On February 9, 2007, the aerosol community lost one of its most respected members. Sheldon K. Friedlander died at age 79 of complications from pulmonary fibrosis at his home in Pacific Palisades. He was the Parsons Professor of Chemical Engineering at University of California, Los Angeles (UCLA).
Sheldon's remarkable career, which spanned six decades, began with a B.S. from Columbia University and a Masters degree from MIT, both in Chemical Engineering. He also worked at the Harvard School of Public Health on an Atomic Energy Commission project regarding control of radioactive aerosols. His time at Harvard catalyzed an interest in aerosols, which he pursued through subsequent Ph.D. research in Chemical Engineering at the University of Illinois at Urbana-Champaign. He then served as a faculty member at Columbia University (1954–1957), John Hopkins University (1957–1964), California Institute of Technology (1964–1978), and UCLA (1978–2007).
Sheldon started his Ph.D. studies at a time when the field of aerosol science was in its early stages of development. Working with H.F. Johnstone, he focused on how particles in turbulent airflow are deposited on the walls of pipes and ducts. Sheldon made important contributions right from the start: he introduced the notion of a “stopping distance” of a particle injected into stagnant air, and then used this concept to predict particle motion through the viscous boundary layer to the surface. His thesis work laid the foundation for much of the later work on deposition of particles in industrial systems as well as dry deposition from the ambient atmosphere, where turbulent eddies impart velocities normal to the mean flow and enable particles to reach the surface.
Sheldon K Friedlander
As a natural extension of this early work, Sheldon then considered the transport of particles to collectors of various geometries, such as cylindrical fibers and spheres. He developed expressions for the deposition of particles of different sizes to these obstacles which became valuable for predicting the efficiencies of fiber filters and collection of airborne particles by cloud droplets.
His work also involved studies of aerosol dynamics in the human respiratory system, including transport through the upper airways down into the lung. These studies informed research by others on both detrimental and therapeutic aspects of aerosols. For example, his early work on aerosol deposition in the lung led to the later development by others of methods to deliver therapeutic drugs directly to the lung.
But Sheldon went far beyond investigations of particle deposition. In what is often acclaimed as one of his most valuable scientific contributions, he theorized that regardless of the initial size distribution, a collection of particles in certain size ranges tends to grow, coagulate, and settle as it ages to form a “self-preserving” size distribution. In other words, the distribution of well-aged particles tends toward a predictable spectrum when expressed in dimensionless form regardless of the initial conditions. The theory of the self-preserving size distribution is based on selected solutions to the General Dynamic Equation (GDE), essentially a mass balance for aerosols that incorporates major mechanisms affecting aerosol formation, growth, transport, and ultimately loss. Since its development, many researchers have used the self-preserving distribution for such applications as the design of controlled aerosol reactor experiments and industrial-scale manufacturing of chemical commodities. The theory greatly simplifies analysis of the motion and transformations of particle clouds, since it is no longer necessary to account for the initial distribution. The theory also permits back-of-the-envelope calculations of concentrations and size distributions, thus avoiding the need for numerical solutions to the GDE.
By the early 1970s, the Clean Air Act established by the newly founded Environmental Protection Agency had focused attention on identifying the sources of particles in the ambient atmosphere. Once again, Sheldon made pioneering contributions to the field. He reasoned that different categories of particle sources such as coal-fired power plants, automobiles, and soil resuspension have different characteristic chemical compositions. The atmospheric aerosol includes mixtures of particles from these and other sources. By measuring the composition of ambient particles and knowing the composition of particles emitted from different sources, it was thus possible to estimate the relative contribution of each source type influencing the measurement site. This “chemical element balance” was the forerunner of Source-Receptor Modeling that has become a major subdiscipline in its own right. The numerous software packages commercially available to conduct this type of modeling demonstrate the power and importance of the method.
When equipment became available to measure number concentrations of very small particles, Sheldon began to study yet another topic within aerosol science: the formation and growth of particles from molecular clusters. Using large Teflon bags on the laboratory roof at Caltech, he and his students studied the oxidation of SO2 to form sulfate aerosol. He identified two different regimes of oxidation: a low humidity process which forms very small particles without photochemistry, and a high humidity mechanism in which SO2 is absorbed into existing particles and oxidized in the presence of liquid water. This work was followed by major studies around the world on the formation of atmospheric aerosol from precursor gases.
Sheldon was a strong believer in the collection of ambient data as a means of understanding aerosols. In this vein, he was co-Principal Investigator of one of the first major urban field campaigns, the California Aerosol Characterization Experiment (ACHEX) in the early 1970s. Many noteworthy accomplishments came out of ACHEX. As one example, Sheldon and his group assembled an inventory of carbon emissions in Los Angeles and used it to apportion sources for the measured carbon aerosol; this resulted in the first carbon balance for an urban airshed. His group also compared particle size distributions measured during ACHEX across the Basin to demonstrate aerosol growth by photochemistry. This work promoted widespread interest in elemental and organic carbon during the 1990s, which included chemical speciation of organic compounds in aerosols.
