Remember the silver lining we all dreamed of, in which—how did it go—the drop in traffic and travel that came with the Covid-19 shutdown meant cleaner air? It was a form of imposed collective action on the climate crisis. We saw signs of an environmentalist utopia. Traffic dropped. Jellyfish zoomed visibly through Venice’s suddenly clear canals. Satellite images of cities showed the toxic orange industrial miasma nearly vanish.
So when a team led by a couple of University of Washington researchers went about measuring the changes in air quality in the U.S. during the coronavirus shutdown, they expected notably cleaner air. They gathered national Environmental Protections Agency air quality data from across the country. “Then I started plotting PM2.5, and it was really surprising,” says Bujin Bekbulat, a UW doctoral student and author on the study.
In fact, PM2.5—a form of pollution that comes from vehicle exhaust, agricultural emissions, fires, and fossil fuel burning—was about 5 percent higher than the model predicted (based on the trend over the last decade). Ozone—which comes from chemicals, power plants, and vehicles—stayed about the same. The team also measured NO2 levels in three cities (Seattle, New York, and Los Angeles) and did find it dropped by about 30 percent after stay-home orders.
Another study, led by UW post-doctoral fellow Jianbang Xiang, gathered data near and on I-5 for five weeks before and after the state’s stay-home order. Xiang found traffic volume dropped by half following the order, and pollutants dropped more consistently: things like black carbon and nitrogen oxide by 20 percent, carbonic oxide by eight percent, and PM2.5 by six percent. (Neither study has finished peer review yet.)
So what’s going on? Less travel should equal consistently less pollution across the board, right?
Not exactly, say Bujin and Julian Marshall, who's the first study’s senior author and a civil and environmental engineering professor at the UW. While NO2 primarily comes from car exhaust, contributions to PM2.5 and ozone are significantly more complex. As is the matter of air quality in general, affected by seasons and weather. Marshall says PM2.5 naturally goes down between winter and spring, while ozone naturally goes up. You can check out some maps of how recent pollutant levels have fluctuated here.
In some ways, our lay-guess—of course the air is cleaner—might point to a certain fallacy in how we consider emissions: We tend to oversimplify our environmental impact into those aspects that are the most visible and related to constant personal choices. We might try to drive less, forgetting perhaps that only a quarter of U.S. greenhouse gas emissions are due to transportation. And with something like PM2.5? “Traffic is only one contributor,” Marshall says. “There’s also power plants. There’s road dust. There’s agriculture. There’s light industry. There are diesel backup generators. There’s wood smoke.”