Over the past decade, new national emission standards have been established in the U.S., and regulations have been adopted in California to reduce the level of emissions generated by heavy-duty diesel vehicles. Particulate matter (PM) and nitrogen oxide (NOx) emissions have been specifically addressed by the most recent regulations for heavy-duty vehicles. Breathing PM pollution is a health concern, and NOx emissions react in the atmosphere to form ozone and PM air pollution. 

In a previous TLT article, a new approach for measuring emissions from heavy-duty vehicles was discussed (1). An On-Road Heavy-Duty Vehicle Emissions Monitoring System was developed that measured the emissions a truck produced as it accelerated through a tent containing the system from a standing start or a crawl. One of the authors of this work indicated that trucks contributed 30% of NOx emission while only representing 2% of the U.S. overall vehicle fleet (including light-duty vehicles) in 2015.

The high NOx emission rate is generally associated with older engines. Thomas Kirchstetter, associate adjunct professor in the civil and engineering department at the University of California, Berkeley, in Berkeley, Calif., says, “We have been analyzing the composition of emissions produced by vehicles operating in California for over 25 years. The concern with NOx and PM emissions led California to enact two regulations (Drayage Truck Rule in 2010 and the Truck and Bus Rule in 2012) that accelerated the modernization of the truck fleet. Two innovations added to trucks in response to new emission standards are a diesel particle filter that is present in trucks operating after 2007 and a selective catalytic reduction system that uses a urea-water mixture that produces ammonia and converts NOx to nitrogen gas. The latter was placed on trucks built after 2010.”

Over the past eight years (2010-2018), Kirchstetter and his colleagues Robert Harley, Carl W. Johnson professor of civil engineering at the University of California, Berkeley, and Dr. Chelsea Preble, postdoctoral researcher at the University of California, Berkeley, have monitored emissions produced by trucks operating in the San Francisco Bay area. 

The researchers studied trucks entering the Port of Oakland, Calif., and the Caldecott Tunnel, which is located between Oakland and Orinda. Kirchstetter says, “The Port of Oakland was a key location for us because most of the vehicles serving it are Drayage trucks that were subjects of the first regulation. We also studied truck traffic operating in the Caldecott Tunnel to get a better sense of emissions from the broader on-road truck fleet that operates on highways throughout the state.”

Plume capture & carbon balance method
The researchers studied truck emissions using a mobile lab (see Figure 1) that was equipped with state-of-the-art instrumentation to sample truck emissions in real time. Preble says, “We were able to outfit a custom designed van with a variety of instrumentation that quickly measured the composition of individual truck emissions as they drive by in succession. The value obtained is a fuel-based emission factor that is reported in grams of pollutant such as NOx, per kilogram of diesel fuel consumed.”


Figure 1. The mobile lab shown was used over an eight-year period to monitor emissions produced by trucks operating in the San Francisco Bay area. (Figure courtesy of the University of California, Berkeley.)

The researchers used the plume capture & carbon balance method to measure the level of NOx emitted by individual trucks. The record of pollutant concentrations shows peaks that correspond to the passing trucks. Preble says, “The ratio of the areas under the curves for NOx and carbon dioxide are multiplied by the carbon fraction for diesel fuel to generate the fuel-based emission factor.”

The individual trucks were identified by their license plates, which are present in a California database listing them by model year. 

The researchers determined that trucks equipped with selective catalytic reduction displayed NOx emissions that were 90% lower than trucks produced prior to 2004. Implementation of the diesel particle filter led to an increase in nitrogen dioxide emissions in trucks that had only this piece of equipment by a factor of three to four times the baseline values. For trucks that also used selective catalytic reduction, the nitrogen dioxide emission factor dropped back to levels seen prior to the use of the diesel particle filter.

Ammonia levels increased for trucks using selective catalytic reduction. Kirchstetter says, “We believe that 95% of the ammonia increase can be attributed to only 10% of the trucks using selective catalytic reduction. Most of the trucks had very low ammonia emission factors. Ammonia is a concern because it can be converted in the atmosphere into constituents of secondary PM, ammonium nitrate and ammonium sulfate.”

Trucks using selective catalytic reduction also exhibited a significant increase in nitrous oxide emissions. Nitrous oxide is a greenhouse gas and contributes to depletion of the ozone in the stratosphere that protects us from ultraviolet radiation.

The research team concludes the use of pollution equipment on trucks led to a significant reduction in NOx emissions that far outweighs the increase in ammonia and nitrous oxide emissions. Kirchstetter says, “Periodic monitoring of emissions can be useful to determine how emissions levels are changing over time and if the emission controls on trucks continue to perform well.”

Additional information can be found in a recent presentation (2) or by contacting Kirchstetter at twkirchstetter@berkeley.edu. Kirchstetter also would like to acknowledge that this study was done in collaboration with the Bay Area Quality Management District and supported by the California Air Resources Board.

REFERENCES
1. Canter, N. (2015), “New method for measuring heavy-duty vehicle emissions,” TLT, 71 (4), pp. 12-13.
2. Preble, C., Harley, R. and Kirchstetter, T. (2018), “Truck Exhaust Plume Capture for Quantification of Nitrogen-Species Emission Rates: Impact of Diesel Particle Filters and Selective Catalytic Reduction Systems,” presented at the American Geophysical Union Fall Meeting on Dec. 12, 2018.