Log in | Subscribe | RSS feed

What’s New

California adopts new emission standards for urban buses

25 February 2000

The California Air Resources Board (ARB) adopted a new regulation to reduce emissions from the state’s transit buses. The bus rule, which will be phased-in beginning in 2002, affects about 8,500 urban buses at approximately 75 California transit agencies.

Various components of the regulation, to be introduced over the next ten years, focus on the reduction of particulate matter (PM) and nitrogen oxides (NOx) emissions. Diesel exhaust PM contributes to mortality and contains substances known to cause cancer, while NOx contributes to ozone, the main harmful component of urban smog. The rule calls for cleaner engines, cleaner diesel fuel, retrofit with diesel particulate filters to reduce exhaust PM emissions from older diesel buses, use of zero emission buses (ZEB) and reduced exhaust PM and NOx from new diesel engines.

The regulation allows transit agencies the flexibility of choosing between either a diesel or alternative fuel "path" to lower air emissions. Agencies may choose to use low-emission alternative fuels such as compressed or liquefied natural gas, propane, methanol, electricity, fuel cells or other advanced technology. Continued use of diesel brings with it a requirement to use low-sulfur (15 ppm) diesel fuel beginning 1 July 2002, and cut PM emissions from new diesel buses down to 0.01 g/bhp-hr beginning in 2004. A new lower NOx standard of 0.2 g/bhp-hr applies to both diesel and alternative fuel bus engines sold to California transit agencies starting in 2007. Today’s bus engine standards are 0.05 g/bhp-hr for PM and 4.0 g/bhp-hr for NOx.

In addition, for both diesel and alternative fuel paths, a NOx fleet average of 4.8 g/bhp-hr begins in 2002, which will require some transit agencies to retire their oldest, highest polluting buses. A requirement to retrofit existing buses with particulate filters or other devices to reduce PM starts in 2003. Large transit agencies with 200 or more buses that choose the "diesel path" and continue to purchase primarily diesel vehicles are required to begin demonstrating the use of at least three ZEBs by 2003.

From model year 2008 through 2015, large transit agencies on the "diesel path" will be required to make ZEBs 15% of their new bus purchases/leases. For large transit agencies on "alternative fuels path", the 15% ZEB rule runs from model year 2010 through 2015.

The new bus emission rule was criticized by the natural gas industry, which expected that the ARB would mandate wider use of alternative fueled buses. Clean diesel technologies, which are allowed for transit agencies that choose the "diesel path", have the potential of achieving emission levels comparable to or cleaner than those from natural gas engines without the high costs of switching to and operating alternative fueled vehicles.

The ARB’s ruling was endorsed by the Engine Manufacturers Association (EMA), California Chamber of Commerce, and Western States Petroleum Association. The EMA made a commitment to meet the 0.01 PM standard by 2002 October 15 months ahead of the ARB’s January 2004 deadline.

Source: California ARB