A University of Washington-led team investigated whether fewer cars on the road due to stay-at-home orders led to cleaner air.
Starting in mid-March, many states issued stay-at-home orders or encouraged people to work from home to try to curb the spread of COVID-19. In cities nationwide, daily traffic dropped.
But did fewer cars on the road lead to cleaner air? Not necessarily, according to a new study led by University of Washington researchers.
Using air quality data from U.S. Environmental Protection Agency monitors across the U.S., the research team looked for changes in two common pollutants: ozone, and fine particulate matter called PM2.5. Compared to the past 10 years, neither pollutant has been consistently lower than expected levels since stay-at-home orders began. But the team found that another pollutant that mainly comes from car exhaust, NO2, was at much lower levels in three cities – Seattle, Los Angeles and New York – (30% lower on average) after stay-at-home orders were implemented.
The paper describing this analysis is not yet peer-reviewed but has been submitted to a journal for consideration. Their study was first posted May 8 to the preprint site ChemRxiv.
What is PM2.5?
“It’s difficult to determine whether the air really is cleaner now because, in general, there is a lot of variability in weather and emissions patterns,” said senior author Julian Marshall, a UW professor of civil and environmental engineering. “You can’t look solely at concentrations today and compare them to the same day a month or a year ago.”
Because pollutant concentrations vary over time and across regions, the researchers developed a method to determine whether pollutant levels in any given week were notably different from normal.
“Let’s say you’re measuring height and you want to know if a difference of 2 inches is a big deal. Well, it depends on how much variability there is,” Marshall said. “If you measured the height of people in a city and then you have someone who’s 2 inches taller than the average height, you’d probably say they’re still about average because people’s height varies by feet. But if you found an NBA hoop rim that was 2 inches different from the average, that’s a big deal. Rim height is so precisely controlled.”
What is ozone?
The team’s “robust differences” metric compares a pollutant’s median concentration during a week in 2020 to its median concentration in the same time period over the past 10 years. The metric also adjusts for whether a region has been getting cleaner or dirtier over the past 10 years, a lot like adjusting prices for inflation. This calculation yields a score where negative numbers indicate that a region was cleaner than expected for that week, and positive numbers indicate that a region was dirtier than expected.
The researchers attempted to calculate the robust differences score for the nearly 1,000 PM2.5 and the more than 1,170 ozone monitors across the U.S. But the team excluded monitors that had less than three years of data or were missing data from at least two of the last three years. West Virginia didn’t have any PM2.5 monitors that qualified, and Rhode Island, South Carolina and Hawaii didn’t have ozone monitors that qualified. For all other states, the team used individual monitor data to calculate state-level scores.