report i environmental remote sensing of earth’s atmowhere · 2018. 6. 13. · number of...

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S P E C I A L R E P O R T .. *

Millimeter-wave Environmental Remote Sensing ,

of Earth’s Atmowhere i I

Alan Parrish

S. Deerfield, MA Millitech Corp t

introduction It is well known to the scientific

community that the spectral properties of the gases that make up the Earth’s atmosphere play a major role in determining the environment in which we live. For example, the temperature at the Earth’s surface is determined by the balance between the solar radiation absorbed at the Earth’s surface and the infrared radiation emitted by the Earth into space. The infrared re-radiation is controlled by the wavelengths and intensities of the absorption lines, and bands of the gases that make up the Earth’s atmosphere. As a second example, the environmental level of ultraviolet radiation in which life has evolved is controlled by the amount of ozone in the stratosphere. Ozone is very effective at absorbing biologically damaging solar ultraviolet radiation between 285 and 315 nanometers, UV-B radiation. Therefore, the environment- determining physical properties of the atmosphere depend on its chemical composition.

Over the past two decades, scientists have realized in ever-increasing numbers that the balance of chemical and physical processes that determine the atmospheric composition, and hence the environment at the surface, is delicate and subject to perturbation by gases released as a result of human activities. This realization has driven greatly expanded efforts to under- stand the physics and chemistry of the atmosphere and predict the ef- fects of man-made pollutants. These efforts include improved computer modelling of atmospheric

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processes and development of improved instrumentation to measure the atmospheric composition as a function of position and time.

The majority of the instruments used for measuring the composition of the atmosphere make use of the spectral properties of its constituent gases, that is, they measure their absorption or emission spectra in the ultraviolet, visible, infrared or mm- wave regions of the electromagnetic spectrum. This paper concentrates on the contribution of mm-wave instruments to the on-going program of atmospheric measurements, particu- larly in the stratosphere.

The atmospheric constituents that have been measured at mm- wavelengths include ozone, chlorine monoxide (CIO), water vapor, nitrous oxide (N20), HO2, CO and HCN in the stratosphere. Programs in the troposphere have concentrated on measuring water vapor at 22 GHz and temperature using the oxygen transitions at 60 GHz for applications in weather forecasting.

In the stratosphere, the measurements of ozone and chlorine monoxide are the most important. Ozone measurements must be made with good calibration stability to measure ozone depletion directly at rates of tenths of a percent per year. Chlo- rine monoxide is involved in all of the reaction sequences in which chlorine, which was originally released in chlorofluorocarbons, breaks down stratospheric ozone. A global picture of its distribution and changes in time is vital to understanding the processes involved.

A mm-wave instrument used for atmospheric spectroscopy funda-

mentally consists of a low noise het- erodyne downconverter feeding a multichannel spectrometer. There are various applicable spectrometer technologies; the simplest being a multichannel filterbank The IF input to the filterbank feeds, in parallel, a number of narrowband filters, each tuned to a slightly different frequency. The output of each filter feeds a square-law detector. The video out- puts are digitized and processed in a computer. Thus, the instrument has the capability of simultaneously measuring the intensity of received radiation at a set of frequencies corresponding to the filter frequencies, as transformed by the various heter- odyne stages in the instrument The frequencies are spaced across the spectral line to be observed. The number, spacing and resolution of the filters are determined by the require- ments of the particular measurement

The receivers and spectrometers used in atmospheric measurements are fundamentally similar to those used by radio astronomers to study the chemical composition and dy- namics of clouds of gas and dust in star-forming regions of the Galaxy. There has been a close association between researchers doing atmospheric measurements and those doing radio astronomy. A number of them have worked in both fields.

The high angular resolution of a large radio telescope is not required for ground-based atmospheric measurements because emission comes from everywhere in the sky rather than from a radio source of small angular diameter. However, moderate angular resolution is use-

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[Continued on page 261

MICROWAVE JOURNAL DECEMBER 1992

26 CIRCLE 13

From page 241 PARRISH

ful in limb-viewing measurements from spacecraft.

The atmospheric measurements are calibrated using the thermal radiation from black body sources at known temperatures. The sources are typically made from microwave absorber material. The techniques are similar to those used for measuring receiver noise temperatures in the laboratory using microwave ab- sorbers and liquid nitrogen. With care, high absolute accuracy can be achieved in the measurements.

