ESTILOS AGROGANADEROS DE LA SAKANA:

Whitby played an active, leadership role in the laboratory and helped to establish the research program and nurture younger faculty members as they joined the laboratory. He served as the laboratory director from 1956 to 1972, followed by Liu (1972–1995) and Pui (1995–present). He was active in the air pollution community and served on the Clean Air Scientific Advisory Committee of the U.S. Environmental Protection Agency (EPA). He was elected to the National Academy of Engineering in 1978 for his contribution to aerosol

instrumentation and air-quality measurement. Whitby’s untimely death in 1983 marked a great loss to the department and the laboratory.

Whitby’s contribution to aerosol research included instrumentation and measurement techniques, as well as research on aerosol filtration and atmospheric aerosol characterization. He was interested in using sensor- based aerosol measuring techniques to study the changing concentration and size distribution of aerosols caused by mixing, transport, sedimentation, and/or coagulation. In 1966, he developed an electrical particle counter for measuring aerosol size distribution in the 0.01 to 1.0 µm range (Whitby & Clark, 1966). The instrument uses a sonic jet ionizer, which he had developed earlier (Whitby, 1961), as a unipolar ion source for charging particles by ion diffusion. The counter then measures aerosol size distributions by electrical mobility, using an electrometer as a charge detector. Figures 1 and 2 show, respectively, a schematic diagram and a photograph of the prototype instrument. Thermo-Systems Incorporated (TSI), later developed the instrument as a commercial product, principally through the effort of G. J. Sem and his colleagues. The instrument was marketed as a Whitby Aerosol Analyzer (WAA) and was used in both ambient and laboratory aerosol studies.

Whitby also experimented with commercially available optical particle counters (OPC) to determine their size response and the effect of particle refractive index and shape on instrument response. He developed techniques to generate chain aggregate aerosols and measured the response of the OPC to the chain aggregate particles (McFarland & Tomaides, 1969a, 1969b). He then combined the measurement capability of the WAA in the 0.01 to 1.0 µm range

Kenneth T. Whitby

with the measurement capability of the OPC in the 0.3 to 10 µm range to measure aerosols in the 0.01 to 10 µm range. With funding from the EPA and other governmental agencies, he developed several such instrument systems to measure atmospheric aerosols in cities across the country. A typical system included a WAA, one or more OPCs, one or more condensation nucleus counters (CNCs), a computer, and additional manual sampling devices; the system could be used to conduct research on the size distribution of aerosols with a time resolution and size range that older, more traditional approaches could not achieve.

The first such system, the Minnesota Aerosol Analyzing System (Whitby et al., 1972b), was used in the Los Angeles Smog Study to characterize the smog aerosol size distribution from August 19 to September 19, 1969. Figure 3 shows the average size distribution of aerosols measured during the 4-week measurement period (Whitby et al., 1972a). The figure also includes similar measurements made in Minneapolis by William E. Clark (1965) and Carl M. Peterson (1967), as well as results reported by other researchers using different

Figure 1. Schematic diagram of the electrical particle counter.

Source: Whitby & Clark (1966). Reprinted with permission from Wiley-Blackwell.

Figure 2. The electrical counter system showing the mobility analyzer, charger, and electric and flow instrumentation.

Source: Whitby & Clark (1966). Reprinted with permission from Wiley-Blackwell.

measurement techniques. Both Clark and Peterson were graduate students working in the Particle Technology Laboratory under Whitby’s direction. He was generally credited for having developed the volume size distribution plot shown in Figure 3 that has helped to explain the nature and sources of atmospheric particles. Large particles with a mode in the 30 to 60 µm range— the “coarse-mode aerosol”—are thought to come from wind-blown dust, or generated by other mechanical processes, such as road dust suspended by vehicle traffic or plant debris. Fine particles with a peak diameter in the 0.1 to 1.0 µm range—the “accumulation-mode aerosol”—are thought to originate from combustion and atmospheric gas-to-particle conversion processes that have accumulated in this range through subsequent coagulation, sedimentation, removal, and other atmospheric transformation processes.

An interesting study in automotive aerosol emission occurred in 1975; the EPA and General Motors sponsored the study, using university and government laboratories as participants to take part in the measurement. This Sulfate Aerosol Dispersion Study was conducted at the General Motors proving ground in Milford, Michigan, in October 1975. In this study, 352 cars were driven around a 10 km test track, while observers made measurements at the roadside. The cars were equipped with catalytic mufflers for exhaust gas treatment, which also converted sulfur in the fuel to sulfate aerosols upon emission. The measured size distribution (Wilson et al., 1977) was found

Figure 3. The bimodal size distribution of atmospheric aerosols.

to be trimodal (as Figure 4 shows), having a pronounced “nuclei mode” at approximately 0.02 µm in diameter.

Whitby had a keen interest in aerosol instrumentation. Earl O. Knutson, a doctoral student working under Whitby’s direction, developed a theory on the transfer function of the differential mobility analyzer (DMA). Knutson and Whitby showed that the DMA had a triangle-shaped transfer function (Knutson & Whitby, 1975a) and was capable of resolving the mobility of uniform polystyrene latex spheres carrying discrete units of electronic charges, as shown in Figure 5 (Knutson & Whitby, 1975b). The DMA subsequently became a key measuring instrument in the aerosol field. Several people contributed to the development of the device in the Particle Technology Laboratory. Liu and Pui (1974a) worked independently to develop the DMA as an electrostatic particle size classifier to generate monodisperse aerosols for instrument calibration and experimental purposes. More information about Whitby and his work can be found in Sem and colleagues (2005).

Figure 4. Trimodal aerosol size distribution measured at the General Motors Proving Ground on October 29, 1975.

Source: Wilson et al. (1977). Reproduced with permission of the Air and Waste Management Association from the Journal of the Air Pollution Control Association via Copyright Clearance Center, Inc.