The Supreme Court justices overruled a decision by the U.S. 9th Circuit Court of Appeals that had upheld the 2-year-old regulation by the South Coast Air Quality Management District. The rule required the owners of diesel buses, garbage trucks, airport shuttles and other fleets to buy vehicles powered by cleaner-burning engines, such as those that use natural gas.
The court, in an 8-1 decision written by Justice Antonin Scalia, said that portions of the regulation appeared to be pre-empted by national anti-pollution laws that prohibit state and local agencies from adopting their own vehicle emission standards. The Supreme Court justice directed the 9th Circuit to review its decision.
"If one state or political subdivision may enact such rules, then so may any other; and the end result would undo Congress' carefully calibrated regulatory scheme," Scalia wrote for the court.
Lawyers for engine makers and the U.S. government had argued that if anti-pollution regulators in Southern California could adopt their own pollution standard for vehicles, then so could local and state agencies in other parts of the country. That would create a huge and complex burden for vehicle and engine makers to comply with the rules, attorneys said.
The Supreme Court rejected the arguments made by lawyers for the AQMD, which had maintained that the rule applied only to purchasing policies and did not impose new regulatory requirements on vehicle makers.
"We are disappointed in this decision by the court," said Barry Wallerstein, executive officer of the South Coast Air Quality Management District, in a statement. "However, we are determined to continue implementing the rules for publicly owned fleets."
The agency will also consider asking the state and the U.S. Environmental Protection Agency to permit it to continue to regulate privately owned fleets, Wallerstein said.
Air quality regulators said 70% of the cancer risk from bad air came from diesel particles. The smog also has been blamed for an increase in asthma, especially among children.
Times staff writer David Savage contributed to this story.