Remember Beijing’s 2013 “air-pocalypse,” when the sky in the Chinese capital turned black as a thundercloud? The levels of microscopic particulates known as PM2.5 reached 900 micrograms per cubic meter in the city, three times the level rated “hazardous” by the U.S. Environmental Protection Agency. 

The city has since made strides in cleaning up its famously polluted air, but other Asian cities continue to suffer suffocating levels of particulates. Dehli, India, for one, is renowned for unhealthy levels of PM2.5. According to Greenpeace Southeast Asia, PM2.5 pollution caused 54,000 deaths in the city last year. 

Now a team of engineers from the University of Minnesota’s Center for Filtration Research has come up with a novel approach to help test cleaning the air of these cities using the world’s largest “air fresheners,” towers that scrub pollution from the atmosphere. 

“It provides another approach for air pollution remediation,” says Qingfeng Cao (Ph.D. ’19), a postdoctoral associate with the team involved with the towers. “I don’t think it’s always easy to limit the air pollution emissions. Yes, people should do that, but it takes time for developing countries to really meet the standards for the factories. I think this approach is another way for people to think about.” 

Three demonstration towers have been constructed so far—two in China, one in Delhi, and another Delhi project is on the way. 

“We’ve done modeling of putting 400 of these units in Delhi. It’s actually quite effective,” says David Pui (B.M.E. ’70, M.M.E. ’73, M.S.M.E. ’73, Ph.D. ’76), who heads the center and the University’s Particle Technology Laboratory. The towers aren’t cheap: roughly $2 million at the scale of the Delhi project. “But certainly, even if you were to build 100 units, it’s not that much, is it?,” Pui says. “I mean in terms of people dying from air pollution and having respiratory diseases. Fixing those medical problems will cost a lot more money than building these.” 

The developers envision that the utility of the towers may not stop at dirty air but could extend to drawing carbon dioxide out of huge volumes of air to blunt global warming. “Just as wind turbines are popping up here and there, you may find that some of these towers are popping up here and there,” says Thomas Kuehn (B.M.E. ’71, M.M.E. ’73, M.S.M.E. ’73, Ph.D. ’76), professor emeritus of mechanical engineering and another member of Pui’s group. “The work has really been focusing on the different applications of these towers.” 

The original concept—which Pui and his team began thinking about in 2014—was a solar-powered tower that would collect air and scrub it with filters. Sunlight would heat a huge area of air and earth under a gently sloping glass roof. The air would rise and increase in velocity as it slid toward a vertical stack and a series of filters would then clean the high-speed air rising in the stack. 

In their original 2014 paper, Pui and Cao were thinking big—really big. Modeling air pollution cleanup in Beijing, they envisioned eight towers, each with a greenhouse base that measured up to three miles across and covered an area larger than the Minneapolis suburb of Richfield. Their calculations showed that these immense towers would move and filter sufficient air to reduce air pollution in China’s capital by between 11 and 14 percent. Says Pui, “The key is the air volume that you can bring through the filter.” 

But the prospect of finding vacant lots the size of Richfield in the vicinity of a teeming city were daunting. A Chinese real estate developer told Pui the land would cost a lot more than the tower itself. 

So they scaled back their tower to dimensions they could more easily shoehorn into an urban environment and found interested partners in the Chinese Academy of Sciences, with whom Pui had collaborated on particulate research for years. 

In 2016 the Chinese government built an air-cleaning tower of Pui’s design in Xi’an, a provincial capital well known for pollution that comes from burning coal, burgeoning numbers of vehicles, and sandstorms that sweep in from western deserts. In fact, the city had introduced a “Three-Year Action Plan for Managing Haze with an Iron Fist and Defending the Blue Sky.” 

With a glass-roofed base measuring a bit smaller than a football field and a 200-foot-tall concrete stack at its center, the tower would of course be too small to solve Xi’an’s air problems. But it did provide an opportunity to study the performance of such a device. 

“It is a good pilot-scale system to demonstrate the concept,” says Pui. A paper by Chinese colleagues showed that city residents were willing to pay a premium in real estate prices to live close to a clean-air tower. 

Another benefit: The tower attracted a lot of attention from scientists and engineers, including a group from India that wanted to build a similar device in Delhi. In 2019, Delhi’s air reached an alarming record point of more than 500 micrograms of PM2.5 per cubic meter. According to the Indian English-language publication The Economic Times, the Indian Supreme Court pressured the government to take effective action to clean up the air. 

In a collaboration between the U of M, IIT Bombay, and Tata Projects Ltd., the Indian government committed to building a demonstration air-cleaning tower in Delhi. Scrubbing existing pollutants was attractive because much of Delhi’s problem doesn’t come from a smokestack or tailpipe, but from farmers burning stubble in their fields after harvest. Says Kuehn, “You either change the lifestyle of the farmers, which is an option but may take a while, or you deal with the air pollution that exists.” 

The original solar tower concept sent clean air out a tall stack “where it’s not going to do anybody any good locally,” says Kuehn. So the U of M team took a different approach to the Delhi tower, installing a bank of 40 giant fans to draw air down the stack, through filters, and out the base to put the clean air near the ground. 

The Delhi project, unveiled this summer and which began operation this fall, directs cleaned air into a pedestrian thoroughfare linking bus, train, and Metro stations. Says Kuehn, “There’s a lot of foot traffic through there, so it would make a lot of sense to try to clean that localized air between these three different facilities.” 

Pui’s team is also responsible for a third test tower, this one in Yancheng Science Park in eastern China. It’s a small structure suitable for air flow and filtration experiments. It’s also distinguished by its filter—which uses falling water inside the stack to wash away pollution. 

Says Kuehn, “We all know that when it rains the rain tends to clean up the air, gets the particles out of the air. So why not use that concept instead of filters? It’s a lot cheaper. You don’t have to buy filters, install them, replace them, dispose of them.” 

The watery filter may also offer another future possibility: By adjusting the chemistry to remove carbon dioxide from large volumes of air, the carbon could be sequestered underground to help stem global warming. 

“The problem with extracting CO2 from the air is that the concentration is so low you have to handle huge volumes of air,” says Kuehn. That has meant spending a lot for electricity to power fans. But in a large solar tower, the sun moves the air for free. 

Some of the drawbacks of using the solar towers for cleaning air wouldn’t apply to carbon capture. The tower doesn’t need to eject clean air near ground level where people can breathe it. It doesn’t need to be near a city but can be sited in a desert or grassland or estuary where land prices are low. 

Says Kuehn, “It doesn’t matter where the air is coming out or when you operate it, as long as you take the CO2 out. It’s much more flexible in that respect.” 

Pui says that in Dehli, a second unit is being built with the idea that additional units may be possible if the first are successful. He’s hopeful that Delhi will build dozens in the coming years. 

Kuehn says the team is also pursuing Chinese government support to install equipment needed at the Yancheng tower to test CO2 removal. And far into the future, who knows? Perhaps giant towers dotting the landscape, functioning as the purifying lungs of a futuristic economy 

“This is what we believe,” says Pui. “However, there are many ways to skin a cat, aren’t there? We feel that this is a very good technology. Whether it will be accepted or not depends on alternative technology and depends on governments’ willingness to invest in such technology.” 

Greg Breining (B.A. ’74) is a freelance writer in the Twin Cities.