stratospheric ozone and hydroxyl radical measurements by balloon-borne lidar

Stratospheric ozone and hydroxyl radical measurementsby balloon-borne lidar

W. S. Heaps, T. J. McGee, R. D. Hudson, and L. 0. Caudill

A balloon-borne lidar system for the measurement of ozone and hydroxyl radical in the stratosphere hasbeen constructed and flown by Goddard Space Flight Center. On this flight ozone concentration at alti-tudes between 20 and 37 km were determined with vertical resolution of -0.5 km. In addition, horizontallyresolved ozone measurements with 0.15-km resolution were obtained over a 2-km range. The temporal vari-ation of the hydroxyl radical concentration was measured in the 34-37-km altitude region, ranging from 40parts/trillion shortly after noon to -5 parts/trillion 2 h after sunset. Improvements in the system are pro-posed to increase the sensitivity of the instrument below the parts/trillion level, permitting hydroxyl deter-minations in the 20-30-km altitude range.

1. Introduction

Stratospheric photochemistry has received muchattention in the past decade since the suggestion thatman's activities might decrease the column content ofozone in the stratosphere, permitting biologicallyharmful middle ultraviolet radiation to reach the sur-face of the earth. Ozone may be quite sensitive tochemical perturbation of its concentration because ofthe existence of catalytic cycles which proceed as illus-trated:

X+O33XxO+ 2 X+0 3- XO+0 2XO+O-X +0 2 XO + 03 -X + 202

netO + 03 202 netO3 + 03 - 302

Both reaction chains convert 03 and 0 to 02 withoutconsuming X. Here X may be any of a number ofspecies including chlorine, Cl, which is transported tothe stratosphere in the form of chlorofluorocarbons ornitrogen oxide, NO, which is injected directly into thestratosphere as a component of aircraft exhaust ortransported up from the ground in the form of nitrousoxide, N2 0.

The hydroxyl radical (OH) is among the most sig-nificant species in the stratosphere because of its par-ticipation in a variety of reactions affecting catalyticdestruction of ozone. First, OH can assume the role of

The authors are with NASA Goddard Space Flight Center,Greenbelt, Maryland 20771.

Received 9 February 1982.

X in the two cycles described above. Second, OH in-creases the amount of free chlorine atoms present in thestratosphere by reacting with the reservoir moleculeHCl via OH + HCl - H20 + Cl. Therefore, the greaterthe hydroxyl concentration, the more serious the threatto 03 posed by chlorofluorocarbons. Finally, OH reactswith NO2 to form the reservoir nitric acid HNO3 via OH+ NO2 + M - HNO3 + M. Thus a larger concentra-tion of stratospheric OH implies a lower risk of 03 de-struction by nitrogen oxides.

Hydroxyl radical concentration has been measuredseveral times in the troposphere and at altitudes from30 to 70 km along with extensive determinations of totalcolumn abundance.1 -9 The stratospheric values de-termined using balloon-borne resonance fluorescenceinstruments range from -2.5 X 106 at 30 km for an 80°solar zenith angle to -4 X 107 at 38 km at a 410 solarzenith angle. No measurements exist in the 15-30-kmaltitude interval. Recent changes in rate constants aswell as indirect experimental observations have indi-cated that hydroxyl radical concentrations in this in-terval are perhaps five times lower than previously es-timated.10 11 If true this would significantly impact theozone column depletion calculations since the ozonemaximum lies in this altitude range. Experimentalconfirmation of this reduced OH level has a high pri-ority.

For stratospheric concentration measurements lidarhas a number of attractive features. The ability torange provides several unique experimental opportu-nities: (1) it permits the measurement to be made inair that is well-removed from the experimental plat-form, greatly reducing the possiblity of self-contami-nation; (2) it permits an investigation of horizontalstructure in species concentration; (3) it allows simul-

15 June 1982 / Vol. 21, No. 12 / APPLIED OPTICS 2265

taneous profile determinations to be made; and (4) itallows measurements to be made at altitudes greaterthan those obtainable by the balloon platform. In ad-dition, lidar has the ability to make full diurnal mea-surements because it is not dependent on sunlight as aspectral source. It has also been pointed out recentlythat fluorescence lidar can be used to obtain remotepressure and temperature data.12

The balloon lidar system made use of two lidartechniques for the measurement of ozone and the hy-droxyl radical. Ozone was measured using differentialabsorption lidar (frequently called DIAL), and the hy-droxyl radical was measured using remote laser-inducedfluorescence. In the DIAL technique two wavelengthsare transmitted simultaneously, or nearly so. Thewavelengths are chosen so that one is absorbed stronglyby the species of interest, while the second frequencyis relatively unattenuated. By measuring the relativestrengths of the return due to Rayleigh scattering of thebeams at the two wavelengths, one can infer speciesconcentration along the optical path. By timing thereturns, the profile of the species concentration alongthe optical path can also be derived.

