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Air pollution levels ease as people stay home, but heat could reverse the trend

Arizona Republic logo Arizona Republic 4/15/2020 Erin Stone, Arizona Republic
a train is parked on the side of a road: As many Phoenicians work from home due to the COVID-19 pandemic, Central Avenue appears empty and void of traffic in downtown Phoenix on March 25, 2020. © Michael Chow and Thomas Hawthorne/The Republic As many Phoenicians work from home due to the COVID-19 pandemic, Central Avenue appears empty and void of traffic in downtown Phoenix on March 25, 2020.

Cities across the globe are seeing drastic reductions in air pollution as the COVID-19 pandemic forces commuters to work from home and brings daily life to a standstill.

In Phoenix the decline in pollution is measurable, according to regional air quality agencies, though not as significant as some bigger cities. Ozone and small dust particles were at unusually low levels in March, but officials say there could be other factors alongside a reduction in traffic.  

The number of vehicles on the road is down nearly 40% since early March and traffic delays have dropped about 57%, according to the Maricopa Association of Governments, the regional planning agency for the metro Phoenix metro area. 

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The agency used travel time and speed data from the analytics company INRIX to measure congestion on all major freeways and most of the arterial streets in Maricopa County before and after the COVID-19 shutdown. After the shutdown, commuters were driving at speeds 9% faster than normal during morning rush hour, and traffic delays more than halved across the board. 

With emptier roadways, there have been noticeable declines in pollutants caused directly by commuter traffic, according to agency air experts.

“Because of the shutdown, even though pollutants increased back up after the rain a few weeks ago, they didn’t go back as high as they would have,” said Ron Pope, an atmospheric scientist with Maricopa County Air Quality Department. “It's the traffic pollutants that you're really seeing decrease.”

Between March 16 and 22, nitrogen oxide emissions declined by about 37% in the Phoenix area compared to the same time in 2019, according to Environmental Protection Agency data analyzed by the Arizona Department of Environmental Quality.

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Nitrogen oxides, or NOx, are primarily created by car and truck exhaust and are a major contributor to ozone pollution in the Valley.

Maricopa County runs four of its own NOx monitors. Pope said that, before the work-from-home recommendations in mid-March, NOx levels were 17% lower than they were last year, a reduction he attributes to the unusually rainy weather in those weeks.

After traffic tapered off in mid-March due to the COVID-19 work-from-home order, NOx levels in his monitors dropped another 3%.

Effects of shutdown still unclear

It’s difficult to tell how much the reduced traffic has contributed to the decline in air pollution because air quality is usually in the good or moderate range due to rainy weather this time of year, Pope said.

Additionally, stricter stay-at-home measures in California have contributed to more dramatic pollution reductions in that state, which means winds are carrying less pollution into Arizona. Officials predict the impacts of less traffic on air quality will become clearer in the next few weeks.  

“Because of the complexity of ozone formation, a reduction of NOx does not necessarily equal a reduction of ozone right away,” said Erin Jordan, Arizona Department of Environmental Quality spokesperson.

“It is something we are watching closely and will be tracking as data continue to be gathered," Jordan said. "But there is not a clear indication if there will be a noticeable drop in the ozone levels as we move into the longer days of spring and summer when there is more sunlight to produce ozone.”

As the days get longer, ozone pollution tends to worsen in Phoenix. Ozone is a colorless, odorless gas that forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in sunlight.

Ozone levels are fluctuating

In Arizona, vehicles are the biggest contributors to the pollutants that make up ozone. Usually the high level of traffic in the sprawling metro area combined with desert sunshine makes a perfect recipe for ozone. That’s part of the reason why Phoenix has some of the highest ozone levels in the country.

“Normally April is when ozone starts to ramp up, and it’s probably a little lower than I would expect right now,” Pope said. “I would expect a lot more moderate days at this time of year than what we're seeing, so I would say ozone is coming down a little bit, but it's not dramatic.”

Over the past 15 years, the average number of moderate ozone days in March is 17 days. This year, if current ozone forecasts hold, Phoenix could see only six days of moderate ozone, which would make it one of the best years for low ozone in the past 40 years, according to ADEQ.

Other dangerous air pollutants are down as well. The Maricopa County Air Quality Department found that compared to the same time last year, particulate matter pollution has also declined.

