Atmospheric Backscatter Data from the 2017 ASCENDS/ABoVE campaign as measured by the NASA-GSFC CO2 Laser Sounder

Cloud and aerosol backscatter and surface reflectance are important factors in the propagation of coherent light for airborne lidar measurements of atmospheric trace gases. As part of the 2017 ASCENDS/ABoVE campaign to measure atmospheric carbon dioxide (CO₂) in the northern latitudes, NASA-GSFC flew their airborne CO₂Sounder instrument, an integrated path differential absorption lidar, aboard NASA's, DC-8 Airborne Science platform. The flights covered over 40,000 km predominantly over northern Canada and Alaska at altitudes up to 13 km. The laser pulses used to measure XCO₂column mixing ratio are also rich in information on the structure and composition of the atmosphere yielding aerosol scattering, cloud properties, planetary boundary layer height and surface reflectance, all in the near infrared at 1.57-µm wavelength.

The CO₂Sounder is a master oscillator power amplifier laser system which produces pulses of 1-µs duration in a 10-KHz pulse train transmitted at near-nadir from the aircraft. The return signals are collected by a 20-cm diameter reflecting telescope and fiber coupled to a low-noise single-photon counting sensitive HgCdTe avalanche photodiode detector array. The laser's output wavelength is tuned across the CO₂absorption line using 30 individual wavelengths and repeated at 330Hz. The time resolved return signal is integrated over 1 µs and the center 10 absorbed wavelengths are not used for the scattering analysis. The data are further averaged over 1 s of ground track and the solar background subtracted. To ensure optimum operation, the instrument is calibrated over desert in clear sky conditions with ~80% transmission and a surface reflectivity of ~40%. The attenuated atmospheric backscatter coefficient is then calculated from the range-corrected, time-resolved backscatter at the non-CO₂absorbed wavelengths.

In this work we will present data showing the mid-summer varied distribution of aerosols, clouds, and atmospheric layering in the northern latitudes. These data provide potentially important ancillary information for understanding greenhouse gas fluxes and budgets in the Arctic. We will also discuss in greater detail the measurement technique, results, and calibration of the instrument.