In this experiment the differential lidar equation canbe written in the following form:

C = InaeA/4ir 3' O(z) exp(-2?[0 3 ])/Z 2 dzI,

where I is the laser flux,n is the number density of scatterers,o. is the scattering cross section,e is the photon detection efficiency,

A is the area of receiver telescope,O(z) is the overlap between laser beam and tele-

scope field of view, [(z) = 1.00 for z > 1km],

a' is the ozone absorption coefficient,[03] is the ozone number density, and

Z is the range.If the signals from two different ranges are ratioed, theconstants in front of the integral cancel out. (In someapplications n is not a constant and may vary with Zrapidly enough to require inclusion inside the integral.)This cancellation of instrumental factors reduces thesystematic errors in the technique, holding promise forconcentration determinations of great accuracy.

Remote laser-induced fluorescence makes use of atransmitted wavelength which is absorbed by thespecies of interest, giving rise to emission at a longerwavelength. By measuring the strength of the fluo-rescence signal relative to the transmitted signal thespecies concentration can be derived. A refinement onthe technique involves tuning the transmitted wave-length on and off an absorption line of the measuredspecies. The difference in the signals, when the laserline is on and off the absorption line, eliminates inter-ference caused by molecules with broadband absorptionwhich also fluoresce in the bandpass of the lidar de-tector.

The lidar equation for remote laser-induced fluo-rescence can be written as

C = InanJeA/4ir O(z) exp[-(a 1 + a 2)Z]/Z2dz,

where I is the laser flux,n is the hydroxyl number density [OH],o- is the hydroxyl absorption coefficient,q is the hydroxyl fluorescence efficiency,c is the photon detection efficiency,

A is the telescope area,O(z) is the overlap between laser beam and the

telescope field of view,a1 is the atmospheric attenuation coefficient

at the transmitted wavelength, anda2 is the atmospheric attenuation coefficient

at the fluorescence wavelength.The major contribution to a, and 2 is ozone absorp-tion, and once the ozone concentration is known theycan be readily evaluated. ij depends on the degree ofatmospheric quenching. It may be calculated usingmeasured quenching coefficients if temperature andpressure are known. O(z) has the functional form kZ2

for z < 300 m, where the overlap coefficient k can bedetermined from the 282-nm returns. Thus fluores-cence lidar data inversion consists simply of evaluatingall the terms in the equation and solving for n.

11. Description of the Instrument

The overall instrument is quite complex and can bedivided into five systems: (1) structure, (2) power andtelemetry, (3) pointing, (4) thermal, and (5) the idar(i.e., the laser transmitter and detector). Each will beconsidered in turn.

A. Structure

The gondola which supported the lidar system was,for the most part, constructed of aluminum (Fig. 1).The outer framework formed a cube -2 m on a side.The girder base and the front face of the cube had acontinuous opening -45 cm wide to allow an unob-structed view for the lidar for elevations from straightdown to 600 up. (Elevation angles >60° would beginto view the base of the balloon.) The framework sup-ported two radiator panels on the front (each side of the45-cm slot) and one on the rear (Fig. 2). Four steel ca-bles, one from each upper corner of the cube, were at-tached to a central fitting suspended from an eyelet onthe lower portion of the azimuth drive motor. Thebaseplate of the cube supported two interior structures.At the rear of the gondola were shelves -30 cm deep and1.2 m high which housed the batteries, telemetry, anda Dasibi ozone analyzer.13 The rest of the base platformwas occupied by a support frame for the gimbal onwhich the instrument compartment could rotate in el-evation.

The lidar system was mounted on three shelves sup-ported in a 1.1-m aluminum box girder cube. This cubein turn was supported up by a magnesium girth ringwhich surrounded the cube in its vertical lateral plane.Aluminum pressure-tight domes slid over the front andrear of the cube and bolted to the girth ring. Thesejoints were sealed with 0 rings. Steel fittings at thecenter of the girth rings mated with the gimbal frame

2266 APPLIED OPTICS / Vol. 21, No. 12 / 15 June 1982

Fig. 1. Frontal view of the gondola showing the overall cubic struc-ture. The front radiator panels are prominant, with the viewing slotfor the telescope. The pressure domes have been left off the pack-

age, permitting viewing of the interior of the experiment.

Fig. 2. Radiator panels have been pulled back to yield a better viewof the interior. The front pressure dome is shown in the fore-

ground.

permitting free rotation about the horizontal lateralaxis. Cooling fluid lines passed through a rotating fit-ting inside one of the supports. Electrical connectionswere made through the girth ring at several points oneach side near the axis of rotation.

B. Power and Telemetry

Power was supplied by batteries (Fig. 3). Lithiumbatteries were chosen to operate the system electronics

Fig. 3. View from the right rear quarter showing the rear radiatorpanel, the battery and telemetry shelf, and part of the rear pressure

dome.

and auxiliary systems because of their high ratio ofstored energy to weight. The high-peak current re-quirements of the laser itself could not be guaranteedby the supplier of lithium batteries, so silver cells wereused for the laser alone. Dc-dc converters provided theregulated voltages for the system components (5, ±15,±28 V). Total system capacity for a 30-h durationflight (-% duty cycle) was 1075 A h, of which 325 A h were needed for the laser.

The command and telemetry equipment was pro-vided by the Goddard Sounding Rocket Division.'4The telemetry used a 50-kbit pulse code modulation(PCM) system. The system collected and transmitteda frame of 512 10-bit (9-bit signal plus parity) wordsnominally ten times/sec. The laser was synchronizedto this pulse repetition rate. For each laser shot 390words were available to transmit lidar returns. The restof the words were used for housekeeping and systemdata (temperatures, battery voltages, on-off status ofpumps, fans, etc.). The command system permitted theclosing or opening of thirty-two relays on board thegondola. These were used for turning the laser elec-tronics on and off, overriding thermal systems, tuningthe laser, and selecting the lidar operating program andthe data sets returned to the ground.

