We implement a new 12-km global nested simulation capability in GEOS-Chem, an open-source global 3-D model of atmospheric chemistry. Compared with the standard 25-km simulation, the 12-km simulation features stronger vertical transport due to better resolved horizontal convergence, along with improved representation of urban NO₂ and ozone titration. Application to methane emission inversion yields higher information content and resolves finer spatial structure in emission sectors.