It makes sense that big trucks would pollute more than little cars. A larger engine, hauling more weight equals more pollution.
But how much pollution and how many people does that pollution affect? And, ultimately, where should elected officials focus their environmental efforts? That’s what a team of researchers at the University North Carolina at Chapel Hill and the Harvard School of Public Health wanted to find out.
The results of their study were published this week in Environmental Research Letters.
What’s new — Sarav Arunachalam is a research professor at UNC Chapel Hill specializing in emissions and air quality monitoring.
“Our study was the first to really break down the health risks by vehicle type, precursor, and pollution type,” Arunachalam tells Inverse.
The team broke down vehicle classes into different groups:
- passenger cars
- light-duty trucks like pickups and SUVs
- medium-duty trucks (think UPS delivery vans)
- heavy-duty trucks like big rigs
Then they compared emissions by magnitude rather than simply by vehicle count to get a count of premature deaths in each state and region from each form of transportation. The study looked at the Northeast and Mid-Atlantic regions of the US, but it’s likely that the broader points can be extrapolated to other areas.
Here’s the background — In one sense, the results are wholly unsurprising: light-duty trucks, including SUVs, are responsible for the most premature deaths from air pollution in the area studied.
There are an awful lot of light-duty trucks and SUVs in the world, far exceeding the number of buses and tractor-trailer trucks, so it makes sense that these would collectively cause more pollution and thus more deaths.
However, this wasn’t true for all geographic regions. In New York City, for example, buses have the largest impact per ton of emissions. In Massachusetts, it’s heavy-duty trucks that make the biggest impact. Highway-happy Virginia marks light-duty automobiles as the biggest polluter.
Why it matters — What it means is that there isn’t necessarily a one-size-fits-all environmental policy for solving vehicle pollution. Recent efforts to electrify New York City’s transit bus fleet appear to be vital for reducing pollution, given the outsized impact they make on the environment and, ultimately, on lives.
Arunachalam says that electrifying a single New York City bus is equivalent to replacing 23 passenger cars with EVs.
“As soon as you remove one combustion source, you clean the air immediately,” says Arunachalam. “There is no lag here. You see reductions in emissions and you’re cleaning the air immediately.”
The study used data from 2016 — the last complete data set with pollution, vehicles, and premature deaths measured — but Arunachalam says there’s nothing to indicate that anything major has changed between then and now, though he notes that 2020 was an outlier because of the economic shutdown.
“We can use numerical models to ask what if we remove all the light-duty automobiles from the world,” he says. “But last year, with the pandemic, we got a real-world study.”
Curiously, it was a mixed bag. Reducing one pollutant, like nitrous oxide from automobile emissions, is great. But removing one pollutant can perversely make room for other pollutants.
With the reduction in nitrous oxide, there was a corresponding increase in ozone in some urban areas of the country because of the reactions between trees and combustion products. “It’s initially clean from a lot of perspectives, but certain pollutants can go up” as a result, Arunachalam says.
They also discovered that on a per-ton basis it’s reducing ammonia, an unregulated emissions pollutant, that has the biggest effect on public health. Ammonia, or NH3, is a direct precursor to the particulate matter, or PM2.5, concentrations examined in the study. Reducing a ton of NH3 emissions from light-duty trucks and SUVs would have a 75 times greater effect in terms of damage reduction than a 1-ton reduction of NOx. From heavy-duty trucks, it’s 90 times greater.
“It’s an unplanned byproduct of combustion,” Arunachalam says. “There’s an additional reaction that produces ammonia” — NH3 — on the way to reducing NOx while releasing CO2 and nitrogen exhaust.
If oxygen is being pulled out of the air to help make explosions in the engine, the corresponding nitrogen and hydrogen need to go somewhere, and some of it goes to creating new ammonia, which eventually creates more fine particles in the air.
Solve one problem, create another.
What’s next — The regional differences in major emitters show the need for additional research and more targeted fixes. Replacing exhaust-belching internal combustion buses in New York City with fancy new zero-emission electric ones makes much more of an impact on more people than a bunch of commuters in Stamford, Connecticut buying Teslas.
While in Virginia, where there simply aren’t as many buses (and more people driving longer distances than in Manhattan), a targeted approach to improve fuel economy and encourage zero-emissions cars might be a better solution.
“In peer review, there were comments about how this should be a good study for policymakers down the road,” said Arunachalam. “There must be a combination of regional and local measures to address it.”
Abstract: On-road vehicular emissions contribute to the formation of fine particulate matter and ozone which can lead to increased adverse health outcomes near the emission source and downwind. In this study, we present a transportation-specific modeling platform utilizing the community multiscale air quality model (CMAQ) with the decoupled direct method (DDM) to estimate the air quality and health impacts of on-road vehicular emissions from five vehicles classes; light-duty autos, light-duty trucks (LDT), medium-duty trucks, heavy-duty trucks (HDT), and buses (BUS), on PM2.5 and O3 concentrations at a 12 × 12 kilometer scale for 12 states and Washington D.C. as well as four large metropolitan statistical areas in the Northeast and Mid-Atlantic U.S. in 2016. CMAQ-DDM allows for the quantification of sensitivities from individual precursor emissions (NO, SO2, NH3, volatile organic compounds, and PM2.5) in each state to pollution levels and health effects in downwind states. In the region we considered, LDT are responsible for the most PM2.5-attributable premature mortalities at 1234 with 46% and 26% of those mortalities from directly emitted primary particulate matter and NH3, respectively; and O3-attributable premature mortalities at 1129 with 80% of those mortalities from NO emissions. Based on a detailed source-receptor matrix of sensitivities with subsequent monetization of damages that we computed, we find that the largest damages-per-ton estimate is approximately $4 million per ton of directly emitted primary particulate matter from BUS in the New York-Newark-Jersey City metropolitan statistical area. We find that on-road vehicular NH3 emissions are the second largest contributor to PM2.5concentrations and health impacts in the study region, and that reducing 1 ton of NH3emissions from LDT is ~75 times and from HDT is ~90 times greater in terms of damages reductions than a 1 ton reduction of NO. By quantifying the impacts by each combination of source region, vehicle class, and emissions precursor this study allows for a comprehensive understanding of the largest vehicular sources of air quality-related premature mortalities in a heavily populated part of the U.S. and can inform future policies aimed at reducing those impacts.