C. Pointing

The system could be pointed in both azimuth andelevation to permit ranging in any desired direction and


to permit minimization of scattered solar flux. Eleva-tion pointing was provided by a harmonic drive systemattached to the instrument girth ring at one of thegimbals. A shaft encoder on the opposite side regis-tered elevation angle, which was transmitted to theground via the PCM system. Control signals for theharmonic drive were generated by a computer on theground and telemetered back to the gondola. Similarlythe azimuth was controlled by rotating the entire ex-perimental package. Azimuth angle was sensed by anonboard magnetometer. A computer on the groundusing the magnetometer data generated control signalsfor the azimuth drive which were transmitted back upto the gondola. The overall pointing accuracy of a fewdegrees was more than adequate for the purposes of thelidar.

D. Thermal Control

One of the most complex aspects of the experimentengineering was the thermal design for the lidar systemoperating in the stratosphere. The design of strato-spheric probes for diurnal operation is made difficultbecause of the great contrast in the environment be-tween day and night. The lidar system is also uniqueamong stratospheric instruments in its requirementsfor power dissipation, for when running the laser con-sumes -1 kW of power of which the transmitted laserpower amounts to only -0.01 W. The rest of the system(electronics, pumps, etc.) generates another 0.5-1.0 kWof heat. The problem is further complicated by theextreme sensitivity of certain components of the systemto temperature. The frequency-doubling and summingcrystals, of which there are four in the system, must bemaintained at constant temperature to within -0.1 Kfor proper operation. Also the dye laser etalon, whichis the ultimate wavelength and bandpass controllingelement of the system, changes frequency with tem-perature.

The thermal control system has not performed ideallyon either of the two flights to date. On the first flightlaser compartment temperatures were too cool, and nouseful geophysical data were obtained; on the secondflight the same area became too warm. Fortunately,extensive thermal instrumentation on the gondola hasyielded a reasonable understanding of the system'sshortcomings, and the ultimate solution appears athand. The original design involved a combination ofactive and passive techniques under automatic andground control to permit adaptation to any operatingcondition. Each sensitive element is mounted in itsown insulated housing with a regulated heating elementand set to operate above the ambient temperature. Theexperiment housing was physically divided into twoseparate compartments insulated from one another.The upper compartment contained the laser head andthe optical components. The lower compartmentcontained the laser electronics and system powersupplies. The bulk of the heat is generated in this lowercompartment. Two separate thermostatically con-trolled cooling loops were provided. For the electronicsa liquid-air heat exchanger in the lower compartment

with two pumps and two fans was used to transport heatto an external radiator at the rear of the gondola. Onepump and one fan alone were sufficient to providecooling at the anticipated rate of dissipation, but theadditional pump and fan provided redundancy andadditional cooling if an exceptional condition were en-countered. The external radiators were flat aluminumplates painted white and insulated on the back.Channels within the plates permitted the flow of cool-ant. Total radiation area provided for the electronicswas -5 m2. A similar system provided cooling for thelaser except that a liquid-liquid heat exchanger wasused to transfer heat from the laser head. The radiatorpanels were provided with resistance heaters on theirrear surfaces to prevent freezing of the coolant duringshutdown periods at night. Resistance heaters and fanswere also provided for the upper compartment but werenot used during the first two flights.

In addition to the active thermal regulation systemsdescribed above, the entire experiment chamber wascovered with 2.5-5.0 cm of white foam insulation tominimize the thermal conductance to and from theexperiment. The batteries, telemetry, commandequipment, and Dasibi ozone analyzer were separatelyinsulated by a rigid foam housing over the entireequipment shelf.

A modification which will be incorporated into thenext system is an active cooling system for the upper(laser) compartment including an air-liquid heat ex-changer and fans. As with the other cooling circuits thiswill be under automatic control, with manual overridefrom the ground if required.

E. Laser Transmitter and ReceiverThe lidar system (Fig. 4) consists of three subsystems:

the source, the detector, and the data-handling system.Each shall be described in turn.

Fig. 4. Schematic shows the components of the lidar system alone.Details of the layout in the actual experiment varied slightly from this

figure.


TO ATMOSPHERE

Fig. 5. Schematic shows the layout of the laser systems and transmitoptics. All wavelengths are in microns.

