The movement of ozone and other pollutants by wind currents - a phenomenon known as interstate transport - can cause pollution levels to be elevated even in areas that do not have emission sources. Multiple actions have been taken to address interstate transport:
- In 1990 Congress created the ten state Ozone Transport Commission (OTC) directing the states in the Northeastern U.S. to collaboratively find a solution to persisitent summertime ozone. The OTC set out to accomplish this goal in part by implementing a regional trading program for NOx (an ozone precursor).
- In 1998, the U.S. EPA finalized the NOx SIP Call, ultimately setting up the NOx Budget Trading Program (NBP), a new program covering 20 states and the District of Columbia. The states in the original OTC trading program joined the NBP.
- In 2005, EPA promulgated the Clean Air Interstate Rule (CAIR) to continue to deal with interstate transport of ozone. Under CAIR, the geographic trading region was again extended to helps states in their efforts to reduce particluate matter emissions, NOx and SO2. CAIR's emission reduction requirements begin in 2009 for the NOx ozone season and and annual programs, and 2010 for the SO2 annual program. CAIR was challenged in U.S. Court of Appeals for the District of Columbia which ultimately sent the rule back to EPA to fix flaws identified by the court, but let the CAIR emission reductions take effect in order to protect the environment.
- In 2011, EPA promulgated the Cross-State Air Pollution Rule (CSAPR) to replace the remanded CAIR rule. EPA again relied on a trading program to implement redcutions, with the geographic region growing to include 28 states. Liake CAIR, CASPR was challenged in the courts. On August 21, 2012 the District Court vacated CSAPR and told EPA to continue to administer CAIR while it dealt with the flaws the court identified. The court did not set a definitive schedule for EPA to replace CSAPR, only directing them to act expidiously.