Selling Clean Air In Costa Rica -- In the third and final part of his series, NPR's John Burnett reports Costa Rica has adopted a concept known as Joint Implementation to help finance the preservation of the country's forests. The idea is for industrial polluters like utility companies to "buy" the carbon dioxide cleansing properties of the country's forests. They can either invest in hydroelectric plants or wind farms to avoid further pollution, or contribute money to directly help preserve the trees. Now Costa Rica has a new twist -- it's sold two million dollars worth of tradable pollution certificates measured in tons of carbon dioxide absorbed by a given number of trees. A Chicago investment firm is even selling the certificates to the public as commodities.