Two transmission wavelengths were required (see Fig.5). The first was a narrowband tunable source of282-nm radiation to excite the hydroxyl radical fluo-rescence. The second was a near-ultraviolet wavelengthof any bandwidth that was not strongly absorbed byozone. A frequency-doubled tunable dye laser was usedto generate the 282 nm. Light from a commercialNd:YAG laser (400 mJ at 1.06 gm in a 15-nsec pulse and10 pulses/sec) was frequency-doubled in a CD*A crystal.The second harmonic was separated from the uncon-verted fundamental 1.06-,um radiation with a dichroicmirror and directed onto a polarization beam splitter.This device used a halfwave plate to rotate the plane ofpolarization of the light and polarizer to split the obliquepolarization into horizontal and vertical components.The horizontal component was reflected from a foldingmirror in the oscillator dye cavity and pumped a dye cellfilled with flowing Rh 6G dye dissolved in ethanolslightly off the axis of the dye oscillator cavity. A pairof prisms behind the dye cell expanded the dye outputhorizontally before it passed through a Fabry-Perotetalon and illuminated a Littrow-mounted gratingwhich served as the rear reflector of the dye laser cavity.The Fabry-Perot was supported in a motorized mountso that the laser could be tuned a few tenths of a nano-meter. The output from the dye laser (564 nm) passedthrough a partial reflector on the other side of thefolding mirror. Meanwhile, the vertically polarizedcomponent of the second harmonic passed through aquarterwave plate, was reflected off a total reflector, andthen passed back through the quarterwave plate. Thisrotated the polarization of the beam with the result thatit now could pass directly through the first polarizer.This beam and the dye laser output beam were co-aligned with a dichroic mirror and directed to the am-plifier dye cell. The resultant dye laser output wassteered to a KDP frequency-doubling crystal whichconverted -10% of the radiation to 282 nm.

For the second wavelength, needed to measure ozone,355 nm was chosen because of the ease with which it

could be produced. The unconverted 1.06-tim radia-tion from the Nd:YAG laser, after separation from thesecond harmonic with the dichroic mirror, passedthrough another CD*A frequency doubler. The outputfrom this doubler (second harmonic and fundamental)was then summed in a KD*P crystal. The resultantoutput wavelength was 355 nm.

A small fraction of the 282-nm radiation is split offfrom the main beam and sent through a cell in whichOH was produced by the photolysis of water with axenon lamp. The etalon could then be fine tuned tomaximize the fluorescence signal from the cell. Thisensured that the dye laser was centered on an OH ab-sorption line and, similarly, that the laser was off theline when purposely tuned away. Known fractions of282 and 355 nm were split off from the main beams andmeasured for the purpose of calibrating the lidar outputintensity. Finally, the two beams were combined usinga dichroic mirror, expanded by a telescope, and trans-mitted along the axis of a Cassegrain telescope whichserved as the collector for the receiver subsystem.

The receiver was composed of the telescope, amonochromator, and the photomultiplier tubes. Thetelescope had a 30-cm diam and an effective speed off/15. Light gathered by the telescope was reflected 900through a hole in the barrel by a 45° flat located be-tween the primary and secondary mirrors. It was thenfocused by an antireflection coated quartz lens on apinhole aperture which restricted the field of view to 1mrad and defined the entrance aperture of the mono-chromator. The focal length and location of the lenswere chosen such that the f/No. of the telescope wasmatched to that of the monochromator. A 1-mm thickSchott UG-1 filter was used to reduce the amount ofscattered visible light in the system.

The monochromator was a 1/4-m Jarrell-Ash with an1180-grooves/mm grating blazed for 3000 A. Three exitslits, 1, 3, and 1 mm wide, were located at the focal planeof the monochromator and situated so that the threepassbands were 280.5-283.5, 305-315, and 353.3-356.6nm. Solid quartz light pipes directed the light fromeach slit to its respective EMI 9814QB photomultipliertube. Overall detection sensitivity of the receivers was-0.15%.

The system electronics were assembled using Camachardware. An onboard LSI-11 computer controlled theoperations during the flight. Several different opera-tional scenarios were available and could be selectedfrom the ground. Because of the large dynamic rangebetween day and night operations, the signal-processingequipment for each of the three detected wavelengthswas of a dual nature. For low-background low-signalregimes a photon counting system was used. In high-background situations fast transient digitizers were usedto convert the analog signals from the photomultipliertubes to digital words.

Ill. Flight History

The lidar flight, designated 1224-P by the NationalScientific Balloon Facility, began at 1035 CDT on 20Oct. 1980 from Palestine, Tex. The balloon volume was


28.05 million cubic ft, and the payload weight was 2337kg (5152 lb), including 567 kg (1250 lb) of steel-shotballast. The balloon reached 65,000 ft (-20 km) at 1204CDT, and the laser was turned on at 1208 CDT. Initiallaser energy and coolant flow rates were excellent. Thefluorescence signal from the hydroxyl cell was strong.The original schedule called for a 33% duty cycle for thelaser, 20 min. on and 40 min. off. However, because ofthe extreme desirability of obtaining measurements inthe 20-30-km region the laser was permitted to runcontinuously. The ascent was unexpectedly slow,however, apparently due to cirrus clouds beneath theballoon which decreased the radiative heating and,therefore, the lift of the balloon. After 1 hr of contin-uous operation, at which time the balloon had ascendedto 27.4 km (90,000 ft), the laser was shut down to coolthe compartment, although all temperatures within thegondola were well within the comfortable range.

At 1340 CDT the laser was restarted. The altitudewas 33.5 km (110,000 ft). Again all energies werenominal. Upper gondola compartment temperatureswere higher than previous values, however. The laserwas shut down at 1410 CDT at an altitude of 36.3 km(119,200 ft).

The third measurement interval was begun at 1450CDT at a float altitude of 36 km (119,000 ft). All sys-tems appeared to be operating properly. Gondolaheating continued, however. The laser was turned offat 1512 CDT.

