ground-based lidar and atmospheric studies

G R O U N D - B A S E D L I D A R AND A T M O S P H E R I C S T U D I E S

I. S T U A R T M c D E R M I D

Table Mountain Facility, Jet Propulsion Laboratory, California Inst#ute of Technology, p.o. Box 367, Wrightwood, CA 92397, U.S.A.

Abstract. This review considers the requirements and possibilities for the development of a ground- based network for long-term observations of the atmosphere. This network would be specifically designed to provide early detection of changes in the composition and structure of the stratosphere. The species and parameters identified as being important and amenable to ground-based measurements are summarized, as are the currently available techniques capable of making the required measurements. Ultraviolet laser remote sensing is identified as the most promising technique for the measurement of ozone and temperature profiles which are considered to have the highest priority for network measurements. The laser techniques, and the research at JPL Table Mountain Observatory, to implement ozone and temperature measurements are discussed in greater detail.

1. Introduction

There exists an active, expanding research program utilizing ground, aircraft, balloon, rocket, shuttle and satellite platforms to study the chemistry and physics of the upper atmosphere. However, the development of a ground-based measuring network for long term observations, designed specifically to provide early detection of changes in stratospheric composition and structure, has only recently been considered. A workshop, sponsored jointly by NASA, NOAA, and CMA, was convened in order to evaluate the feasibility of such a network and to identify and prioritize the key species and parameters to be measured. Much of this introduction is based on the report of this workshop issued by NASA-UARP (1986).

A ground-based measurement station has the unique capability of being able to make frequent observations, daily if the weather permits, thus allowing the accumulation of long-term data bases which are essential for statistical trend analysis, Hilsenrath (1985). Ground stations also avoid some of the problems associated with flight platforms such as size, power and weight restrictions, balloon and rocket failures, and limited launch opportunities. Since the ground station is stationary the same atmospheric volume may be viewed for long continuous periods. The long integration times possible can be used to increase sensitivity and precision. The global coverage obtained by satellites is, in principle, to be preferred over the limited geographic coverage that can be achieved with a ground network. However, in addition to their high cost, long lead time, and limited lifetime, some satellite instruments have been subject to sensitivity changes during operation in space. These factors limit their use for trend detection during the next decade.

Surveys in Geophysics 9 (1987) 107-122. �9 1987 by D. Reidel Publishing Company.

108 L STUART McDERMID

TABLE I

Measurement priorities

1. Column Ozone 2. Ozone profile (0-70 km) 3. Temperature profile (0-70 kin) 4. C10 vertical profile 5. Water vapor vertical profile 6. Aerosol vertical distribution 7. NO2 vertical profile or column 8. Stratospheric column of HC] 9. Vertical profile of long-lived tracers,

CH4 and N20 10. Other species (HNO3, OH, CIONO2)

The measurements that were identified by the workshop as being of the highest priority for a network are listed in Table I in order of importance. In arriving at this set, consideration was given to the progress of current research efforts.

Some of the justifications for the measurements listed in Table I are as fol- lows. The role of atmospheric ozone in absorbing ultraviolet radiation from the Sun and thus preventing this radiation from reaching the Earth's surface is very important. Changes in the total column concentration of ozone could have serious impact on human health, and terrestrial and oceanic ecosystems. Ozone absorption of solar energy also provides the main soure of heating in the stratosphere and thus the vertical distribution of ozone is important in determining the temperature structure and dynamics of the stratosphere. This, in turn, is critical in controlling connections between stratospheric and tropospheric weather and climate which can occur through both radiative and dynamical coupling.

The temperature of the stratosphere controls the rates of chemical reactions and thus governs the efficiency of the various catalytic cycles affecting the ozone abundance. Knowledge of the temperature profile can also be used to make corrections to experimental measurements, e.g. for the temperature dependence of absorption cross sections, and thus improve the accuracy of the measurement.

