Simulation studies for a space-based CO2 lidar mission

We report results of initial space mission simulation studies for a laser-based, atmospheric CO₂ sounder, which are based on real-time carbon cycle process modelling and data analysis. The mission concept corresponds to the Active Sensing of CO₂ Emissions over Nights, Days and Seasons (ASCENDS) recommended by the US National Academy of Sciences’ Decadal Survey. As a pre-requisite for meaningful quantitative evaluation, we employ a CO₂ model that has representative spatial and temporal gradients across a wide range of scales. In addition, a relatively complete description of the atmospheric and surface state is obtained from meteorological data assimilation and satellite measurements. We use radiative transfer calculations, an instrument model with representative errors and a simple retrieval approach to quantify errors in ‘measured’ CO₂ distributions, which are a function of mission and instrument design specifications along with the atmospheric/surface state. Uncertainty estimates based on the current instrument design point indicate that a CO₂ laser sounder can provide data consistent with ASCENDS requirements and will significantly enhance our ability to address carbon cycle science questions. Test of a dawn/dusk orbit deployment, however, shows that diurnal differences in CO₂ column abundance, indicative of plant photosynthesis and respiration fluxes, will be difficult to detect.