At 1608 CDT the laser was turned on for a fourthmeasurement period. Laser energy levels did not comeup to previous levels, however, and signals from the OHwavelength reference cell stayed below normal operat-ing threshold. Examination of housekeeping data in-dicated that ambient temperatures within the lasercompartment had risen above the doubling-crystalcontrol temperatures. This was in sharp contrast withexperience on a previous flight, at which time the uppercompartment temperatures had remained excessivelycool. At 1623 CDT the laser was shut down without anymeasurements having been taken. Solar heating ap-peared to be a more significant source of the overheatingproblem than laser operation based on the temperaturehistory of the gondola. It was decided to rotate thepackage to place the gondola in the shadow of the rearradiator panel in an attempt to accelerate the cooling.At -1640 CDT it was determined that neither the azi-muth nor the elevation drive system was functioning.In its uncontrolled rotation we observed the receiver topoint very nearly toward the sun. Apparently nodamage was suffered by the receiver at this time. Sinceobservation of the sunset behavior of OH was a primegoal the system was restarted at 1736 CDT, althoughinterior temperatures had not returned to desired levels.Laser operation did not appear to contribute signifi-cantly to heating, so the laser was left on. At 1803 CDTaltitude was 35.7 km (117,000 ft). Local sunset on theballoon occurred at -1830 CDT. The laser continuedto run in the data-taking mode with laser energy aboutone-fifth of the morning values. At 1909 CDT altituderead 35 km (115,000 ft). At 1928 CDT we shut down

the laser after almost 2-h continuous operation at lowoutput power. The upper compartment had begun tocool after sunset even with the laser running.

By 2000 CDT temperatures were within acceptablelevels. The laser was restarted at 2004 CDT, but thelaser energy remained low and no signal appeared in thehydroxyl reference cell. Because we had been informedthat the flight path had turned toward the Gulf ofMexico and would, therefore, have to be terminatedwithin a few hours, the laser was permitted to run con-tinuously. The telemetry experienced occasional in-terruptions from 2100 CDT on. Laser energy climbedsteadily, and occasionally signals were observed fromthe OH-wavelength reference cell. Pressure had re-mained relatively stable within the gondola throughoutthe flight. This was taken as a strong indicator that nofrequency drift had occurred within the dye laser de-spite the fact that laser power was too low to generatereference signals. At 2127 CDT we were warned of theimminent need to terminate the flight, so all systemswere shut down. The termination command was sentby NSBF at 2130 CDT. It was immediately obvious tothe NSBF operators that the payload had fallen. Thenext day it was learned that the gondola had crashed ina field outside Bogalusa, Louisiana. It had burned onimpact and was essentially destroyed. Failure inves-tigations by NSBF and Goddard concluded that thegondola had experienced the shock of the parachuteopening, with an asymmetric load on the cables,snapping one of them and resulting in subsequentseparations of the suspension at the other three cornersof the gondola.

IV. Discussion of Results

A. Ozone ConcentrationAlthough the balloon lidar was capable of making a

dual-wavelength DIAL measurement, our data showed,as expected, that the stratosphere was largely uniformin the density of backscatterers for horizontal propa-gation of the laser beams. This meant that ozone pro-files could be obtained using only the signals at 282 nm.The results of the analysis are presented in Figs. 6-9.Figure 6 shows the ozone profile determined during theascent. The balloon lidar results are plotted as crosseswith the horizontal crossbar extending 2 on either sideof the measured value. The averaging period for eachmeasurement was 6-7 min, during which time the bal-loon ascended of the order of 500-700 m. Each pointis plotted at the midpoint of the corresponding altituderange. The solid line is a plot of the value obtained bya Dasibi in situ ozone analyzer located aboard thegondola. The agreement between the two instrumentsis good, although there does appear to be a slight dis-crepancy in the 26-28-km region. The laser was shutdown during the ascent between 28 and 34 km to permitcooling.

Figure 7 shows the lidar and Dasibi results obtainedduring the descent phase of the flight. Note that thehorizontal scale has been expanded 2.5 times with re-spect to Fig. 8. Again the agreement is reasonably good,


ALT

TUDE

Kri1

BALLOON LIDAR OZO1NE ASCENT DATA

ALT

T

DE

KM

.2OZONE CONCENTRATION

Fig. 6. Ozone profile measured by the lidar and the Dasibi on ascent.The concentration scale is linear. The error bars represent two

standard deviations.

RANGE RESOLVED OZONE RETURNSALTITUDE IS 22.5 TO 23.0 KM

lEl

C0NCENTRATI0N

2250RANGE (METERS)

Fig. 8. Range-resolved ozone measurement. The local time is 1222CDT. Azimuth ranged from 78.80 to 98.40. Error bars represent two

standard deviations.

with the lidar showing some tendency to obtain a lowerresult than the in situ instrument. Dumping of ballastat 33.6 km resulted in a renewed ascent back to 34.6 kmat the end of this data set. This is visible on the in situinstrument profile only as a slight thickening of thelower portion of the plot attesting to the excellent re-producibility of the Dasibi instrument. The lidar val-ues also exhibit reproducibility here well within its ex-perimental error bars.

