Vertical 2-μm heterodyne differential absorption lidar measurements of mean CO₂ mixing ratio in the troposphere

Vertical mean CO₂ mixing ratio measurements are reported in the atmospheric boundary layer (ABL) and in the lower free troposphere (FT), using a 2-μm heterodyne differential absorption lidar (HDIAL). The mean CO₂ mixing ratio in the ABL is determined using 1) aerosol backscatter signal and a mean derivative of the increasing optical depth as a function of altitude and 2) optical depth measurements from cloud target returns. For a 1-km vertical long path in the ABL, 2% measurement precision with a time resolution of 30 min is demonstrated for the retrieved mean CO₂ absorption. Spectroscopic calculations are reported in details using new spectroscopic data in the 2-μm domain and the outputs of the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5). Then, using both aerosols in the ABL and midaltitude dense clouds in the free troposphere, preliminary HDIAL measurements of mean CO₂ mixing ratio in the free troposphere are also presented. The 2-μm HDIAL vertical measurements are compared to ground-based and airborne in situ CO₂ mixing ratio measurements and discussed with the atmospheric synoptic conditions.