Sensitivity Analysis and Error Budget For Carbon Dioxide and Water Vapor Simultaneous and Independent Measurement Using 2-Micron Triple-Pulsed IPDA Lidar

Abstract:

A new 2 μm triple-pulse Integrated Path Differential Absorption (IPDA) lidar for measuring weighted-average column dry-air volume-mixing ratios of water vapor (H₂O) and carbon dioxide (CO₂) is under development at NASA Langley Research Center. This instrument is a technological update to the already demonstrated 2 μm CO₂ double-pulse airborne IPDA lidar system at NASA Langley Research Center. The lidar consists of a direct detection system and a 2 μm triple-pulse laser transmitter. The transmitter generates three consecutive pulses at three different locked wavelength, separated by approximately 200 microseconds, for each pump pulse at 50 Hz pulse repetition rate. Optimized wavelengths tuning and locking for each of the pulses are selected such that H₂O interference is minimized from CO₂ measurement, and CO₂ interference is minimized from the H₂O measurement. This innovative technique allows simultaneous and independent measurement of H₂O and CO₂ optical depths from an airborne platform. Focusing on optical depth measurements, total errors in H₂O and CO₂ retrievals result from both random and systematic sources. Random error is associated with the IPDA lidar detection system. Systematic errors includes both atmospheric and IPDA lidar transmitter uncertainties. Atmospheric systematic error results from a combination of sensitivities to meteorological data and molecular interference. Transmitter systematic errors result from laser spectral quality and wavelength locking control. IPDA sensitivity analysis results in CO₂ differential optical depth total error of 0.45% for single shot measurement dominated by the receiver. This will reduce to 0.21% for 500 shot average dominated by atmospheric pressure uncertainty. For H₂O differential single-shot optical depth measurement, the 0.58% error is dominated by atmospheric effects governed by temperature and molecular interference at lower and higher ground elevations, respectively. The error will reduce to 0.48% by averaging. This presentation will illustrates the feasibility of implementing the 2 μm pulsed lidar technology for obtaining accurate H₂O and CO₂ airborne column measurements from an airborne platform. The technologies and techniques being developed are critical and enabling for a potential space based column measurement of CO₂ and H₂O.