Coronavirus Has Slashed Air Pollution. This Interactive Map Shows How.

March 25, 2020 TH Author headline,virus,science

The covid-19 pandemic has changed the world, grinding to a halt increasingly large geographic areas and portions of the economy in an effort to slow the virus’ spread.

The impacts have been profound on the ground, but government-mandated lockdowns have also remade the atmosphere. Satellite data from China, the first epicenter of the outbreak, and Italy, the second hot spot, have shown big drops in pollution following lockdowns that limited the movement of people and goods and factories’ ability to produce stuff. With the pandemic now becoming increasingly prevalent in the U.S., Americans have already started moving less as mayors and governors have turned to similar measures.

In an effort to track the impacts, Earther assembled an interactive map to explore the changes in air pollution not just in the U.S. but globally. The map runs on Google Earth Engine and uses data collected by the European Space Agency’s Sentinel-5P satellite, which circles the Earth capturing various types of data. It includes four snapshots from December 2019 through March 20, 2020. The Sentinel satellite data shows nitrogen dioxide, which is a handy proxy for human activity.

“Nitrogen dioxide is produced by fossil fuel burning and therefore often used as an urban pollution tracer,” Barbara Dix, an atmospheric researcher at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder, told Earther in an email. “Burning fossil fuels directly emits a lot of nitric oxide and a little nitrogen dioxide (often referred to as NOx together), but the nitric oxide is rapidly converted into nitrogen dioxide in the atmosphere. Nitrogen dioxide can easily be measured by satellite.”

Given that fossil fuels power everything from cars to electricity, nitrogen dioxide satellite imagery really does show the impact covid-19 is having on society like no other dataset. There are clear signs of the virus’ impact all around the world, and we’ll dive into some U.S. examples below. But it’s also important to note a few small caveats as you scroll around the map and look at the before and after images.

The data presented here is a series of single-day snapshots. Weather patterns can blow pollution around and disperse it while rain and even the level of sunshine can further change readings taken by Sentinel-5P. There are also natural sources of nitrogen dioxide that can affect readings. The data in Google Earth Engine isn’t necessarily quality filtered. Dix noted that means clouds can mess with readings, which may be why on the interactive map there are some rougher-looking areas like northern New England in March or signs of pollution in the Seattle area where there may not be much. It will take researchers time to really dive into the data and filter it to get a fine-scale understanding of covid-19’s impact on air pollution. Despite these caveats, the trends in many major cities around the U.S. are staggering and clearly at least in part tied to the changes forced by the covid-19 pandemic.

“The rapid decrease we see in nitrogen dioxide due to covid-19 is unprecedented,” Dix said. “We are now witnessing a global experiment where one emission source is rapidly turned down (NOx), while other sources are still up or will decrease more slowly. A lot of atmospheric science will come out of this.”

“I imagine that the air pollution monitoring data collected during the covid-19 shutdown will be useful to test our fundamental understanding of the sources of pollutants (economic sectors, natural emissions, etc.), the chemistry of nitrogen dioxide, ozone and particulate matter, and short term health and ecosystem effects of air pollution,” Viral Shah, a postdoctoral researcher at Harvard, told Earther in an email.

California

California became the first state in the U.S. to issue a shelter-in-place order effective on March 19, though many cities made their decision to shut down earlier. The resulting drop in pollution from January to March in the state’s major metro areas is stark. Los Angeles is a huge source of pollution owing to its car culture, and the city’s precipitous drop in pollution is clear. Traffic reports back up the satellite data. The Bay Area and San Diego also saw pollution disperse in the face of a shelter-in-place order.

The impacts extended across the border as well. Tijuana—which is intimately linked to San Diego—saw pollution dissipate to practically nothing. Ditto for El Paso and Juarez visible further east. The Mexican and American governments have agreed to partially close the border in an effort to stop the spread of covid-19.

Even though they’re not in states with lockdowns, Salt Lake City, Phoenix, and Las Vegas all have varying degrees of restrictions on residents and businesses. And once again, the map makes clear that’s likely having an impact on pollution.

The Northeast Corridor

The populous area stretching from Boston to Washington, DC, is the epicenter of the U.S. coronavirus outbreak and also a hub of economic and political activity. It’s also an area where states and cities have moved quickly to shut down non-essential services to slow the virus’ spread.

Though New Yorkers aren’t as car-dependent as their Los Angeles counterparts, there are still plenty of vehicles normally on NYC roads and dense clusters of buildings emitting pollution. Ditto for Boston, Philadelphia, Baltimore, and Washington, which normally form a daisy chain of pollution along Interstate 95. With covid-19 lockdowns, the chain has been broken.

Midwest

The area from Minnesota to West Virginia forms the biggest cluster of states with lockdown orders in effect. Six governors have already put their states on lockdown are or will do so shortly.

The data here is a bit noisier, but some clear trends are apparent. Car-centric Detroit has a big dip in pollution. Sprawling Chicago’s pollution also disappears.

The U.S.-Canadian border, like its southern counterpart, is also closed outside of essential services and trade. The province of Ontario called for a lockdown late last week, and the impacts of these moves also show up on the map. Pollution also gets wiped out in Detroit’s neighbor, Windsor, and Toronto further east.

One odd blip is a hot spot of pollution in Kansas near Emporia. That could be one of the data artifacts Dix mentioned or nitrogen dioxide emissions possibly tied to fires that burn through the state’s grasslands each spring.

Texas and parts of the South

Even though the South is has been slow as hell to act, and Texas’ lieutenant governor spent Monday actively advocating for letting old people to die to save shareholder value, the impacts are still notable there. While the data is a bit noisy, nitrogen dioxide emissions in Houston, a hot spot for the petrochemical industry, appear to have declined. New Orleans—another petrochemical hot spot that’s located the only southern state to call for a total lockdown—appears to have seen a drop in pollution as well.

That may be tied to the fortunes of fossil fuels, which have dropped due to a price war sparked by Saudi Arabia and plummeting demand as the world economy slows. Sharp dives are also visible in Mexican cities like Monterrey, which may be tied to the border closure. Explore the full map here.

Updated, Wednesday, March 25 11:40 a.m.: This post has been updated to reflect that nitric oxide can be converted into nitrogen dioxide. The last map label in this post has also been updated after a furious editorial discussion on whether Texas is a part of “the South.”