The day was like any other in summer on the Lake Michigan coast: hot. But Charles Stanier remembers the breeze.
It was the summer of 2017, and he’d been working up a sweat in a trailer in Illinois Beach State Park. A professor of chemical and biochemical engineering at the University of Iowa, Stanier spent the morning checking instruments, climbing up and down ladders, and wondering if anything would come of all his team’s work. A cool breeze crept in from the lake, and the computers flew into a frenzy.
“You’re like, ‘Oh, we’re getting so much interesting data,’” Stanier said.
The data he was there to collect was related to the invisible ozone lingering in that wind. Stanier helped design the multi-agency project to study it: the 2017 Lake Michigan Ozone Study, which surveyed the complexities of ozone pollution in the region, one of the country’s hotspots. With that data, researchers can better model ozone, informing forecasts, regulatory planning and climate predictions.
The latest study to emerge from the LMOS examines the connection between lake breezes and high-ozone days. Led by Patricia Cleary, a chemist at the University of Wisconsin-Eau Claire, the climate study found the majority of high-ozone days occur when there’s a breeze that travels inland from the lake.
“Lake breeze is very often associated with high-ozone events,” she said. “It can be as much as 85%.”
High in the atmosphere, ozone blocks harmful solar radiation. But near the ground, it’s the main ingredient in smog. The pollutant harms the breathing systems of both plants and humans. Ozone irritates and damages the lungs, causing coughing, wheezing or shortness of breath. The most vulnerable groups include children, older adults, outdoor workers and people with asthma. Research also indicates exposure to ozone worsens COVID-19 mortality.
According to an October report from the Wisconsin Department of Natural Resources, the state’s air quality continues its 20-year upward trend. But much of the Wisconsin shoreline, from Kenosha to Door County, is stuck in nonattainment, meaning certain areas don’t meet the Environmental Protection Agency’s ozone standards.
That’s due to the lake’s meteorology.
Lake Michigan’s ozone path
Ozone isn’t emitted to the air directly. It’s born in the atmosphere when emissions from vehicles, factories and power plants mix into a chemical soup, which the sun cooks into ozone. Hot, sunny days are most likely to spawn unhealthy ozone levels.
Overnight, winds shuttle ozone-forming pollutants offshore—from Chicago, Milwaukee and other industrial sources—over the lake. Trapped above water, under the glare of the sun, they react to produce ozone.
Due to temperature differences between the lake and shore, winds can form that carry the ozone inland. Land heats faster than water. In summer, as daytime temperatures rise with the sun, the land grows warmer than the lake. When hot air rises over land, it sucks in cool, ozone-rich air from the lake. This lake breeze transports ozone up and down Lake Michigan’s western coast.
But how much ozone wafts onto shore, versus one or two miles inland, was unclear. One key LMOS goal was to provide that data by car, ship and plane. Cleary’s analysis also took advantage of long-term monitors run by the Wisconsin DNR.
For instance, one blazing June day during the study, researchers drove north up the lakeshore from Zion, Illinois, to Wisconsin, passing through Chiwaukee Prairie, Kenosha and Racine. Along the way, they took regular detours across east-west streets and stopped every five minutes to record measurements.
Ozone levels peaked by the shore and dropped significantly the farther inland they went.
“A 5 parts per billion change is a lot,” said Stanier, also one of the study co-authors. “They were seeing 20 parts per billion changes within 2.5 miles.”
Between 15-20% of those high-ozone days, Cleary said, are like this. Despite a lake breeze, the ozone doesn’t travel far inland, sticking to the shore. The particulars of the gradient depend on overall wind patterns across the region.
The sharp gradient means communities living on the shoreline face the brunt of ozone, but air quality is an environmental justice issue, Cleary noted. An Asthma and Allergy Foundation of America 2020 report stated Puerto Rican Americans and Black Americans are almost twice and 1.5 times more likely to have asthma than white Americans, respectively. Health factors like asthma compound the impacts of poor air quality.
Climate change worsens the mix
The wealth of on-the-ground data from the LMOS offers scientists the chance to test their ability to predict ozone behavior on a given day. Daniel Horton, a climate scientist at Northwestern University, uses such data to develop climate models and make air quality predictions.
Climate change can exacerbate ozone in several ways, Horton said.
Ozone season overlaps with the summer months, when sunlight is plentiful and temperatures are warmest. Warming temperatures associated with climate change raise ozone levels by accelerating ozone formation. And, “as the spring and fall get warmer, the ozone season’s going to expand,” Horton said.
Hotter summers also bring more stagnant days: rainless days with light winds and halting air. Stagnation traps air pollutants near the ground, encouraging more ozone buildup.
These outcomes “all depend on how we change or do not change how we emit in the atmosphere,” Cleary said.
Air quality and climate change are closely linked. Climate change worsens high-ozone days, and emitting pollutants into the air impacts the climate.
“When we attempt to limit global warming, we could also improve air quality,” Horton said. “It goes both ways.”
People can sign up for EPA air quality alerts. Those who are concerned or sensitive to poor air quality may choose not to go outside on high-ozone days.
This fall, the EPA announced it will review whether changes should be made to the current 70 parts per billion ozone standard, which was set in 2015 and upheld in 2020—a decision that invoked numerous legal challenges. The agency expects to reach a decision by the end of 2023.
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Featured image: The aftermath of high tides can be seen along Lake Michigan’s shoreline at Kohler-Andre State Park. Recent years have had record-breaking water levels across the Great Lakes region. (Photo Credit: John McCracken)