The modeling of dispersion of reacting pollutants in the atmosphere requires proper description of both the chemistry involved and the transport of pollutants. We present here highly reduced chemical schemes for rural and urban dispersion problems based on the Computational Singular Perturbation (CSP) technique. CSP identifies the steady-state species for removal, by resolving the "fast" and the "slow" chemistry components. Here, the CSP method is employed for the reduction of the Regional Atmospheric Chemistry Mechanism (RACM), which includes 77 species and 237 reactions. Three cases were tested: (1) a polluted scenario including ground emissions, (2) a relatively cleaner environment with 10% of the emission levels of case 1, and finally (3) rural conditions without any emissions, for which the effects of emissions on the reduction procedure were directly studied. It was found that reduced mechanisms with high degree of reduction, down to a 16 step mechanism, i.e., 56 species in steady state, produced excellent results for most species for all cases. These reduced mechanisms did not change much between urban or rural conditions and the inclusion of emissions did not affect significantly the selection of major species. This indicates the persistence of a well-defined fast subset of the mechanism, suggesting that the produced reduced mechanisms are valid for a wide range of conditions in the atmosphere.