This paper analyses the consequences of incomplete environmental model specification on policy design in a multi-pollutants framework. Focusing on the correlations between aerosols and greenhouse gases, the paper compares abatement and stock targets setting for various cases of incomplete model specification. Both cases of lax and stringent regulation, compared to what is socially optimum, may arise. Furthermore, the lax/stringent nature of the policies may evolve over time, so that the time horizon of policy design matters in assessing the impact of incomplete model specification.