Virgil A. Marple

16 August 1939 – 24 December 2017

The impact of Virgil Marple’s work in the field of Aerosol Science, starting in 1970, can be summarized with the following quotations:

“Of the works done lately at your laboratory I have been most impressed by that of Marple. – Please tell him about my high opinion of this classical work” (N. Fuchs, personal communication, 1975). And Fuchs later stated to Dr. Benjamin Liu: “Whereas there exists a substantial theory of jet impactors based chiefly on the brilliant theoretical and experimental work performed under your guidance by V. Marple, our knowledge of the wide flow impaction on various objects is still insufficient although much work has been done on this subject” (N. Fuchs, personal communication, 1980)
“His creations for the inhalation community include the adult and pediatric Marple-Miller impactors and most recently the Next Generation Pharmaceutical impactor (NGI). The quality and performance of the NGI has won over the inhaler testing community, well past the original dreams of the NGI consortium, to the benefit of the thousands of users and ultimately to the benefit of the populations worldwide needing inhalers to manage respiratory diseases” (Daryl Roberts and Jolyon Mitchell, 2018)

The most important contribution of Virgil Marple’s work is the fundamental understanding of inertial impactors (Ensor, 2011). This work started with his Ph. D thesis entitled A Fundamental Study of Inertial Impactors (Marple, 1970), where he conducted numerical fluid computations (CFD) to solve the Navier Stokes equations for the flow field around both rectangular and round jet impactors. He then calculated single particle trajectories to determine weather the particles struck the impaction plate or exited with the flow leaving the impactor. The computer simulations were used to predict collection efficiency curves as a function of Reynolds number, jet-to-plate distances, and throat lengths. These results were published in several papers (Marple et al., 1974; Marple and Liu, 1974, 1975, Marple and Willeke, 1976). Marple and Rader (1985) later added the effect of ultra-Stokesian drag and refined the computational grid to update the efficiency curves.

Virgil Marple became a professor of the Mechanical Engineering Department at the University of Minnesota in 1970. Virgil was a faculty member of the Particle Technology Laboratory under the direction of professors Kenneth Whitby and Benjamin Liu. He was also the director of the Engineering Co-op program for 25 years. Virgil was also a co-owner of MSP Corporation (now a division of TSI Inc.), a company that commercialized many of his impactor designs.

Marple worked on the design and development of many impactors including (1) the Micro-orifice uniform deposit impactor (i.e. MOUDI), with an ingenuous mechanical design to slowly rotate the impaction plates to obtain quasi-uniform particle deposits on the impaction plate (Marple et al. 1991), with the aim to improve the X-ray fluorescence chemical composition analysis conducted on the particle deposits; (2) the nano-MOUDI that extends the lower impactor cutpoint down to 10nm (Marple and Olson, 1999); (3) the Marple personal cascade impactor (Rubow et al., 1987), initially designed to measure the size distribution and concentration of wood dust aerosols; (4) the respirable impactor, a single-stage impactor that mimics the respirable curves as defined by the American Conference of Governmental Industrial Hygenists (ACGIH) or the British Medical Research Council (BMRC) (Marple, 1978; Marple and McCormack, 1983); (5) the personal dust sampler, a 2-Lmin personal sampler with a 0.8-mm stage to separate coal dust particles from diesel exhaust particles in a coal mine (Marple et al., 1995a); (6) the Marple-Miller impactor (MMI), designed for the testing pharmaceutical inhalers (Marple et al., 1995b); (7) the next generation pharmaceutical impactor (NGI), a cascade impactor capable of operating at any flow rate between 15 and 100 L/min, with calibration curves measured experimentally following Good Laboratory Practices (GLP) (Marple et al., 2003a, 2003b, 2004); (8) the personal environmental monitor (PEM), a single-stage impactor sampler with cutpoints of 2.5 or 10-mm at 2, 4 or 10 L/min (Marple, 1989), and (9) the Quartz Crystal Microbalance Cascade Impactor (QCM MOUDI), a 6-stage multi-nozzle cascade impactor with real-time QCM mass sensors integrated on the impaction stages (Chen et al., 2016).

