12 May 2008

The US Environmental Protection Agency (EPA) has released a report from the Kansas City PM Characterization Study. The study, conducted in 2004 and 2005, evaluated particulate matter (PM) emissions from light-duty gasoline vehicles.

Consistently with past studies, it was found that high emitting vehicles provide high contribution of the PM inventory—50% of the PM emissions came from 13% of the vehicles. It was also found that light trucks had slightly higher PM emissions than cars.

The results from Kansas City indicate that 5% of the vehicles tested (in the summer phase) were found to be “smokers”. The average PM emissions of these vehicles were 43 mg/mi (for comparison, the Tier 2 Bin 5 PM limit is 10 mg/mi). Though smoking vehicles have higher emissions on average, it was found that not all of the highest emitters were smoking, and not all of the smokers were emitting very high emissions. Only 2 of the 14 highest emitters in the summer were visibly smoking.

The emission trends show a clear drop in emissions levels (for PM as well as HC, CO, NOx) with later model year vehicles. However, it has not been determined whether the drop is due to technology changes (compliance with tighter standards) or whether it reflects varying levels of vehicle deterioration. It is likely a combination of both of these factors.

According to the study, the contribution of gasoline vehicles to the nationwide PM emission inventory is higher than previously estimated. Applying the Kansas City data in a draft version of EPA’s MOVES model results in an estimated average nationwide increase of light-duty gasoline PM emissions of about 1.6 times compared to the MOBILE6.2 model. Emissions are generally higher than MOBILE6.2 in winter months and lower than MOBILE6.2 in summer months. Overall annual emissions are expected to be significantly higher in areas with colder winters, even while summer emissions in those areas may be lower compared to MOBILE6.2.

Studies in the past—for example the 1998 Northern Front Range Air Quality Study by the Colorado State University—have indicated that the contribution of gasoline vehicles to ambient PM inventories may have been underestimated, especially in cold climates during winter. The relative contribution of gasoline vehicles may become even more significant as diesel cars and trucks adopt particulate filters for PM emission control (as suggested by the clean tailpipes in filter-equipped diesels, compared to blackened tailpipes in gasoline cars).

Source: US EPA