 km²) global wind archive from the NASA GEOS-FP meteorological data assimilation system. We evaluate this 12-km configuration of GEOS-Chem by comparison with the standard 25-km nested configuration in simulations of transport tracers, oxidant-aerosol chemistry, and inversions of satellite data using the Integrated Methane Inversion (IMI). The 12-km simulation features stronger vertical transport (up to 20 % lower surface ²²²Rn concentrations)because it better captures eddy fluxes both spatially and temporally. Aerosol deposition and stratosphere–troposphere exchange are similar at the two resolutions. The 12-km oxidant-aerosol chemistry can better simulate urban observations of NO₂, with stronger ozone urban titration but slightly higher surface ozone background due to enhanced vertical transport. 12-km and 25-km inversions using the IMI show highly consistent results on the regional scale, but the 12-km inversion provides greater information and improved spatial detail to resolve emissions from different sectors.