During the development of all these impactors, Marple continued to address fundamental impaction related issues pertinent to the use of multiple round nozzles, needed to reduce the impactor pressure drop and achieve lower cutpoints. For example, during the development of the MOUDI, Fang et al. (1991a) studied the effect of the nozzle cluster diameter on the cross-flow effects that can result in abnormal or shallow impaction efficiency curves, and a new cross-flow parameter threshold value was determined experimentally to ensure proper impactor performance in multi-nozzle impactor stages. Fang et al. (1991b) also studied the effect of relative humidity changes in the impaction region, determining that the effect was negligible. The effect of gravity on the performance of inertial impactors was also studied by Marple using CFD methods. This work resulted in the addition of the Froude number as a parameter to determine the effect of gravity on the collection efficiency curves (Marple et al., 1992). More recently, Marple also experimentally studied the jet-to-jet interactions that can occur when impactor nozzles are close to each other, leading to secondary line deposits on the impaction plate and on the back of the nozzle plate (Rocklage et al., 2013).

Marple also conducted fundamental research to understand the performance of virtual impactors. Marple and Chien (1980) conducted a parametric CFD study of virtual impactors, evaluating the effect of all the important design parameters including the Reynold number, the minor to total-flow ratio, the separation between the nozzle and the collecting probe, the diameter of the collecting probe, the nozzle throat length, and the collection probe inlet design. These CFD simulations lead to the design of virtual impactors with sharper cuts, lower minor flow ratios and lower particle losses than previous virtual impactor designs. The 2.5-mm high-volume virtual impactor (HVVI) was developed for the EPA to separate fine wood smoke from fugitive dust (Marple et al., 1990). A special version of this virtual impactor with a cut of 1-mm, led to the high-volume PM10/2.5/1.0 trichotomous sampler, retrofitted to an Andersen 10-mm high-volume sampler (Marple and Olson, 1995; Lungdren at al., 1996).

Virtual impactors are also used as aerosol concentrators to sample and detect biological or chemical warfare agents. A dual stage virtual impactor with a 2-mm cutpoint and a 300 to 1-L/min flow ratio (Romay et al., 2002) was developed by Marple at MSP Corporation (Shoreview, MN), as part of the US Army Chemical and Biological Mass Spectrometer Block II. This aerosol concentrator was later extended to 1-mm cutpoint to address concerns with smaller biological threats, but it has also been used to conduct ice nucleation studies in forest ecosystems (Tobo et al., 2013).

Parallel to the extensive work on impactors, Dr. Marple also conducted research for the US Bureau of Mines Mineral Technology Center for Respirable Dust. This work involved the development of instruments and sampling techniques for measuring coal dust aerosols responsible for the occurrence of black lung disease in coal miners. The fluidized bed dust generator (Marple et al., 1978) was developed to generate dusts from a fluidized bed of brass beads. The dust was fed through a chain conveyer, with the brass beads breaking up any dust agglomerates, and the primary dust particles were released in an elutriation chamber directly above the fluidized bead. For instrument evaluation purposes, Marple designed a large dust test chamber with internal baffles to achieve uniform flow, and a rotating base to ensure all instruments were exposed to the same aerosol concentration (Marple at al., 1992). The work for the mining industry also included several field studies to measure the particle size distribution in underground coal mines (Rubow and Marple, 1988). The MOUDI impactor was critical to separate coal dust from diesel exhaust particles in mines equipped with diesel engines, resulting in a bimodal distribution clearly distinguished with this cascade impactor.

Another important device that improves the monodispersity of calibration aerosols is the multiplet reduction impactor (MRI), designed to be attached to the exit of the vibrating orifice monodisperse aerosol generator (VOAG). The MRI (Siegford, et al., 1994) has an adjustable and sharp cutpoint, capable of removing doublets and triplets formed after the generation of the primary particles by the VOAG, without decreasing the aerosol concentration in a significant way. The MRI combined with the VOAG, or with the more recent flow-focusing monodisperse aerosol generator (FMAG) results in a nearly perfect monodisperse aerosol of know size, which is critical for the correct calibration of high-resolution impactors and other aerosol spectrometers.

