Great visibility on a blustery spring day in the Washington region on April 10. (chasingmailboxes/Flickr)

Washingtonians are breathing the cleanest spring air they have in decades, probably a side effect of orders to stay at home during the ongoing coronavirus crisis, along with favorable weather, air quality experts say.

The pollution decline was set into motion by decades of national, state and local reduction measures. Data shows local pollution amounts are at their lowest levels in at least 25 years.

The drop in local pollution mirrors trends seen elsewhere in the United States and other countries where the pandemic has curtailed economies and taken thousands of polluting vehicles off the roads.

Washington region Air Quality Index 1995 to 2020. (EPA AirData via Daniel Goldberg)

In the Washington region, traffic along major interstate highways has decreased by about 50 percent while electricity demand has dropped by about 7 percent, according to Jennifer Desimone, the air program chief with the Metropolitan Washington Council of Governments.

Multiple pollutants have shown decreases in recent weeks.

Ryan Stauffer, an air pollution specialist at NASA, has documented reductions in both PM2.5 (fine particulate matter) and nitrogen dioxide (NO2) using measurements from satellites and ground-based monitors.

It is “the first time since records began in 1999 that the DC area ... has had all ‘Good’ PM2.5 air quality days over the past month,” Stauffer wrote in an email.

Air quality levels in Washington March 20 to April 20 based on amount of fine particulate matter (PM2.5) in the air. The number of days with “moderate” air quality have gone down while “good” air quality has gone up. (Ryan Stauffer/NASA)

He added that a ground-based monitor at the River Terrace Elementary School in the District shows NO2 levels during the past month were about 33 percent lower than the 2010-to-2019 average for this time of year.

Concentration of the pollutant nitrogen dioxide (NO2) March 20 to April 20, 2020 compared to 2010 to 2019 average. (Ryan Stauffer/NASA)

A Council of Governments briefing on covid-19 and air quality provided to The Washington Post indicated NO2, PM2.5 and ozone pollution all “seem a bit lower” since the lockdown began.

But the improvement in air quality cannot all be tied to the lockdown. The contribution from weather also “needs to be investigated,” the Council of Governments briefing stated.

Daniel Goldberg, a research scientist at George Washington University, explained that wet and windy weather helps wash away and disperse pollutants, thereby improving air quality. In contrast, dry, stagnant weather makes air quality worse. This spring has been windy and wet, “which could be a supporting factor as to why the air quality has been particularly good,” Goldberg wrote in an email.

Taking into account the weather, the coronavirus-related drop in pollution may not be that substantial, he said.

“It’s unclear what fraction of the improvement is due to emissions changes related to covid-19, and what fraction is due to favorable weather,” Goldberg wrote.

As the weather heats up and the air turns more stagnant into the summer months, and air quality worsens, the effect of the lockdown on pollution levels may become clearer, both Goldberg and the Council of Governments’ briefing said.

Goldberg stressed that national, state, and local government regulations implemented since the 1970s are most responsible for the clean air Washingtonians are now breathing.

“[D]ecades’ worth of thought-out policies are much more effective at improving our air pollution than a forced ‘lockdown,’ ” Goldberg wrote.

The Council of Governments’ Desimone wrote in an email that while reduced traffic and energy consumption associated with covid-19 have helped improve the air, “pollution levels are expected to rebound once restrictions are lifted” and “the region has still not met the health-based air quality standards for ozone.”

The cleaner air that has resulted from this unintended experiment should motivate Washingtonians to seek more significant and sustained improvements in air quality, said Susan Anenberg, a professor at George Washington University’s Milken Institute School of Public Health and a colleague of Goldberg’s.

“The fact that our air pollution levels are not dropping even lower right now shows that to protect the air we breathe, we need to clean up emissions from trucks, industrial sources, and power plants, in addition to passenger cars,” Anenberg said in an email.

She continued: “This is a time to consider whether we would like to return to the status quo. Air pollution appears to be a risk factor for increased severity of covid-19, among a variety of other diseases. By reducing air pollution, we can improve the overall health status of the population, and ensure that people are more resilient to unforeseen risk factors like viruses.”

Goldberg echoed the call for long-term commitments to reduce pollution.

“The visibility has improved in many cities throughout the globe, and we can now see natural features that were once obscured by pollution. Hopefully this is something the general public would like to continue to see going forward,” Goldberg wrote.