Chlorine can destroy ozone in the atmosphere through the following catalytic cycles,

C1+O3 ~ C10 + O2 CIO + O ~ Cl + 02

Net: O + 03 ; 202

and, at lower altitudes,

C I + O 3 ~ C 1 0 + O 2 OH + 03 :, HO2 + 02

GROUND-BASED LIDAR AND ATMOSPHERIC STUDIES 109

CIO + HO2 ~ HOC1 + 0 2

HOC1 + hv ~ OH + C1 Net: 203 ~ 302.

The C10 radical is the chlorine species responsible for directly catalyzing the destruction of ozone. Due to the continuing release of chlorofluorocarbons (CFC's), the atmospheric concentration of C10 is predicted to be increasing at a significant rate.

Water vapor is the dominant source of the hydroxyl radical in the stratosphere through the following reaction scheme,

03 + hv ~ O(1D) + 0 2 O(ID) + H20 ~ 2 OH.

Hydroxyl radicals (and HO2) play a critical role in all stratospheric chemical cycles either directly, as a reactant, or through control of the partitioning between reactive and stable species within each of the chemical families, C1Ox, NOx, and HOx. The concentration of stratospheric water vapor is expected to increase since it is produced in the methane oxidation cycle,

CH 4 + OH J) CH 3 4- H20

and the methane concentration is steadily increasing. Aerosols are included in the list even though they are not thought to play a

major role in the chemistry of the unperturbed stratosphere. However, they could be important radiatively, for example following volcanic eruptions, and chemically, for example by supporting heterogeneous reactions in polar stratospheric c10uds (PSC's). The presence of significant aerosol layers can also affect the interpretation of data from many optical sensors both on the ground and in space.

In the nitrogen family NO2 is directly involved in the catalytic destruction of ozone,

NO + 0 3 ~ NO2 4- 02 N O 2 + O ~ NO4-O 2

Net: O + O 3 ~ 202

and,

N O + O 3 ) NO2+O2 NO2 + 03 ~ NO34- 0 2

NO3 4- hv ) NO 4- O2 Net: 203 ~ 302.

If the C10 concentration is increasing as predicted then the concentration of NO2 could be reduced by its conversion into chlorine nitrate,

C10 + NO2 + M ~ CIONO2 + M

110 I. STUART McDERMID

However, this could be offset by increases in N20 which is the precursor of stratospheric NO and NO2.

HC1 is believed to be the major inorganic chlorine compound in the stratosphere and is the longest-lived reservoir species in the C1Ox family. Some of the major reactions, which control the partitioning of HC1 into C10, are:

HC1 + O H ~ H 2 0 + C1

C1 + C H 4 ..... ) HC1 + C H 3

C1 + HO2 ,, ~ HC1 + 0 2

C I + O 3 ~ C 1 0 + O 2

C10 + N O ~ C1 + NO2

C 1 0 + O > C1 + O2.

By monitoring both C10 and HC1 it will be possible to see whether the partitioning of species within the chlorine family is changing with time.

There are several long-lived (source) trace gaseous species, including N20 and CH4, which are primarily of tropospheric origin and which have long stratospheric lifetimes. Since the removal mechanisms for these species are relatively simple and well characterized, namely photolysis for N 2 0 and reaction with OH for CH4, their latitudinal and vertical distributions are strongly dependent on transport processes. This makes them ideal tracers for studying changes in atmospheric circulation.

