The coronavirus kills people but there is an upside.

Pandemic panorama: Skies were clear above San Francisco, on March 25, about a week after California’s stay-at-home order took effect.

The Wall Street Journal

Coronavirus Offers a Clear View of What Causes Air Pollution

By Jim Carlton

May 3, 2020

The coronavirus shutdowns are giving scientists an opportunity they never thought they would have: to see what would happen to the planet if the world’s economy went on hiatus.

The result has been drops in air pollutants to levels not seen in at least 70 years, easier breathing for people with respiratory ailments and consistently clear views of landmarks often obscured by smog, such as the Hollywood sign in Los Angeles and the Manhattan skyline.

“There have been some interesting pseudo-experiments, like when Beijing closed its plants for the [2008 Summer] Olympics, but only for a few days,” said Melissa Lunden, chief scientist for Aclima, a San Francisco company that measures pollution with street-level sensors. “Now, everyone on the planet can see the changes.”

The reasons, experts say, is that factories have shut down, and people who can’t go to the office, church or restaurants have stopped driving. Vehicle traffic in Los Angeles and New York has plunged about 90% from levels in January 2020, according to data analytics firm StreetLight Data Inc.

One of the biggest airborne pollutants to fall off has been nitrogen dioxide, which is a byproduct of fossil-fuel emissions that most scientists believe is contributing to climate change. Satellite data from the National Aeronautics and Space Administration show NO2 levels in the Northeastern U.S. dropped 30% during March from the previous four-year average for the month.

San Francisco-based Aclima has compiled data that shows the NO2 readings dropping in lockstep as the coronavirus swept first from China in January to Europe in February and the U.S. in March. Scientists said it was the first time they could remember so many cities going clean all at once.

Aclima’s most comprehensive data is for its own backyard, the San Francisco Bay Area, home to about eight million people. The region recorded a 31% decline in NO2 during the 10 weekdays ended April 6 compared with the previous three-year average for the same time. In addition, Aclima found a 39% drop in particulate matter such as from smoke and a 41% plunge in soot created by diesel fumes and other human sources. The company’s scientists say they believe those are the lowest levels since the first half of the last century.

“We knew how bad the pollution was,” said Davida Herzl, Aclima’s co-founder and chief executive officer. “What we didn’t know was how significantly it could drop.”

The reductions in air pollution weren’t evenly distributed in the Bay Area, however. In the northwest corner of Oakland known as West Oakland, the level of human-caused carbon dioxide fell 56% over that April 6 time period compared with 43% for the Bay Area as a whole. Aclima officials attribute the drop to the fact that West Oakland, a majority African-American neighborhood, is in an industrial zone surrounded by ships, trucks, trains and factories.

Environmental activists say the readings support what they have long argued, that lower-income communities such as West Oakland suffer from more pollution than other parts of cities. The rate of childhood asthma in West Oakland is five times higher than the statewide average, according to Brian Beveridge, co-director of the West Oakland Environmental Indicators Project.

“This tells me, look, there really is a direct correlation between the health outcomes in communities like West Oakland and the levels of pollution,” Mr. Beveridge said.

Pollution levels will inevitably rise in the U.S., as they have in other parts of the world that have begun to return to normal, Ms. Herzl said. But she said it was likely to be more gradual than the drop, as the economy won’t restart as quickly as it slowed.

Write to Jim Carlton at jim.carlton@wsj.com