Air pollution unaffected by traffic decrease, data show

Air pollution unaffected by traffic decrease, data show
Data from regional air quality agencies shows no correlation between particulate matter levels in the air and the decrease in traffic due to the state’s stay-home order. Photo:

One of the many arguments proponents have made for either a regional or statewide low carbon fuel standard (LCFS) to reduce the amount of CO2 emissions from vehicles is that it will improve air quality and particulate matter. In a 2019 technical analysis, the Puget Sound Clean Air Agency (PSCAA) cited “positive health impacts” of its proposed LCFS. The agency has since suspended indefinitely its work on its LCFS.

However, recent data from two regional clean air agencies shows local air quality has been unaffected by the significant reduction in driving that’s come as a result of Governor Jay Inslee stay home order.

A report by the Washington State Department of Transportation (WSDOT) showed that in late March statewide state highway traffic levels had fallen by 63 percent compared to the same day last year. Between March 15-22, there was a 44 percent decrease in vehicle traffic; during that time the PSCAA reports air particulate matter in the cities of Seattle and Tacoma were at their highest levels in the past four years. Those pollution levels dropped dramatically later in March, when traffic levels remained significantly lower than the same days last year.

“What you find is the air isn’t really cleaner,” Washington Policy Center (WPC) Environmental Director Todd Myers said during a June 10 virtual summit.

The lack of correlation has also become apparent as highway traffic has increased. According to Apple’s COVID-19 mobility trends that uses anonymized Apple Map data, driving among iPhone users in both cities has reached the baseline that existed prior to the COVID-19 outbreak. Statewide, traffic levels as of June 11 are now 18 percent below 2019 levels.

Yet PSCCA data for recent weeks shows particulate matter levels have remained stable since decreasing in late May.

The disconnect between air pollution and road activity has also been noticed by the Olympic Regional Clean Air Agency (ORCAA), which covers Clallam, Grays Harbor, Jefferson, Mason, Pacific, and Thurston counties. At the ORCAA agency’s May 13 meeting, Air Monitoring Supervisor Odelle Hadley reported that the state’s stay-home order had no effect on air quality in April in terms of particulate matter levels.

“We really just don’t see anything statistically significant or different in any of our data,” she said at the meeting, adding that any significant changes in air pollution levels were due to below-average temperatures or wind speeds.

Myers told Lens that the sudden change in particulate matters levels due to current weather or an increase in wood stove use undermines a possible counter-narrative that more time is needed before the impacts of vehicle traffic reductions can be noticed.

“In 24 hours, you see a change in particular matter just from a temperature inversion,” he said. “Saying it takes months is wrong, (though) it is fair to say that March-June may not be a useful period of time to judge, because it is spring and rainy. This might be the time where you are least likely to see an impact.”

However, other state public policy decisions further indicate that vehicle emissions have a minimal effect. The state Department of Ecology in January ended its 38-year vehicle emissions check program that drivers had to conduct before getting their tabs renewed. The state legislature in 2005 voted to end the program by 2020 based on recommendations from Ecology.

Ecology Air Quality Manager Kathy Taylor said in a December agency statement that “today, new cars are much, much cleaner than in decades past.” However, the agency statement did note that the program was “aimed at toxic forms of air pollution like carbon monoxide, rather than the carbon dioxide and other greenhouse gases.”

Myers has advocated replacing older, less efficient wood stoves as a more effective way to reduce air pollution. He said that what often complicates discussions around public policy proposals like an LCFS is the nuance between related elements such as greenhouse gases or CO2 emissions and particulate matter. Additionally, other contributing factors can make it difficult to determine how direct an impact certain policies have on the environment.

“If the science is murky…then job creation or social justice or whatever other value you have – that’s what’s really making the (public policy) decision,” Myers said. “The biggest problem is the people who pay the price for that failure…are not the politicians. We have to find a way to ground the political discussions that we have…in science.”

WPC hosts a webinar each Wednesday.


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