There are a number of other species whose measurement would provide important information on changes in the composition of the stratosphere

TABLE II

Instrumentation

Species Instrument Status (1986)

03 column Dobson 03 0-20 km YAG lidar

15-45 km Excimer lidar 25-75 km Microwave

Temperature Lidar C10 25-45 krn Microwave H20 0-30 km Balloon hygrometer

Microwave Aerosols 0-30 km Lidar NO2 strat col UV/vis spectrum HC1 strat col IR CH 4 strat col IR N20 20-50 km Microwave HNO3 strat col IR C1ONO 2 strat col IR OH UV fluorescence

Excimer lidar

Operating Under development, ready 1 yr Under development, ready 1 yr Ready to proceed Demonstrated Demonstrated Operating Proposed (>35 km), demonstrated >45 km Demonstrated Demonstrated Demonstrated Proposed Proposed Proposed Proposed Proposed, research mode Proposed, minimum 3 yr to feasibility

GROUND-BASED LIDAR AND ATMOSPHERIC STUDIES 1 1 1

including, but not Iimited to, CIONO2, OH, HO2NO2, and H202. At present, measurements of these species are thought to be either of a lower priority or not possible with the current state of development of measurement systems.

Table II shows the set of instruments and their status of development for achieving measurements of the species given in Table I. Even for those instruments that are indicated as having been demonstrated there have been no long- term operational tests and some development will probably be required for network implementation. For optimum operation of most of these instruments it is desirable to minimize the atmospheric column, water vapor, aerosols and density, through which they must probe. This implies that operation from dry, high altitude sites is preferred if available. From the point of view of global coverage it was considered that a minimal network would require six stations with locations at mid- and high-latitudes in both northern and southern hemi- spheres, in the tropics, and at a polar site (probably Antarctica).

2. JPL Table Mountain Observatory

A research effort is underway at JPL-TMO to try to demonstrate lidar measurements of ozone and temperature profiles in a mode suitable for long-term observations and for eventual network implementation. TMO is located in the San Gabriel Mountains in Southern California. It is at an elevation of 2300 m and at latitude 34 ~ N; the atmospheric 'seeing' conditions are excellent for most of the year.

3. Ozone DIAL

The capability to make range-resolved measurements of atmospheric ozone using lidar has been demonstrated by several groups in Europe and in Japan. In France, an ozone lidar system has been in use at the Observatoire de Haute Provence since 1981 and has undergone much development since then, see Pelon and Megie (1982a, b), Megie and Pelon (1984), Megie et al. (1985), Pelon et al. (1986). At Zugspitze, in the German Alps, an excimer lidar system has been operated, by Werner et al. (1983) and Rothe and Walther (1986), making ozone measurements up to 50 km. Much of the pioneering work in using excimer lasers for atmospheric ozone measurements has been carried out by the group at Kyushu University in Japan, see Uchino et al. (1978, 1979, 1983). However, to date none of these groups has implemented a ground-based ozone lidar with the precision and form required for the proposed NASA network for the detection of stratospheric change.

Differential absorption lidar (DIAL) requires two (or more) laser wavelengths which are chosen such that one coincides with a region of high absorption,

l 12 I. STUART McDERMID

~:: .... : ~ : ~ : ~ : ~ : ~ : ~

)" w 5! ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

S (x w, R) S ( x o, R) ~;;;;;;;;~/;;i~:~;::~;i;:~;~;; ::;:::::, =====================================================

- - - - - - 7 - - ' : ' ! '?"[ : ! ' ! '? ' ; : : ' ! ' ; '"?! '? ' '/! i'!'!" �9

8< xw, R+~R)~ ,.9.( x o, ~+ARi::::;;;;;i!iiiii~iiii;~il-i~!iiiiiiii

Fig. I. Schematic of the DIAL technique.

R R+AR

!iiii;~ ........

iliil :iii~iii!!!Si;!jiiillli~'iif~i ::illi: i;)i;; :!!ii?iii!iiiiii;iil;;ili;ii;i;2i;i;~;;i;i!il :!;~iii:. iii~iii ~i!iiS~iiiii/iiiii~!iiiiii~!iiiiiit:i-~iiii !ii~i!

specific to the species being measured, and the other is tuned into the wings

of this region, to a wavelength with m u c h lower absorpt ion.

