Cost-effective policies allow for minimizing the compliance costs associated with reaching a desired environmental quality target. In this paper a conceptual model has been developed to examine the compliance costs under an intra-plant emission trading system for a non-uniformly mixed assimilative pollutant. The model incorporates the number of emission sources, the concentration of pollutants emitted at each source, the marginal cost of abatement for each source, the transfer coefficient that relates emission at each source with the impact on ambient air quality, and the desired ambient air quality target. The model is applied to an integrated steel plant in India. Results of this study demonstrate that emission trading is more cost effective than the existing regulatory system. Further, intra-plant trades would result in significant savings to the steel plant while securing an improvement in ambient air quality in the studied geographical area.