In summary, Dr. Virgil Marple, known by his friends and colleagues as the “impactor man”, dedicated his whole academic career to study the fundamental understanding of inertial separation devices, but his mechanical creativity was also evident in all the impactor geometries shown in his numerous inventions. This mechanical design virtuosity probably came from his youth on the family farm in western Minnesota, where he learned to fix things and keep them running, but also from his love for automobiles, and his impressive collection of more than 100 antique cars, kept on his farm and residential property in Independence, Minnesota (Romay and Pui, 2019). Professor Virgil Marple can be considered as a true aerosol pioneer for his outstanding contributions to the field of Aerosol Science.

References

Chen, M., Romay, F. J., Li, L., Naqwi, A., & Marple, V. A. (2016). A novel quartz crystal cascade impactor for real-time aerosol mass distribution measurement. Aerosol Science and Technology, 50(9), 971-983.

Ensor, D. S. (Ed.). (2011). Aerosol Science and Technology: History and Reviews. RTI Press. pp 166-173.

Fang, C. P., Marple, V. A., & Rubow, K. L. (1991). Influence of cross-flow on particle collection characteristics of multi-nozzle impactors. Journal of Aerosol Science, 22(4), 403-415.

Fang, C. P., McMurry, P. H., Marple, V. A., & Rubow, K. L. (1991). Effect of flow-induced relative humidity changes on size cuts for sulfuric acid droplets in the microorifice uniform deposit impactor (MOUDI). Aerosol Science and Technology, 14(2), 266-277.

Lundgren, D. A., Hlaing, D. N., Rich, T. A., & Marple, V. A. (1996). PM10/PM2. 5/PM1 data from a trichotomous sampler. Aerosol Science and Technology, 25(3), 353-357.

Marple, V. A. (1970). A fundamental study of inertial impactors (doctoral dissertation). Department of Mechanical Engineering, University of Minnesota. https://www.osti.gov/biblio/4095434

Marple, V. A., Liu, B. Y., & Whitby, K. T. (1974). Fluid mechanics of the laminar flow aerosol impactor. Journal of Aerosol Science, 5(1), 1-16.

Marple, V. A., & Liu, B. Y. (1974). Characteristics of laminar jet impactors. Environmental Science & Technology, 8(7), 648-654.

Marple, V. A., & Liu, B. Y. (1975). On fluid flow and aerosol impaction in inertial impactors. Journal of Colloid and Interface Science, 53(1), 31-34.

Marple, V. A., & Willeke, K. (1976). Impactor design. Atmospheric Environment, 10(10), 891-896.

Marple, V. A. (1978). Simulation of respirable penetration characteristics by inertial impaction. Journal of Aerosol Science, 9(2), 125-134.

Marple, V. A., Liu, B. Y., & Rubow, K. L. (1978). A dust generator for laboratory use. American Industrial Hygiene Association Journal, 39(1), 26-32.

Marple, V. A., & Chien, C. M. (1980). Virtual impactors: a theoretical study. Environmental Science & Technology, 14(8), 976-985.

Marple, V. A., & McCormack, J. E. (1983). Personal sampling impactor with respirable aerosol penetration characteristics. American Industrial Hygiene Association Journal, 44(12), 916-922.

Marple, V. A. (1989). PEM development, fabrication, evaluation and calibration. Unpublished, Report, US Environmental Protection Agency, Office of Research and Development, Research Triangle Park, NC.

Marple, V. A., Liu, B. Y., & Burton, R. M. (1990). High-volume impactor for sampling fine and coarse particles. Journal of the Air & Waste Management Association, 40(5), 762-767.

Marple, V. A., Rubow, K. L., & Behm, S. M. (1991). A microorifice uniform deposit impactor (MOUDI): Description, calibration, and use. Aerosol Science and Technology, 14(4), 434-446.

Marple, V. A., Olson, B., & Rader, D. (1992). The effect of gravity on particle collection efficiency of inertial impactors. In R. L. Frantz & R. V. Ramani (Eds.), Generic Mineral Technology Center for Respirable Dust Publications 1990, 11, 26–29.

Marple, V. A., & Rubow, K. L. (1983). An aerosol chamber for instrument evaluation and calibration. American Industrial Hygiene Association Journal, 44(5), 361-367.

