Commodity market
In this market, the atmosphere's capacity to absorb certain amounts of pollutants is measured, divided into units, and traded amongst various market players.  Those who emit more SO2 must pay those who emit less.  Critics of such schemes argue that unauthorized or unregulated emissions still happen, and that "grandfathering" schemes often permit major polluters, such as the state governments' own agencies, or poorer countries, to expand emissions and take jobs, while the SO2 output still floats over the border and causes death.