As temperatures gradually rise above freezing in Ann Arbor, melting snow indicates the end to winter is near. But underneath the slush lining city streets lies months of accumulated urban pollution, which is released into the air as the snow melts.
In an interview with The Michigan Daily, Mark Flanner, associate professor of climate and space sciences and engineering, said snow doesn’t just passively sit on the ground during the winter; it also acts as a sponge, absorbing pollution in the form of particulate matter.
“The snowpack acts as a big storage container for atmospheric pollutants collected throughout the winter,” Flanner said. “Falling snowflakes collect particles and chemicals in the air, and can do so more effectively than raindrops because they have much larger surface area than raindrops.”
That means vehicle exhaust, heavy metals from automobile parts and road salt all settle into snowbanks along roads and sidewalks. While frozen, these particles remain trapped and attached to ice crystals.
Kerri Pratt, professor of chemistry and earth and environmental sciences, wrote in an email to The Daily that this buildup is often visible to the naked eye.
“Pollutants, in the form of gases and atmospheric particulate matter (i.e. particles, PM), fall onto the snow over the winter,” Pratt wrote. “For example, we can see snow darken from vehicle soot and road dust coating the snow. Most of these pollutants stay on the snow until snowmelt, but some can undergo reactions.”
As snow partially melts during the day and refreezes overnight, crusted layers form. Flanner said, under certain conditions, these layers can break apart and be lifted by wind or traffic, sending microscopic particles back into the atmosphere.
“The dirty layers at the top of the nearly melted snow and on the newly exposed ground become sources of particulate matter entering the atmosphere and degrading air quality,” Flanner said. “When there is not much wind to carry the pollution away and mix it with cleaner air, as happened in mid-February, the air quality can become hazardous.”
Pratt wrote this phenomenon can help explain the foggy conditions during snowmelt as relative humidity increases. Odors during this period can arise from a mix of factors, including the release of trapped pollutants as well as gases from thawing organic matter and urban runoff.
One culprit behind that odor is ammonia, which is emitted from sources such as vehicle exhaust, waste and regional agricultural activity. When trapped ammonia is released during snowmelt, it can react with acidic gases produced by vehicle exhaust — such as nitric and sulfuric acids — and form ammonium salts. These salts make up fine particulate matter in the atmosphere, small enough to enter the lungs and bloodstream. Particulate matter of this size is associated with increased risk of respiratory and cardiovascular diseases. Another set of reactions involves road salt, which, when spread to de-ice roads, can contribute to the formation of reactive chlorine-containing compounds under certain atmospheric conditions.
Scientists fear global warming might worsen air quality, as warmer temperatures and less snowfall won’t necessarily lead to less pollution. Instead, climate change is expected to increase winter temperature variability in the Midwest, leading to more frequent freeze-thaw cycles.
More frequent freeze-thaw cycles can intensify the concentration of pollutants near the surface of snowbanks and thus create more opportunities for particles to become airborne. In other words, the release of pollutants doesn’t only depend on how much snow falls, but also how variable winter temperatures become.
In January, Ann Arbor had an average air quality index of 52 and 9.9 micrograms per cubic meter. In February, the city had 15 days above normal AQI — between zero and 50 — with a high of 111 on Feb. 15. According to the U.S. Environmental Protection Agency, when the AQI falls between 51 and 100, air quality poses risk to unusually sensitive individuals. Between 101 and 150, children and members of sensitive groups, such as those with preexisting heart or lung conditions, may experience health effects.
In an interview with The Daily, Dr. Toby Lewis, a pediatric pulmonologist, said low air quality presents worsening symptoms for vulnerable patients.
“From a respiratory perspective, we worry most about asthma and (chronic obstructive pulmonary disease) and cardiovascular events, such as heart attack and stroke, but there is a lot of literature about impacts on other conditions as well,” Lewis said.
In an interview with The Daily, LSA junior Celina Cookson said she notices her asthma symptoms flare up during freeze-thaw cycles from changes in humidity.
“Every single time around this time of year, I go to (University Health Service) for some sort of cough, and I have no other symptoms,” Cookson said. “I think it’s also the change in humidity, because when it snows, and then it melts, that changes the humidity too.”
In an interview with The Daily, Public Health junior Simran Sinha said snowmelt during the transition period from winter to spring exacerbates her asthma symptoms.
“As it gets colder, I tend to get sick more and then with walking outside in the cold, traveling on campus, I do tend to get more shortness of breath and need my inhaler more,” Sinha said. “I definitely think the transition from winter to spring — that exacerbates my asthma more. I also need to use my inhaler more.”
Lewis said healthy adults are not immune either, particularly those who spend long hours outdoors or engage in strenuous activity.
“For otherwise healthy adults, the risks often have to do with degree of exposure,” Lewis said. “If you work or do a lot of physical activity outside, you have more time outside and being exposed and also tend to be breathing more deeply and faster due to the intensity of your activity — more breathing outside equals more exposure,” Lewis said.
Daily Staff Reporters Emma Spring and Sophia Jayasekera can be reached at sprinemm@umich.edu and sophiaja@umich.edu.
