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Smog-Causing Pollutant Way Down In Mass. — For Now

With many residents limiting travel due to the coronavirus pandemic, the number of highway trips in Massachusetts has plummeted. (Jesse Costa/WBUR)
With many residents limiting travel due to the coronavirus pandemic, the number of highway trips in Massachusetts has plummeted. (Jesse Costa/WBUR)

Traffic has dropped by about half in Massachusetts during the coronavirus shutdown, and that means cleaner air, at least for now.

NASA data indicates that the level of the pollutant nitrogen dioxide (NO2), which comes mostly from cars and trucks, has dropped about 30% in the Northeastern United States. And that regional drop seems to be playing out locally, says Jeffrey Geddes, an assistant professor of earth and environment at Boston University.

Nitrogen Dioxide, March 2015-2019 average, Northeast USA (NASA)
Nitrogen Dioxide, March 2015-2019 average, Northeast USA (NASA)
Nitrogen Dioxide, March 2020, Northeast USA (NASA)
Nitrogen Dioxide, March 2020, Northeast USA (NASA)

Geddes points to ground station measurements collected in Kenmore Square as one example. The graph shows a drop in nitrogen dioxide compared to previous years — especially around what used to be rush hour.

"To me, this is the most convincing figure because this is what's measured at nose level, where we're breathing the air," Geddes says. "If you're crossing a street in Kenmore Square this year, you're probably exposed to at least half — if not less — of the pollution than we would have seen last year."

Nitrogen dioxide measurements in Kenmore Square, based on preliminary data from the EPA and OpenAQ. (Courtesy Jeffrey Geddes)
Nitrogen dioxide measurements in Kenmore Square, based on preliminary data from the EPA and OpenAQ. (Courtesy Jeffrey Geddes)

Nitrogen dioxide is regulated by the federal Environmental Protection Agency, and Geddes says the overall levels have decreased in recent years. NO2 pollution rarely exceeds national standards, and is generally less of a problem than other air pollutants, like ultra-fine particles.

However, NO2 will pose more of a concern come summer, when it combines with other chemicals and sunlight to produce ground level-ozone. Unlike ozone in the upper atmosphere, which shields us from ultraviolet radiation, ground-level ozone is a key component of harmful smog.

"At the ground level, ozone is a really bad molecule for our health and for the environment," Geddes says, and "the summer is our ozone season."

Geddes says that while tailpipe emissions may rise and fall, ozone levels don't always follow in lockstep. Heat and stagnant air can accelerate ozone production, so we may see high levels this summer even if traffic remains lighter than usual. On the flip side, if the wind blows in our favor, clear skies may remain.

Geddes says it's hard to know what the current drop in air pollution will mean for environmental health long-term. It may depend on which workplace changes stick after the stay-at-home orders ease.

"People are learning they can do more from home, so maybe there'll be less commuting over the long term," Geddes says. "But I also worry about the other scenario, where perhaps people are a little bit scared to take public transit. And so once people go back to work, more people will be taking their cars, causing more congestion and more pollution."

"It could really cut both ways," he says.

Related:

Barbara Moran Twitter Senior Producing Editor, Environment
Barbara Moran is the senior producing editor for WBUR’s environmental vertical.

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