18 February 2005

The California ARB has proposed amendments to the limit on emissions of nitrogen dioxide (NO2) in the diesel emission control strategy verification procedure. Effective 2007, all verified emission control systems would not be allowed to increase NO2 emissions by more than 20% (as a percentage of the total NOx by mass) above the engine baseline level.

To ensure the effectiveness of emission control devices for use in California diesel retrofit programs, the ARB developed the verification procedure in 2002. The original procedure included an NO2 emission limit defined as 20% of the total baseline NOx emission. This limit, however, proved to be too restrictive resulting in manufacturers not being able to verify catalytic emission control devices, such as particulate filters. In 2004, the ARB suspended the NO2 limit for the period of 3 years, which allowed for verification of a number of systems.

The old NO2 limit did not account for variability in engine-out NO2 between different engines. Engine-out NO2 levels can vary from around 5% of the total NOx to as much as 15%. Higher NO2 levels are usually seen in newer turbocharged models than in old naturally aspirated engines. As a result, the NO2 limit made it impossible to verify devices which use active platinum-based catalysts, such as most catalyzed particulate filters, the CRT filter, or several oxidation catalysts. While effective in reducing PM and other emissions, these devices increased the NO2 portion of the total NOx above the ARB limit.

The current proposal introduces a more relaxed limit, defined as a maximum incremental increase of 20% by mass over the baseline NO2 emission level. For instance, for an engine with a baseline NO2 fraction of 10%, this corresponds to total NO2 emissions of 30% of the NOx. The proposal also introduces requirements to pre-condition devices before testing to remove any stored PM (which could react with NO2 and lower its tailpipe levels).

Diesel emission control systems that do not meet the proposed NO2 limit would not be eligible for verification beginning 1 January 2007. Previously verified systems that do not meet the limit would no longer be verified beginning the same date.

The environmental impact of increased NO2 remains largely uncertain. Nitric oxide (NO) and nitrogen dioxide, the two compounds of regulated engine NOx emissions, undergo transformations in the atmosphere to reach equilibrium depending on the atmospheric conditions. Environmental regulations have traditionally set limits on the total NOx, not on NO2 alone. Increased NO2 emission, however, may be important in near-source exposure, as NO2 is a more toxic gas than NO. NO2 levels have been always explicitly regulated in occupational health legislation.

The new proposed limit, although more relaxed, will still present a challenge for technologies that rely on active oxidation catalysts.

The proposed amendments will be discussed during a public workshop to be held in El Monte, CA, on 22 March 2005.

The verification program for diesel emission control devices by the US EPA/ETV has no limits on NO2 emissions.

Source: California ARB (workshop notice)