In the coming decade, the most important intellectual challenge for the atmospheric sciences will be understanding the interaction between changes on the global scale and those on the regional scale. The problem is challenging due to the inherent nonlinearities in the photochemistry at all spatial scales, the difficulties in simultaneously representing atmospheric fluid dynamics at global and regional scales within a numerical model, the large demands placed on computational resources by the photochemical kinetics of all the relevant molecular species, and the limitations of observations which are too sparse, infrequent, and inaccurate, to adequately constrain our understanding. We attempt to address these issues through field, lab, and satellite measurements, as well as through modeling. Our two main focuses are Tropospheric Ozone and Oxidation and Aerosols, Water, and Climate.