Three options for the flexible operation of the cap-and-trade System of the air pollutants currently in operation in the Seoul Metropolitan Area are discussed by adding greenhouse gases. The three options are 1) linkage between the cap-and-trade system for air pollutants and the Korea Voluntary Emission Reduction Program, 2) permitting both the intra-trading among the air pollutants (NOx and SOx, and possibly PM) and inter-trading between the air pollutants and GHGs, and 3) introducing the project-based certificationmechanism of pollutants reduction and extending the participants. 1) most of facilities which register the KVER program in the SMA is also the target facilities of the cap-and-trade system of air pollutants in the SMA . Thus, if both programs are linked, it is expected that the facilities do not need to respond to two programs separately and can reduce expanses. 2), 3) After applying these options, participants, tradable species, offset species, seller, attainment evaluation would be extended. It might encourage active participation and the market active.