Figure 8 presents a range-resolved ozone profile takenon the ascent at an altitude of 22.5 km. Ozone con-centration is presented at 150-m intervals out to a dis-tance of 2 km from the gondola. The horizontal lineacross the plot is the weighted average ozone concen-tration from 900 to 2250 m. The error bars shownrepresent two standard deviations. The error bar at 150m represents two standard deviations in the weightedaverage. The gondola ascended -0.6 km during themeasurement period, which was of the order of 6 minlong. At the same time the azimuth angle varied in an

OZONE CONCENTRATION

Fig. 7. Ozone profile measured on descent. Note that the concen-tration range is 2.5 times less than on the previous slide, and the

vertical scale is reduced.

RANGE RESOLUED OZONE RETURNSALTITUDE IS 36.6 KM

IEl3i

C0NCE

TRATI0N

T I. . . 5 8 . . . . .0 750 1500 2250

RANGE (METERS>

Fig. 9. Range-resolved ozone measurement. The local time is 1400CDT. Azimuth ranged from 90.00 to 112.40.

arc 10-20° wide. The results of these motions are suchthat the shape of the measurement volume for eachrange cell is complicated. Each is a right cyclinder, witha base defined by two concentric arcs and the perpen-diculars joining their ends. The arcs are 150 m apart,and the height of the cylinder is -600 m. The variationof the 03 concentration in the horizontal direction issomewhat surprising. Although considerable verticalstructure has been seen in 03 profiles by in situ in-struments, a horizontal structure of such a magnitudeis not expected.1 5'16 Indeed it is difficult to imagine amorphology of the ozone concentration field that wouldyield such a structure.

In Fig. 9 we see another range-resolved profile takenat an altitude of 36.5 km. Here the horizontal structureis much less pronounced. Generally the profiles in the20-27-km altitude region exhibit structure and otheraltitudes do not. We believe that the fact that thevertical structure observed by others also exhibits thisaltitude behavior is strong evidence that this severesmall-scale horizontal structure is real.


-. 1, + II

BALLOON LIDAR OZONE DESCENT DATA

+ I t-r It0

B. Hydroxyl Radical Concentration

The signal detected when the laser pulse is propa-gated is composed of three parts. First, the photonsarising from fluorescence by the hydroxyl radical; sec-ond, the photons generated by other species whichfluoresce in the receiver bandpass; and third, solarphotons scattered into the field of view of the receiverby the atmosphere. We can represent these by thesimple expression C = F + I + B, with C for counts, Ffor fluorescence, I for interfering fluorescence, and Bfor background. Information about the hydroxyl rad-ical concentration is contained in the F component only,which must be extracted from the total count C beforefurther analysis is possible.

Data were collected at the fluorescence wavelengthfor 300 psec after the laser pulse. At the observed levelsof signal there was a negligible probability of detectingfluorescence photons beyond -30 usec; thus the pho-tons detected from 30 to 300 sec on each laser shotconstituted a measurement of B.

The magnitude of the interfering fluorescence I wasdetermined by noting the excess counts above thebackground B detected when the laser was tuned off thehydroxyl absorption line. It is assumed that any in-terfering species absorbing at 282 nm and fluorescingat 305-315 nm would have a relatively flat absorptionspectrum which would exhibit no significant change inmagnitude over the few thousandths of nanometers thatthe laser was tuned to go off the OH line. Thus thesignal return can be represented as C = I + B.

An average value of F was determined by averagingcounts for on- and off-line laser pulses in some shorttime interval (typically 300 shots on-line, 100 shots off).The background average B is determined for each in-terval and subtracted as indicated:

P + 7 = - (on-line),

7 = - (off-line).

Both I and F are proportional to the average trans-mitted laser power, which is not usually the same forboth data intervals. Thus the off-line value of isscaled by the ratio of laser power on-line to off-line andthen subtracted from the on-line count to yield P.Values of F determined in this fashion are then aver-aged (weighted for laser power) over the whole data-taking interval, which ranged from 10 to 30 min.

During daytime measurements B was the dominantcomponent of C, and there was a relatively large un-certainty in the determination of F arising from thedifferencing of two large numbers. For nighttimemeasurements B was negligible, and the uncertainty inF arises simply from counting statistics. In both in-stances, however, the statistical SNR was consistently>1 only within the first 2 usec following the laser shot.The values of F for these 2 sec were individually pro-cessed by the techniques described above to yield avalue of the hydroxyl concentration and associateduncertainty for the 15-150-m range interval and the150-300-m interval. These values, weighted by theirrespective uncertainties, were then averaged to generatethe concentration measurements presented here.

BALLOON LIDAR HYDROXYL DATA

I EHVDR0

LIEl

C0NC

lE51 . 1 1208 1800 2400

LOCAL TIME (CDT)Fig. 10. Diurnal variation of hydroxyl concentration. Altitude is34-36 km. Error bars shown are one standard deviation due tocounting statistics. The width of each bar represents the temporal

averaging period.

Figure 10 shows the measured hydroxyl radical con-centration vs local time (CDT). The error bars shownare for statistical uncertainty and represent one stan-dard deviation, hence the data at 1400 and 1850 areupper limits only. The width of each point representsthe time interval over which data were averaged to ob-tain the measurements.

