We describe and evaluate a system for regional modeling of atmospheric composition with the Met Office Unified Model (UM), suitable for climate, weather forecasting and air quality applications. In this system, named NUMAC (`Nested UM with Aerosols and Chemistry'), a global model provides boundary conditions for regional models nested within it, using the Met Office's Regional Nesting Suite for multi-scale simulations. The regional models, which can run at convection-permitting or cloud-resolving scales, use the same code as the global model. The system includes double-moment prognostic aerosol microphysics with interactive chemistry of sulfur species, ozone, NO_x and CO as in the UK Earth System Model (UKESM). Double-moment prognostic cloud microphysics is optional. To test NUMAC, we compare simulations to surface and aircraft measurements from NASA's KORUS-AQ campaign over South Korea. The performance of the regional model, which we run at 5 km resolution, is similar to the well-evaluated global model when the regional and global models use the same emissions. Most species such as ozone, NO_x, OH, or PM_2.5 are simulated within a factor of 2 of observations most of the time, though they are biased low compared to sensors in polluted areas (observed surface dry PM_2.5 averages 28ugm^-3 but we simulate 17ugm^-3). Meteorology and clouds are represented satisfactorily. With higher-resolution emissions, many of the low model biases are reduced, but a tuning was required to keep NO concentrations realistic, indicating shortcomings in the chemistry scheme. We demonstrate the potential of NUMAC for studies of aerosol-cloud interactions.