The difference in the absorpt ion cross section at these on- and off-line wave-

lengths is called the differential absorpt ion cross section and it is the magni tude

o f this pa rame te r which determines the sensitivity of the me thod as will be

shown later. The two wavelengths are t ransmit ted into the a tmosphere , s imul-

taneously if possible, over the same path and the intensity of the elastically

backscat tered radiat ion is moni to red for each wavelength as a function of t ime (range). This is indicated schemat ical ly in Figure 1. The on- and off-line signals

TABLE III

Lidar equation

S = E. e(R). e()O. ~ �9 (3R +/3A ) a R

where E = laser energy e (R) = telescope/laser overlap e(2) = receiver efficiency A = telescope area R = range 3R = rayleigh backscatter coefficient BA = aerosol backscatter coefficient ~R = rayleigh extinction coefficient ~A = aerosol extinction coefficient ~o3 = ozone extinction coefficient


TABLE IV

DIAL equation

S(Xo, R).S(Xw, R + AR) N(R) In , ~ , S(20, R + AR). S(2 w, R)

+ In fl(20, R + AR). fl(2w, R) /3(~ 0 , R)./3(&., R + AR)

- 2AR[~(20, R) - o~(2w, R)]}

where a= o(20)- o(;t~.) /~=/3A +/3R

are then compared and the absorption by the species of interest is separated from other attenuation processes in the atmosphere.

The elastic backscatter lidar equation for a single wavelength, 2, and range, R, can be written for the received signal, S, as shown in Table III.

An expression for the number density, N(R) , of the species of interest aver- aged over the range cell AR can be obtained by taking the logarithm of the ratios of the received signal from range R and R + AR for each of the on- and off-line wavelengths, and recalling that the extinction coefficient is given by the product of the number density and the absorption cross section. The solution then is given in Table IV.

Thus, in order to determine the species concentration in a DIAL experiment it is necessary to know the values of the differential atmospheric transmission and backscattering terms and also the magnitude of the differential absorption cross section for the species of interest. While the differential absorption cross section can be measured in laboratory experiments it is much more difficult to determine values of the atmospheric backscatter and extinction. For this reason it is frequently assumed that the spectral dependence of the backscatter and extinction cross sections are small over the wavelength range covered by the on- and off-line wavelengths and hence these terms reduce to zero.

4. DIAL Model

To assess the potential of various experimental arrangements and specifications a simple model has been developed. This model uses the ozone concentration distribution for a mid-latitude, taken from the U.S. Standard Atmosphere (1976). The atmospheric molecular composition and density are also taken from this source. To describe the aerosol density as a function of altitude the composite distribution from Wright et al. (1975) was used, without modifica- tion for cloud or layers of volcanic origin. The Deirmendjian (1969) tabulations

1 14 I. STUART McDERMID

of extinction coefficients and phase functions for the Haze L type aerosol distri-

bution were used as this most closely represents a continental type aerosol.

The absorption and backscatter coefficients, which are fixed in this model,

are given in Table V. The model will accept various different experimental

parameters and then estimate the error in the ozone concentrat ion measure-

ment by solving the lidar equations for the given parameters. It is assumed that

for a nighttime measurement the signal is limited by the shot noise. The error

formulae are then shown in Table VI. The model was used to assess the sensi-

tivity of the experiment to various parameters such as laser energy, laser repeti-

tion rate, receiver efficiencies and apertures, and integration time.

Based on the results and the availability and cost of components , a lidar

system, as described below, was designed. The predictions from the model for

this system are also given later and are based, in part, on actual component

specifications.