Estimates of the systematic error due to each aspectof the system calibration are as follows:

Photon flux, I ±50%Telescope area, A <1%Fluorescence efficiency, a +10%Absorption cross section, cr +25%Ground state population, :3 +5%310-nm detector efficiency, 3 1 0 +5%Range, Z <1%Ozone attenuation, exp(-axZ) 1%Overlap coefficient, k ±15%The daytime values shown in Fig. 10 are two to three

times lower than expected from 1-D model calculationsof hydroxyl concentration and three times lower thanAnderson's measurements of 14 July and 20 Sept. 1977.The postsunset measurements, on the other hand, aresomewhat higher than would be expected from astraightforward modeling of OH-HO 2 chemistry.

The low OH measurements could be caused by anyof the following instrumental phenomena: (1) on-linedata taken off the peak of the absorption line; (2) off-line data taken in the wings of the absorption line; (3)laser flux saturating the transition; (4) overlap functionless than the calibration value; (5) laser energy less thanthe monitor indicates; and (6) frost on the windows ofthe system which reduce both transmitted laser energyand the photon detector efficiency 3 10 -

Errors (1) and (2) can be eliminated by using the OHreference cell to tune the laser from the ground if itshould drift off the line center. Saturation is eliminatedby expanding the laser beam to reduce energy densityand by using short pulses to preclude the formation oflongitudinal modes in the dye laser. The overlapfunction is measured continuously using the 282-nmRayleigh scattering return.


Errors (5) and (6) remain as possibilities. The precisemeasurement of UV laser light is difficult because thehigh flux of energetic photons is frequently able tomodify the response of the detector. Postflight cali-bration of the sensor is very desirable but obviously notpossible in this case. Frosting of the window should bedetectable as a decrease in the apparent solar back-ground as the window transmission decreases. Watervapor in the gondola was removed prior to launch bycirculating the interior air over a liquid nitrogen cooledheat exchanger. However, there was a rapid 20% de-crease in solar flux -45 min into the flight which mayindicate moisture on the window. This too could havebeen confirmed if the package had been recovered in-tact. The decrease in solar flux that was observed,however, was too small to account for the low observa-tions.

Laser-produced OH can cause a high measurementof hydroxyl.1 7-2 0 This OH is produced by the followingmechanism:

03 + hvlaser 0 02 + O(1 D),

O('D) + H20 - OH* + OH*,

OH* +M-OH+M.

In the troposphere, where H20 and 03 concentrationsare high and the chemistry is rapid, this process is verysignificant, and the second half of a single laser pulsedetects OH produced by the first half of the pulse. At35 km the total OH yield in a single laser pulse is cal-culated to be <105 hydroxyl radicals/cm3, and thenumber thermalized within 10 nsec is even smaller. Foran ozone concentration of 1 X 1012 molecules/km 3 theyield of O(1D) for a millijoule laser pulse is 4.2 X 109/mJ/cm path length. The fraction converted to OH at35 km is -9 X 10-5, giving a total OH yield of 3.8 X105/mJ/cm of path length. The laser beam has an ini-tial diameter of 5 cm and a divergence of 0.3 mrad.Thus the 3.8 X 105 hydroxyls/cm are spread over a20-cm2 column, yielding a single-shot concentrationincrement of 1.9 X 104 cm- 3 .

If the laser is fired repeatedly into the same columnof atmosphere, the concentration of OH would build upuntil the loss balanced the production. For a pulse of0.05 mJ/cm 2 the steady-state concentration of OH couldbe as high as 1.4 X 107 /cm3 ; however, this requires over500 pulses into the same volume of air. Since this vol-ume is only 10-7 sr in angular extent, the probability ofholding the gondola steady for this period of time is verysmall. Wind shear could also be expected to signifi-cantly disrupt this process.21

Since the attitude control system was not functioning,some rocking and rotation about the vertical axis wasexpected. If the gondola rotates once per hour (6280mrad/hour), two adjacent laser shots would be 0.17mrad apart, thus two adjacent shots would overlap<50%, and alternate shots would not overlap at all. Ifthe gondola is also rocking and rotating, the overlapwould be even less. Thus it does not appear thatlaser-produced OH was a problem in this experiment.

V. Conclusions

We believe it has been demonstrated that the lidartechnique is effective both for vertical profiling and forinvestigating small-scale structure in stratosphericozone. Long-term monitoring of ozone by satellite in-struments would be significantly enhanced by im-provements in ground-truth instrumentation. Theranging capability of lidar should permit such mea-surements to be made on a geographic scale more nearlycompatible with the FOV of the satellite instruments.When more detailed investigations of ozone are un-dertaken using lidar, performance can be improved bythe selection of different operating wavelengths.Maximum SNR is obtained when the e-folding range(two-way) lies in the range interval over which theconcentration is being determined. A multiple-wave-length system would generate the best possible range-resolved concentration determination. Since the ulti-mate accuracy of the concentration determination isdependent only on the accuracy of the range determi-nation (all other instrument parameters cancel out),greater attention could be given to the timing accuracyof the system. Although the timing errors on the sys-tem described in this paper were only a few tenths of apercent, faster electronics which yield smaller timingerrors are available.

