ES&T
FEATURES Here is a menu of new and expanded environmental applications for the technology of
Airborne remote sensing Glenn E. Schweitzer U.S. Environmental Protection Agency Environmental Monitoring Systems Laboratory Las Vegas, Nev. 89114
During the past several years, a number of new environmental appli cations of active and passive airborne remote-sensing technologies have been successfully demonstrated. Engineer ing sophistication in designing re mote-sensing programs, and analytical skills in interpreting acquired data have developed concurrently with im provements in hardware systems. Federal agencies have a major role in further development of the remotesensing state of the art, as well as in assurance of the quality of data ac quired in this manner, that are used for environmental decision making. Also, on both the federal and state levels, consideration of the use of remote sensing has become an integral part of the process of planning environmental assessments. Moreover, commercial firms can now provide a wide range of services in both acquisition and inter pretation of data. Traditionally, pollution monitoring has been conducted by collecting samples for later analysis in a labora tory. In some cases, automated meth ods were developed to provide contin uous measurements at specific moni toring sites, and to telemeter data to analytical facilities. Remote sensing supplements these "contact" moni toring techniques. It provides the speed, perspective, and mobility nec essary for the collection of large amounts of environmental information 338A
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that would otherwise be prohibitively expensive or even impossible to obtain by contact monitoring alone. EPA's Environmental Monitoring Systems Laboratory, headquartered in Las Vegas, which also has facilities in Warrenton, Va., has significantly ex panded the use of remote sensing in the environmental field. Building on technologies developed by the De partment of Defense, NASA, the U.S. Geological Survey, and other agencies, as well as commercial firms, the labo ratory has applied multispectral scanning (MSS), laser technology, and aerial photography to a wide range of environmental problems throughout the country. Figure 1 identifies re mote-sensing projects carried out during 1981. Multispectral data
Many applications of airborne multispectral scanning for environWhat is multispectral scanning? Multispectral scanning (MSS) is a system that uses no cameras. Rather, it has radiation detectors that pick up visible and invisible light reflected from the earth. One major application of MSS is found in the LANDSAT satellite. Equipment in that spacecraft converts light intensities to digital signals that are sent to earth at the rate of 15 mil lion bits/s. At ground stations, a com puterized signal-deciphering system translates the data into images on photographic film. The resulting im ages reveal details of earth not ob tainable by conventional aerial pho tography.
mental monitoring and assessment have been demonstrated during the past decade. A number of federal and state agencies have used data acquired from satellite and aircraft platforms in order to address a variety of land-use and resource problems. However, only during the past several years has EPA begun to use MSS technology to ad dress a significant number of impor tant environmental problems. Table 1 identifies projects employing this technology, which were conducted last year. The airborne multispectral scanner used by the laboratory, a Daedalus DS-1260, acquires data at altitudes ranging from 500-20 000 ft above ground level. The 11-band system records radiant energy data in the ul traviolet through thermal infrared portions of the electromagnetic spec trum. As shown in Figure 2, the scan ner has a rotating mirror that scans across the ground scene, perpendicular to the line of flight. Radiant energy from the ground surface is reflected through focusing optics onto a beam splitter that diverts the visible radia tion (0.38-1.10 μτή) to a 10-channel spectrometer, and the thermal infrared radiation (8-14 μιτι) to a solid-state detector. Electronic signals from the 11 detectors are digitized and recorded on magnetic tape in a high-density format. The scanner is equipped with internal visible and thermal reference sources for data calibration. With the aid of a digital analysis system, the data are calibrated, and geometric corrections are applied to rectify scan-line distortions. Primary data analysis and processing involves classifying pixels (individual picture elements) in categories of energy re flectivity. Single- or multichannel
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© 1982 American Chemical Society
