Tunnel construction jobs are major undertakings that improve mobility, access and efficiency for infrastructure and transportation systems. But more recently, tunnels have served as micro-laboratories, delivering new insights on emissions performance from all the vehicles that travel through them.

Two recent studies examining emissions data from heavy-duty vehicles in two different tunnel settings led to the same conclusion: Real-world vehicle emissions have dropped considerably over time, and a small minority of vehicles can contribute a disproportionate amount of emissions. Researchers attributed this to advancements in diesel engines and emissions controls delivering measurable emissions reductions, including decreases in black carbon emissions (particulate matter or PM 2.5), nitrogen oxides (NOx) and greenhouse gases (GHG).

Since 2011, the newest-generation commercial trucks, powered by diesel engines that incorporate selective catalytic reduction systems (SCR) and diesel particulate filters (DPF) are helping cities, municipalities and states achieve significant clean air improvements. These trucks achieve near-zero levels of emissions, meeting standards set by the U.S. Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) for ozone precursors (NOx) and PM 2.5 emissions. Relative to previous generations of technology, these newest engines reduce emissions by 98%. Consider that it would take more than 60 of today’s Class 8 trucks to equal the emissions from a single truck of the 2000 vintage.

Study #1: California’s Caldecott Tunnel

A study sponsored by CARB over a span of eight years (2010 to 2018) gathered emissions data from heavy-duty vehicles passing through the Caldecott Tunnel, located just outside San Francisco. Pollutants measured were PM 2.5, NOx, ammonia (NH3) and nitrogen dioxide (N2O). The fleet sampled included short-haul drayage trucks, dump trucks, cement mixers, tractor-trailers, flatbed trucks and construction equipment, spanning in age from model years 1985 to 2017. Researchers directly correlated the measured emissions with the vehicles passing through the tunnel and cross-checked the age and compliance status of those vehicles.

The study came to several surprising conclusions.

• Under real-world conditions, DPFs reduce PM 2.5 emissions by 97% compared to pre-2004 engines. When an SCR system is added, heavy-duty diesel engines cut PM 2.5 by a further 81%, compared to pre-2010 engines.
• Model year 2010 and newer trucks, which incorporate DPF and SCR systems, deliver emissions levels below today’s EPA/CARB requirements (0.02 g/kg of PM 2.5).
• The dirtiest 9%of the fleet accounted for more than 70% of the measured PM 2.5 and NOx.
• Diesel trucks are a minor source of NH3 and N2O emissions, minor contributors to GHG creation; SCR systems increased these emissions. But the reduced PM 2.5 emissions delivered by DPFs, combined with the fuel economy gains delivered by the SCR, far outweigh the effect of N2O global warming increase.

Study #2: Maryland’s Fort McHenry Tunnel and Hong Kong’s Shing Mun Tunnel

Between 1992 and 2015, the Health Effects Institute (HEI) conducted a tunnel study at the Fort McHenry Tunnel, located outside Maryland’s Port of Baltimore. Researchers compared these results with a similar emissions inventory conducted between 2003 and 2015 at the Shing Mun Tunnel in Hong Kong. The results from this study were just recently made public.

HEI sought to validate estimates made by the EPA MOVES air quality computer model, used to inform U.S. air quality regulations. According to HEI’s analysis, this popular emissions model substantially overestimates real-world emissions for most pollutants in Baltimore’s Fort McHenry Tunnel by factors ranging from two to 100 times.

Alongside this conclusion, the study found a substantial decrease in transport-related emissions from both light-duty (non-diesel) and heavy-duty (diesel) vehicles traveling through the Fort McHenry Tunnel—a 78% to 96% reduction between 1992 and 2015. The study saw a similar decrease in Hong Kong’s Shing Mun Tunnel—an 80% decrease in PM 2.5 emissions between 2003 and 2015, most likely due to Hong Kong’s adoption of Euro V standards and mandated SCR retrofitting.

The Bottom Line: Why This Matters

So why do these two studies, conducted on opposite coasts of the United States, matter? Their findings highlight just how effective the newest emissions control technologies and cleaner fuels are for diesel-powered commercial trucks. They also highlight how a small population of older or improperly maintained trucks, and those that emissions controls have been removed or tampered with, can have significant impacts on overall emissions.

Analyses like these show, in real-world working conditions, that the newest-generation diesel trucks achieve the low-emissions performance demanded by regulators and promised by manufacturers. Rather than being judged by the oldest, highest emitting technologies, diesel ought to be evaluated on the evidence supplied by these new, near-zero emissions innovations.

These studies also indicate a need for a greater policy focus on gross emitters, emissions control system tampering and in-use emissions. As was shown at California’s Caldecott Tunnel, it is often a small number of vehicles that are responsible for a majority of the emissions. And when the primary motor vehicle emissions model in the U.S. (MOVES) substantially overestimates actual measured emissions by factors ranging from two to 100 times for a number of pollutants, something needs to change.

The new generation of advanced diesel technology delivers a positive climate impact because it is near-zero emissions and is more fuel efficient, which translates into lower GHG emissions. Beyond these advancements, when coupled with low-carbon, advanced renewable biodiesel fuels, existing diesel technologies yield immediate clean air and climate benefits. Accelerating the adoption of these new-model diesel technologies offers a means to achieve rapid and large-scale emission reductions. And let’s not forget about validating what we think the models are telling us against the real world, and getting the gross emitters and bad actors fixed or off the road; both are bad for everyone.