Synergising cross-sectoral efforts to secure cleaner, safer air for all

Synergising cross-sectoral efforts to secure cleaner, safer air for all

The air we breathe has a significant impact on our health. Air pollutants such as fine particulates, ozone, volatile organic compounds and toxic gases are notorious for their links to a wide range of health issues—not only respiratory illnesses, but also cardiovascular and neurological diseases. Air is essential for our lives, but human activities in modern societies often result in the release of fumes and harmful gases into the atmosphere. Although clean air has become an increasingly important asset, not everyone has access to it.

In indoor settings such as homes and offices, air purifiers are particularly helpful in improving the air quality and reducing the concentrations of air contaminants. But what if gigantic versions of these purifiers were erected in cities to achieve cleaner air outdoors?

Standing out among the houses and other buildings in China’s third-most populous city, Xi’an, is a 20-storey air purifier. This skyscraper-sized air cleaning system, dubbed the world’s largest air purifier, was unveiled in 2016 to fight the persistent air pollution plaguing the city. The chimney-like tower draws in polluted air at the bottom; filters out particulate matter like PM2.5; and releases clean air at the top. To date, a total of four towers of similar kind have been built in Xi’an and Yancheng, China, and Delhi, India. Among the experts who have made these giant air purifiers possible is Prof. David Y. H. PUI, PAIR Senior Fellow. He designed the towers with his marvelous hands. His collaborators in Minnesota include Prof. Emeritus Thomas KUEHN, Charles LO, and former student/post-doc Dr Qingfeng CAO and Prof. Shawn CHEN of Virginia Commonwealth University. The local Principal Investigators are Prof. Junji CAO in Xi’an, Prof. Jing SUN in Yansheng, and Profs. M. SAHU and Y.S. MAYYA in Delhi.

In this Issue, PAIR is privileged to interview Prof. Pui, Regents Professor at the University of Minnesota, USA, about the tower development and the keys to achieving clean air, based on his decades-long experience in aerosol and air filtration research. Prof. Pui is Director of the Center for Filtration Research and the Particle Technology Laboratory at the university.

The smog battle: Giant air purifiers that protect our blue skies

Following the debut of the Solar-Assisted Large-Scale Cleaning System (SALSCS) in Xi’an, Prof. Pui and his team have continued to enhance the system’s design and performance. Over the years, the group has developed three generations of air cleaning towers, including the first one in Xi’an, a second-generation tower in Yancheng, and two units of a third-generation system in Delhi, India. In general, these systems are composed of three main structures, a large solar collector, a chimney, and a filtration system for removing pollutants. From one generation to the next, new technologies have been incorporated into the systems to enhance their cleaning capacities.

“The first generation makes use of solar heating to drive polluted air into the system, where a filter array is set up to remove the polluted air. The clean air then exits through the tower to the surrounding area,” Prof. Pui explained. “In the second generation, installed in Yancheng Science Park, the filter array is replaced with a water spray array to scrub out the polluted air. This second generation can also be used to take out carbon dioxide by dissolving NaOH in the water. The sprayed NaOH droplets can take out the CO2. Two units of the third generation, installed in Delhi, India, make use of fans to drive clean air from the tower to the surrounding environment.”

From drawing to building, from design to construction

A large tower in the backyard can change the look of a neighbourhood, especially in low-rise communities. Furthermore, just like household air purifiers, the large-scale systems can produce some noise during operation. Building giant facilities in public spaces often creates dissent and resistance from the local community. However, in the case of SALSCS, there does not seem to be much concern about noise or other potential impacts of the tower. By contrast, people would rather spend more money to live in the surrounding area.

“When the Xi’an tower was constructed and put into operation, an economics professor from Shanghai Jiaotong University led a team to survey the residents living near the clean air tower. They found that the residents were willing to pay 4% more in real estate value to live close to the tower,” Prof. Pui explained.

When asked which factors determined the towers’ location, Prof. Pui expounded on the importance of government support. “The first tower cost $2 million US dollars to build. This is a large amount that many companies cannot afford, so the implementation really depends on support from the government,” he said. “We were very fortunate to have the support of the Xi’an government in building the first tower, the Yancheng Science Park for the second one, and the Delhi government and the central government of India for the two third-generation systems.”

“For small cities like Hong Kong, where severe land and housing shortages have been thorny issues for years, building such large purifiers can be even more difficult.”

