International agreement on CO2 emissions control policy is contingent inter alia on a clear understanding of the geographical disposition and regulation mechanisms governing regional sources and sinks of atmospheric CO2. The current state of knowledge concerning the carbon cycle suffers from over-reliance on data provided by a sparse global surface network of about 120 air sampling stations because there is a current lack of any uniform, globally-dense sampling alternative. One consequence of the current severe undersampling is that significant uncertainties are propagated into the inferred CO2 fluxes and consequently obscure much of the fine-scale variability which must be resolved in order to identify and quantify regional sources and sinks of atmospheric CO2.
Resolution of CO2 spatial gradients is necessary in order to define sources and sinks of CO2 with sufficent accuracy that they may be quantified and separated from the 1.4% seasonal fluctuation component. This requires a measurement with a precision equivalent to 1-2 ppmv from an instrument in earth orbit. The data must be collected over a wide distribution of latitude, with a spatial resolution sufficient to provide global monthly mean values on a scale of order 106 km2. Currently, there is no available remote sensing instrumentation capable of providing these high-accuracy CO2 mixing ratio measurements with the vertical and horizontal spatial resolutions that have been identified by the U.S. Carbon Cycle Science Plan.
A collaborative proposal submitted by the JPL Laser Remote Sensing Group and Coherent Technologies, Inc. (CTI; Boulder, Colo.) has been selected by the NASA Earth Science Enterprise Instrument Incubator Program to develop a new airborne active laser sensor – the Laser Absorption Spectrometer (LAS) – using the integrated path differential absorption (IPDA) approach to attain high precision measurements of atmospheric CO2, both from aircraft altitudes and eventually from Earth orbit. The measurement emphasis for this instrument development is on the distribution of lower atmospheric CO2 abundance, particularly the lower 3-5 km of the troposphere. Instruments considered for global-scale measurements must be able to sense this portion of the atmosphere, with measurement precision of 1-2 ppm CO2, since it is in this region that the “signals” due to spatially and temporally varying source and sink mechanisms are at their most undiluted. The LAS approach utilizes a laser transmitter frequency offset from the line-center of a CO2 absorption feature with appropriate line strength, along with a separate transmitter frequency in the “window region” between lines, to measure differential absorptance. Surface backscatter will provide the return signal and off-line-center tuning will be used to “weight” the response of the instrument to various altitudes.
Application of this approach for atmospheric profiling of trace gases dates to the mid-1970s (Menzies and Chahine, 1974) and an aircraft instrument utilizing this approach to measure regional ozone transport was described by Shumate et al. (1981). This instrument employed discretely tunable cw CO2 lasers, whereas the current tropospheric CO2 mixing ratio measurement application will utilize cw solid-state Tm,Ho:YLF lasers operating in the 2.05-mm region. Extensive theoretical evaluation of numerous candidates demonstrated that this wavelength yields weighting functions whose response peaks in the atmospheric boundary layer (Chiao et al., 2001).
In a related activity, CTI is also leading a collaborative effort with JPL directed toward the development of high-power Tm,Ho:YLF laser technology which would ultimately enable global-scale laser absorption spectrometry of the tropospheric CO2 mixing ratio from Earth orbit. This work is supported by the NASA Cross-Enterprise Technology Development Program (CETDP).
References
R. T. Menzies and M. T. Chahine: Remote Atmospheric Sensing with an Airborne Laser Absorption Spectrometer. Appl. Opt., 13(12), 1974, 2840-2849.
M. S. Shumate, R. T. Menzies, W. B. Grant, and D. S. McDougal: Laser Absorption Spectrometer: Remote Measurement of Tropospheric Ozone. Appl. Opt., 20(4), 1981, 545-552.
M. P. Chiao, R. T. Menzies, and D. M. Tratt: Differential Laser Absorption Spectrometry for Global Profiling of Tropospheric Carbon Dioxide: Selection of Optimum Sounding Frequencies for High Precision Measurements (unpublished report, 2001).