Reflection on 50 Years of Friendship and Collaboration on Aerosol Science and Technology

I started my master’s degree in 1971 and completed Ph.D. degree in 1976 under the mentorship of Prof. Benjamin Liu. During these years, we worked on bipolar charging and established the criteria to neutralize charged aerosols (Liu and Pui, 1974a, 1974b) (Fig. 1). An electrical aerosol analyzer (EAA) was developed to measure atmospheric particle size distributions and led to the successful commercialization of TSI 3030 EAA (Liu and Pui, 1975). TSI founding aerosol instrument manager Gilmore Sem wrote that without the success of the EAA, TSI would have gotten out of the aerosol instrument business (Schmidt et al., 2022). Professor Kenneth Whitby and Dr. William Wilson of EPA invited me to participate in the LA smog measuring campaign (Whitby et al., 1975) which provided results, together with other field measurements, to help EPA set up the PM2.5

academic. Yan was a senior director of Applied Materials and held 100 patents before his early retirement. He is now developing a sensor for real-time detection of nanoparticles and biological particles in air and liquid (Ye and Pui, 2021). Francisco worked at MSP/TSI for 25 years as a senior product manager before returning as a senior research engineer at the Center for Filtration Research (CFR). We published a series of papers on aerosol transport, deposition and charging (Pui et al., 1987;Tsai et al., 1990;Pui et al., 1990aPui et al., , 1990bRomay et al., 1991;Ye and Pui, 1990;Ye et al., 1991;Romay and Pui, 1992). During this period, I have started intense collaboration with Prof. Heinz Fissan, not only on research but also on organizing aerosol associations, IARA, IAC, and several major conferences and workshops. Our collaboration has built a strong basis that led us receive the Max Planck Research Award-the highest award for Engineers and Scientists in Germany, Humboldt Research Award for Senior U.S. Scientists (Pui), and the establishment of the Fissan-Pui-TSI Award for International Collaboration presented every 4 years during the International Aerosol Conference (http://iara.org/FissanPuiTsi.htm). Our families are close friends (Fig. 6). In fact, together with Fissan, Kousaka, Pourprix and Szymanski, we sent our daughters to each other's families every summer for several years. We now meet every two years to continue our friendships in Kyoto (2013)

Decade of 1990's
Chair Professor Da-Ren Chen started his Ph.D. study in the early 1990's. During his dissertation research and post-doctoral years and subsequent tenure as the PTL Manager, we together developed many exciting new aerosol technologies. Our two seminal papers on Electrospray (Chen et al., 1995a;Chen and Pui, 1997) have received approximately 900 citations. We also explored the technology for aerosolizing nanoparticles in the aerosol form for measuring liquidborne particles using the more sophisticated aerosol instruments. Following the development of a technique to produce nanoparticle medicines, we co-founded a start-up Nanocopoeia in St. Paul, Minnesota (https://nanocopoeia.com). Da-Ren also modeled and designed the Nano-DMA, the workhorse for making nanoparticle measurements. Prof. Fissan and his student Dr. Detlev Hummes also were involved in this important development (Chen et al., 1998). Prof. Ben Liu and I also started the Center for Filtration Research (CFR) in 1991, which is still going strong with 20 leading international filter manufacturers and end users. During the early years, we also worked with Dr. Wilson Poon (WL Gore), Dr. Scott Earnest (NIOSH Division Director), and Dr. Shintaro Sato (Hitachi) on a variety of filtration projects. Besides Scott as a NIOSH director, we have 4 other CFR Ph.D.s, Drs. Chaolong Qi, Liming Lo, Seungkoo Kang, and Drew Thompson working at NIOSH, an affiliated member of CFR and a good resource for our research. Prof. Da-Ren Chen has been a key contributor to CFR. One of the early projects of great interest to CFR members was the design of the pleated filters. Da-Ren performed a detailed study using the finite-element numerical method to provide the design guidelines (Chen et al., 1995b). Many of his students also contributed to the CFR as student investigators or post-doc at UMN, including Dr. Chaolong Qi (at NIOSH), Dr. Lin Li (at MSP/TSI) and Dr. Qisheng Ou (CFR lab manager). Dr. Ta-Chih Hsiao performed CFR research at WashU and is now a Professor at the National Taiwan University. Other CFR early graduates included Dr. Ming Ouyang (Cummins), Dr. Bruce Forsyth (Boston Scientific), and Dr. Hee-Siew Han (TSI). A master's student Xiang Zhang that I mentored is now the President of the University of Hong Kong. I also enjoyed helping Prof. Chuen-Jinn Tsai on Taiwan TAAR, Prof. Kangho Ahn on Korea KAPAR, and Prof. Junji Cao on CAAR during the startup of their respective aerosol associations. I have appreciated many years of friendship with Prof. Dr. Bernd Sachweh, who was a post-doc at UMN with Prof. Peter McMurry (1992-93). Bernd invited me to BASF's "Meeting with Professors" in Germany for several years and also to BASF conferences in Beijing and Singapore. He is currently Vice President, Special Projects Asia, at BASF (China) Co. Ltd. in Shanghai, China.

