Connecticut has signed on to a regional effort to reduce greenhouse gas emissions and generate revenue. The Transportation and Climate Initiative Program will require gasoline and diesel suppliers to purchase emission allowances for air pollution created by the fuels they sell. The plan would gradually reduce the number of allowances sold each year. Supporters of the plan say that will reduce the amount of greenhouse gases generated. That could also make each allowance sold at auction more valuable. Its opponents say it will raise gas prices. The Connecticut Energy Marketers Association issued a statement calling the program a “money grab.” The TCI program also involves Massachusetts, Rhode Island, and the District of Columbia.