A broad band lidar for precise atmospheric CO2 column absorption measurement from space

Global measurement of carbon dioxide column with the aim of discovering and quantifying unknown sources and sinks has been a high priority for the last decade. In order to uncover the “missing sink” that is responsible for the large discrepancies in the budget the critical precision for a measurement needs to be on the order of 3 ppm. That number comes from the transport inversion model experiments, which indicate that global column data with a precision better than 1% (3ppmv on the 380 ppmv background) on a time scale of one month is the science requirement to improve surface flux estimates [1]. No species has ever been measured from space at such a precision. In recognition of the importance of understanding the CO2 budget and in order to evaluate its impact on global warming the National Research Council (NRC) in its recent decadal survey report (NACP) to NASA, recommended planning for a laser based total CO2 mapping mission in the near future [2]. That’s the goal of Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission -to significantly enhance the understanding of the role of CO2 in the global carbon cycle. Existing passive sensors suffer from two shortcomings. Their measurement precision can be compromised by the path length uncertainties arising from scattering within the atmosphere. Also passive sensors using sunlight cannot observe the column at night. Both of these difficulties can be ameliorated by lidar techniques.