Decade of 2000's
One of the major programs during this period was funded by Intel on Extreme UV Lithography (EUVL) Mask Study. The objectives were to develop methods to evaluate and control particulate contaminant generation, transport and deposition in a mask handling system (Fig. 7). Prof. Fissan was a key investigator, and his student Dr. Christof Asbach, a CFR post-doctoral research associate, was a key contributor and is now the President of GAeF (Gesellschaft fuer Aerosolforschung)the oldest aerosol association in Europe and the world. Other students/post-docs working on the project included Prof. Se-Jin Yook, Prof. Jung-Hyeun Kim, Prof. Jing Wang and others. We have developed thermophoretic technique to protect the masks, and injection system to evaluate particle deposition under vacuum conditions (Asbach et al., 2006). We also deposited known size nanoparticles on the masks as calibration masks. In all, we published 16 peer reviewed journal papers (Kim et al., 2006a). Many of the techniques we have developed are now industry practice for EUVL system. I also had the opportunity of mentoring Dr. Seungki Chae who became VP and Sr. VP of Samsung Electronics and Samsung Display. During this period, long-time collaborator Dr. George Mulholland worked with us on certifying NIST 60 nm and 100 nm primary standard particles using the DMA technique (Mulholland et al., 2006). He worked with Jung-Hyeun to obtain the slip correction in the large Knudsen number regime (Kim et al., 2005) and published a series of papers with Prof. Weon-Gyu Shin on agglomerate particles characterization (Shin et al., 2010). In 2006, I attended Prof. Jing Wang's Ph.D. final defense in Aerospace Engineering and Mechanics (AEM). I was so impressed with his thesis research in fluid mechanics that I immediately recruited him to join my group as a post-doctoral research associate to work on aerosol and filtration research. In just a few years, he published a series of 35 papers focusing on EUVL and filtration research (Wang et al., 2007). He also helped mentoring junior students, Dr. Tze Yan Ling (Intel) and Weon Gyu Shin (Changnam National University). He has continued to contribute a great deal to CFR, even after he left to join ETH Zurich in 2010.
I am most grateful to have received the Fuchs Memorial Award during the 2010 International Aerosol Conference in Helsinki, Finland. The award is sponsored jointly by the American Association for Aerosol Research (AAAR), the Gesellschaft fuel Aerosolforschung (GAeF), and the Japan Association of Aerosol Science and Technology (JAAST). An Award Committee of the International Aerosol Research Assembly (IARA, www.iara.org), consisting of 18 international member associations, selects the winner(s) every four years. The co-winner of this 2010 Fuchs Memorial Award was Prof. Markku Kulmala of the University of Helsinki. IARA website stated, "The Fuchs Memorial Award recognizes outstanding original research contributions to the field of aerosol science and technology. It is considered the highest honor for researchers in the field. Presented every four years at the International Aerosol Conference, the award memorializes the late Professor Nikolai Albertovich Fuchs, the great Russian scientist who is regarded by many as the "father of aerosol science"".

