### LaserAbsorption Spectrometer Conceptfor Global-Scale Observations ofAtmospheric Carbon Dioxide

Robert T. Menzies, DavidM. Tratt, MengP. Chiao, and ChristopherR. Webster Jet PropulsionLaboratory, CaliforniaInstitute ofTechnology 4800 Oak Grove Drive, Pasadena, CA 91109, USA Tel. 

## +1-818-354-3787



### ABSTRACT

A laser absorption spectrometerconceptis describedforhigh precision measurements ofCO2

### mixing ratio in the troposphere. The approach is based on measurement of integrated path differential absorption at selected frequencies within a CO2 

### absorption band near 2.05 J..lm. Coherent detection is usedto attain high sensitivityto the return signal backscatteredfrom the surface.LaserAbsorptionSpectrometer Conceptfor Global-Scale Observations ofAtmospheric CarbonDioxide

Robert T. Menzies, DavidM. Tratt, Meng P. Chiao, and ChristopherR. Webster JetPropulsion Laboratory, California Institute ofTechnology 4800 Oak Grove Drive, Pasadena, CA 91109, USA

Introdnction

The global carbon cycle is attracting increasing attention both scientifically and politically. The chief societal objective is to stabilize greenhouse gas concentrations, with particular attention given to CO2• This demands accurate assessment ofthe total source of CO2 to the atmosphere. Attempts to improve understanding ofthe spatial structure ofcarbon sources

are inherent in this assessment. Scientific objectives include four categories that can be addressed using space-based observations on a global scale [Sarmiento and Wofsy, 1999]: (1) oceanic vs. terrestrial sinks of CO2; (2) net sources (or sinks) ofCO2 over continental scale regions and the various oceans; (3) interhemispheric transport ofatmospheric CO2; and (4) interannual variability in regional exchange ofCO2

.Each objectiverepresents akey component ofcoupled carbon models that is notpresently well constrained by existing carbon dioxide measurementdata. Observations ofcarbon dioxide mixing ratios from Earth orbit, primarily in the lower and middle troposphere with measurement precision equivalent to about 2 ppmv, are desired to define spatial gradients of CO2, from which sources and sinks can be derived and quantified. Data would be needed over a wide distribution oflatitude, with spatial resolution sufficient
to provide global monthly mean values on a spatial scale oforder 106

- 107 km2• An active sensing approach using tunable IR lasers offers several advantages over other potential techniques. Passive techniques for retrieving carbon dioxide distributions may include spectral radiometers operating in the CO2 thermal IRbands, or spectrometers observing scattered solar radiation in the near infrared absorption bands. The former technique is extremely sensitive to uncertainties in knowledge of the temperature profile, and a cooled instrument would be required, butthe instrumentcould operate day and night. The latter technique [Rayner and O'Brien, 2001], making use ofobservations in the sun-glintregions over the oceans, has the potential to achieve the required signal-to-noise ratio (SNR). Obviously this would be limited to daytime observations over the oceans and would require a more lengthy period to build up complete maps overthe oceans. Lasertechniques include integratedpath differential absorption, using the surface
to provide the backscattered signal, or full-up DIAL, using the atmospheric aerosol to provide the backscatter in arange-gated mode. Here we concentrate on the former approach and describe a laser absorption spectrometer instrument that has the potential to achieve the required precision. Analysis of this approach for atmospheric profiling oftrace gases dates to the mid-1970's [Menzies and Chahine, 1974], and an aircraft instrument utilizing this approach to measure regional ozone transport was described in Shumate et

### at. [1981]. There exist several challenges to global-scale Earth-orbiting observations of carbon dioxide, and in addition extensive correlative measurements using wellcalibrated ground-based and airborne sensors would be required in order to transformmeasurementprecision into

an acceptable level ofaccuracy. The presence ofclouds limits the observing opportunities, and thin cirrus in particular can potentially delude the retrieval algorithms. Aerosol scattering in the boundary layer, and in elevated layers, complicate the measurement retrieval interpretation. A DIAL instrument with sufficient sensitivity to obtain the required measurement precision would detect even sub-visible cirrus in the path. Obviously the aerosol scattering layers would appear also. Howeverthe DIAL approach, being directlydependent on the aerosol backscatter, is influenced by small-scale inhomogeneities in the aerosol spatial distributions. For all approaches the temperature profile must be well known. Also the surface pressure, or alternatively the profile of oxygen or nitrogen density, must be well known.

