Atmospheric aerosol backscatter measurements using a tunable coherent CO_2 lidar

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  • Atmospheric aerosol backscatter measurements usinga tunable coherent CO2 lidar

    Robert T. Menzies, Michael J. Kavaya, Pierre H. Flamant, and David A. Haner

    Measurements of atmospheric aerosol backscatter coefficients, using a coherent CO2 lidar at 9.25- and 10.6-,um wavelengths, are described. Vertical profiles of the volume backscatter coefficient fl have been mea-sured to a 10-km altitude over the Pasadena, Calif., region. These measurements indicate a wide range ofvariability in 13 both in and above the local boundary layer. Certain profiles also indicate a significant en-hancement in fl at the 9.25-jm wavelength compared with 13 at the 10.6-jim wavelength, which possibly indi-cates a major contribution to the volume backscatter from ammonium sulfate aerosol particles.

    I. IntroductionThe measurement of atmospheric aerosol backscatter

    coefficients at CO2 laser wavelengths has been receivingincreased attention during the past few years. Thereare several reasons for this interest. Quantitativemeasurements of aerosol volume backscatter coeffi-cients in the 10-,um spectral region, for a variety of at-mospheric conditions, are important for the assessmentof the feasibility of various CO2 lidar remote sensingconcepts. The use of the CO2 differential absorptionlidar (DIAL) technique for measurements of atmo-spheric species has been discussed and reported by alarge number of workers, but only a few have success-fully measured range-gated mixing ratios of trace gasspecies using the atmospheric aerosol to provide thebackscatter signal. Measurements of water vapor",2and ozone in an urban photochemical smog3 have beenreported using this technique. Advances in CO2 lasertechnology and in coherent detection techniques shouldprompt more widespread use of the aerosol backscatterDIAL at CO2 laser wavelengths. Coherent CO2 Dopplerlidar can be used to measure wind velocity and turbu-lence for a variety of applications.4'5 The applicationof an earth-orbiting Doppler lidar to the measurementof tropospheric wind fields on a global scale has prob-ably been the major factor in providing the impetus tomeasure atmospheric aerosol backscatter coefficientsrecently. The assessment of the feasibility of this

    The authors are with California Institute of Technology, Jet Pro-pulsion Laboratory, Pasadena, California 91109.

    Received 5 December 1983.0003-6935/84/152510-08$02.00/0. 1984 Optical Society of America.

    technique, which has the potential to improve signifi-cantly forecasting ability by providing data required fornumerical weather prediction from sparsely populatedareas (for example, above the Pacific ocean),6 dependscritically on knowledge of the aerosol volume back-scatter coefficient fl, as a function of altitude at least upto the tropopause at a number of globally distributedlocations. Measurements of aerosol backscatter coef-ficients at various wavelengths in the 9-11-Am regioncan also be used in studies of large aerosol particle for-mation, transport, and removal processes, especiallywhen they are conducted using a mobile lidar, such asa shipborne or airborne instrument.

    Although there is now a large body of informationregarding atmospheric aerosol backscatter coefficientsin the visible, resulting from the use of visible lidars,only a few groups have reported measurements of atCO2 laser wavelengths thus far. Schwiesow et al.measured vertical profiles of : at 10.6 m above east-central Colorado on several days during the winter of1978, using an airborne (side-looking) focused cw lidar. 7These profiles ranged up to 4 km above the surface. Postet al. reported measurements of aerosol backscatterprofiles taken during the spring-summer 1981 period,using a ground-based pulsed CO2 lidar near Boulder,Colo.8 More recently, Post has reported results ofmeasurements taken during 1982 and early 1983, usingthe same lidar system.9 Jones and colleagues at NASAMarshall Space Flight Center have recently reportedlidar measurements of aerosol backscatter coefficients,using an airborne focused cw system, during flights overvarious regions of California.10 [All these backscattermeasurements were made at the 10P(20) CO2 laser lineat 10.6 Aum.] Steinvall et al. have discussed measure-ments of boundary layer backscatter coefficients andhave correlated simultaneous measurements of aerosolbackscatter and extinction using the 1OR(18) and

    2510 APPLIED OPTICS / Vol. 23, No. 15 / 1 August 1984

  • PULSED TEA CO, LASER

    I Ad O 1PTOACOUSTICe A 4-r--4- DETECTOR

    Fig. 1. Coherent TEA CO2 lidar block diagram.

    19R (20) lines near 10.25Aum.1 1 To date, the only dataset which is large enough to permit the application ofstatistical analysis to determine both the mean andrange of variability of : at several altitudes in the tro-posphere is that of the NOAA group over Boulder, Colo.Similar data sets taken over other locations, samplingdifferent tropospheric air masses, would be quite in-formative.