Decade of 2010's and Beyond
Two milestones during this period were being named a member of the U.S. National Academy of Engineering (NAE) in 2016 and a Regents Professor at the University of Minnesota in 2019. There is a fixed number of 30 Regents Professors among 4,000 faculty members at UMN. We have explored industrial applications of filtration research during this period. Three major contributors during this period are Prof. Sheng-Chieh (Shawn) Chen, Dr. Seong Chan Kim and Dr. Qisheng Ou. They all have served a term as the manager of PTL/CFR. Shawn was Chuen-Jinn's Ph.D. student and came as a post-doc. He has worked on several topics: 1. evaluating membrane filter efficiency using sub-10 nm quantum dots-a collaboration with Prof. Doris Segets (Chen et al., 2016), a former post-doc at U of Erlangen-Nuremberg with Prof. Wolfgang Peukert; 2. co-authoring a PM2.5 review paper which has received 700 citations in a few years; 3. exploring the Electret filter applications-collaborating with Prof. Ziyi Li of the University Science and Technology Beijing on Zeolite coated Electret Media. Seong Chan spent 15 years at CFR separated by 5-year stay at Entegris as a contamination engineer. He came as a post-doc from Prof. J.K. Lee at Pusan National University. He worked on agglomerates generation and characterization, and health effects of nanoparticles. He performed in-vitro studies with Prof. Gunter Oberdorster at the University of Rochester (Kim et al., 2010), and many filtration applications projects, particularly in respirators/masks and contamination transport problem to mitigate Covid spreading (Kim et al., 2006b). I am pleased that Seong Chan has now started to work as a defect/contamination specialist at ASML, a major manufacturer of EUVL systems. Dr. Qisheng Ou started as a post-doctoral research associate from the Washington University of St. Louis (with Da-Ren as his Ph.D. advisor) and is now manager of CFR/PTL. He has performed research on filtration topics and developed systems to: 1. produce high temperature agglomerates to evaluate engine exhaust filters (Ou et al., 2019); 2. develop methods for coating nanoparticle membrane on wall filters to improve efficiency; and 3. evaluate respirator/masks, dental tools dispersion, and decontamination methods (Ou et al., 2020(Ou et al., , 2021. He mentored several Chinese scholars who became professors in Chinese universities, including Prof. Xinjiao Tian, Prof. Qiang Lv, and Prof. Cheng Chang, and recent students Dr. Chenxing Pei (Midea group), Weiqi Chen and Dongbin Kwak. Dr. Ou is currently focusing on starting an indoor air quality program. Two Japanese scholars came to out group during 2015-2017, Dr. Shigeru Kimoto and Dr. Maromu Yamada, and contributed to the CFR program. Dr. Kimoto worked on contamination control and instrument calibration. Dr. Yamada came from JNIOSH and returned to be a Senior Research Fellow in the Work Environment Research Group. Another Japanese scholar I remember well is Dr. Yoshiyuki Endo who sent me his growing family photo each year. One other major contributor was Dr. Zhili Zuo, who started our bioaerosol program. I have also enjoyed my frequent visits with Professor Pratim Biswas over the years, first as an Advisory Board member for his Energy, Environmental and Chemical Engineering Department at the Washington University in St. Louis, and now as an Advisory Board member for his College of Engineering at the University of Miami.
Two recent Korean graduates helped to develop new fields in my group. Dr. Changhuk Kim made use of the soft X-ray technique to detect sub-ppb concentration of airborne molecular contaminants in real-time (Kim et al., 2015). He is now an Associate Professor at the Pusan National University. Dr. Handol Lee started a systematic study on liquid filtration (Lee et al., 2017) and is now an assistant professor at Inha University. The latest Ph.D. to graduate from my group is Dongbin Kwak who did a fundamental study on particle formation, transport, deposition and filtration for semiconductor applications (Kwak et al., 2021).
There were many other former students/post-docs/scholars who have contributed to my career, and they are listed below. I like to tell my group that we should be kind to each other because aerosol/particle discipline is a relatively small and specialized community. Sooner or later, we will cross each other's path. Collaboration and friendship can not only increase productivity in the profession but also bring happiness to colleagues and families (Fig. 8)  currently working with me on designing and operating 2 Air Cleaning Towers in Delhi, India, for the health and wellbeing of the residents. The Delhi Towers are the third generation Air Cleaning Tower (Fig. 9). Starting in 2015, Qingfeng and I published 5 papers on developing solar-assisted large scale cleaning system (SALSCS) to removed PM2.5 in urban atmosphere (Cao et al., 2015(Cao et al., , 2018. We collaborated with Prof. Junji Cao of the Chinese Academy of Sciences (IEECAS), Prof. Wenquan Tao of Xi'an Jiaotong University, and Dr. Ningning Zhang of CAS, to construct the first generation

Center for Filtration Research (CFR)
We have just completed the 62 nd Semi-Annual Review Meeting of the Center for Filtration Research. During the past 31 years, we have performed extensive filtration research and graduated a large number of students. Currently, we have 20 leading filtration manufacturers and end users (Fig. 10). Besides supporting UMN researchers, we are also funding subcontracts to former CFR researchers and collaborators, who are now renowned experts in the field, including Profs. Da-Ren Chen    I often like to end my speech by showing the attached graph. An integrative approach, from collaborations among academia, government, and industry, can accelerate the solution to the PM2.5 problem in the world (Fig. 11). These three sectors of academia, government and industry represent three gears driving the wheel of progress: Sources ⇒ Effects ⇒ Regulation ⇒ Control. Fig. 11. An integrative approach, involving Academia, Government, and Industry to address PM2.5 issues (Pui et al., 2014).

Outreach Activities and Fingerson/TSI Distinguished Lectures
The academia can most effectively address the sources (coal burning and vehicle emissions) and their effects (visibility and health). To protect the public and environment health, the government can progressively set stricter regulations for PM2.5 and vehicle emissions standards. The industry can respond by developing novel control technologies for baghouse filters and diesel/gasoline particulate filters, which will reduce pollutions sources. Further, the academia can also help the government to set the regulations and the industry to develop novel technologies. This will enable the development of green technologies to provide sustainable environment for the world.
I am deeply gratified that AAC organizers set up the commemorative session to honor my 50 years of service to the aerosol discipline. I hope that this will start other commemorative sessions for many other well deserving colleagues who have made major contributions to the aerosol discipline.