It is a pleasure to honor Professor Peter McMurry by dedicating to him this special issue of Aerosol Science and Technology on the occasion of his retirement from the University of Minnesota. Peter has made incalculable contributions to the field of aerosol science through his research, his leadership in the scientific community, his teaching and mentoring, and his activities in the American Association for Aerosol Research (AAAR).
Peter was born in Pennsylvania and earned his B.A. degree in Physics from the University of Pennsylvania in 1969. He then served in the Peace Corps in Liberia and Uganda until 1971, when he entered Caltech, earning his M.S. (1973) and Ph.D. (1977) degrees in Environmental Engineering Science under Sheldon Friedlander. After leaving Caltech he moved directly to a faculty position in the Department of Mechanical Engineering at the University of Minnesota, where he was also a member of the Particle Technology Laboratory established by Ken Whitby. For more than 40 years, during which time he published approximately 275 papers and advised 28 Ph.D. and 35 M.S. students, Peter’s research focused primarily on studies of the nucleation and growth of atmospheric aerosols and measurements of aerosol physical and chemical properties. He has been the recipient of numerous awards, among them the 1996 AAAR David Sinclair Award and the 2006 Fuchs Memorial Award from the International Aerosol Research Assembly, the highest award given by the aerosol community for outstanding research. Peter has also served AAAR in many different capacities throughout his career, most notably as a member of the Board of Directors from 1987–1995, as President from 1994–1995, and as Editor-in-Chief of Aerosol Science and Technology from 2008–2016.
A hallmark of Peter’s research has always been his remarkable ability to identify an important problem of the future and then position himself to contribute to its solution, often while employing creative new approaches. At different times his research has involved the development of new instruments, methods, or theory; or the establishment of collaborative relationships. For example, his work on new particle formation (a topic that has consumed Peter’s attention since he first encountered it during his graduate work at Caltech) led to his development of the ultrafine condensation particle counter (UCPC), an instrument that is now commercially available. The UCPC extended the lower limit for particle detection from ∼10 nm down to ∼3 nm, much closer to the size of freshly formed nuclei, and was initially employed (using a pulse-height analysis technique to measure particle size distributions in the 3–8 nm range with exceptional sensitivity) to make the first atmospheric measurements of new particle formation and growth rates. These studies showed that the rates of these processes could not be explained solely on the basis of sulfuric acid-water chemistry, which at the time was believed to be the predominant mechanism for atmospheric nucleation. The results spurred interest in the potential roles of chemical components such as ammonia, amines, and other organics in these processes, a topic that is currently at the forefront of research on new particle formation. Although Peter was not trained as a chemist, he knew that to make further progress in this area it was necessary to be able to measure the chemical composition of the species involved in new particle formation. He therefore established collaborations with chemists, and together they combined nanoparticle sizing and counting techniques with mass spectrometry in laboratory and field studies and developed new theoretical models, allowing Peter to continue to pursue his dream of advancing the understanding of the atmospheric processes involved in the transition from molecules to clusters to particles.
Another notable body of research grew out of Peter’s pioneering applications of the tandem differential mobility analyzer (TDMA) to aerosol measurements. In this method, particles of a desired size are selected with a DMA, subjected to a variable environmental parameter (e.g., temperature, humidity, reactant gas composition or concentration), and then re-sized with a second DMA. Fundamental particle properties can be determined from the relationship between particle size and parameter values. A major advantage of this method over those commonly used at the time was that it allowed measurements to be made on real, suspended aerosol particles rather than on deposited particles or bulk materials, where the measurement can be influenced by the sampling process and supporting surface. In addition, this real-time method took advantage of the high precision of the DMA and the detection sensitivity of condensation particle counters, allowing analyses to be performed routinely at ambient atmospheric particle concentrations. Over many years Peter applied the method to measurements of aerosol volatility, the uptake of reactive gases by particles, and particle hygroscopicity and mixing state. The TDMA is now widely used in the aerosol community for these and other applications.
Peter’s work on instrumentation has also had an enormous impact on aerosol mass spectrometry, which in the last two decades has revolutionized the field of aerosol chemistry. His seminal contribution in this area came about as part of an effort to develop an instrument for measuring freshly nucleated particles in low-pressure semiconductor processing equipment, and involved the development of the aerodynamic lens. This device, which is now the standard inlet for almost every commercial and home-built aerosol mass spectrometer, is remarkable in its simplicity and in its ability to aerodynamically focus particles with a wide range of sizes into a very narrow, low-divergence beam. Such beams can then be used to transport particles with near-unit efficiency from atmospheric pressure into a high-vacuum chamber where they can be vaporized and mass-analyzed in the absence of contaminating gases. The aerodynamic lens is critical for achieving the sensitivity needed for ambient aerosol analysis. Considering that earlier sampling devices, such as capillaries, provided efficiencies of only a few percent, it is no exaggeration to say that without the aerodynamic lens aerosol mass spectrometry could not have achieved anywhere near the impact it has had in aerosol science.
Although Peter has retired, his legacy will live on through the instruments and methods he developed and which are now used in standard aerosol practice, through his contributions to scientific understanding of aerosol properties and behavior, and through his commitment to mentoring and promotion of graduate students, postdocs, and colleagues. He leaves all of us in this community with his own unique example of integrity, humility, and scientific excellence to follow.
Many more people would happily have contributed articles to this issue. However, in consultation with Peter it was decided that because of the limited space available (10 papers) the work presented would be that of selected research groups headed by individuals (including the two of us) who worked closely with Peter as students, postdocs, or collaborators. The articles cover topics that are all near and dear to Peter’s heart, including new particle formation; instrument development including design, theoretical analysis, and characterization; aerosol physical and chemical properties; and ambient aerosol measurements.