    An experimental study is under way at JPL to de-termine the range of variability of : profiles over thePasadena, Calif., region, using a coherent CO2 lidar attwo wavelengths: the 1OP(20) laser line at 10.6 Am andthe 9R (24) laser line at 9.25 Am. The air masses sam-pled in the free troposphere over Pasadena are oftenrepresentative of maritime air masses, with westerlytrajectories from over the Pacific. This provides anopportunity to compare aerosol backscatter profileswith those obtained over Boulder, Colo., and elsewhere,where the tropospheric air masses have been over con-tinental regions for a few days.

    This is the first series of measurements in which dataare reported at a CO2 laser wavelength near 9 um, andcomparisons are made between : profiles at this wave-length and : profiles at 10.6 Aim. An enhancement in/ has been predicted for wavelengths near 9 Am in cer-tain conditions when ammonium sulfate is a majorconstituent of the aerosol particles.12 13 The laboratorybackscatter measurements of Mudd et al. 14 clearly in-dicate that for pure ammonium sulfate particles thereis nearly an order-of-magnitude increase in volumebackscatter coefficient when going from 10.6 to 9.2,um.For other common aerosol materials, the spectral de-pendence of backscatter is much less. The most recentfeasibility studies of a satellite-borne CO2 Doppler lidarinstrument for measurement of global winds in the

    troposphere assume an operational transmitter wave-length of 9.11 Atm [R(20) of 2C1802 ] and model abackscatter enhancement factor (globally averaged) of1.5 when compared with the backscatter which wouldexist at 10.6 Am.15 Actual measurements of backscatterfrom the tropospheric aerosol, such as those reportedhere, provide a timely test of the validity of such anassumption.

    II. Experimental SystemThe coherent TEA CO2 lidar system which has been

    constructed for measurements of aerosol backscattercoefficient vs altitude is depicted in Fig. 1. This systemis wavelength tunable, which permits measurements ofbackscatter and attenuation at various wavelengths.The TEA laser optical cavity is an unstable resonator,composed of a Littrow-mount reflection grating and agermanium meniscus-convex output coupler. The in-side and outside surfaces are AR coated, with the ex-ception of the central 15-mm square area of the insidesurface, which is uncoated, yielding an intensity re-flection coefficient for normal incidence equal to -40%at 10 ,um. The output coupler is mounted on a PZTstack for cavity length tuning. The TEA laser is in-jection locked to produce tunable single frequencypulses for coherent detection. The injection oscil-lator (cw waveguide laser) and the local oscillator (alow-pressure cw laser in an Invar-rod cavity structure)are both grating tunable, with PZT elements for cavitylength control. The block diagram depicts the front-end injection configuration. The waveguide laser ra-diation is weakly focused and directed through a smallhole in the mirror shown before striking the small el-liptical mirror, which directs it through the centralpatch of the TEA output coupler along the axis of the

    1 August 1984 / Vol. 23, No. 15 / APPLIED OPTICS 2511

    TELESCOPE ASSEMBLY(RECEIVE) (TRANSMIT)

    I t

    PZT

    ALIGNMENTHe-Ne LASERS

    PZT

    INJECTIONOSCILLATORWAVEGUIDE CO2LASER

  • TEA cavity. A portion of the oppositely directed cwlaser radiation, which is partially composed of multiplereflection components, is intercepted by the same smallelliptical mirror and directed to the HgCdTe detectorwith the shutter in front of it. In this manner, the de-tector output displays a fringe pattern when the fre-quency of the injection oscillator is swept through sev-eral TEA laser cavity resonances. In normal operationthe frequency of the injected radiation is optogalvani-cally stabilized with a small FM dither applied to it,16and the TEA laser cavity length is controlled througha servo loop which responds to the dither-synchronoussignal from the detector to maintain a TEA cavity res-onance at the same frequency as that of the injectedradiation. The dither is blanked (and the shutter isclosed) immediately prior to firing the TEA discharge,and then applied again after an adjustable delaytime.

    An optoacoustic detector (OAD) is sometimes in-serted in the path of the injection laser. This providesan alternative method of stabilizing the injection laserfrequency when the OAD cell is filled with CO2 gas ata reduced (50-100-Torr) pressure.17 Ordinarily opto-galvanic stabilization is used by applying the FM ditherto the laser and using a phase synchronous demodulatorto provide an error signal in a feedback loop. The OADwith CO2 provides a dispersion curve when the FM laseroutput is tuned through the CO2 absorption line, andthis dispersion curve can also be used to provide an errorsignal when the injection oscillator frequency departsfrom the CO2 line center frequency.

