Regulatory Background on the U.S. Mobile Source Emission Control Program

U.S. EPA Light-Duty Tier 2 and Gasoline Sulfur Rulemaking

Since the mid-1970s, U.S. federal and California light-duty motor vehicle tailpipe emission regulations have been continually pushed to lower levels in response to air quality concerns. The current light-duty vehicle emission standards are the U.S. Environmental Protection Agency's (EPA) Tier 2 and the California Air Resource Board's (ARB) Low Emission Vehicle II (LEV II) programs. California acted first, adopting their LEV II program in late 1998, followed by EPA finalizing the Tier 2 regulations in December 1999. Both the ARB LEV II regulations and the EPA Tier 2 regulations began their phase-in with the 2004 model year. Phase-in for the LEV II program was completed with the 2006 model year while the Tier 2 program is not fully phased-in until model year 2009. Complete details of the Tier 2 program are available from the EPA website at: Similarly, more complete information regarding the ARB LEV II regulations is available at:

These EPA and ARB light-duty emission programs require significant reductions in hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx) emissions relative to vehicle emission requirements associated with the regulations that precede each of these new emission programs (e.g., EPA's Tier 1 or California LEV I regulations). The LEV II regulations, for example, maintain tight hydrocarbon emission levels established in the LEV I program (adopted in 1990; implementation began with the 1994 model year), but significantly reduce NOx emission requirements compared to LEV I requirements. The Tier 2 program draws from both the California LEV I and LEV II programs in significantly tightening both HC and NOx tailpipe emissions relative to Tier 1 regulations that were first implemented with the 1994 model year. Compared to pre-controlled vehicles sold in the U.S. prior to 1975, today's Tier 2 and LEV II cars and trucks are meeting emission standards that require reductions of up to 98+% with respect to VOCs, 96% for CO, and 98% for NOx.

Full useful life tailpipe emission standards for the fully phased-in U.S. EPA Tier 2 and California LEV II programs are summarized in Tables 1 and 2, respectively. Each of these programs provides auto manufacturers with several different certification categories to choose from for their light-duty vehicle fleet. Tailpipe emissions are measured on a chassis dynamometer using the U.S. Federal Test Procedure (FTP, a vehicle speed vs. time driving cycle). The concept of multiple certification categories was first introduced with ARB's LEV I program with Transitional Low Emission Vehicle (TLEV), Low Emission Vehicle (LEV), and Ultra-Low Emission Vehicle (ULEV) certification options that varied by vehicle weight class (e.g., passenger car and light-duty truck weight classes). The EPA Tier 1 light-duty emission regulations also had weight class specific emission regulations but only one set of emission standards for each gasoline vehicle weight class.

The Tier 2/LEV II programs have several common features that are also significant changes from either Tier 1 or LEV I requirements:

  1. fuel neutral requirements (emission standards are equivalent for gasoline and diesel-fueled vehicles);
  2. 120,000 mile full useful life durability; and
  3. a single set of standards that does not vary with light-duty vehicle weight class (up to 8500 lb. gross vehicle weight for all passenger cars and light-duty trucks; up to 10,000 lb. for medium-duty passenger vehicles [MDPVs]).

Treating passenger cars and light-duty trucks on an equivalent emissions basis is an important focus for both the Tier 2 and LEV II programs. Both of these programs place a premium on cold-start emission performance and high emission system efficiencies with respect to NOx emissions.

In both the Tier 2 and LEV II programs, manufacturers, besides certifying vehicles in one of the available categories detailed in Tables 1 and 2, must also meet a corporate average emission requirement for the entire fleet of vehicles sold in a given model year. In the California program, this corporate average emission requirement is based on non-methane organic gas (NMOG) emissions, while NOx emissions are used for fleet averaging in the federal Tier 2 program. Fleet average NMOG emissions have been set by ARB on a declining scale for each model year to gradually force manufacturers to produce more and more of their California vehicles in the lower emission certification categories. For example, the fleet average NMOG requirement for vehicles up to 3750 lbs. loaded vehicle weight (LVW) is 0.053 g/mi NMOG (based on the 50,000 mile emission requirements) in model year 2004, declining to 0.035 g/mi (based on the 50,000 mile emission requirements) in model year 2010. In comparison, the fully phased-in Tier 2 program has a single 0.07 g/mi fleet average NOx requirement (based on full useful life limits) for all light-duty vehicles produced by a given manufacturer that fall under the Tier 2 requirements (the Tier 2 NOx fleet average requirement is fully phased-in with model year 2009 vehicles).


Table 1. California LEV II 120,000 mile tailpipe emission limits


Certification Level NMOG (g/mi) CO (g/mi) NOx (g/mi)
LEV-2 0.090 4.2 0.07
LEV-2/LDT2* 0.090 4.2 0.10
ULEV-2 0.055/td> 2.1 0.07
SULEV 0.010 1.0 0.02

* the LEV-2/LDT2 certification category is limited to no more than 4% of the LDT2 light-duty truck production for a given manufacturer



Table 2. U.S. EPA Tier 2 120,000 mile tailpipe emission limits


Certification Level NMOG (g/mi) CO (g/mi) NOx (g/mi)
Bin 1 0.0 0.0 0.0
Bin 2 0.010 2.1 0.02
Bin 3 0.055 2.1 0.03
Bin 4 0.070 2.1 0.04
Bin 5 0.090 4.2 0.07
Bin 6 0.090 4.2 0.10
Bin 7 0.090 4.2 0.15
Bin 8 0.125 4.2 0.20


Both ARB and EPA have included a clean fuel component in their LEV II and Tier 2 regulatory programs with respect to gasoline sulfur levels. ARB established a 30 ppm sulfur average for gasoline as a part of their California Phase II reformulated gasoline requirements. This sulfur level was further reduced to an average of 15 ppm sulfur starting in 2004 with the introduction of California Phase III reformulated gasoline regulations. Similarly, the EPA included gasoline sulfur level regulations as an integral part of their Tier 2 regulatory package with the phase-in of 30 ppm average S levels started in 2005.