Diesel Oxidation Catalysts:
Informational Update
The U.S. Environmental Protection Agency (EPA) is providing
this information to help explain the operation of diesel oxidation
catalysts (DOCs) used for retrofit purposes and address questions
about emissions of NO2 and ultrafine particles. EPA may prepare
similar updates for other retrofit technologies. For further information
visit EPA's National Clean Diesel Campaign.
• Diesel oxidation catalysts verified by EPA and/or the California Air Resources
Board (GARB) are available for nonroad and highway diesel engines from a
wide range of model years. See the list of technologies verified by EPA and
those verified by GARB. EPA and GARB have a reciprocity agreement, under
which EPA accepts and permits the nationwide use of retrofit technologies
that are verified by GARB.
• Diesel oxidation catalysts verified by EPA and/or GARB are typically effec'
tive at removing 20-40 percent of the total particulate matter (PM) mass.
The PM removed by DOCs is largely the soluble organic fraction (SOF or
organic material) that comes from unburned fuel and lube oil.
• While available data show that DOCs do not remove elemental carbon PM,
they remove many other particles and components including substantial
organic hydrocarbons, many PM precursors and many toxics including
polyaromatic hydrocarbons. For example, in a study performed by the
Manufacturers of Emission Controls Association (MECA), DOCs removed
54-68 percent of 18 polyaromatic hydrocarbons, 68-91 percent of total
hydrocarbons and a similar amount of carbon monoxide (CO).1
1 MECA Report: Demonstration of Advanced Emission Control Technologies Enabling
Diesel-Powered Heavy-Duty Engines to Achieve Low Emission Levels (June 1999)
www.meca.org/galleries/default-file/SwRIHDE.pdf
SEPA
United States
Environmental Protection
Agency
Office of Transportation and Air Quality
EPA-420-F-07-068
November 2007
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• The PM removed by DOCs contributes to ambient PM2 5 levels and the hydrocarbons
removed contribute to ozone formation. Retrofit projects with DOCs can begin reduc-
ing emissions immediately and can help state and local governments reduce emissions
of PM2 5 and VOCs in the near term. These reductions can play an important role in
reducing air pollution and helping nonattainment areas meet the National Ambient Air
Quality Standards for Ozone and PM25,
• EPA is aware of concerns that DOCs may produce some ultrafine particulates. Such
concerns are associated with sulfur levels in diesel fuel and the potential for sulfur to
accumulate in the DOC and then be released as sulfate particles (sulfates). EPA now
requires highway diesel fuel to be ultra-low sulfur diesel fuel (ULSD) which contains 15
ppm sulfur or less. When used with ULSD, EPA does not believe DOCs increase ultra-
fine PM. Additionally, a DOC may be formulated to help reduce the potential to create
sulfate particles. Because nonroad diesel fuel is not required to meet ULSD sulfur levels
until 2010, nonroad equipment equipped with DOCs should be fueled with highway fuel
or the DOCs should be properly formulated to reduce the potential for sulfate make,
• EPA is also aware of concerns that DOCs may increase the NO2 fraction of total NOx
emissions. The NO2 produced by a DOC is dependent on the catalyst formulation,
GARB has established a limit on incremental NO2 from diesel retrofit devices and all
DOCs on its list of verified products comply with this limit. Data EPA has reviewed to
date indicate that EPA verified DOCs also comply with the GARB limit. Data supplied
by DOC manufacturers to both GARB and EPA show that, for some verified DOCs,
the NO2 fraction of total NOx actually decreases slightly. DOCs do not raise total NOx
levels. EPA is continuing to analyze NO2 data from verified technologies and is looking
to implement an NO2 requirement for verified technologies that would maintain harmo-
nization with California,
• DOCs can be a cost effective way to remove PM from diesel exhaust on both highway
and nonroad vehicles. Cost effectiveness is influenced by such factors as vehicle age,
remaining vehicle life and vehicle usage. EPA's highway cost effectiveness study showed
a range of about $11,000 to $50,000 per ton of PM removed for school buses and Class
8b trucks. For 10 year old school buses or Class 8b trucks, the cost effectiveness was
about $30,000 per ton.2 Cost effectiveness ranges for a variety of nonroad vehicles can be
found in EPA's non-road cost effectiveness study.3
• Total PM removal by DOCs (or other exhaust aftertreatment devices) can be augmented
by the addition of a closed crankcase ventilation (CCV) system. A number of CCV
2 Technical Report: An Analysis of the Cost Effectiveness of Reducing Particulate Matter Emissions from
Heavy-duty Diesel Engines Through Retrofits, EPA420-S-06-002, March 2006.
www.epa.gov/cleandiesel/documents/420s06002.pdf
3 Technical Report: An Analysis of the Cost Effectiveness of Reducing Particulate Matter and Nitrogen
Oxides Emissions from Heavy-Duty Nonroad Diesel Engines Through Retrofits, EPA420-R-07-005, May 2007.
www.epa.gov/cleandiesel/documents/420r07005.pdf
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systems have been verified by EPA and GARB. CCV systems remove additional HC
and CO emissions by routing crankcase blowby gases back into the engine intake. They
also reduce oil consumption and oil spotting by separating liquid oil from the gases and
returning it to the crankcase,
• EPA National Ambient Air Quality Standards establish mass based emission standards
for PM25. The ambient PM25 has many components including sulfates, nitrates, and car-
bonaceous compounds. EPA's various diesel exhaust emission standards are based on the
total mass of diesel particulate which is almost entirely carbonaceous and includes both
organic and elemental components. Ongoing research is evaluating the relative health
effects of the different components of ambient PM. This work will show which compo'
nents of ambient PM (and possibly which components of diesel PM) are important to
control,
• While DOCs can play a very important role for many engines and may be the only
option for certain older or high emitting engines, EPA encourages selection, when
feasible, of technologies that yield greater PM control such as diesel particulate filters
and partial-flow filters. These devices are more effective at reducing diesel particulates
of all sizes and composition. In selecting a retrofit technology, the intended application
should be carefully evaluated for such factors as exhaust temperature, duty cycle, engine
condition and past and future maintenance practices. These parameters are important
in determining whether a vehicle or piece of equipment can support a particular retrofit
^Q technology.
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