the practice of buying or selling emission credits 排放交易〔有害物质的排放配额的买卖〕
From Longman Business Dictionary
emissions tradingeˈmissions ˌtrading
FINANCEthe activity of buying and selling emission credits (=a tax credit which a company that does not send harmful gases or other substances into the air can sell to a company that does) or emission units (=an official right allowing a company to send a specific amount of harmful gases or other substances into the air)