correspondence Airborne Lidar Plume and Haze Analyzer (ALPHA-1) Edward E. Uthe, Norman B. Nielsen, and Walter L. Jimison, Atmospheric Science Center, SRI International, Menlo Park, Calif. 94025 Abstract A new two-wavelength airborne lidar system has been constructed and field-tested. The system was designed to observe the distribution of particle concentrations over large regional areas. During a one-week field-test program, the system was used to observe boundary layer structure over the Los Angeles area and the downwind structure of particulate plumes from the Navajo (Page, Ariz.) and Four Corners (Farmington, N.Mex.) power plants. Data examples presented show the importance of terrain features in influencing particle concentration distributions over regional areas. A new airborne lidar system has been constructed and field-tested by SRI International (SRI) under contract to the Electric Power Research Institute (EPRI). This two-wavelength system operates simultaneously in the visible (0.53^) and near-infrared (1.06 j^m). Only 100 mj of laser energy at the IR and 20 mj at the green are transmitted for complete eye safety to observers at ground level when the aircraft is operated from an altitude of 3 km. Particle backscatter signatures are processed by a two-microcomputer data system for realtime graphic display on a facsimile recorder and are also recorded on magnetic tape for use in subsequent data-analysis programs. Table 1 presents system specifications. Additional details of the lidar system, including associated video camera, supplementary sensors, and installation on the SRI Queen Air aircraft, are presented by Uthe et al. (1980), while general descriptions of the lidar technique have been presented by Collis and Russell (1976) and Collis (1970). 0003-0007/80/091035-09$06.25 1980 American Meteorological Society FIG. 1. Los Angeles area flight paths, 16 December 1979. A 1-week period of field tests of the ALPHA-1 system included boundary layer flights over the Los Angeles area and plume tracking flights downwind of the Navajo and Four Corners Power Plants. All data collection was performed from an aircraft altitude of about 3 km above sea level. Locations of the Los Angeles flight paths are shown in Fig. 1. Figure 2 shows lidar cross sections plotted from 1.06-^m wavelength logamplitude data stored on magnetic tape during traverses made during early morning and afternoon flights. In these presentations, darker gray levels indicate increased particulate pollution and the very dark area (followed by both dark and light patterns) near the bottom of the picture represents the earth's surface. The morning cross section (Fig. 2(a)) shows a pollution layer extending to only about 250 m above the surface. The afternoon cross section (Fig. 2(b)) reveals a pollution cloud of greater density over the city and extending to greater heights than in the morning. Elevated aerosol layers in the vicinity of the San Gabriel Mountains result from air flow up the mountain slopes and an elevated wind reversal. Bulletin American Meteorological Society 1035
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1038 Vol. 61, No. 9, September 1980 Figure 2(c) shows the green-wavelength (0.53 data for comparison with the 1.06 wavelength data presented in Fig. 2(b). As expected, because of the smaller ratio between particle and molecular scattering at the visible wavelength, less contrast between the dense aerosol layer and clear-air background was observed. Figure 3 presents data taken from the real-time facsimile plot (1.06 ^m) for the flight path that extends over Catalina Island as shown in Fig. 1. Industrial pollution plumes were detected at several altitudes. Fog patches and layers were also observed. The reversal of gray scales (from black to white) for large aerosol returns is explained by the plotting of the less significant bits of eight-bit data records on a four-bit display device. (The causes of the vertical interference patterns and gray scale reversals for large plume and surface returns h^ve since been corrected.) On 17 and 18 December, the ALPHA-1 system was used to observe the downwind structure of the plume from the coal-burning Navajo Power Plant. The particulate plume was visible to the eye only at ranges of less than 1 km from the plant; however, the N0 2 plume was visible at greater distances. Figure 4 presents the location of ALPHA-1 flights for the morning (approximately 0800 MST) of 18 December 1979. As shown by the data presented in the cover figure and in Fig. 5, the plume remained relatively concentrated to a downwind distance of at least 37 km. The 37 km cross section shows FIG. 4.. Navajo ^. Power ^ Plant area flight a- paths and a locations \ of that the R plume was being s channeled by / the Vermillion plume returns, 18 December 1979. Cliffs. The 56 km cross section shows that the plume had Transmitter Wavelength Pulse energy Pulse length Pulse repetition frequency Beam divergence Receiver Telescope Field of view Optical filtering Detectors Logarithmic amplification Data system Backscatter digitization TABLE 1. Simultaneous 1.06 nm (infrared) and 0.53 pm (green) 100 mj at 1.06 /xm 20 mj at 0.53 nm (adjustable from 0 to 20 mj) 15-17 ns at 50% peak amplitude 10 pps maximum 2 mrad 14-inch Schmitt-Cassegrainian Adjustable 1-5 mrad Dichroic filter/long wave pass narrowband filters: 1.06/mi: 45% max. transmission 4.6 nm wide 0.53/nm :45% max. transmission, 0.86 nm wide Silicon avalanche 40 db dynamic range, 40 MHz Simultaneous two-wavelength sampling ALPHA-1 system specifications. Processing Program storage Recording Display Visual scene Data Sample interval: 0.01 /*s, Resolution : 8 bits Dual microcomputers (LSI-11/2) Dual floppy disk unit 9-track magnetic tape 4-bit analog facsimile hard copy Real-time two-channel A-scope Real-time 16 gray scale analog facsimile Recording: video cassette Display: 9-inch TV monitor Two-wavelength lidar backscatter signatures Aircraft location Date and time Laser energy Real-time program control switch readings 16-channel 10-bit low-speed A/D inputs (supplementary data sensors) Pyranometers (upward and downward looking) Nephelometer Turbulence
Bulletin American Meteorological Society 1039 FIG. 5. ALPHA-1 lidar cross sections of subvisible Navajo Power Plant plume 9.3, 18.5, 37, and 56 km downwind from plant, approximately 0800 MST, 18 December 1979.
