The market based SO2 allowance trading component of the Acid Rain Program was intended to allow utilities to adopt the most cost effective strategy to reduce SO2 emissions. Every Acid Rain Program operating permit outlines specific requirements and compliance options chosen by each source. Affected utilities also were required to install systems that continuously monitor emissions of SO2, NOx, and other related pollutants in order to track progress, ensure compliance, and provide credibility to the trading component of the program.