With Fewer Cars On The Road In Mass., Emissions Are (Slightly) Down
People still driving to work face a surreal scene on their commutes: no traffic.
Massachusetts Transportation Secretary Stephanie Pollack said this week that highway congestion has, "in many cases literally evaporated overnight." Recent data from the DOT shows travel times are way down: a third shorter than average on I-93, and about half as long on I-90 west of Boston.
The coronavirus shutdown provides a "natural experiment" to scientists who study greenhouse gas emissions — what happens when most of the cars suddenly vanish from the roads? The surprising answer: not much. At least not yet.
Preliminary data from scientists at Boston University and Harvard show no significant decrease in carbon dioxide (CO2) or carbon monoxide (CO) emissions around Boston.
"So far it's a pretty small signal," says Lucy Hutyra, an associate professor of earth and environment at Boston University. One exception: an obvious spike in emissions at the BU collection site when students moved out of their dorms, packing Commonwealth Avenue with cars and vans.
Hutyra, who has been collecting data on emissions from sites around Boston for almost a decade, notes that only about a quarter of the city's greenhouse gas emissions come from transportation. The rest come from industry and buildings. Plenty of those are still up and running, and people are still burning fuel at home.
"The transportation sector is important, but it doesn't control everything, especially for CO2," said Steven Wofsy, a colleague of Hutyra's and a professor of atmospheric and environmental science at Harvard. "Most of the other sectors are toodling right along."
Another factor: spring. As soil warms up, it starts to release CO2, further muddying the data.
The scientists agree that they may eventually see a more noticeable decrease in the coming months. Hutyra's preliminary projections are that the coronavirus shutdown may lead to a 30% reduction in traffic emissions, which would translate into about a 10% reduction in overall emissions.
"The changes we've seen up until the present time have been relatively modest," Wofsy said, "but those reductions are obviously going to be more and more significant over time."
There is a twist to this story. Preliminary data from satellite images show that one pollutant is noticeably down — nitrogen dioxide (NO2). NO2 comes mostly from tailpipes — not factories or warming soil — so it offers scientists a quicker, clearer picture of emission trends.
"It tends to be able to pick up pretty quick changes in traffic emissions, especially over big cities," said Jeffrey Geddes, assistant professor of earth and environment at Boston University.
Looking at satellite images of Boston before the shutdown, Geddes notes that the NO2 concentration over Boston, "really pops out in color, because that's where most of the emissions are."
Now, not so much.
"There's a pretty convincing signal of a decrease in nitrogen dioxide region-wide," he says.
Geddes notes that the satellite data is preliminary, and can be distorted by weather and cloud patterns. As more data comes in over the next few weeks and months, he expects to get a clearer picture of nitrogen dioxide reduction, and what that says about transportation emissions overall.
Hutyra says it will be interesting to see what happens when the economy kicks back in and people return to work. Perhaps there will be long-term behavior changes — like more remote learning and virtual meetings — that alter emissions permanently. And this unplanned experiment can help scientists better quantify what those changes may mean.
Harvard's J. William Munger, who studies carbon emissions at Harvard Forest, offered one preliminary conclusion: "Viruses are not a great solution to greenhouse gas emissions and air pollution problems."