Instrument description

An Earth-orbiting instrument concept will be described, followed by briefcomments on an analogous aircraft instrument. The integrated path differential absorption (IPDA) technique can utilize either CW or pulsed transmitter; we emphasize CWhere because ofits relative simplicity. For an instrument operating from low Earth orbit (LEO) the required power-aperture product ranges from 0.5

- 1.5 W_m2 depending on altitude. Figure 1 indicates required transmitter power overocean surfaces assuming a O.S-m diameterreceiver aperture, along with typical coherent detection efficiency factors. The two altitudes span the range oftypical LEO altitudes, with the lower end oftherange obviously being favorable in terms ofrequired active sensing instrument mass and power.

### The assumed round-trip atmospheric transmittance is 0.25 and thepointing is 3-degrees off-nadir. We assume sufficient SNRto achieve a0.3% measurementprecision from the instrument itself, not including other contributions to the error budget. The global average surface wind speed is near 7 mls. 

### Middle and high latitude surface wind speeds are usually higher. At a minimum the instrument should be able to operate with surface wind speeds up to 10 mls. 

### The ocean surface retroreflectance for this value of surface wind speed is

near 0.1 sr-I

. Over land surfaces the (pin) 

### factor is near 0.1 S{I or above for vegetation in semi-arid regions, dry soils, and deserts [Kaufman etat., 

### 1997; Kaufman etat., 

### 2000], and for snow and ice. In lidar applications the "hot spot" effect (retroreflectance enhancement) often adds significantly to the reflectance that one would

assume based on the albedo of the surface and a Lambertian model.

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### Figure 1. 

### Dependence of threshold ocean surface retroreflectance on transmitter power, altitude, and surface wind speed, U10, 

### for the assumed instrumentparameters.

To achieve sufficient sensitivity, narrow receiver bandwidth is essential. Here we have assumed

a bandwidth of 10 kHz, compared with a 2-kHz lower limiton the signal spectral width due to the decorrelation time of the surface reflection signal. This places a requirement on the transmitter short-term (msec)jitteras well as the angular rate of change of the spacecraft attitude. Spacecraftangular drifteffects can beremoved (compensated) by use of a frequency-agile RF local oscillator (LO) in the second-IFdomain. Table 1 provides a summary of the salient characteristics ofthe LAS CO2 

### instrument, assuming a 450-km altitude. The concept includes two transmitter lasers, the second being at the fixed off-line frequency with output power requirement reduced by the CO2 

### optical depth factor. 5 

### PARAMETER VALUE Spectral Region 2.05,2.07 11m TransmitterPower 2W Receiver aperture 0.5 m Beamdivergence 10 J.lrad Receiver bandwidth 10kHz Along-trackresolution 100km

Table 

### 1. LAS instrumental operating characteristics.

Required ancillarydata

To achieve overall measurementprecision atthe 0.5% level, the atmospheric columnbeingprobed mustbe well characterized. The measurement objective is the carbon dioxide volume mixing ratio. A sufficiently
accurate spectroscopic measurement must rely on an accurate temperature profile, since the line intensity and linewidth are temperature dependent. An accurate temperatureprofile as well as an accurate surfacepressure

can be combined to provide the pressure and density profiles, assuming hydrostatic equilibrium. In lieu of surfacepressure, an accurate profile ofoxygen or nitrogen may serve as aproxy for an atmospheric density profile. Only a very few lines in atypical P- orR-branch ofaband provide sufficiently low susceptibility to uncertainties in temperature over arange oftropospheric temperatures to maintain this source oferror at areasonably small fraction ofthe total errorbudget. (This is the subject ofa separate paper.) The best temperature profiler available in the present era, the Atmospheric Infrared Sounder [AIRS; Aumann and Pagano, 1994], is expected to provide accuracy to 1 Krms with 1km vertical resolution. (AIRS is scheduled to be launched in late 2001, beginning a 5-yr lifetime mission.) Including an AIRS-class instrumentin the payload would increase mission costs substantially. An alternative is to formation fly with AIRS or an equivalentcapability AIRS follow-on. Anotheralternative is to rely on the numerical weather prediction model analyses, which should provide profiles with ±2 K accuracy over most ofthe globe during the next decade when an AIRS-class sounder is in orbit. The same NWP analyses would provide the surface pressure maps, Surfacepressure accuracy is expected to be about 2 hPa

rms over most ofthe globe, i.e., 0.2% rms. The ideal spectral line to use for the differential absorption sounding will provide the optimum combination of optical depth and insensitivity to temperature profile uncertainties, for the offset tuning required to provide a weighting function peaking at the desired middle or lower tropospheric altitude. This translates into a requirement on the CO2 