images as well as enhanced and clas sified images can be studied on a video monitor. In addition, statistical pa rameters computed from the data can be extracted for detailed analysis. Hard copy records that can be gener ated at this stage in data processing include black and white and color film images, electrostatic paper plots from each channel, and statistical print outs. Further processing is required be fore the final product can be made. Data are color-coded, and enlargement as well as reduction factors are com puted. Optional programs in this phase include a geographic rectification routine to match the image to selected map projection scales, as well as input image annotations. Final products include hard copy color or black and white film images (positive or negative) and electrostatic paper plots. Applications An early application of the M S S system was for inference of selected water quality parameters and trophic indices to indicate the trophic state of lakes. Demonstration projects in sev eral states have shown that MSS data, when coupled with water quality measurements at the surface that could serve as "ground truth," can provide a good indication of relative lake conditions. One would compare spectral changes during and after restoration efforts, and compare re stored lakes with those not being re stored. Figure 3 presents maps devel oped for classifying spectral differ ences associated with water quality variations in a Montana lake. Colors show nitrate-nitrogen distribution and temperature patterns. Shades of white and gray represent different land classifications. Another early MSS application using the thermal band involved the measurement of temperature gradients associated with industrial discharges into waterways. MSS technology can be very effective in delineating thermal plumes attributable to point sources, and in ascertaining surface tempera ture pattern characteristics of water bodies. A recent use of MSS technology is in the evaluation of applications for permits to discharge sewage and as sociated wastes into coastal areas. MSS imagery can readily identify such discharge areas in relation to other point-source discharges. Also, this technique provides excellent perspec tives of discharge areas in relation to shoreline residential areas, beaches, kelp beds, fishing grounds, and other
FIGURE 1
More than 100 remote sensing projects nationwide8
o General land use studies • Hazardous waste site investigations A Spill prevention countermeasures and control Δ Air quality assessments α Wetlands/coastal area analyses * Septic system surveys • Environmental impact statement reviews ® Water quality studies
Two additional hazardous waste site investigations were conducted at Prudhoe Bay and Kenai, Alaska
. Ground data (simultaneous) were collected and analyzed by the Uni versity of Montana Biological Station (Big Fork).
Nitrate-nitrogen distribution in μglL
Approximate scale: 1:150 000
Surface temperature patterns in C
Approximate scale: 1:50 000
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alone. Variations in emission intensity indicate a change in the transmission of light when it passes through the water. With the ratio of the chlorophyll a or DOC fluorescence signals to the Raman signal for water, one can obtain a new fluorescence indicator that is insensitive to changes in optical transmission. Results of one flight over a 10-km section of Lake Mead are shown in Figure 5. The continuous profile produced by the laser system is compared to the concurrent chlorophyll a ground truth data obtained from 28 fixed sampling sites on the water surface under the flight path. A high degree of correlation between the ground truth and airborne data was achieved. With accurate navigation data, it should be possible to produce maps showing isopleths of chlorophyll a or DOC fluorescence for the surface layers of lakes, rivers, and coastal waters.
FIGURE 4
Operation of airborne laser fluorosensor
Pulsed laser beam
Telescope field of view
Fluorescence and Raman emission from surface water volume
One optical attenuation length
FIGURE 5
Raman-corrected fluorescence of chlorophyll a, compared to ground truth data
Flight of June 7, 1979, over Las Vegas Bay, Lake Mead, Nev. Airborne laser fluorosensor profile of ( P F / P R ) ratio of chlorophyll a fluorescence to water Raman signals Chlorophyll a data for surface water samples normalized to airborne data
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Air quality monitoring Current methods of monitoring air pollution with ground-based instruments are not able to provide rapid, area-wide measurements of atmospheric pollution effectively. Such area-wide measurements are necessary to understand the long-range transport of pollutants. They can provide input information to mathematical models. Also, they can reveal the structure of urban plumes, point-source plumes, and their interactions. Finally, they can provide quantifiable information on specific pollutants and their precursors. Downward-looking laser systems called "lidars" have been built and tested, and their success has been demonstrated as a means of providing these difficult measurements and characterizations of atmospheric pollution. Researchers at the Las Vegas laboratory have developed devices that map the aerosol distribution in the atmosphere (2). Data obtained have been used to define mixing-layer depths; to describe point-source plume dimensions; and to characterize large polluted air masses. Airborne lidars have also been used to position other aircraft, equipped with in situ measuring and sampling equipment, in plumes and in air masses. Table 2 lists several recent air quality monitoring projects that the laboratory carried out. Measurements are made by observing the relative back-scattering of the intense, extremely short pulse of laser light as it interacts with the suspended particles and droplets below the aircraft. Electronic analyzers on board monitor the elapsed time be-