It is important to be able to determine the distribution of the constituent of interest as a function of altitude. This can be done, even with aground-based, upward looking instrument, by making use of the fact that the spectral lines of interest are pressure broadened for the atmospheric pressures corresponding to altitudes below 70 km. Intermolec- ular collisions perturb the energy levels of the emitting molecules, and hence the frequencies of the emitted quanta. The pressure broadening is largely independent of the line frequency, unlike the Doppler broadening resulting from the ran- dom motion of the molecules, which is proportional to frequency and temperature. As a rule of thumb, the pressure broadening is typically of the order of 3 MHz per millibar of

Fig. 1 A hypothetical example illustrating how the altitude distribution of a trace constituent in the atmosphere

is retrieved from the details . of the pressure-broadened

line shape.

atmospheric pressure for molecules of interest. For lines with frequencies of the order of 100 GHz, this implies that the pressure broadening of the spectral line is greater than the thermal broadening for altitudes below 70 km. The lower limit of altitude coverage is set by the bandwidth covered by the instrument, and also the altitude distribution of the constituent being observed. The spectrometers of instruments presently in use typically have bandwidths of 500 MHz and can make useful measurements down to 20 km.

Because of the pressure broadening, the shape of the spectral line contains information on the altitude distribution of the emitting molecule. Figure 1 shows conceptually how the altitude distribution is retrieved from the observed line shape. In this figure, a hypothetical atmosphere has been assumed in which the molecule of interest is concentrated in two thin layers, one at an altitude where the pressure is 10 millibars and the other at 100 millibars. The layers are assumed to be optically thin so that there is negligible absorption of the signal from the upper layer in the lower layer. In this case, the signal seen at the ground is es- sentially the sum of the contribution from the two layers. The contribution from each layer is proportional to the product of the molecular density in the layer and the path length along the line of sight through it. The difference between the intensity measured at f l and that measured at f2 is nearly entirely controlled by the density in the upper layer. The intensity difference between f2 and f3 is nearly entirely controlled by the density in the lower layer. The den- sities of the two layers can be determined from the measured intensities at fl, f2 and f3.

In practice, the molecule being observed is continuously distrib- uted instead of the two discrete layers used in the hypothetical example. The retrieval algorithm has to deal with a large number of layers. Although the related mathematical techniques are beyond the scope of this paper, it is still possible to re- trieve the altitude distribution of the observed species with a vertical resolution that is approximately the pressure scale height of the atmos-


MICROWAVE JOURNAL 0 DECEMBER 1992

[From page 261 PARRISH

phere, that is, the altitude at which the pressure is 1 /e of the pressure at the surface, or 10 km.

Because of the pressure broadening, the contributions from low altitudes are greatly smeared out and the peak intensity is low. For this reason, trace constituents in the troposphere generally can not be observed using microwave techniques. However, constituents present in sig- nificant quantities, such as water vapor and oxygen, produce useful spectral lines. Measurements of the tropospheric thermal emission at one frequency near the center of the 22.2 GHz water vapor line, and at another frequency well away from the line center, such as 31 GHz, have been used to measure the precipitable water vapor in the atmosphere. Oxygen has a set of lines near 60 GHz and individual lines at 118 GHz and higher frequencies. The atmosphere is opaque at the center frequencies of the oxygen lines because oxygen is a major atmospheric component. However, it becomes increasingly transparent at frequencies away from the line center. Because the mixing ratio of oxygen in the atmosphere is

constant, the emissivity at frequencies on the oxygen line wings is pri- marily controlled by the atmospheric temperature distribution. Tempera- ture profile monitoring can be done using spectral measurements of the oxygen line wings. These weather forecasting applications have been discussed in more detail previous1y.l

A much more favorable viewing geometry is available if the receiver- spectrometer is deployed above the

Fig. 2 Geometry for measurements of spectral line emission from the

atmosphere above the limb of the Earth using a balloon- or satellite-borne instrument.

part of the atmosphere to be observed, for example on a spacecraft. As shown in Figure2, it can then observe the line in emission through the limb of the Earth, with the line of sight passing through a tangent to the atmosphere. This maximizes the path length through the atmosphere at a particular altitude, and hence the signal contribution from that altitude. If the instrument contains an antenna with a narrow beamwidth, the vertical resolution of the retrieved profile is enhanced com- pared to the ground-based geometry because the contributions from altitudes below the tangent altifade are sharply cut off by the at%&#% beam and those from above the tangent altitude are attenuated by the viewing geometry.

