Abstract
The capability of an airborne 2-μm integrated path differential absorption (IPDA) lidar for high-accuracy and high-precision active remote sensing of weighted-average column dry-air volume mixing ratio of atmospheric carbon dioxide (XCO2) is demonstrated. A test flight was conducted over the costal oceanic region of the USA to assess instrument performance during severe weather. The IPDA targets CO2 R30 absorption line using high-energy 2-μm laser transmitter. HgCdTe avalanche photodiode detection system is used in the receiver. Updated instrument model included range correction factor to account for platform attitude. Error budget for XCO2 retrieval predicts lower random error for longer sensing column length. Systematic error is dominated by water vapor (H2O) through dry-air number density derivation, followed by H2O interference and ranging related uncertainties. IPDA XCO2 retrieval results in 404.43 ± 1.23 ppm, as compared to 405.49 ± 0.01 ppm from prediction models, using consistent reflectivity and steady elevation oceanic surface target. This translates to 0.26% and 0.30% relative accuracy and precision, respectively. During gradual spiral descend, IPDA results in 404.89 ± 1.19 ppm as compared model of 404.75 ± 0.73 ppm indicating 0.04% and 0.23% relative accuracy, respectively. Challenging cloud targets limited retrieval accuracy and precision to 2.56% and 4.78%, respectively, due to H2O and ranging errors.
Highlights
Atmospheric carbon dioxide (CO2) is a dominant greenhouse gas that contributes to global warming and climate change, while influencing the carbon cycle on Earth
Ground and airborne testing demonstrated the capability of the triple-pulse integrated path differential absorption (IPDA) technique, using fixed 6.5-GHz on-line for CO2 measurements resulted in a lower differential optical depth while losing the tuning capability
This paper presents the XCO2 measurement results using the 2-μm pulsed IPDA lidar during a local four-hour flight conducted on 17 September 2019
Summary
Atmospheric carbon dioxide (CO2) is a dominant greenhouse gas that contributes to global warming and climate change, while influencing the carbon cycle on Earth. Based on National Research Council recommendation, NASA initiated the Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission, which involves several groups for developing different CO2 IPDA lidar instruments [7,9] These instruments use either pulsed or continuous wave laser transmitters operating at 1.6- or 2.0-μm wavelengths. High variability of atmospheric H2O influences IPDA CO2 measurement through direct molecular interference, spectral line broadening and dry-air number density derivation [6] To address this issue, the same technology was upgraded to produce three successive pulses, for each pump pulse, for simultaneous measurements of CO2 and H2O [18]. Ground and airborne testing demonstrated the capability of the triple-pulse IPDA technique, using fixed 6.5-GHz on-line for CO2 measurements resulted in a lower differential optical depth (about 0.1) while losing the tuning capability. Moderate-state ocean surface was targeted, during clear sky conditions, off-shore Wilmington between the spirals
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