The California Air Resources Board (ARB) has reached an agreement in principle with the USEPA and most manufacturers of heavy truck engines that will lead to the federal agency adopting tighter standards to cut smog-forming emissions from big-rig trucks and buses by over 50 percent.

The Statement of Principles between USEPA, ARB and the engine manufacturers outlines a joint understanding that USEPA will finalize a regulation during 1996 that sets a national emission standard for smog-forming oxides of nitrogen and hydrocarbons of 2.4 grams per brake horsepower-hour for new truck and bus engines sold in 2004. Current emission standards for those pollutants are slightly over 6 grams per brake horsepower-hour.

The state's clean air plan, adopted last November, called for adoption of more stringent emission standards for heavy-duty trucks as essential to California's effort to achieve federal clean air standards throughout the state by 2010. Despite major efforts by engine manufacturers which have substantially reduced heavy-duty vehicle emissions, big-rigs remain one of the largest emission sources. The tougher, nationwide standard is also needed to help cut emissions from out-of-state trucks that travel through California. ARB data shows that out-of-state trucks account for over 20 percent of all truck emissions. ARB chairman John Dunlap said, "We are very pleased with this agreement that will not only help to clean California's air but also will help lower pollution levels throughout the nation.

 While some of these engines are already controlled, ARB analysis shows that without further emission limits they will contribute about one quarter of Southern California's total smog-forming emissions from motor vehicles in 2010. California's 1994 State Implementation Plan (SIP), the state's blueprint for attaining and maintaining healthy air quality, includes provisions for cuts from off-road diesels that the SOP now covers.

 "The ARB has long sought to reduce emissions from off-road heavy-duty equipment in California. Off-road heavy-duty equipment is a serious source of air pollution that becomes an ever larger concern in our state as we reduce pollution from other sources," said Dunlap.

Many of the most common types of heavy-duty farm and construction equipment use engines with horsepower levels that range between 50hp and 175hp. The SOP includes emission controls for that equipment which is preempted from state control and can only be regulated by USEPA.

In 1995 the ARB, in conjunction with USEPA and engine manufacturers, signed an SOP for heavy-duty on-road vehicles, such as long-haul trucks and interstate buses. This standard cut on-road emissions from those vehicles by more than half. With both the on-road and off-road SOP's signed, emissions will be cut significantly from both in-state emission sources and out-of-state vehicles and equipment that make their way to California.