CO2 Sounder lidar multi-wavelength approach: Minimizing biases and extracting vertical CO2 information

The NASA Goddard Space Flight Center CO2 Sounder measures the CO2 mixing ratio using a pulsed multi-wavelength integrated path differential absorption (IPDA) approach. Compared to passive CO2 sensors, such as those aboard GOSAT and OCO-2 satellites, active remote sensing measurements offer many advantages including high-latitude and night coverage, robustness against aerosol and cloud contamination, precise range resolution, and column measurements to cloud tops. The CO2 Sounder instrument employs a multi-wavelength sampling approach, which helps bias correction and providing vertical CO2 information. Biases in remote sensing measurements from satellites such as GOSAT and OCO-2 have been found to be one of the main factors limiting the accuracy of CO2 flux estimate with the data. While lidar XCO2 retrievals do not incur biases from aerosols, they are still subject to biases from other sources such as Doppler shifts and the wavelength response of the optical filter used to block out the solar background from the detector. The CO2 Sounder multi-wavelength approach helps identify and correct such biases. We will discuss some biases observed in past airborne campaigns and ways to mitigate them. The multi-wavelength approach combined with the high spectral resolution of lidar measurements also provides the vertically-resolved CO2 information by taking advantage of the altitude-dependent pressure-broadening and sharp vertical weighting function. Simulations of the CO2 Sounder approach shows that with sufficient averaging to get the required signal-to-noise ratio, one can resolve at least two pieces of vertical information, corresponding to the columnaveraged CO2 mixing ratio and the CO2 vertical gradient. However, compared to the column-averaged CO2 mixing ratio, the higher order terms are more sensitive to systematic errors and other effects. We will show simulation results and some preliminary airborne vertical gradient retrievals from past field campaigns.