TABLE V

Ozone DIAL model fixed constants

Ozone cross sections 308 nm 1.28 )< 10 - t9 c m -2

353 nm 2.00 x t0 -22 cm -2

R a y l e i g h c ros s s e c t i o n s 3 0 8 n m 5 .54 x 10 .27 c m 2 sr -1

353 n m 3.21 • 10 .27 c m 2 sr -1

Aerosol backscatter coefficients 308 nm 1.35 x 10 .5 km -1 sr -1 353 nm 1.10 • 10 .5 km -1 sr -1

A e r o s o l e x t i n c t i o n coe f f i c i en t s

308 n m 5.01 • 10 -4 k m -I

353 n m 4 . 9 0 ;,< 10 -4 k m -I

TABLE Vl

Ozone DIAL error formulae

- Single shot SNR, E z w = S z w

- Error in DIAL ratio, for N pulses,

- E r r o r in o z o n e c o n c e n t r a t i o n m e a s u r e m e n t

[03 ] l [ 0 3 ] - 2 A ~ 1 2 A R

- - -Er,tio A~jz = (oh - ~2) [03]


5. Ozone Lidar Systems

5.1. STRATOSPHERE

A block diagram of the laser transmitter system is shown in Figure 2. This system uses xenon chloride (XeC1) excimer lasers which operate at a fundamental wavelength of 308 nm. This represents the on-line wavelength and the off-line, reference wavelengths, 339 nm and 353 nm, are generated by stimulated Raman scattering (SRS) in high pressure deuterium and hydrogen cells respectively, see Bischel [1983].

For first Stokes shifts such as these, this is a fairly efficient process, with typical conversion of about 50% being achievable, see Falsini et al. (1985), Baranov et al. (1985). Since a free running XeC1 laser produces an output spectrum with two lines it is advantageous to tune the system to give a single laser line. This is done by tuning a low power oscillator with a wavelength selective device (grating, prism or etalon) inside the cavity and then using this tuned output to control the high power amplifiers, see Pacala et al. (1982, 1984). The oscillator-amplifier configuration can also lead to better beam quali- ty which is important for the Raman shifting process and for aligning the laser with the telescope receiver. In its final form the laser system at TMO should produce 1.5 J/pulse at 308 nm and 150 mJ/pulse at 339 nm and 353 nm all simultaneously and at a repetition rate up to 150 Hz. The laser spectral linewidth will be in the range 0.01-0.1 nm with a beam divergence of <0.5 mrad.

X' TUNEOOSC'L ToRt l PREAMPLIFIER

POWER POWER AMPLIFIER AMPLIFIER

POWER POWER AMPLIFIER AMPLIFIER

308 nm =

-\

i

t J 353 n m HYDROGEN SRS

308 n m ,,

339 nm = DEUTERIUM SRS

Fig. 2. Ozone DIAL laser transmitter system.


308 nm 339 nm

IF1 ~IF2

PMT 353 n m

BS1 BS2 IF3 Fig. 3. Ozone DIAL receiver system.

The receiver system is shown in Figure 3. The telescope (Space Optics Re- search Laboratories) has a 90 cm aperture and an effective efficiency, including obscuration by the secondary, of 70%. The different laser wavelengths are separated by long-wave pass dichroic beamsplitters (Technical Optics Ltd) which have a reflectivity of >99% for the selected wavelength and a transmission of about 80% for the longer wavelengths.

The calibration curve for one of these beamsplitters, BS1 in Figure 3, is shown in Figure 4. To further increase the rejection of background radiation and other laser wavelengths, a narrow-bandwidth (0.2 mn FWHM) interference filter (Spectro-Film Inc.) is placed in front of the photomultiplier. The interference f~lters have a transmission of about 40% and the quantum efficiency of the bi-alkali photocathodes (EMI 9883QB) is about 25%. Thus the overall receiver efficiency is on the order of 7% for the first channel, 308 nm, and 6% and 5% for the reference channels, 339 nm and 353 nm.

It is the goal of this project to make ozone concentration profile measurements with a precision of 1% up to 50 km altitude. The lidar model has been used, with the system specifications described above, to predict the performance of the system. Figure 5 shows a plot of the measurement uncertainty as a function of altitude above 35 km and for range elements of 1, 2, and 5 km. For the stratosphere below 35 kin, a precision of 1% should be readily achievable even for range elements less than 1 km. Figure 5 shows that the experiment degrades very rapidly above 40 km. There are three main factors acting to reduce the precision in this range; the atmospheric density is decreasing thus reducing

GROUND-BASED LIDAR AND ATMOSPHERIC STUDIES 1 1 7

i00

338nm

20

308n m

I V I ~ ~ I ~ l I ~ I I

300 400 500 540 Wavelength, nm

Fig. 4. Beam splitter calibration.

