The California Air Resources Board has approved a multi-pronged regulation that will significantly reduce NO_x emissions from heavy-duty diesel trucks.

The “Heavy-Duty Low NO_x Omnibus Regulation” will require manufacturers to comply with tougher emissions standards, overhaul engine testing procedures, and further extend engine warranties to ensure that emissions of NO_x (oxides of nitrogen, a key component of smog) are reduced to help California meet federal air quality standards and public health goals.

All components of the new rule will be phased in, allowing engine manufacturers time to prepare for compliance. The NO_x standards that engines must meet will be cut to approximately 75% below current standards beginning in 2024, and 90% below current standards in 2027.

Even as California ramps up the numbers of zero-emission electric and fuel-cell trucks on our roads over the next decade and beyond, tens of thousands of new internal combustion trucks will still be sold in our state. This regulation ensures that conventional diesel trucks will run as cleanly as possible at every point in their duty cycle. It takes a significant bite out of smog-forming pollution in every region in the state, and will make a major contribution to cleaning the air in communities close to ports, railyards and distribution centers that are now most heavily impacted by pollution from heavy truck traffic.

—CARB Chair Mary D. Nichols

The regulation is expected to have a significant impact on communities adjacent to railyards, ports and warehouses that typically experience heavy truck traffic. These trucks often idle, move slowly and make frequent stops—all actions that increase NO_x emissions. Today’s heavy-duty trucks do not control NO_x effectively during such low-load conditions. The new standards will reduce NO_x emissions by 90% or more when trucks are operating under these low load real-world operations.

Once it is fully phased in by 2031, the rule is expected to reduce NO_x emissions in California by more than 23 tons per day. These NO_x reductions are the equivalent of taking 16 million light-duty cars off the road in 2031. (For context, California currently has 26 million registered light duty vehicles).