The microwave limb sounder (MLS), built at the Jet Propulsion Laboratory of the California Institute of Technology, was launched on the upper atmosphere research satellite (UARS) in September, 1991. It uses the limb viewing geometry shown in Figure 2. Its antenna has a vertical dimension of 1.6 m, which at its op-


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erating frequency of 205 GHz, yields ~ a half power beamwidth of 0.07: This beamwidth corresponds to a vertical distanceof 3.5 km at the limb and the vertical resolution of the MLS retrieval is similar.

Measurements of stratospheric ozone began in the 1920s using an ultraviolet spectrophotometer developed by G.W.B. Dobson at Oxford University? Thirty of these instruments have records dating back to the late 1950s and early 1960s. These simple, ground-based instruments have provided an extremely important historical record. British Antarctic Survey scientists also used the instruments to make the measurements that led to the dis- covery of the Antarctic ozone hole, reported in 1985.4

A variety of other instruments have been development for measurements of ozone and other trace constituents during the last few decades. Many of these instruments are deployed on satellites. Each has technical strengths and weaknesses related to the technique used to make the measurement. For example, an advantage of mm-wave measurements is that mm-waves are not significantly scattered by dust and aerosols in the atmosphere, so mm-wave measurements are unaffected by changes in atmospheric aerosol loading because the mm-wavelength is so much greater than the particle size. There-

Fig. 3 First observations of chlorine monoxide over Antartica made with a ground-based mm-wave instrument

in 1986. The diurnal variation of the CIO line is shown.

Reprinted from Nature, copyright 1987 Macmillan Magazines Limited?

fore, UV radiation is strongly scattered by aerosols, and measurements made using UV techniques are significantly affected by volcan- ic eruptions, such as the eruption of Mt. Pinatubo in 1991. Another advantage is that the spectral lines of the important species ClO, which are most favorable for observation, are at mm-wavelengths.

Remotely sensed measurements of stratospheric CIO were first made in the 1980s with a ground-based,

mm-wave instrument operating at 204 GHz.5 A successor to that instrument was used to observe the CIO line at 278 GHz and to make occasional measurements from the summit of Mauna Kea (1 4,000 ft.) in Hawaii throughout the 1980s, and was used to make the first CIO meas~rements~~~ in Antarctica during the National Ozone Expedition (NOZE I) from August to October, 1986. Figure 3 shows observations of the diurnal variation of the CIO

twenty-first century.

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MICROWAVE JOURNAL DECEMBER 1992 CIRCLE 58 ON READER SERVICE CARD 31

line made with this instrument. Dur- ing.the day, the line shape is similar *

to-that shown in the hypothetical case in Figure 1 because two nearly distinct layers of CIO can be found. The narrow peak at the line center is due to a peak in the CIO distribution at 40 km altitude. This peak was expected from atmospheric models available at the time and also is similar to the CIO line observed at mid- latitudes. The broad underlying curve is due to CIO at lower altitudes of 20 km and in exactly the region where balloon-borne sondes were showing that ozone was being de- pleted. At night, the signal de- creases markedly as photolysis ceases and the chlorine is bound in reservoir species. The amount of daytime CIO at 20 km altitude was on the order of 100 times what was expected from the photochemistry as understood at the time. The mm- wave observations were repeated8 and independently confirmed the following year by direct in situ sam- pling from a ER-2 (converted U2) aircraft flying into the polar vortex at altitudes up to 19 km (60,000 ft.). The in-situ measurementsg also showed detailed correlation between loca- tions of high CIO and low ozone on a fine spatial scale, providing extremely strong evidence for a cause-and-effect relationship between high CIO and ozone depletion over Antarctica.