Marple, V. A., & Olson, B. A. (1995). A high volume PM10/PM2. 5/PM1. 0 trichotomous sampler. In In: Particulate matter: health and regulatory issues: proceedings of an international specialty conference; April; Pittsburgh, PA. Pittsburgh, PA: Air & Waste Management Association (pp. 237-261).

Marple, V. A., Rubow, K. L., & Olson, B. A. (1995). Diesel exhaust/mine dust virtual impactor personal aerosol sampler: design, calibration and field evaluation. Aerosol Science and Technology, 22(2), 140-150.

Marple, V. A., Olson, B. A., & Miller, N. C. (1995). A low-loss cascade impactor with stage collection cups: calibration and pharmaceutical inhaler applications. Aerosol science and technology, 22(1), 124-134.

Marple, V. A., & Olson, B. (1999). A microorifice impactor with cut sizes down to 10 nanometers (Final Report for Grant Number G1135242, G1145242, G1155242/2782). Morgantown, WV: West Virginia University, National Research Center for Coal and Energy, Generic Mineral Technology Center for Respirable Dust.

Marple, V. A., Roberts, D. L., Romay, F. J., Miller, N. C., Truman, K. G., Van Oort, M., … & Hochrainer, D. (2003). Next generation pharmaceutical impactor (a new impactor for pharmaceutical inhaler testing). Part I: Design. Journal of Aerosol Medicine, 16(3), 283-299.

Marple, V. A., Olson, B. A., Santhanakrishnan, K., Mitchell, J. P., Murray, S. C., & Hudson-Curtis, B. L. (2003). Next generation pharmaceutical impactor (a new impactor for pharmaceutical inhaler testing). Part II: Archival calibration. Journal of Aerosol Medicine, 16(3), 301-324.

Marple, V. A., Olson, B. A., Santhanakrishnan, K., Roberts, D. L., Mitchell, J. P., & Hudson-Curtis, B. L. (2004). Next generation pharmaceutical impactor: a new impactor for pharmaceutical inhaler testing. Part III. Extension of archival calibration to 15 L/min. Journal of Aerosol Medicine, 17(4), 335-343.

Rader, D. J., & Marple, V. A. (1985). Effect of ultra-Stokesian drag and particle interception on impaction characteristics. Aerosol Science and Technology, 4(2), 141-156.

Rocklage, J. M., Marple, V. A., & Olson, B. A. (2013). Study of secondary deposits in multiple round nozzle impactors. Aerosol Science and Technology, 47(10), 1144-1151.

Roberts, D. L. & Mitchell J. P. (2018). Humble giant of cascade impactors, Virgil A. Marple August 16, 1939 to December 24, 2017. Inhalation Magazine, Back Page.

Romay, F. J., Roberts, D. L., Marple, V. A., Liu, B. Y. H., & Olson, B. A. (2002). A high-performance aerosol concentrator for biological agent detection. Aerosol Science & Technology, 36(2), 217-226.

Romay, F. J., & Pui, D. Y. (2019). Professor Virgil Alan Marple: August 16, 1939 to December 24, 2017. Aerosol Science Technology, 53(6), 728-729.

Rubow, K. L., Marple, V. A., Olin, J., & McCawley, M. A. (1987). A personal cascade impactor: design, evaluation and calibration. American Industrial Hygiene Association Journal, 48(6), 532-538.

Rubow, K. L., & Marple, V. (1988). Determining the size distribution of coal/diesel aerosol mixtures with the microorifice uniform deposit impactor. In International symposium on respirable dust in the mineral industries, University Park, PA.

Siegford, K.L., Marple, V. A., and Rubow, K. L. (1994). A multiplet reduction impactor for the vibrating orifice aerosol generator. Journal of Aerosol Science, 25(S1):113–114.

Tobo, Y., Prenni, A. J., DeMott, P. J., Huffman, J. A., McCluskey, C. S., Tian, G., … & Kreidenweis, S. M. (2013). Biological aerosol particles as a key determinant of ice nuclei populations in a forest ecosystem. Journal of Geophysical Research: Atmospheres, 118(17), 10-100.

Biography prepared by Francisco J. Romay