    The TEA laser pulse is directed into a reflective,off-axis transmit telescope, which has a 15-cm outputdiam. Two large mirrors are used as a periscope to di-rect the expanded beam through a rotatable dome onthe roof of the laboratory. The receiver telescope is a15-cm diam f/18 reflecting Newtonian telescope with asmall central obscuration. The focal plane of thistelescope is imaged onto a cooled HgCdTe photomixerusing a relay lens. The local oscillator is stabilized toa 30-MHz offset frequency with respect to the injectionlaser through the use of the second photomixer and anassociated stabilization loop centered at 30 MHz. Thereceiver which amplifies and filters the output from thelidar signal photomixer is centered at the 30-MHz offsetfrequency.

    The characteristics of the coherent lidar system aresummarized in Table I.Ill. Calibration and Measurement Methodology

    To use a coherent TEA CO2 lidar system to measureatmospheric aerosol backscatter as a function of alti-tude, it is necessary to calibrate the system responsivity.This requires knowledge of what may be called thetelescope overlap function and its dependence on range,i.e., the percentage of transmitted laser radiation at agiven range which is within the field of view (FOV) ofthe receiver, such that backscattered radiation will mixefficiently with the local oscillator at the detector plane.Once the telescope overlap function is known, a cali-brated target can be placed at a known range and the

    Table 1. Summary of CO2 Lidar Characteristics

    Transmitter:TEA CO2 laser, grating tunable, injection-controlledCavity: Positive-branch unstable resonator.

    Pulse energy:

    Pulse width:Repetition rate:Beam diameter:Beam divergence:

    Receiver:Newtonian telescope

    diameter:Photomixer:Receiver:

    High-speed signalaverager:

    Signal processing:

    M = 2.21-3 J, single-mode (depends on

    transition)1-2,jsec0.1 Hz15 cm (after expansion)0.2 mrad (full angle, after beam

    expansion)

    15cm

    HgCdTe, PV30-MHz center frequency, 10-MHz

    bandwidth, 0.1-,sec rise-time lineardetector

    1-,sec minimum sample spacing 12-bit resolution

    HP 1000 model 45 computer

    system responsivity can be measured in an absolutesense. Return signals from hard targets have been usedto characterize the optical efficiency and receiver gainof the coherent lidar system. Targets of flame-sprayedaluminum, sandblasted aluminum, and 400-grit siliconcarbide sandpaper have been studied with the lidar, andlaboratory comparisons with Lambertian flowers ofsulfur surfaces have been conducted to determine theirangular and spectral reflectance properties.18 Basedon information obtained from these studies, it wasconcluded that several candidate target materials wereunsatisfactory for a wavelength tunable lidar calibrationtarget because their reflectance characteristics not onlydepend on wavelength in the 9 -11-,m region but alsoshow some variability in their spectral characteristics,depending on factors involving the manufacturingtechniques which are difficult to control. The prop-erties of sandblasted aluminum, for example, dependnot only on the grit specification but also on the oper-ator's technique during the sandblasting operation.The cause of its spectral variability in the 9-11-m re-gion is thought to be due to the residual embedded sil-icate material. This probelm is alleviated to a largeextent when the overcoat of aluminum is depositedduring the flame-spray process. However, there re-mains some variability even among flame-sprayedaluminum samples, which may be due to the sample sizeand the operator technique. For these reasons, theflame-sprayed aluminum target material which is usedin the field to calibrate lidar responsivity should becompared in the laboratory with a standard such asflowers of sulfur, which can be fabricated with repea-table characteristics with a surface which is Lambertian.Since it is Lambertian, the flowers of sulfur sample re-flectance can be measured using an integrating sphere,and then its biconical or directional-conical reflectanceat any angle can be calculated and used as a standardfor comparison with other surfaces for which the re-flectance in conditions which duplicate the lidar ge-ometry is the desired quantity.18

    2512 APPLIED OPTICS / Vol. 23, No. 15 / 1 August 1984

  • The instantaneous received power from a hard targetat distance R, can be written as

    P8 (t) = PT t -- R .* A. 7 O(R.)

    X exp s2 a j] (1)

    and the instantaneous power received from the back-scatter of a distributed aerosol at distance Rb (assumingthe aerosol backscatter in the vicinity of Rb, over thedistance covered by c/r2, r being the pulse duration, canbe represented by a spatial average ) can be writtenas

    Pb(t) = if, PT(t')dt' 3(Rb) 4c *. 2 O(Rb)X exp 1-2 f ab(r)dr . (2)

    In these equations, P8 (t) and Pb(t) are the receivedpowers for the two cases, PT is the transmitted pulsepower, r is the pulse duration, A is the receiver col-lecting aperture area, l7 is the optical efficiency, O(R)is the telescope overlap function at range R, a, (r) andab(r) are the atmospheric attenuation coefficients atthe wavelength of transmission over the paths to thehard target and to the aerosol volume, respectively, /is the aerosol volume backsca...

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