Past and future considerations for heavy-duty diesel engine emissions

In her Commentary in the June issue of EHP, "Diesel Exhaust: A Moving Target," Janet Arey (2004) makes a strong point for standardizing diesel exhaust reference material for future research due to the changes in diesel technology and resulting emissions.

Arey (2004) appropriately recognized that diesel exhaust particulates are a small category of emissions in what is truly a complex mixture of ambient particles. According to the U.S. Environmental Protection Agency's (EPA's) most recent emissions inventory (U.S. EPA 2003), emissions from all diesel sources (on-road light and heavy-duty, off-road, marine and rail) in 2001 accounted for 4.37% of the nation's fine particle inventory.

Recognizing the shift toward dramatically lower emissions and potential changes in the composition of those emissions, diesel engine manufacturers have initiated a unique broad-ranging stakeholder project known as the Advanced Collaborative Emissions Study, or ACES (French 2003). The objective of this collaborative government, academic, and industry research program is to develop the necessary data to assess and characterize in a timely manner (i.e., in the 2007-2008 timeframe) the emissions and any potential health effects from real-world exposures to exhaust from advanced prototype 2007-2010 heavy-duty engines, after-treatment systems, and reformulated fuels. This effort includes 794 emissions characterizations, chronic animal exposure studies, and short-term studies on allergic responses, and it is expected to publish findings in 2009-2010 (Warren 2004).

In further noting Arey's appeals on the "need to understand the atmospheric chemistry of diesel and other vehicle exhaust" (Arey 2004), alternative fuels should also be evaluated. Now, more than ever, this is of particular significance as the use of alternative-fueled vehicles has increased, but the understanding and research of these emissions in the atmosphere have not kept pace.

A number of recently published studies have assessed the emissions from alternative-fueled heavy-duty vehicles. In a study of school buses running on diesel and compressed natural gas (CNG), low-emitting clean diesel technology had the lowest level of both U.S. EPA regulated emissions and toxic air compounds as defined by the California Air Resources Board (Ullman et al. 2003).