Double-pulse 2-μm integrated path differential absorption lidar airborne validation for atmospheric carbon dioxide measurement

Abstract

Field experiments were conducted to test and evaluate the initial atmospheric carbon dioxide (CO₂) measurement capability of airborne, high-energy, double-pulsed, 2-μm integrated path differential absorption (IPDA) lidar. This IPDA was designed, integrated, and operated at the NASA Langley Research Center on-board the NASA B-200 aircraft. The IPDA was tuned to the CO₂ strong absorption line at 2050.9670 nm, which is the optimum for lower tropospheric weighted column measurements. Flights were conducted over land and ocean under different conditions. The first validation experiments of the IPDA for atmospheric CO₂ remote sensing, focusing on low surface reflectivity oceanic surface returns during full day background conditions, are presented. In these experiments, the IPDA measurements were validated by comparison to airborne flask air-sampling measurements conducted by the NOAA Earth System Research Laboratory. IPDA performance modeling was conducted to evaluate measurement sensitivity and bias errors. The IPDA signals and their variation with altitude compare well with predicted model results. In addition, off-off-line testing was conducted, with fixed instrument settings, to evaluate the IPDA systematic and random errors. Analysis shows an altitude-independent differential optical depth offset of 0.0769. Optical depth measurement uncertainty of 0.0918 compares well with the predicted value of 0.0761. IPDA CO₂ column measurement compares well with model-driven, near-simultaneous air-sampling measurements from the NOAA aircraft at different altitudes. With a 10-s shot average, CO₂ differential optical depth measurement of 1.0054±0.0103 was retrieved from a 6-km altitude and a 4-GHz on-line operation. As compared to CO₂ weighted-average column dry-air volume mixing ratio of 404.08 ppm, derived from air sampling, IPDA measurement resulted in a value of 405.22±4.15 ppm with 1.02% uncertainty and 0.28% additional bias. Sensitivity analysis of environmental systematic errors correlates the additional bias to water vapor. IPDA ranging resulted in a measurement uncertainty of <3 m.

Space-based active optical remote sensing of carbon dioxide column using high-energy two-micron pulsed ipda lidar

“…Measurements can be optimized by tuning on-lines based upon ground target scenarios, environment and science objectives. With 10 MHz detection bandwidth, surface ranging with an uncertainty of <3 m can be achieved as demonstrated from earlier airborne flights [6].…”

A Singular Value Decomposition framework for retrievals with vertical distribution information from greenhouse gas column absorption spectroscopy measurements

“…2. Perfect knowledge of the optical path length with a clear atmosphere -Lidar instruments are pulsed (Abshire et al, 2014;Refaat et al, 2016), or alternatively have some modulation (Lin et al, 2015)), and simultaneously measure the surface elevation (via ranging) and thus the precise light path length. In addition, this ranging capability enables the time-gating of the surface returns so as to exclude aerosol backscatter, a common cause of bias.…”

“…Sufficient number of wavelength samples -Due to complexities in generating precisely tuned laser light for wavelength samples, many lidar GHG sensing instruments (Refaat et al, 2016;Lin et al, 2015;Menzies et al, 2014) use only 2 wavelength samples. Here, we assume at least a few wavelength samples across the absorption line.…”

“…Atmospheric measurements have also been made using airborne integrated path differential absorption (IPDA) lidar instruments (Abshire et al, 2014;Lin et al, 2015;Menzies et al, 2014;Refaat et al, 2016). While column-averaged mixing ratios are retrieved from measurements using methods ranging from simple differential absorption ratioing (Refaat et al, 2016), least-squares line-fitting (Wunch et al, 2011) and in Rodgers (2000) where the input prior covariance matrix is informative (i.e., the prior covariance matrix has at least one finite eigenvalue). In contrast, the Singular Value Decomposition (SVD) approach Hansen (1990) can extract vertical information from the measurement without assuming any prior GHG distribution, opening the possibility of an unbiased retrieval.…”

Column carbon dioxide and water vapor measurements by an airborne triple-pulse integrated path differential absorption lidar – novel lidar technologies and techniques with path to space

“…Based on the successful demonstration of the double-pulse IPDA lidar, the triple-pulse IPDA lidar transmitter generates three successive laser pulses for every pump pulse at pump repetition rate of 50 Hz [12]. Table 1 compares the double-pulse and triple-pulse 2-m IPDA lidar transmitters [1-2, 6, 12].…”