### band strength. The 2vI 

### + V3 

### band in the 2.05-2.07 /lm region is at theupper end ofthe useful range ofband strengths. (The 2v\


### + 2V20 

### + V3 band near 1.58 !lm is at the lower end.) Suitable lines in this band can be used to provide weighting functions which effectively interact with the lowertroposphere, peaking at or near the surface. Clouds and aerosol layers ofsignificant optical thickness must be recognized in the instrumentfield-ofview. They effectively alter the atmospheric sounding thickness of the column through the extinction and backscatteringprocesses, renderingperturbations to the altitude weighting function. A climatological model of the boundary layer aerosol may suffice to maintain this potentialerror source within bounds, butfurther study of this approach is necessary. Adding a laser altimeter to the payload would provide a means of "flagging" the conditions for which accurate retrievals of the lower tropospheric carbon dioxide are not possible. A visual

scene camera and a thermal IR imager would be an important addition to provide the necessary contextual information. An oxygen A-band spectrometer might be sufficient to provide the cloud and aerosol information during daytime portions of the orbits. Alternatively a pulsed version of the CO2 LAS instrument, using an injection-seeded transmitter, may be employed. The effectiveness of various instrument combinations in discerning the clouds and aerosol layers must be carefully modeled in simulations.

Summary

A laser absorption spectrometer instrument employing a coherentdetectionreceiver appears to be a feasible means for providing global-scale carbondioxide mixing ratios in the lower troposphere with sufficient precision to contribute to the carbon cycle scientific objectives. It is likely that other sensors in the same payload, and possibly formation flying with a satellite carrying a high accuracy temperature profiler, will be necessary in order to achieve the precision requirement and minimize potential biases. Transformingprecision to accuracy will require extensive correlative measurements using ground-based and airborne CO2 measurement techniques with an historical record of demonstrated accuracy. An aircraft instrument would permit the demonstration ofthe LAS approach under a variety of atmospheric and surface conditions. A 100-200 mW transmitter would suffice for such an instrument. This would be viewed by many as an essential stepping-stone in the development ofa credible approach to a globalscalemeasurement.

Acknowledgment This work was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under
contract with the National Aeronautics and Space Administration.

References

Aumann, H. H., and R. J. Pagano: Atmospheric Infrared Sounder on the Earth Observing System. Opt. Eng., 33, 776-784 (1994).

Kaufman, Y. J., A. Wald, L. A. Remer, B. -C. Gao, R. -R. Li, and L. Flynn: Remote sensing of aerosol over the continents with the aid ofa 2.2 !lmchannel. IEEE Trans. Geosci. RemoteSensing, 35, 1286-1298 (1997).

Kaufman, Y. J., A. Karnieli, and D. Tanre: Detection of dust over deserts using satellite data in the solar wavelengths. IEEE Trans. Geosci. Remote Sensing, 38, 525-531 (2000).

Menzies, R. T., and M. T. Chahine: Remote atmospheric sensing with an airborne laser absorption spectrometer. Appl. Opt., 13, 2840-2849 (1974).

Rayner, P. J., and D. M. O'Brien: The utility ofremotely sensed CO2 concentration data in surface source inversions. Geophys. Res. Lett., 28, 175-178 (2001).

Sarmiento, J. L., and S. C. Wofsy, co-chairs: 

### A U.S. Carbon Cycle Science Plan:A Reporto/the Carbon and Climate Working Group (1999). U.S. Global Change Research Program, 400VirginiaAvenue, SW, Suite 750, Washington, DC 20024.

Shumate, M. S., R. T. Menzies, W. B. Grant, and D. S. McDougal: Laser absorption spectrometer: remote measurement oftropospheric ozone.Appl. Opt., 20, 545­ 553 (1981).