The SALSCS brings new hope to cities battling urban air pollution. However, for small cities like Hong Kong where severe land and housing shortages have been thorny issues for years, building such large purifiers can be even more difficult. Perhaps more time is needed to address the city’s long-standing problems before a similar tower can be built.

“Unlike cities in mainland China which have many parks situated between building clusters, Hong Kong tends to have buildings and houses squeezed into congested areas. There are few parks among the building clusters in Hong Kong, and the parks are relatively small. Given the traffic congestion in Hong Kong, it would be good to have towers built in areas where people living nearby are inhaling a lot of the combustion particles from cars. Small particles released from cars, especially those in the ultrafine particle size range, are particularly harmful. They have greater effects on human health due to their large surface areas. During my sabbatical leave at The University of Hong Kong (HKU) in 2014, I worked with Prof. Sun KWOK at HKU and Prof. Chak CHAN at City University of Hong Kong (CityU) to file a patent on a ‘Freeway Air Cleaning System’ to clean polluted air along freeways and roadways. We have not been successful in getting government funding to develop it.”

Tripartite efforts in untangling the knotty air problem

Climate change and air pollution are deeply intertwined. Greenhouse gases cause a warmer climate, increase the production of ozone that can manifest into haze, and exacerbate wildfire risks, resulting in a vicious cycle. The cleaning system designed by Prof. Pui represents a powerful new solution for climate action.

Turning engineering drawings into built structures requires collaborative efforts by governments, architects and construction companies. Throughout the interview, Prof. Pui humbly reiterated the importance of cross-sectoral efforts. “My team served as consultant in the projects. We designed the tower and provided the drawing to governments and construction companies who put it into practice,” he said.

“Academia, government and industry are three gears driving the wheel of progress in addressing climate change and PM2.5 problems.”

“We need to have government support first, and then feasibility studies by experts, before industry can participate. Still, the companies need to be reimbursed. Where does this money come from? I think it depends on the government after all.”

Academia, government and industry are three gears driving the wheel of progress in addressing climate change and PM2.5 problems. To Prof. Pui, the three sectors have distinct roles at different stages in the process. “Academia can most effectively address the sources (coal burning and vehicle emissions) and their effects (visibility and health). To protect public and environmental health, the government can set progressively stricter regulations for PM2.5 and vehicle emissions standards. Industry can respond by developing novel control technologies for baghouse filters and diesel/gasoline particulate filters. Furthermore, academia can help the government to set the regulations and help industry to develop novel technologies,” he explained.

Lessons from the pandemic for air filtration research

The field of air filtration has gained greater public interest due to increasing awareness of airborne diseases in recent years. Although the COVID-19 virus continues to circulate, the situation may not be as intimidating as it was several years ago.

When the virus first emerged to cause widespread pandemic, the market saw a surge in demand for approved respirators. In response, Prof. Pui’s team evaluated all available respirators and face masks, finding that Electret (charged-fibre) filters have significantly higher filtration efficiencies than mechanical filters. Amid the global panic buying, particularly when there was a scarcity of facial masks alongside an influx of counterfeit, sub-standard and non-equivalent N95-like respirators, Prof. Pui’s team developed various cleaning methods for re-using the respirators and masks. The group also developed a “Particle Decay Model” for evaluating indoor air cleanliness. “I was told that the University of Minnesota has a lower infection rate than other Big Ten universities because facility engineers can use our model to evaluate classroom cleanliness,” he remarked.

Should scientists become entrepreneurs?

“Dedication to the advancement of knowledge and society is the fundamental scientific virtue that drives a scientist’s endeavours”

Over the years, Prof. Pui has developed over 40 patented technologies, and some have developed into commercial instruments that have propelled businesses to grow into leading manufacturers.

In the 2000s, he co-founded a start-up that applies the electrospray technologies that he and his former student Da-Ren CHEN (now Chair Professor of Virginia Commonwealth University) developed in biopharmaceutical products. After the initial set-up, Prof. Pui has not been heavily involved with the company. The convergence of science and entrepreneurship is a growing trend, with increasing numbers of scientists transitioning into entrepreneurial roles to commercialise their research. When asked whether he aspires to pursue parallel paths in academia and entrepreneurship, Prof. Pui’s response reflected the fundamental virtue that drives a scientist’s endeavours: dedication to the advancement of knowledge and society. “I put a wall between the company and academic research. If there is even a perception that you are doing something to benefit your company, this is not good. I hope the start-up will be successful and will save many lives along the way. Similarly, my development of the SALSCS will also save many lives.”