Abstract
AbstractWe present an evaluation of airborne intensity‐modulated continuous‐wave (IM‐CW) lidar measurements of atmospheric column CO2 mole fractions during the Atmospheric Carbon and Transport–America (ACT‐America) project. This lidar system transmits online and offline wavelengths simultaneously on the 1.57111‐μm CO2 absorption line, with each modulated wavelength using orthogonal swept frequency waveforms. After the spectral characteristics of this system were calibrated through short‐path measurements, we used the HITRAN spectroscopic database to calculate the average‐column CO2 mole fraction (XCO2) from the lidar‐measured optical depths. Using in situ measurements of meteorological parameters and CO2 concentrations for calibration data, we demonstrate that our lidar CO2 measurements were consistent from season to season and had an absolute calibration error (standard deviation) of 0.80 ppm when compared to XCO2 values calculated from in situ measurements. By using a 10‐s or longer moving average, a precision of 1 ppm or better was obtained. The estimated CO2 measurement precision for 0.1‐, 1‐, 10‐, and 60‐s averages was determined to be 3.4, 1.2, 0.43, and 0.26 ppm, respectively. These correspond to measurement signal‐to‐noise ratios of 120, 330, 950, and 1,600, respectively. The drift in XCO2 over 1‐hr of flight time was found to be below 0.1 ppm. These analyses demonstrate that the measurement stability, precision, and accuracy are all well below the thresholds needed to study synoptic‐scale variations in atmospheric XCO2.
Highlights
Atmospheric carbon dioxide (CO2) is a crucial part of the Earth's carbon cycle and one of the major greenhouse gases (GHGs) in the Earth's climate system
We present an evaluation of airborne intensity‐modulated continuous‐wave (IM‐CW) lidar measurements of atmospheric column CO2 mole fractions during the Atmospheric Carbon and Transport–America (ACT‐America) project
We have demonstrated a technique for measuring XCO2 with an IM‐CW lidar system through flight testing with very good results
Summary
Atmospheric carbon dioxide (CO2) is a crucial part of the Earth's carbon cycle and one of the major greenhouse gases (GHGs) in the Earth's climate system. Comprehensive measurements of regional atmospheric CO2 distributions are urgently needed to develop a more complete understanding of CO2 transport and sources and sinks This is one of the key reasons NASA has been carrying out the Atmospheric Carbon and Transport–America (ACT‐America) suborbital mission (Davis et al, 2017). Two NASA aircraft (C‐130 and B‐200) were deployed during each field campaign Both aircraft were equipped with GPS for geographic location, altitude, and time information and in situ sensors for the measurement of atmospheric CO2, other GHGs, and meteorological variables such as temperature, pressure, humidity, and wind.
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