Less Traffic: Cleaner Skies?

With the substantial decrease in travel due to stay-at-home orders in response to the coronavirus, many urban areas have seen declines in the byproducts of combustion. But has this decline translated into a significant improvement in our air quality? And how does Seattle’s improvement compare to other cities across the world?

The pollutant around the world with the most dramatic drop in levels due to the coronavirus has been nitrogen oxide. Nitrogen oxide is formed during the combustion of fossil fuels due to the resulting heat allowing oxygen and nitrogen to bond.

However, air pollution is not simply a factor of the pollutants emitted into the air; it is also related to the weather patterns that an area experiences. For example, the inversions we typically see a few times during the winter trap moisture and pollutants near the surface, enshrouding Western Washington in a thick layer of smog while higher elevation spots like the Issaquah Alps bask in sunshine and unseasonably warm temperatures.

Governor Inslee’s “Stay Home, Stay Healthy” order was issued on 3/23, but traffic began a long-term downtrend during the first week of March. It bottomed out around the end of March and has been very slowly rising over the past month. Interestingly, in Seattle, nitrogen oxide concentrations only began to decrease at the end of March despite the month-long decrease in transportation, and this was likely due in part to a shift in the weather pattern to cleaner, onshore flow towards the end of the month.

Still, the change from average has been slight, with nitrogen oxide concentrations only about 10–20% less than usual. In other words, the decrease in traffic has decreased the amount of nitrogen oxide over Seattle, but the decrease has been relatively slight and was only realized after a shift in the weather pattern.

The decrease has been much more dramatic for other cities. Los Angeles often sees a ton of nitrogen oxide to hefty transportation emissions, but they were 40% below their baseline as of mid-April. At Wuhan, the center of the outbreak, declines were even more stark, with nitrogen oxide levels reaching nearly 1/3rd of the 2015–2019 average during the lockdown. Restrictions on travel have eased since, and levels are now only 10–20% below average.

With all the hardship and suffering caused by the coronavirus, finding silver linings can take quite a bit of work. But hopefully we can rejoice in knowing that the air we breathe is cleaner in a socially distant world, even if only a little bit.

~Charlie Phillips

Charlie Phillips, a Madrona resident, received his B.S. in atmospheric sciences from the University of Washington and works in Portland as a meteorologist. Check out his weather website at charlie.weathertogether.net.