6O

O

4o [..

100

80

60

40

20

5 I t

A 4 / j >_3 Z

r162 i , i

~ 2 g

/> /

[

O~ ~< 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

ALTITUDE (kin) Fig. 5. Ozone DIAL uncertainty plot.


TABLE VII

Ozone DIAL anticipated performance

Altitude Precision Range resolution

<35 km 1% 1 km 35-40 km 1% 2 km

2% 1 km 40-45 km 1% 5 km

2% 2 km 45-50 km 2% 5 km

the laser backscattering, the ozone concentration is falling reducing the absorption and hence the difference between the on- and off-line signals, and the range is greater reducing the signal due to the R 2 term. The anticipated performance of the JPL-TMO ozone DIAL system in its final form is summarized in Table VII. Both Figure 5 and Table VII show the performance for a four hour integration time at a repetition rate of 100 Hz, i.e. 1.4 million pulses. The reliability of the laser systems when accumulating this number of pulses on an almost daily basis is still to be determined and some development in this area may be desirable.

5.2. TROPOSPHERE

To probe the tropospheric part of the ozone column it is not necessary to penetrate the ozone maximum, at about 20-25 km, and hence wavelengths with a stronger absorption by ozone can be utilized. Previous studies, e.g. Proffitt (1986), and Megie and Pelon (1984), have shown that the optimum wavelengths for tropospheric measurements are between 280 nm and 295 nm. Due to the aerosol loading in the troposphere the on- and off-line wavelengths must be much closer together, within 5 nm, in order to justify the assumption that the particulate back-scattering cross sections are the same for both wavelengths, see Megie and Pelon (1984).

Some potential lasers and their wavelengths, in nm, for a tropospheric ozone DIAL system are shown in Table VIII. For the JPL-TMO system Raman shifting the fourth harmonic of Nd: YAG has been selected as the optimum approach The on-line wavelength, 288.9 nm, is generated by the first Stokes Raman shift in deuterium and the reference wavelength, 294.2 nm is generated by the first Stokes shift in HD; see Komine (1986). A block diagram of the custom, dual-beam Nd: YAG laser system (Quantel) is shown in Figure 6. This system will produce 1.2 J/pulse in each beam in the fundamental at 1060 nm and will give 100 mJ/pulse/beam in the fourth harmonic at 266 nm and at 10 Hz. Thus, for 50% Raman conversion efficiency the system will provide 50 m J/pulse simultaneously at 294.2 and 288.9 nm. It is planned to use a much smaller

GROUND-BASED LIDAR AND ATMOSPHERIC STUDIES

TABLE VIII

Tropospheric ozone DIAL potential lasers

119

KrF Nd: YAG (IV) XeC1

Fundamental 248 266 308 H 2 (lst stokes) 276.5 299.1

(2nd stokes) 312.4 341.5 HD (lst stokes) 272.3 294.2

(2nd stokes) 301.9 329.0 D 2 (lst stokes) 267.8 288.9

(2nd stokes) 291.1 316.2

aperture telescope, 40 cm diameter, than for the stratospheric measurement . Due to the closeness of the two wavelengths it is not possible to use dichroic beamsplit ters to separate the two wavelengths but they can be isolated with dispersive optics or by dividing the beam and using narrow bandpass line filters.

The model predictions for the t ropospheric ozone DIAL system are shown in Figure 7 for range elements of 1 and 2 km. It can be seen that a precision of 1% should be achievable up to 15 km alti tude for integration periods of 1 hour at 10 Hz, i.e. 36 000 pulses. As for the stratosphere, the uncertainty plots show a sharp rise at higher altitudes. In this case this is due to the severe at tenuation of the laser and backscattered radiation by the ozone maximum. In fact the laser beam is effectively totally absorbed in this region.

