Effects of Temperature and Relative Humidity on Filter Loading by Simulated Atmospheric Aerosols & COVID-19 Related Mask and Respirator Filtration Study
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Effects of Temperature and Relative Humidity on Filter Loading by Simulated Atmospheric Aerosols & COVID-19 Related Mask and Respirator Filtration Study A DISSERTATION SUBMITTED TO THE FACULTY OF THE UNIVERSITY OF MINNESOTA BY Chenxing Pei IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Dr. David Y.H. Pui, Advisor August 2020 © Chenxing Pei 2020 Acknowledgments First and foremost, I am deeply indebted to my advisor, Professor David Y. H. Pui, for his continuous guidance, mentoring, and inspiration throughout my undergraduate and graduate studies. He provided me with the encouragement and freedom to work on any filtration related studies that interested me. His support, patience, and trust have made my years spent in the Particle Technology Laboratory an enjoyable, enriching, and memorable experience in my life. I would like to express my deepest appreciation to my committee members: Prof. Thomas Kuehn, Prof. David Kittelson, and Prof. Kevin Janni for reviewing my dissertation, offering insightful comments and suggestions, and serving on my Ph. D. exam committees. I am extremely grateful to my committee members not only for their time and extreme patience, but for their intellectual contributions to my development as a scientist. I would also like to thank Dr. Qisheng Ou, whose help cannot be overestimated. Dr. Ou provided me with the tools that I needed to choose the right direction and successfully complete my dissertation. Special thanks to Dr. Young H. Chung, who led to many interesting and motivating discussions. His help and advice were invaluable for my research and life. Thanks also to Weiqi Chen, who collaborated with me on the smart sensor project. I am also grateful to Kai Xiao for enlightening me the first glance of research. I would also like to extend my deepest gratitude to my current and former colleagues in the Particle Technology Laboratory: Dr. Young H. Chung, Dr. Lin Li, Dr. Sheng-Chieh Chen, Dr. Qisheng Ou, Dr. Shigeru Kimoto, Dr. Min Tang, Dr. Yan Ye, Dr. i Seong Chan Kim, Dr. Qingfeng Cao, Dr. Handol Lee, Dr. Ziyi Li, Dr. Tsz Yan Ling, Dr. Zhili Zuo, Dr. Chang Hyuk Kim, Dr. Drew Thompson, Dr. Nanying Cao, Dr. Seungkoo Kang, Dr. Lipeng Su, Dr. Xinjiao Tian, Dr. Qiang Lyu, Kai Xiao, Gustaf Lindquist, Swathi Satish, Weiqi Chen, Dongbin Kwak, Tao Song, Jihe Chen, Ren Jeik Ong, Qi Heng Lee, Jonathon Anderson, Kaushik Kanna, Dr. Mamoru Yamada, Dr. Dongbin Kim, Dr. Yun Liang, Dr. Deqiang Chang, Dr. Cheng Chang, and Dr. Ningning Zhang. Their company, discussion, and assistance have been invaluable. I would like to express my gratefulness to ME Connect Seminar Series committee members and our mentor Prof. Suhasa Kodandaramaiah. It was rewarding and interesting to work with all of them to organize seminars every week and learn others’ research in the Department of Mechanical Engineering. I would like to acknowledge members of the Center for Filtration Research for their financial support, comments and interests in my studies: 3M Company, Applied Materials, Inc., BASF Corporation, Boeing Company, Corning Incorporated, Cummins Filtration Inc., Donaldson Company, Inc., Entegris, Inc., Ford Motor Company, LG Electronics Inc., Parker Hannifin, Samsung Electronics Co., Ltd., Shengda Filtration Technology Co., Ltd., Shigematsu Works Co., Ltd., TSI Inc., W. L. Gore & Associates, Inc., WatYuan Filtration System Co., Ltd, Yancheng Environmental Protection Science and Technology City, and the affiliate member National Institute for Occupational Safety and Health (NIOSH). Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. Last but not least, my deep and sincere gratitude to my wife, Kunlin Wang. Understanding me best as a Ph.D. herself, Kunlin has been my best friend and terrific ii companion, who gives me continuous, unparalleled, unconditional love and support. I am also extremely grateful to my parents and parents-in-law, who give me opportunity to explore my new possibilities and support me emotionally and financially. It is their unreserved love, unbounded trust, endless patience, and timely encouragement make me who I am today. Thanks also to my other family members and friends along my way, who bring me love and support. In addition, My thanks go to my 5-year-old twin cats, Laobing and Bantang, who have grown up as the best ‘coworkers’ during my Ph.D. study. iii To my parents, my love, and my future children iv Abstract This dissertation focuses on the effects of temperature and relative humidity on filter loading by simulated atmospheric aerosols and COVID-19 related mask and respirator filtration study. There are two objectives