With the launch of the Upper At- mosphere Research Satellite (UARS) in September, 1991, a con- tinuous, nearly global picture of CIO is being obtained by the microwave limb sounder2 (MLS) on board. The MLS observes CIO and ozone lines at frequencies near 205 GHz, and ozone and water vapor lines at frequencies near 183 GHz. The na- tionally publicized picture of CIO over the Arctic regions and northern Europe was obtained with this instrument.13

Given the importance of the measurements, the accuracy required for long term ozone trend detection, and the relative strengths and weaknesses of various techniques, redundant measurements using a variety of techniques and platforms are needed to assure that erroneous measurements are elim- inated and an accurate picture is obtained. While a single instrument carried on a satellite in a highly in-

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clined orbit can measure the global distribution of ozone or other constituents, an undetected drift in the calibration of this instrument will cause a false picture of ozone trends for the entire world. Likewise, a trend error will be produced if the instrument fails and is replaced by another with slightly different calibration errors.

For this reason, a new, international network of ground-based ob- servatories is being developed to complement the tools aboard UARS and other satellites. This network for detection of stratospheric change will ultimately have five stations, in- cluding stations at northern and southern high latitudes, and northern and southern mid-latitudes, and a tropical station. Sites in Europe, Hawaii and New Zealand are presently being developed. Each site will have spectrometers for UV to visible, infrared and mm-wavelengths, and for light detection and ranging (lid- ars) for measurement of ozone, temperature and aerosols.

A family of mm-wave spectrometers has been developed for the network to measure ozone and CIO. Figure4 is a photograph of the ozone instrument. These instruments are designed to make measurements around the clock under computer control with a minimum of operator attention. The ozone instrument has been in operation since July, 1989 at the JPLTable Mountain Observatory in California and was recently transferred to the network site being developed at Lauder, New Zealand. Data from the ozone instrument is downloaded to at the NASA- Langley Research Center, where it is processed.I0 CIO instruments were installed at network sites in Hawaii and France this year.

An ozone lidar developed at the Jet Propulsion Laboratoryll has been in operation at Table Mountain since 1988, making possible a long term comparison with the mm-wave ozone spectrometer. The lidar transmits two beams in the UV, at 308 and 353 nm, which are elasti- cally bac kscattered to p hotom u I ti - plier detectors at the focus of a 90 cm diameter telescope. The 308 nm beam is much more strongly absorbed by ozone than the 353 nm beam, but other absorption and scattering in the atmosphere is nearly the same at the two wave-

lengths. Therefore, ozone density as a function of altitude can be derived from the difference in the received photon counts as afunction of delay. The lidar instrument is operated typically several nights each week, and the mm-wave instrument acquires data continuously during good weather. Figure 5 shows a time series of the lidar and mm-wave data for altitudes of 30 and 40 km. The two data sets show the 30 percent seasonal ozone variation at 30 km and also are correlated at the level of the day-to-day natural ozone variations on the order of 5 percent. Figure 6 shows that the co-located lidar and microwave instruments are in good agreement regarding variations in ozone, also of the order of 5 percent between one year and the next. The anthropogenic contribution to these interannual differences is expected to be small.

improvements in mm-wave technology will make possible observations at increasing frequencies from spacecraft The upper frequency limit for useful ground-based observations is about 300 GHz, set by absorption by tropospheric water vapor. The availability of sub-mm-wavelength receivers on spacecraft will make possible observations of the first rotational transition of HCI, which produces a spectral line at 637 GHz. A sub-mm-wave receiver has been incorporated into a JPL balloon pay- load as a step in the development of a sub-mm-wave instrument for the earth observing system spacecraft (EOS)* This instrument can measure CIO and HCI simultaneously, allow- ing direct measurement of the parti-


Fig. 4 Field installation of mm-wave ozone instrument

at Table Mountain. It looks out through microwave windows

in the frame attached to the wall. The cryogenically cooled

receiver is at the lower right.



Fig. 5 A time series of the lidar and mm-wave ozone observations made of Table Mountaln.‘2

Fig. 6 lnterannual ozone changes, the fractional difference between two

periods In 1990 and the corresponding periods in 1989 as measured

by the JPL lidar and the microwave instrument at Table Mountain.12

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tioning of stratospheric chlorine between ozone-destructive and inac- tive species.

Conclusion Millimeter-we technology has

made substantial contributions to the program of measurements that is fundamental to understanding the atmosphere and predicting future changes in it These contributions will continue as new satellites and the remaining networks stations are developed.