1040 Vol. 61, No. 9, September 1980 FIG. 6. Four Corners Power Plant area flight paths and locations of plume returns, 20 December 1979. elongated, becoming less concentrated and extending over Marble Canyon south of the Vermillion Cliffs. The ALPHA-1 was flown to the site of the Four Corners Power Plant and was used on 20 December 1979 to track the Four Corners plume during the return flight to California. Location of the flight paths and observed plume returns are shown in Fig. 6. The first four passes of the plume made at downwind distances to 54 km showed a relatively concentrated plume tilted upward toward the north with vertical mixing to the surface of the lower plume particulates. This is consistent with earlier lidar observations of the Keystone plume, showing that fumigation that occurs during midmorning hours starts at the bottom left edge of the plume looking downwind (Johnson and Uthe, 1971). Figure 7(a) presents a plume cross section made at about 100 km downwind of the plant. The plume remains tilted upward toward the north and intersects the ground level over a relatively long distance. Figure 7(b) shows a very dense aerosol layer north of the plume cross section extending above the maximum height displayed on the facsimile plot. The dense layer, which probably resulted from earlier emitted plume particulates, contacts the surface at the higher terrain points. The aircraft altitude was increased from 3 to 3.6 km (above sea level) and a heading was taken toward California while collecting the data shown in Fig. 8. The data show that the terrain restricts passage of the hazy air mass that probably resulted from the Four Corners emissions. The initial data collected with ALPHA-1 during this flight test program illustrate that it is an ideal tool to investigate the structure of power plant plumes and downwind low-visibility air masses. The data show the importance of terrain features in determining particle distribution in the boundary layer. We plan to develop fully the two-wavelength capability for quantitative measurements of aerosol parameters relating to visibility studies.
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Bulletin American Meteorological Society 1043 Acknowledgments. This work was supported by the Electric Power Research Institute under contract RP 1308-2. The authors wish to thank Bill Evans, Earl Scribner, Larry Brieger, William Dyer, and Jan van der Laan for aiding in the design and construction of ALPHA-1, William Hovelman for piloting the aircraft, Joyce Kealoha for aiding with the data presentations, Warren Johnson for discussions on application of ALPHA-1, and Ralph Perhac and Glenn Hilst of EPRI for their interest in the program. References Collis, R. T. H., 1970: Lidar. Appl. Opt., 9, 1782-1788., and P. B. Russell, 1976: Lidar measurement of particles and gases by elastic backscattering and differential absorption. In Topics in Applied Physics, Vol. 14, edited by E. D. Hinkley, Springer-Verlag, Berlin. Johnson, W. B., and E. E. Uthe, 1971: Lidar study of the Keystone stack plume. Atmos. Environ., 5, 703-724. Uthe, E. E., W. L. Jimison, and N. B. Nielsen, 1980: Development of an airborne lidar for characterizing particle distribution in the atmosphere. Final Report, (EPRI) Contract No. RP 1308-2, SRI International, Menlo Park, Calif. A two-volume chronicle and chronology of the important meteorological events of early winters in the Northeast, the Old South, and the Old Northwest, from Samuel Champlain's first season on the Atlantic seaboard (1604) to the "first" blizzard (March 1870). A m e r i c a n ^ isfm^isro^ Early American Winters 1604-1820, (1966), 285 pp., clothbound $12.00. Early American Winters II 1821-1870, (1968), 257 pp., clothbound $12.00. Set of Both Volumes $22.00 Send orders to: AMERICAN METEOROLOGICAL SOCIETY, 45 Beacon St., Boston, Mass. 02108