The importance of atmospheric carbon dioxide (CO2) as a key contributor to greenhouse effect and so to global warming and climate change is widely documented by the scientific community [IPCC 2001]. In this respect, the monitoring of atmospheric CO2 is essential for a better understanding of CO2 concentration time and space changes at different scales. Identification of atmsopheric CO2 is a matter of special importance in the investigation of the global carbon cycle. One might also wonder what the evolution of such surface flux in the general context of global warming will be in the next decades for climate prediction and the best politic, economic and human life adjustements.
As it stands today, the most reliable monitoring activity is conducted from the ground using in situ sensors and instrumented towers in the framework of regional networks [Conway et al. 1994, Lambert et al. 1995]. Despite additionnal airborne measurements which are conducted in a regular basis in some locations [Matsueda and Inoue, 1996; Loyd et al. 2001; Schmitgen et al. 2004; Bakwin et al. 2003], the vertical dimension of CO2 concentration and flux fluctuations is still poorly addressed.
Fig. 1: Surface flux measurements (flux chamber, flux tower, biomass inventory) and estimates (atmospheric inventory) and gap between the different scales which can be filled by Lidar observations.
Ecosystem carbon exchanges on spatial scales of approximately one km2 can be well documented using the eddy-covariance (EC) technique [e.g. Wofsy et al., 1993; Baldocchi et al., 1996; Valentini et al., 2000]. However, little data exists on the carbon budget at the regional scale, ranging from a few tenths to few hundreds of km2 (Fig. 1). At these scales, the carbon fluxes can be estimated either by upscaling pointwise flux measurements using for instance airborne flux transects, biomass inventory, biophysical models and remote sensing informations [Turner et al., 2004, Miglietta et al., 2006] or by inverting atmospheric concentration measurements using a fine scale mesoscale transport model [Uliasz et al., 2005] (Fig. 1). This latter method however is still under development and requires very dense atmospheric observation datasets [Dolman et al., 2006] which is far from the current ability of the global surfec network (~ 100 stations). The intrinsic limitations in space and time call for a significant improvement of the overall global observational capability. Global monitoring, ultimately from space, is foreseen as a mean to quantify sources and sinks on a regional scale and to better understand the links between the various components of the carbon cycle. A vertical profile would be ideal, but a column-integrated amount or column-weighted amount is also valuable, provided that the lower troposphere contributes significantly.
Active remote sensors like lidar can complement the existing ground-based network and could ultimately be operated in space [Flamant et al. 2005]. However, a necessary step prior to any deployment in space for global coverage measurements is a convincing demonstration of the capability of the CO2 differential absorption lidar (DIAL) either from the ground or an aircraft platform. Since Koch et al.  and Gibert et al. (2006), several teams in the world developped DIAL lidars for CO2 atmospheric measurements (see Abshire et al.  for a review).
Geophysical goals of LIDAR-ABC group
The LIDAR group research projects concern the atmospheric branch of carbon cycle, more precisely carbon dioxide (CO2) and methane (CH4). The goals are to close the gaps that exist in current measurements or estimates of concentration and fluxes of CO2. Identification of sources and sinks and understanding of vertical and horizontal transport are particularly addressed. Coherent DIAL systems are currently developped to make:
3-D measurements of CO2 concentration in the boundary layer to address the issue of spatial representativity of in-situ sensor.
Simultaneous range-resolved concentration and velocity measurements and direct turbulent flux measurements using the eddy-covariance technique. This will enable to study CO2 diffusivity and transport in the troposphere.
IPDA (Integrated Path Differential Absorption) measurements to demonstrate operation from space which is seen as an answer to the needs of the carbon cycle scientific community for a global monitoring of CO2 mixing ratio on the Earth surface.
In this context, new Differential Absorption Lidars were (EMIL-LIDIA) and are currently (COWI) in developpement at LMD.