References D.C. Hcgg. M.T. Decker, EO. Guiraud, KB. Earnshaw, OA Merritt Kt? Moran, W.B. Sweezy. R.G. Strauch, ER. Westwater and C.G. Little, “An Automatic Profiler of the Temperature, Wind and Humidity in the Troposphere,” Journal of Applied Meteroology, Vol. 22,1983, pp. 807-831. J.W. Waters, “Microwave Limb-Sounding of Earth’s Upper Atmosphere,” Atmos- pheric Research, Vol. 23,1989, pp. 391- 41 0. G.M.B. Dobson, “Forty Years Research on Atmospheric Ozone at Oxford, A His- tory,” Applied Optics, Vol. 7, 1968, pp. 387-405. J.C. Farman, B.G. Gardinier and J.D. Shanklin, “Large Losses of Total Ozone

5.

6.

7.

8.

9.

10.

11.

12.

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in Antartica Reveal Seasonal CIO/NO Interaction.” Nature, Vol. 315, 1985,

A. Parrish, RL deZafra, P.M. Solomon, J.W. Barrett and ER. Carison, “Chlorine Oxide in the Stratospheric Ozone Layer: Ground-based Detection and Measure- ment,” Science, Vol. 21 1,1981, pp. 11 58- 1160. R.L deZafra, M. Jaramillo, A. Parrish, I? Solomon, B. Connor and J. Barrett, “High Concentrations of Chlorine Monoxide at Low Variations in the Antarctic Spring Stratosphere: Diurnal Variation,” Nature. Vol. 328,1987, pp. 408-41 1. P.M. Solomon, B. Connor, R.L deZafra, A. Parrish, J. Barrett and M. Jaramillo, “High Concentrations of Chlorine Monoxide at Low Altitudes in the Antarctic Spring Stratosphere: Secular Variation,” Nature, Vol. 328.1987, pp. 41 1-413. R.L deZafra, M. Jaramillo, J. Barrett, LK Emmons, P.M. Solomon and A Parrish, “New Observations of a Large Concen- tration of CIO in the Springtime Lower Stratosphere Over Antarctica and Its Im- plications for Ozone-Depleting Chemis- ‘ try,” Journal of Geophysical Research, Vol. 94,1989, pp. 1 1423-1 1428. JG. Anderson, D.W. Toohey and W.H. Brune, “Free Radicals Within the Antarc- tic Vortex The Role of CFCs in Antarctic Ozone Loss,” Science, Vol. 251, 1991, pp. 39-46. BJ. Connor. A Parrish and JJ. Tsou, “Detection of Stratospheric Ozone Trends by Ground-based Microwave Observations,” Proceedings of SPlE Conf. 1491, Remote Sensing of Atmos- pheres, Society of Photo-optical Instru- mentation Engineers, Bellingham, WA, 1991. I.S. McDermid. S.M. Godin and LO. Lindqvist, “Ground-based Laser DIAL System for Long Term Measurements of Stratosheric Ozone,” Applied Optics, Vd. 29.1990, pp. 3603-361 2. k Parrish, BJ. Connor, JJ. Tsou, I.S. McDermid, W.P. Chu and RE. Siskind, “Results from Two Years of Ozone Data Taken with a New, Ground-Based Micro- wave Instrument: An Overview,” Roc. of the Quadrennial Ozone Symposium, Charlottesville,VA, June4-13,1992,tobe published by NASA-Goddard Space Flight Center. lime, February 17,1992, p. 60.

pp. 207-21 0.

Alan Panish received his BEE degree from Yale University in 1965 and his PhD degree from Cornell University in 1971, concentrating in radio astronomy. Between 1971 and 1978, he worked at the Arecibo Observatory. the National Radio Astronomy Observatory and the Research Laboratory for the Electronics at MIT on projects in research and instrumentation. From 1978 to 1988, he worked with researchers at the State Unkrsity of New York, Stony Brook, demloplng instrumentation for ground-basedmeasurements ofstra- tospheric C/O. He joined Millitech Corp. part- time in 1982, and presently wrks there and at the University of Massachusetts, Amherst specializing in stratospheric remote sensing. He is a member of the American Geophysical Union and the International Scientific Radio Union.


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