Space-borne remote sensing of CO2, CH4, and N2O by integrated path differential absorption lidar: a sensitivity analysis

Abstract

CO 2 , CH 4 , and N 2 O are recognised as the most important greenhouse gases, the concentrations of which increase rapidly through human activities. Space-borne integrated path differential absorption lidar allows global observations at day and night over land and water surfaces in all climates. In this study we investigate potential sources of measurement errors and compare them with the scientific requirements. Our simulations reveal that moderate-size instruments in terms of telescope aperture (0.5-1.5 m) and laser average power (0.4-4 W) potentially have a low random error of the greenhouse gas column which is 0.2% for CO 2 and 0.4% for CH 4 for soundings at 1.6 µm, 0.4% for CO 2 at 2.1 µm, 0.6% for CH 4 at 2.3 µm, and 0.3% for N 2 O at 3.9 µm. Coherent detection instruments are generally limited by speckle noise, while direct detection instruments suffer from high detector noise using current technology. The wavelength selection in the vicinity of the absorption line is critical as it controls the height region of highest sensitivity, the temperature cross-sensitivity, and the demands on frequency stability. For CO 2 , an error budget of 0.08% is derived from our analysis of the sources of systematic errors. Among them, the frequency stability of ± 0.3 MHz for the laser transmitter and spectral purity of 99.9% in conjunction with a narrow-band spectral filter of 1 GHz (FWHM) are identified to be challenging instrument requirements for a direct detection CO 2 system operating at 1.6 µm.

Performance simulations for a spaceborne methane lidar mission

“…The present study masks out all profiles with total optical depth larger than 1 and does not exploit laser return signals from cloud tops. Ehret et al [2008] showed that reflectances from tops of water clouds with optical depth >5 are large enough to be used if the cloud boundary is sharp. Such signals could give partial vertical methane column contents down to the top of the clouds, which would be of particular value wherever the cloud tops are at low level.…”

“…Occasional ambiguities by surface pressure uncertainties or the presence of near-surface clouds can be circumvented by accurate ranging (in the order of several meters), use of a high-resolution digital elevation model, and accurate pointing knowledge. Recent studies assessed the potential of spaceborne IPDA lidars: On board a low-polar orbit satellite, this method is able to provide column carbon dioxide or methane measurements with an accuracy (systematic uncertainty; bias) of better than 1%, a precision (random uncertainty; noise) of around 1%, and global coverage between 82°S and 82°N, largely independent of aerosol load or sunlight [Dufour and Bréon, 2003;Ehret and Kiemle, 2005;Ehret et al, 2008;Amediek et al, 2009;Kawa et al, 2010;Kiemle et al, 2011]. First airborne IPDA lidar deployments recently corroborated the studies' results [Abshire et al, 2014].…”

“…Previous studies concentrated on the assessment of IPDA lidar accuracy [Dufour and Bréon, 2003;Ehret and Kiemle, 2005;Ehret et al, 2008] and instrument parameter sensitivity [Kiemle et al, 2011]. They used global average values to describe relevant parameters in the geophysical measurement environment.…”

Atmospheric CO2 measurements with a 2-μm DIAL instrument

“…It is defined as the ratio of the energy contained in a narrow spectral bandwidth to the total output energy. The purity requirements for spaceborne greenhouse gases remote sensing are extremely high with a ratio better than 99.9 % [3]. The spectral purity of the parametric MOPA source can be monitored with the system depicted in Fig.…”

“…In the 2 µm spectral region, the gases of interest present particularly well-suited lines for spaceborne applications which can be efficiently addressed by solid state laser technologies [3,4]. In this paper we present long-range absorption measurements on the R30 2-µm absorption line of the CO 2 .…”