Lidar has also been shown capable of measuringstratospheric hydroxyl radicals with considerable sen-sitivity. There are several areas in which slight changesin the system could greatly improve the performancewith respect to sensitivity, accuracy, or both. Im-provements in the ability to measure high-power ul-traviolet radiation would greatly reduce systematicerrors. Higher laser power results directly in largersignals (provided that problems with saturation andlaser-produced hydroxyl are avoided).

Improvements in the design parameters of the in-strument should be concentrated in two areas: in-creasing the photon detection sensitivity, and reducingthe uncertainty due to solar background. The value ofE31 0 on the last flight was -0.0015. The quartz lightpipes used to direct the monochromator output to thePM tubes appear to have been the least efficient com-ponents. The detection system is presently being re-designed using dichroic mirrors to separate the wave-lengths. Because of the higher innate efficiency of thesemirrors compared to the grating and the simpler opticalpath with fewer elements, the theoretical efficiency ofthis system should exceed 10%. If this were achieved,there would be almost a factor of 100 improvement insignal and a tenfold improvement in SNR.

Several techniques are available for reducing theuncertainty due to solar background. The simplest islonger integration time in the determination of B.Since the first step in solvingfor the fluorescence signalF is subtracting the average B from the total count, theuncertainty in F can be reduced considerably by mea-suring B sufficiently well that its uncertainty is negli-gible. This can be achieved simply by increasing thecounting period after each laser shot from 300 to -105usec, the total time available between shots. This


would reduce the uncertainty in B by more than a factorof 50. B itself can also be reduced in several ways. Atballoon altitudes the largest part of the backgroundarises from single scattering of the solar flux, with theresult that this flux is highly polarized. Since the flu-orescence is not polarized, insertion of a polarizer in thedetection optics reduces the fluorescence by a factor of2, while the solar background will be reduced by a muchgreater amount depending on the rejection ratio of thepolarizer as well as the degree of polarization of thescattered light.

A second way to reduce the solar background is todecrease the field of view of the receiver telescope, re-ducing the size of the aperture at the focal plane. Onthis flight a large aperture was used to increase theoverlap between the field of view of the telescope andthe laser beam at short ranges. A better approach is tofocus the telescope at a finite range by translating theaperture stop further back in the optical path. Thisincreases the rate of overlap at short ranges while re-ducing the overall field of view of the telescope. Towork over the broadest range of distances, future sys-tems should have focusing optics.

Finally, it should be noted that the observations onthe last flight were made viewing horizontally. The skyis always brighter in this direction because there is moreatmosphere to scatter light toward the observer. Ex-perimental observations have shown an eightfold re-duction in background signal when the telescope istilted up at a 400 elevation. A background reductionof a factor of 8-10 is anticipated. The net effect of allthese changes is to yield an improvement of between 1and 2 orders of magnitude in the sensitivity. Thisshould be sufficient to permit measurements of thehydroxyl radical in the 20-30-km altitude range.

References1. J. G. Anderson, J. Geophys. Res. 76, 4645 (1971).2. J. G. Anderson, J. Geophys. Res., 76, 7820 (1971).3. J. G. Anderson, Geophys. Res. Lett. 3, 319 (1976).4. C. R. Burnett, Geophys. Res. Lett. 3, 319 (1976).5. D. D. Davis, W. Heaps, and T. McGee, Geophys. Res. Lett. 3,331

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A. Volz, Geophys. Res. Lett. 3, 466 (1976).7. D. D. Davis, W. Heaps, D. Philen, and T. McGee, Atmos. Environ.

13, 1197 (1979).8. C. C. Wang, L. I. Davis, Jr., P. M. Selzer, and R. Munoz, J. Geo-

phys. Res. 86, 1181 (1981).9. C. R. Burnett and E. B. Burnett, J. Geophys. Res. 86, 5185

(1981).10. W. F. J. Evans, J. B. Kerr, D. I. Wardle, J. C. McConnell, B. A.

Ridley, and H. I. Schiff, Atmosphere 14, 189 (1976).11. R. P. Turco, R. C. Whitten, 0. B. Toon, E. C. Y. Inn, and P.

Hamill, J. Geophys. Res. 86, 1129 (1981).12. T. J. McGee and T. J. McIlrath, Appl. Opt. 18, 1710 (1979).13. L. Zafonte, W. Long, and J. N. Pitts, Jr. Anal. Chem. 46, 1872

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Extremely Strong Fluctuating Ozone Profiles in the LowerStratosphere Based on Daily Balloon Radiosonde Soundings,"in Proceedings, Quadrennial International Ozone Symposium,Vol. 2, J. London, Ed. (NCAR, Boulder, 1980), p. 1112.

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Obviously an experiment of this complexity is notpossible without the efforts of a great many people.Special thanks go to Paul Spadin, Ed Johnson, and BillSchaeffer of the Goddard Electro-Optical SystemsEngineering Section; to Walt Nagel, Joe Modlin, andLen Arnowitz of the Goddard Sounding Rocket Divi-sion; and to George Newton of NASA Headquarters,who served as manager for the project in its initialphases.

The staff of the National Scientific Ballooning Fa-cility in Palestine, Texas, were especially helpful in allphases of the field operations. Thanks also to MaryCampbell and Roberta Duffy for their careful typing ofthis manuscript.


stratospheric ozone and hydroxyl radical measurements by balloon-borne lidar

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