Laser Absorption Spectrometer Concept for Global-Scale Observations of Atmospheric 


Carbon Dioxide 


Robert T. Menzies, David M. Tratt, Meng P. Chiao, and Christopher R. Webster 

Jet Propulsion Laboratory, California Institute of Technology 


4800 Oak Grove Drive, Pasadena, CA 91109, USA 


Tel. +1-818-354-3787 


ABSTRACT 

A laser absorption spectrometer concept is described for high precision measurements of CO2 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 used to attain high sensitivity to the return signal backscattered from the surface. 
Laser Absorption Spectrometer Concept for Global-Scale Observations of Atmospheric Carbon Dioxide 

Robert T. Menzies, David M. Tratt, Meng P. Chiao, and Christopher R. Webster 


Jet Propulsion Laboratory, California Institute of Technology 


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 of the total source of 

CO2 to the atmosphere. Attempts to improve 

understanding of the spatial structure of carbon 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) of CO2 over continental 

scale regions and the various oceans; (3) 

interhemispheric transport of atmospheric CO2; and (4) 

interannual variability in regional exchange of CO2. Each 

objective represents a key component of coupled carbon 

models that is not presently well constrained by existing 

carbon dioxide measurement data. 

Observations of carbon 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 of latitude, with spatial resolution sufficient 

to provide global monthly mean values on a spatial scale 

of order 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 IR bands, 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, but the instrument could operate day and night. 

The latter technique [Rayner and O'Brien, 2001], 

making use of observations in the sun-glint regions 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 over the oceans. Laser techniques include 

integrated path differential absorption, using the surface 

to provide the backscattered signal, or full-up DIAL, 

using the atmospheric aerosol to provide the backscatter 

in a range-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 of trace 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 transform measurement precision into 

an acceptable level of accuracy. The presence of clouds 

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. 

However the DIAL approach, being directly dependent 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 brief comments on an analogous 

aircraft instrument. 

The integrated path differential absorption 

(IPDA) technique can utilize either CW or pulsed 

transmitter; we emphasize CW here because of its 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 over ocean surfaces 

assuming a O.S-m diameter receiver aperture, along with 

typical coherent detection efficiency factors. The two 

altitudes span the range of typical LEO altitudes, with the 

lower end of the range obviously being favorable in terms 

of required active sensing instrument mass and power. 
The assumed round-trip atmospheric transmittance is 

0.25 and the pointing is 3-degrees off-nadir. We assume 

sufficient SNR to achieve a 0.3% measurement precision 

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 et at., 1997; Kaufman et at., 

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. 

w 

c..> 

z Altitude (km): 400 -­ I I ;: ~OOfrl ~ : ...J 

U. ,I'"-'U-'0-=-5-m-'/~~r--. W : 

I ~:: _______ II: 

o ---;... ; II: 

I­ I ------'---­ w I I I II: - - - - - ~ - - - - -1- - - - - - r - - - - - l - - - - ­

Z 


0.1 

U) 

c..> 

o 

o 

...J 

o 

:r: 

(J) 

w 

~ 0.01 '--__-L-___L-__-L-___L-___ 


I- 0 2 3 4 


1RANSMITTER POWER (W) 

Figure 1. Dependence of threshold ocean 

surface retroreflectance on transmitter power, 

altitude, and surface wind speed, U 10, for the 

assumed instrument parameters. 

To achieve sufficient sensitivity, narrow 

receiver bandwidth is essential. Here we have assumed 

a bandwidth of 10 kHz, compared with a 2-kHz lower 

limit on the signal spectral width due to the decorrelation 

time of the surface reflection signal. This places a 

requirement on the transmitter short-term (msec) jitter as 

well as the angular rate of change of the spacecraft 

attitude. Spacecraft angular drift effects can be removed 

(compensated) by use of a frequency-agile RF local 

oscillator (LO) in the second-IF domain. 

Table 1 provides a summary of the salient 

characteristics of the 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 

Transmitter Power 2W 

Receiver aperture 0.5 m 

Beam divergence 10 J.lrad 

Receiver bandwidth 10kHz 

Along-track resolution 100km 

Table 1. LAS instrumental operating characteristics. 

Required ancillary data 

To achieve overall measurement precision at the 

0.5% level, the atmospheric column being probed must be 

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 

line width are temperature dependent. An accurate 

temperature profile as well as an accurate surface pressure 

can be combined to provide the pressure and density 

profiles, assuming hydrostatic equilibrium. In lieu of 

surface pressure, an accurate profile of oxygen or nitrogen 

may serve as a proxy for an atmospheric density profile. 

Only a very few lines in a typical P- or R-branch of a band 

provide sufficiently low susceptibility to uncertainties in 

temperature over a range of tropospheric temperatures to 

maintain this source of error at a reasonably small fraction 

of the total error budget. (This is the subject of a 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 K rms with 1 km vertical resolution. (AIRS 

is scheduled to be launched in late 2001, beginning a 5-yr 

lifetime mission.) Including an AIRS-class instrument in 

the payload would increase mission costs substantially. 

An alternative is to formation fly with AIRS or an 

equivalent capability AIRS follow-on. Another alternative 

is to rely on the numerical weather prediction model 

analyses, which should provide profiles with ±2 K 

accuracy over most of the globe during the next decade 

when an AIRS-class sounder is in orbit. The same NWP 

analyses would provide the surface pressure maps, 

Surface pressure accuracy is expected to be about 2 hPa 

rms over most of the 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 the 
upper end of the useful range of band 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 

lower troposphere, peaking at or near the surface. 

Clouds and aerosol layers of significant optical 

thickness must be recognized in the instrument field-ofview. They effectively alter the atmospheric sounding 

thickness of the column through the extinction and 

backscattering processes, rendering perturbations to the 

altitude weighting function. A climatological model of 

the boundary layer aerosol may suffice to maintain this 

potential error source within bounds, but further 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 coherent detection receiver appears to be a 

feasible means for providing global-scale carbon dioxide 

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. Transforming precision 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 of the 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 of a credible approach to a globalscale measurement. 

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 of a 2.2 !lm channel. IEEE Trans. 

Geosci. Remote Sensing, 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 of remotely 

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 Report o/the Carbon and 

Climate Working Group (1999). U.S. Global Change 

Research Program, 400 Virginia Avenue, SW, Suite 750, 

Washington, DC 20024. 

Shumate, M. S., R. T. Menzies, W. B. Grant, and D. S. 

McDougal: Laser absorption spectrometer: remote 

measurement of tropospheric ozone. Appl. Opt., 20, 545­

553 (1981).