YG330-D B

A) Q-SWITCHED OSCILLATOR 7 x 115 Nd:YAG B) PRE-AMPLIFIER 7 x I]5 Nd:YAG C) FINAL AMPLIFIERS 9~ x 115 Nd:YAG D) SECOND HARMONIC GENERATORS E) FOURTH (OR THIRD1 HARMONIC GENERATORS F) WAVELENGTH SEPARATION

Fig. 6. Dual-beam Nd: YAG system schematic.


2.0

1.5

i km

�9 2kin

l o

0 2 4 6 8 10 12 14 16 18 20 ALTITUDE (kin)

Fig. 7. Tropospheric ozone D U A L uncertainty plot.

6. Temperature-Rayleigh Lidar

The pioneering work for temperature lidars was carried out at the Observatoire de Haute Provence by Hauchecorne and Chanin (1980). Since 1981 these researchers have made regular measurements of the atmosphere between 30 and 90 km, see Hauchecorne and Chanin (1982, 1983), Chanin and Hauche- come (1984). In regions of the atmosphere where the aerosol content is negli- gible, i.e. above 30-35 km, the backscattered lidar signal can be assumed to be due to pure Rayleigh scattering from molecules with no contribution from Mie scattering from aerosols. Thus, the lidar signal is proportional to the molecular density of the atmosphere. In order to invert the density profile to give the temperature profile it must be assumed that the atmosphere obeys the ideal gas iaw and is in hydrostatic equilibrium as is stated algebraically in Table IX.

The atmosphere is then divided into layers such that it can be assumed that the temperature and the acceleration due to gravity are constant within a given layer. The pressure at the top of the uppermost layer must be known. The value could be obtained from some independent measurement but it is normally adequate to use a model for the corresponding month and latitude. Expressions can then be written for the pressures at the top and bottom of each layer, P(Ri-t--ARi/2) and P(Ri-ARi/2). The formula for the temperature in the ith layer shown in Table X; the derivation has been given by Chanin and Hauche- come (1984).

The contribution of the uncertainty of the pressure boundary at the top of the


TABLE IX

Ideal gas law, hydrostatic equilibrium

P(R) = ~.%(R) T(R) M

dR(R) =-p(R)g(R) dR

where P(R) = air pressure at range R p(R) = air density at range R T(R) = temperature at range R 3 = universal gas constant g(R) = acceleration due to gravity

TABLE X

Temperature equation

T(Ri)= M g(Ri)AR { P(Ri)g(Ri)AR }

Rlog 1~ P(Ri+ARi/2 )

profile to the uncertainty in the temperature measurement decreases rapidly from the top; even for a 15% uncertainty, the temper ture measurement can be better than 1% at 20 km from the top.

Since the Rayleigh scattering cross section varies inversely with the fourth power of the wavelength, the op t imum wavelength is that which is just long enough to avoid severe at tenuat ion by atmospheric ozone. The second and third harmonic wavelengths of Nd: YAG, 532 and 355 nm, have both been used successfully. As with all lidars the accuracy of the measurement depends on the number of return photons received. For the system at O H P which generates 400 mJ/pulse at 532 nm and 150 mJ/pulse at 355 nm, at 10 Hz, and which uses an 80 cm diameter telescope it has been shown that a temperature measurement to _+ 1 K can be made at 40 km with a 1 km range element in an integration t ime of 1 hr. With the telescope and Nd: YAG system at T M O a factor of up to five increase in the signal level should be expected. This could be used to reduce the integration time significantly while maintaining the same accuracy.

Acknowledgment

The work described in this paper was carried out at the Jet Propulsion Labora- tory, California Institute of Technology, through an agreement with the Na- tional Aeronautics and Space Administrat ion.


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ground-based lidar and atmospheric studies

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