PM2.5, one of the world’s most dangerous air pollutants, is derived from automobile exhaust, industry, and burning wood and coal. The tiny particulates are 2.5 micrometers or smaller in diameter, a fraction the size of a human hair.

In Arizona, PM2.5 is generally caused by woodburning during the winter. The particles are so tiny they can enter deep into the lungs, making them particularly harmful to health. This month, PM2.5 levels were 20% lower than last year, though that is likely primarily due to rainy weather, Pope said. 

Dust particles called PM10, which measure 10 micrometers in diameter or smaller, had been 24% lower this year than last year. Since the Arizona Department of Health Services urged businesses to tell their workers to work remotely in mid-March, PM10 concentrations have declined another 6%, Pope said.

Numbers are lower, but still a risk

Data from the Environmental Protection Agency show that ozone and PM2.5 pollution since the shutdown are lower in metro Phoenix than they've been in the past 10 years.  

Ozone, PM2.5 and PM10 are all dangerous to human health. An average person breathes seven to eight liters of air per minute, which means with each breath of ozone or particulate matter, the risk for respiratory illness, heart disease, and even premature death increases.

Children are even more at risk: They breathe 50% more air per kilogram of body weight and their lungs are still developing and thus more sensitive to respiratory diseases like asthma.

a view of a city street filled with traffic next to a highway: Air pollution is seen over a hazy downtown Phoenix on Jan. 2, 2020. © Thomas Hawthorne/The Republic Air pollution is seen over a hazy downtown Phoenix on Jan. 2, 2020.

“It remains essential that we continue every effort to maintain and improve air quality in the Phoenix and Tucson areas and across the state,” Jordan said. “Ozone in the air we breathe can be harmful to our health and worsen bronchitis, emphysema, asthma and other respiratory issues, including those symptoms related to COVID-19.”

A new study from researchers at Harvard’s school of public health found that people who lived in counties with elevated levels of PM2.5 in the air were more likely to die from the virus.

Regional air experts are pleased with the good air quality and hope that one silver lining from the pandemic could be lower ozone pollution. But there are economic costs to the slowdown.

“With reduced traffic 10 years ago due to the Great Recession, there was a drop in ozone levels. However, we certainly don't want a return of those economic conditions,” Jordan said.

“Simply put, there is no clear trend at this time, and we need to gather more data and do more analysis," she said. "In the long term, the continued growth of teleworking opportunities may help reduce traffic (and therefore pollution) while maintaining a healthy, growing economy.”

Although Phoenix's ozone pollution is among the highest in the U.S., data provided by the Maricopa County Air Quality Department show that the area's ozone reading fell 11.2% on average from 1990 to 2018.

Much of the reduction over the decades has come because of landmark federal regulations like the Clean Air Act, as well as improved tailpipe emissions standards set by the federal government, Pope said.

“Pollution has been coming down every year for the most part — the cars are getting cleaner, businesses are getting cleaner, and there’s been much improvement over the last 30 years,” Pope said. “Is this year going to make that jump greater? I’m sure it will, but we've been reducing emissions year over year, and this year we're reducing emissions even further because of this unfortunate event.”

On March 31 in the midst of the new coronavirus outbreak, the Trump Administration rolled back those clean car standards. Critics noted that the virus is particularly dangerous for people with respiratory diseases and the rule change is expected to be challenged.

Still, regional planners hope the pandemic can result in some lasting changes.

“Since half of the nitrogen oxide emissions that contribute to ozone formation are from cars and trucks, we hope that a reduction in all types of trips will have a positive impact in the upcoming ozone season,” Kelly Taft, communications director for Maricopa Association of Governments, wrote in an email to The Republic.

“Once the pandemic is over, we will continue to encourage people to telework and to avoid single occupant vehicle trips as effective air quality measures. This unprecedented period may have demonstrated to employers and employees alike that teleworking is an effective alternative to work trips, or how some nonessential trips can be combined to reduce overall driving.”

Erin Stone covers the environment for The Arizona Republic and azcentral.com. Send her story tips and ideas at erin.stone@arizonarepublic.com and follow her on Twitter @Erstone7.

Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Follow The Republic environmental reporting team at environment.azcentral.com and @azcenvironment on Facebook, Twitter and Instagram.

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This article originally appeared on Arizona Republic: Air pollution levels ease as people stay home, but heat could reverse the trend

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