Laser Sounder for Global Measurement of CO2 Concentrations in the Troposphere from Space

We report progress in assessing the feasibility of a new satellite-based laser-sounding instrument to measure CO2 concentrations in the lower troposphere from space. INTRODUCTION CO2 measurements from ice cores show that atmospheric CO2 concentrations are higher now than they have been in the past 400,000 years. It is becoming increasingly important to understand the nature and processes of the CO2 sinks, on a global scale, in order to make predictions of future atmospheric composition. Accurate measurements of tropospheric CO2 abundance with global-coverage, 300 km spatial and monthly temporal resolution are needed to quantify processes that regulate CO2 storage by the land and oceans [1]. The NASA Orbiting Carbon Observatory (OCO) is the first space mission focused on atmospheric CO2 for measuring total column CO2 and O2 by detecting the spectral absorption in reflected sunlight. The OCO mission is a key first step, and will yield important new information about atmospheric CO2 distributions. However there are unavoidable limitations imposed by its measurement approach. These include best accuracy only during daytime at moderate to high sun angles, interference by cloud and aerosol scattering, and limited signal from CO2 variability in the lower tropospheric CO2 column. The recent NRC Decadal Survey for Earth Science [2] has recommended addressing these un-met needs in a laser-based CO2 measuring mission called ASCENDS. We have been in developing a laser technique for the remote measurement of the tropospheric CO2 concentrations from orbit [3-6]. Our initial goal is to demonstrate a lidar technique and instrument technology that will permit measurements of the CO2 column abundance in the lower troposphere from aircraft at the few ppm level. Our final goal is to develop a space instrument and mission approach for active CO2 measurements. This would allow continuous measurements of CO2 mixing ratio from orbit, both day and night, over land and ocean surfaces, and under realistic atmospheric scattering conditions.