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The focus of this study is to model and validate the performance of intensity-modulated continuous-wave (IM-CW) CO_2 laser absorption spectrometer (LAS) systems and their CO_2 column measurements from airborne and satellite platforms. The model accounts for all fundamental physics of the instruments and their related CO_2 measurement environments, and the modeling results are presented statistically from simulation ensembles that include noise sources and uncertainties related to the LAS instruments and the measurement environments. The characteristics of simulated LAS systems are based on existing technologies and their implementation in existing systems. The modeled instruments are specifically assumed to be IM-CW LAS systems such as the Exelis’ airborne multifunctional fiber laser lidar (MFLL) operating in the 1.57 μm CO_2 absorption band. Atmospheric effects due to variations in CO_2, solar radiation, and thin clouds, are also included in the model. Model results are shown to agree well with LAS atmospheric CO_2 measurement performance. For example, the relative bias errors of both MFLL simulated and measured CO_2 differential optical depths were found to agree to within a few tenths of a percent when compared to the in situ observations from the flight of 3 August 2011 over Railroad Valley (RRV), Nevada, during the summer 2011 flight campaign. In addition, the horizontal variations in the model CO_2 differential optical depths were also found to be consistent with those from MFLL measurements. In general, the modeled and measured signal-to-noise ratios (SNRs) of the CO_2 column differential optical depths (τ_d) agreed to within about 30%. Model simulations of a spaceborne IM-CW LAS system in a 390 km dawn/dusk orbit for CO_2 column measurements showed that with a total of 42 W of transmitted power for one offline and two different sideline channels (placed at different locations on the side of the CO_2 absorption line), the accuracy of the τ_d measurements for surfaces similar to the playa of RRV, Nevada, will be better than 0.1% for 10 s averages. For other types of surfaces such as low-reflectivity snow and ice surfaces, the precision and bias errors will be within 0.23% and 0.1%, respectively. Including thin clouds with optical depths up to 1, the SNR of the τ_d measurements with 0.1 s integration period for surfaces similar to the playa of RRV, Nevada, will be greater than 94 and 65 for sideline positions placed +3 and +10  pm, respectively, from the CO_2 line center at 1571.112 nm. The CO_2 column bias errors introduced by the thin clouds are ≤0.1% for cloud optical depth ≤0.4, but they could reach ∼0.5% for more optically thick clouds with optical depths up to 1. When the cloud and surface altitudes and scattering amplitudes are obtained from matched filter analysis, the cloud bias errors can be further reduced. These results indicate that the IM-CW LAS instrument approach when implemented in a dawn/dusk orbit can make accurate CO_2 column measurements from space with preferential weighting across the mid to lower troposphere in support of a future ASCENDS mission.

We report on airborne CO_2 column absorption measurements made in 2009 with a pulsed direct-detection lidar operating at 1572.33 nm and utilizing the integrated path differential absorption technique. We demonstrated these at different altitudes from an aircraft in July and August in flights over four locations in the central and eastern United States. The results show clear CO_2 line shape and absorption signals, which follow the expected changes with aircraft altitude from 3 to 13 km. The lidar measurement statistics were also calculated for each flight as a function of altitude. The optical depth varied nearly linearly with altitude, consistent with calculations based on atmospheric models. The scatter in the optical depth measurements varied with aircraft altitude as expected, and the median measurement precisions for the column varied from 0.9 to 1.2 ppm. The altitude range with the lowest scatter was 8–10 km, and the majority of measurements for the column within it had precisions between 0.2 and 0.9 ppm.

The 2007 National Research Council (NRC) Decadal Survey on Earth Science and Applications from Space recommended Active Sensing of CO_2 Emissions over Nights, Days, and Seasons (ASCENDS) as a midterm, Tier II, NASA space mission. ITT Exelis, formerly ITT Corp., and NASA Langley Research Center have been working together since 2004 to develop and demonstrate a prototype laser absorption spectrometer for making high-precision, column CO_2 mixing ratio measurements needed for the ASCENDS mission. This instrument, called the multifunctional fiber laser lidar (MFLL), operates in an intensity-modulated, continuous wave mode in the 1.57 μm CO_2 absorption band. Flight experiments have been conducted with the MFLL on a Lear-25, UC-12, and DC-8 aircraft over a variety of different surfaces and under a wide range of atmospheric conditions. Very high-precision CO_2 column measurements resulting from high signal-to-noise ratio (>1300) column optical depth (OD) measurements for a 10 s (∼1  km) averaging interval have been achieved. In situ measurements of atmospheric CO_2 profiles were used to derive the expected CO_2 column values, and when compared to the MFLL measurements over desert and vegetated surfaces, the MFLL measurements were found to agree with the in situ-derived CO_2 columns to within an average of 0.17% or ∼0.65  ppmv with a standard deviation of 0.44% or ∼1.7  ppmv. Initial results demonstrating ranging capability using a swept modulation technique are also presented.