tween the firing of the laser and the scattering returns, and thereby "range" or measure the distances to the aerosol layers. The laboratory's latest, and now operational, lidar is a two-frequency system consisting of a neodymiumYAG ( y t t r i u m - a l u m i n u m - g a r n e t ) laser transmitter, a Newtonian tele scope receiver, and an electronics sys tem that provides a real-time display of the aerosol cross-section data. The two wavelengths—one in the green portion of the spectrum (0.53 μιη), and the other in the near-infrared (1.06 μιή)—provide a means to differentiate particle size ranges associated with the two major aerosol sources, natural and man-made. This latter feature was designed to permit the measurement of the relative contribution of individual aerosol plumes within an aged air mass. Specialized high-speed digital electronics are needed to take the sig nals from the photomultiplier tubes, convert them to digital format, correct them for background and instanta neous laser peak-power values, display the sounding data on an oscilloscope, and store the data in a buffer memory. Timing restrictions are handled through a bit-sized microprocessor subsystem developed for signal con ditioning, which uses an instruction set specific for this type of application. The real-time display subsystem is combined with the systems controller, and consists of a microprocessor with various peripheral devices. Lidar applications Two examples of lidar applications consist of delineation of a power plant plume, and characterization of a re gional pollution problem. The lidar provides air quality scientists and modelers with fairly high-resolution maps of the mixing layer for the pol lutants, and the aerosol distribution over the geographic area of interest. The system plays a key role in moni toring plume trajectories and disper sion characteristics, and provides data useful for model development and validation efforts. It does not, however, measure discrete particle size, or pro vide size distribution data beyond the two general ranges described earlier. Those measurements would require the interpretation of the differences in backscatter intensity from several la sers operating simultaneously at dif ferent wavelengths. A pollutant-specific remote moni toring system based on laser technol ogy, which is still under development and shows great promise, is the dif ferential absorption lidar (DIAL).
FIGURE 6
SO2 and O3 absorption coefficients for UV-DIAL system
FIGURE 7
Comparison of UV-DIAL and aircraft in situ measurements
This system also uses two lasers very close in frequency output. One is tuned to an absorption peak for a specific molecule, and the other is tuned off the absorption peak. The scattering cross-section is the same for the two laser pulses as they traverse the at mosphere, while the absorption crosssection of one is related to the specific molecule. The difference in the return
signal is a direct measure, therefore, of the concentration of the molecular pollutant. Figure 6 shows the differ ential absorption concept for two air quality criteria pollutants—ozone (O3) and sulfur dioxide (SO2). A high spatial resolution airborne system designed to measure O3 and aerosols has already been tested by Browell et al. of N A S A , and has Environ. Sci. Technol., V o l . 16, No. 6, 1982
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yielded results of the type displayed in Figure 7 (3). The graph of altitude vs. concentration shows an excellent cor relation between the profile, as mea sured with the DIAL system, and that measured with in situ instrumentation on board an aircraft flying a spiral pattern, both measurements being made at the same time. The lowest concentration measured, that is, ap proximately 35 ppb, does not indicate the detection limit for this system, but, rather, the lowest level encountered. In practice, the system would provide a three-dimensional map of a measur able pollutant over the study area from beneath the aircraft to the ground. The Jet Propulsion Laboratory de veloped a laser absorption spectrome try system that has expanded the technology for using the infrared DIAL system for specific pollutants (4, 5). CO2 waveguide lasers provide the impulse source, and heterodyne methods (use of more than one fre quency at a time) are employed for increasing the detection sensitivity. Originally developed for space appli cations, this technique has been tested and used in aircraft. EPA has conducted research on in frared DIAL systems with pulsed CO2 lasers for column concentrations that use earth reflectance (