Later in his career, Sheldon devoted considerable attention to the creation of synthetic aerosols. He explored the potential of aerosols to make useful products, and looked into the development of aerosol reactors to generate particles with carefully controlled characteristics. Sheldon also studied the properties of nanoparticles, and postulated a revolutionary relationship on how agglomerates sinter to become compact particles. His work greatly simplified the description of non-spherical particle dynamics at high temperatures, placing the design and safe operation of aerosol reactors on a firm scientific basis. Sheldon also discovered the elastic, rubber-like behavior of micrometer-long chains of ceramic nanoparticles 10 nm in diameter. Such characteristics of nanoparticles can affect their useful properties and also their impacts on human health.
These examples are but a small sample of Sheldon's original scientific contributions during his productive career. He published many seminal archival papers. In addition, he wrote the influential book Smoke, Dust, and Haze, which was published in 1977 by John Wiley and Sons and later published in a modified second edition in 2000 by the Oxford University Press. This book is widely used by aerosol scientists and educators around the world.
Although his technical contributions have had a major effect on the field of aerosol science, perhaps his most significant accomplishment was training and inspiring others. His brilliant insights motivated those working with him, and his fascination with aerosols was contagious. He supervised numerous Ph.D. students, postdoctoral fellows, and visiting scientists during his career, as well as a large number of undergraduates and MS students. He also worked with several dozen aerosol scientists at his own and other institutions.
Sheldon's vision went beyond the training of other individuals to continue research in the field. He recognized the importance of establishing a community that could not only advance the understanding of aerosol science but also provide credibility for advocates of the discipline to be of service to society. He envisioned a world where aerosol scientists offered their expertise to solve a myriad of problems and improve people's lives in many ways.
With this vision in mind, Sheldon was one of the original founders of the American Association for Aerosol Research (AAAR). From its humble beginning as a small group in Santa Monica in 1982, the Association has grown to nearly 1000 members. Along with the European Aerosol Association, the AAAR has become a flagship of a network of aerosol associations worldwide. Sheldon served as President of AAAR during 1984–1986, and remained active in the association until the time of his death.
Sheldon also believed in the importance of linking science to public policy. He served on the advisory committee to the National Institutes of Health that shaped the first agenda for the National Institute of Environmental Health Sciences in 1969. He also served as the first chair of the EPA Clean Air Science Advisory Committee in 1978–1982, and was a member of the EPA Science Advisory Board Executive Committee during this period. He was chair of the National Research Council (NRC) Panel on Abatement of Particulate Emissions from Stationary Sources, and chair of the NRC Subcommittee on Photochemical Oxidants and Ozone. In addition, he spent time at other Universities; for example, he was a Fulbright Scholar in 1960 and a Guggenheim Fellow in 1969, both at the University of Paris.
As a result of his contributions and leadership in aerosol science, Sheldon received many honors throughout his career. For example, he received five awards from the American Institute of Chemical Engineers: the Colburn Award in 1959, the Alpha Chi Sigma Award in 1974, the Walker Award in 1979, the Cecil Award in 1995, and the Lifetime Achievement Award of the AIChE Particle Technology Forum in 2001. He also received the Humboldt Senior Scientist Award from the West German Government in 1984–1985. He presented several invited lectures around the world, and more recently he was the first recipient of the Christian Junge Award of the European Aerosol Association in 2000 and the Aurel Stodola Medal of ETH in Zurich in 2004.
Two more awards, however, deserve special comment. Sheldon was elected to the National Academy of Engineering in 1975, “in recognition of his contributions to the understanding of the origin and control of pollution by particulate matter.” In addition, Sheldon was the first individual to receive the Fuchs Memorial Award in 1990, which is the highest honor that can be bestowed on an aerosol scientist, presented by the AAAR, Gesellschaft für Aerosolforschung, and the Japan Association for Aerosol Science and Technology. The Fuchs Award is presented only once every four years. In the mid-1990s, the AAAR recognized Sheldon's impact in the field by creating the Sheldon K. Friedlander Award for an outstanding Ph.D. dissertation “in any discipline in the physical, biomedical, or engineering sciences in a field of aerosol science and technology.” The world community of aerosol researchers is indebted to Sheldon Friedlander, outstanding researcher, educator, mentor, and role model who was one of the giants in aerosol science. His contributions to the field will be long remembered, and his wisdom will remain with us in his scientific papers. Most important, his inspiration will continue through his many students and colleagues, as we attempt to emulate the character of a great scientist and a great human being.
Sheldon Friedlander's doctoral students were invited to contribute to this obituary and be co-authors, and those who were successfully contacted and who accepted this invitation are included in the author list. Many other individuals consider Sheldon as one of their primary mentors, although their names are not included as co-authors to keep the list manageable.