DOI: 10.1007/s00340-007-2892-3 Appl. Phys. B 90, 593–608 (2008) Lasers and Optics Applied Physics B g. ehret 1, ✉ c. kiemle 1 m. wirth 1 a. amediek 1 a. fix 1 s. houweling 2 Space-borne remote sensing of CO 2 , CH 4 , and N 2 O by integrated path differential absorption lidar: a sensitivity analysis 1 Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR) e.V., 82234 Oberpfaffenhofen, Germany 2 National Institute for Space Research (SRON), Utrecht, The Netherlands Received: 15 March 2007/Revised version: 26 November 2007 Published online: 17 January 2008 • © Springer-Verlag 2008 ABSTRACT CO 2 , CH 4 , and N 2 O are recognised as the most im- portant 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 simula- tions reveal that moderate-size instruments in terms of telescope aperture (0.5–1.5 m) and laser average power (0.4–4 W) poten- tially 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 μ m, 0.4% for CO 2 at 2.1 μ m, 0.6% for CH 4 at 2.3 μ m, and 0.3% for N 2 O at 3.9 μ m. Coherent detection instruments are generally limited by speckle noise, while direct detection instru- ments 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 challeng- ing instrument requirements for a direct detection CO 2 system operating at 1.6 μ m. PACS 42.68.Wt; 95.75.Qr 1 Introduction Long-lived atmospheric species such as carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O) have been recognised by the International Panel of Climate Change as the most important greenhouse gases, the concentrations of which increase rapidly due to human activities since the industrial revolution [1]. In order to better predict the be- haviour of the climate system and to help constrain political conventions on greenhouse gas avoidance, a more accurate knowledge of the sources and sinks of these gases in terms ✉ Fax: +49-8153-28-1271, E-mail: gerhard.ehret@dlr.de of location, magnitude, and variability on a global basis is essential. Greenhouse gas fluxes at the Earth’s surface exhibit a com- plex pattern in space and time and cannot be directly measured by satellite observations. Concentration measurements of the vertical total column may be used to infer surface sources and sinks by means of inverse models that describe atmo- spheric transport and mixing [2]. Initial estimates reveal that the required level of measurement accuracy is exceptionally high and cannot be provided by the current global observ- ing system [2, 3]. The main drawback of passive sounders in the infrared spectral region is related to their atmospheric weighting functions which favour the middle and the upper troposphere (e.g. 5 km and above) rather than the lower tropo- sphere where the sources and sinks reside [4, 5]. In the solar backscatter region major limitations arise due to atmospheric aerosol interference [6] and from the fact that these systems lack sensitivity at high latitudes due to the unfavourable Sun angle [7, 8]. High measurement sensitivity is expected from making use of integrated path differential absorption (IPDA) lidar, where the strong lidar echoes from cloud tops or the Earth’s surface can be used to infer the trace gas column from sound- ings at two frequencies in the vicinity of an absorption line [9]. The possibility for minimising potential sources of systematic errors which may arise from unknown temperature profiles, water vapour interference, and aerosols is a further advantage of this measurement technique. In addition, sounding in the wing of an absorption line would enable high sensitivity in the low troposphere. There are a few publications reporting successful meas- urements of atmospheric CO 2 columns by ground-based in- struments using laser transmitters at wavelengths near 2.0 μ m and 4.8 μ m [10–13]. In the case of active remote sensing of CH 4 there are several operational instruments for gas leak detection operating in the 3.3-μ m or 1.6-μ m spectral regions [14–17]. Various laser transmitters are employed such as optical parametric oscillators (OPOs), CO:MgF 2 lasers, DF lasers, Ti:sapphire lasers with Raman shifting, or harmonic generation of CO 2 lasers [18]. Differential ab- sorption lidar (DIAL) measurements of range-resolved CO 2 profiles using a pulsed single-frequency Tm:Ho:YLF laser at 2.05-μ m wavelength in combination with heterodyne detec-