of this dissertation: the first one is to fill the gap between lab filter testing and actual filter operation, and the second one is to help curb the COVID-19 spreading from filtration perspective. Filter life is an important criterion to evaluate air filters, and longer life is preferred before reaching the replacing pressure. Filter life could be evaluated in the laboratory by loading lab-generated contaminants on the test filter sample, typically at high concentrations to accelerate the testing process. However, the current filter loading testing protocols and standards use the non-hygroscopic micron dust or less-hygroscopic lab salt particles (NaCl, KCl) as challenging particles. Meanwhile, the testing environment, especially relative humidity, have not been strictly controlled. With increasingly more stringent pollution control requirement in place in past decades, sub-micron particles are becoming more important, such as inorganic salts ((NH4)2SO4, NH4NO3), soot, organic compounds, which are more hygroscopic than lab salts. Therefore, there have been discrepancies between lab filter testing and actual filter operation, including challenging particle species, testing relative humidity and temperature. It is vital to understand how air filters perform in the actual environment so that both filter recommendations and optimization could be done confidently and wisely. This dissertation thrives on the effects of temperature and relative humidity on filter loading by simulated atmospheric aerosols. A Lab-simulated Ambient Environment Air v Filter Test Rig was firstly built with controlled temperature and relative humidity testing environment, simulated atmospheric aerosols generation, and advanced filter testing capability. The testing temperature and relative humidity are well controlled to simulate different filter operating conditions. The techniques to generate atmospheric aerosols were developed, including inorganic salt generation and dry/wet state control at various testing relative humidities, fresh soot and aged soot by organic compounds coating. The versatile atmospheric aerosols generation techniques enable filter loading tests with “real” atmospheric aerosols rather than lab salts. The temperature effect on filter loading was studied, and it is found that temperature is a minor factor affecting filter life, compared to relative humidity. The relative humidity effect on filter loading was then investigated with conventional cellulose filter and nanofiber coated cellulose filter. The testing relative humidity covered from below salts’ efflorescence relative humidity to above their deliquescence relative humidity, and both dry and wet particles at different relative humidities were loaded on test filters. It is found that, in general, the higher the loading relative humidity, the longer the filter life. But the filter life also depends on the filter structure and challenging particle state. Filtration efficiency evolution was also studied. To better simulate atmospheric aerosols, test filters were loaded with (NH4)2SO4 and NH4NO3 mixture particles and soot aged by organic compounds coating separately at various relative humidities. It is found that relative humidity is a very strong factor affecting filter life, particularly for salt mixtures. There is a discrepancy between fresh soot and aged soot, and aged soot should be used in filter loading since it can represent atmospheric soot more realistically. These studies reveal that actual filter operation with vi atmospheric aerosols is complex, and the filter recommendation should be carefully made, and the actual operating conditions should be considered as much as possible. The above research was conducted at constant loading relative humidity, which is not possible in actual filter operation. Hence the effect of relative humidity change on hygroscopic salt loaded filters was studied. This study broadens the understanding of the loaded filter behavior in varying relative humidity which could benefit potential techniques to prolong filter life or to clean filters by reverse pulsing. The ultimate goal of the first objective is to recommend filters based on operating locations. Whereas it is still challenging to achieve filter selection and optimization based on all research mentioned above. We developed and established a Global Air Pollutant/Meteorological Condition Database and a Smart Sensor for Filter Performance Monitoring System. For a city of interest, this Database can report historical pollutants speciation and concentration, historical monthly average temperature and RH, and pollutant concentration trend in past years. The Smart Sensor could monitor filtration efficiency, the differential pressure across the filter, and operating temperature, RH in real- time. With millions of the