22 December 2007
The US Environmental Protection Agency (EPA) has established a nitrogen dioxide (NO2) limit for diesel retrofit technologies verified under the EPA’s National Clean Diesel Campaign (NCDC) Retrofit Technology Verification Program.
Effective January 1, 2009, the EPA is implementing an NO2 increase limit that is harmonized—in stringency and timing—with the requirements for retrofit technologies by the California Air Resources Board (ARB). This requirement limits the increase in NO2 emissions associated with retrofit technologies to levels no greater than 20% above baseline engine levels.
The announcement was made in an EPA “Dear Manufacturer” letter dated December 20, 2007. The letter also states that, while it is appropriate to avoid potential increased NO2 levels as greater numbers of engines are retrofitted, “the EPA does not have an immediate pressing concern for NO2 emissions from retrofit technologies”. This is somewhat peculiar and contrary to the established EPA methods, as the agency is adopting an emission requirement which is not based on an assessment of the potential health and environmental effects.
To evaluate compliance with the NO2 requirement, the EPA is adopting the ARB test procedures at Title 13 Chapter 14 Section 2706 of the California Code of Regulations. As a result, manufacturers must use the same procedures for any new EPA and ARB verifications.
For existing EPA verified technologies, manufacturers must submit NO2 data to EPA prior to November 1, 2008 or those technologies will be removed from the Verified Technology List. Data previously submitted and approved by ARB may also be submitted to EPA to demonstrate compliance with these limits. EPA will update the Verified Technology List to indicate which technologies have demonstrated compliance with the NO2 limit.
Many catalyst-based diesel emission control devices can oxidize nitric oxide (NO) to NO2, thus increasing the NO2 proportion in the total NOx emissions. While NO2 is a more toxic gas than NO, the environmental significance of this effect is under dispute, as the two gases reach a chemical equilibrium after being discharged into the atmosphere. It is generally believed that the NO/NO2 shift is more critical when the exposure occurs in immediate vicinity of the vehicles, for instance in occupational health environments with engines operated in enclosed spaces. A study by France’s Afsset-AIRPARIF has shown that drivers may be also exposed to elevated NO2 concentrations under highway traffic conditions (see also auto-moto.com).
Diesel retrofit technologies verified in California must not increase NO2 emissions by more than 30% over the engine baseline effective 2007, and by more than 20% effective 2009.
Source: US EPA