NOx SIP Call
The most actively traded emission markets in the United States are the federal nitrogen oxides (NOx) and sulfur dioxide (SO2) markets.
An EPA program intended to mitigate NOx emissions, one of the chemical precursors of ozone, the Federal NOx Budget Trading Program, or NOx State Implementation Program (SIP) Call, became effective on May 1, 2003. The EPA introduced the NOx Budget Trading Program rule for states which opted to meet the requirements of the NOx SIP Call through a cap-and-trade program.
NOx SIP Call allowances are currently tradable in 22 states and the District of Columbia.
Note: the NOx SIP Call will be replaced with the Clean Air Interstate Rule, or CAIR.