Data from air-quality sensors shows double-digit reductions in air pollution since millions of commuters in the San Francisco Bay Area are off the roads during the state’s coronavirus stay-home order.
Early sensor data seems to show that removing millions of cars from some of the most congested freeways in the United States has a major impact on air quality.
That’s what has happened in California's Bay Area region, where stay-at-home directives to control the spread of the novel coronavirus has sidelined traffic in the seventh most congested region in the country.
Nitrogen dioxide — known as NO2 — and other pollutants like fine particulate matter and carbon dioxide all get spewed into the atmosphere from fossil-fuel-burning cars and other vehicles on the roadways. Looking at readings of these pollutants from March 9 to March 20, the output of these exhausts declined from 16 percent for carbon monoxide to 29 percent for “black carbon,” the sooty particles emitted from diesel engines, according to a blog published by Aclima, a Bay Area company operating a fleet of roving air-quality sensors in the region.
The output of carbon dioxide (CO2) fell an average of 10 percent across the Bay Area, compared to averages for readings taken by the Air Quality and Meteorological Information System (AQMIS) during the same period in 2017, 2018 and 2019.
Researchers at Aclima, as well as officials with the California Air Resources Board (CARB), which sets policy related to curbing pollution and greenhouse gases, stress that one or two weeks is not enough time to get a thorough understanding of the effects of greatly reducing traffic.
“There are many factors that determine pollution levels day-by-day,” said Dave Clegern, public information officer for Climate Change Programs at CARB. “The lower vehicle counts are a recent development, and we can't really say at this point that fewer cars on the road is having much impact on air quality either way.”
However, the coronavirus-related traffic reductions “creates an unprecedented natural experiment and helps us understand how pollution levels are impacted by large-scale behavioral change,” said Meg Thurlow, vice president of sensing and applied sciences at Aclima.
“There will be quite a bit of data to review, and CARB staff will be looking at it with great interest from both a public health and climate perspective,” said Clegern.
The early air-quality data from Aclima and other sources could prove to be a starting point for policymakers as they continue to explore options to reduce traffic congestion and vehicle miles traveled, which could lead to reduced levels of climate warming gases. Policy proposals advocating for remote-working or expanded transit availability could also benefit from data showing what happens when a large proportion of commuter traffic disappears.
“This unprecedented and entirely unexpected traffic slowdown resulting from response to COVID-19 does present scientists and other researchers with a unique view into what happens when we take drastic and rapid action to reduce emissions at a regional scale,” said Thurlow.
“It’s as though someone snapped their fingers and many of the cars disappeared off the road. There are some folks who are looking at those unintended consequences as a positive," she added.
“I think on the whole, everyone can agree that it’s a terrible crisis, that it’s impacting the most vulnerable communities the most. And it shouldn’t take a pandemic, and the economy grinding to a halt, for us to protect those communities, and for us to care about those communities,” said Fort.
Traffic in the Bay Area, home to more than 7.7 million people, has been ranked as the seventh most congested in the country, according to the recently released INRIX Global Traffic Scorecard, where commuters waste 97 hours a year to traffic congestion.
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