Biosphère Climat

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In the context of the current global warming, it is fundamental to understand the surface-atmosphere exchanges of atmospheric CO 2 ( IPCC 2014 ). The study of the spatial and temporal heterogeneity of these exchanges is necessary to move from the small scale (in situ observation) to the medium scale (mesh of a surface / atmosphere model) and have better precision on climate forecasts. In situ observation of CO 2 exchanges in a complex environment (area with high orography, urban area, partially flooded area, volcanic area) is complicated if not impossible and requires a capacity for remote observation.

In addition, following COP21 and the Paris agreements, each country is asked to quantify its own CO 2 emissions of anthropogenic origin. With this request, the question arises of the reliability of emission inventories. Uncertainty increases with space-time resolution and can be significant for developing countries.

LIDAR makes it possible to carry out maps of the CO 2 density over key areas and thus to respond to the above issues. The quantification of CO 2 sources and sinks requires exceptional precision and accuracy for a remote sensing instrument (<1%). However, thanks to technological developments (laser source and detector), this type of measurement will be available in the coming years with high spatio-temporal resolution (100 m - 1 s) and over a long distance (1-10 km).

Figure 1   Main geophysical research objectives for atmospheric CO 2 and possibilities for measuring ground, airborne and space lidar

First tests of the LIDAR COWI (CO 2 & Wind) in horizontal firing configuration were carried out in July 2013 in Palaiseau, France. LIDAR delivers a 2-D (distance-time) mapping of the CO 2 diurnal cycle and the radial wind speed over the École Polytechnique campus. The dry air CO 2 mixing ratio is estimated from LIDAR measurements and additional spectroscopy and meteorology data. The measurements are compared with simultaneous in situ measurements ( PICARRO instrument ).

The measurements show the variation of CO 2 in the surface layer in peri-urban areas (close to sources of anthropogenic emissions) during a diurnal cycle with some characteristic signals: advection of a plume of CO 2 from the thermal power plant on campus (1 h), emissions from morning road traffic (6 h), concentration jump correlated with a sudden change in wind related to the passage of a thunderstorm on the site (12 h) [Gibert et al . 2015]

Figure 2   Study of the variability of atmospheric CO 2 in the surface layer, above the Ecole Polytechnique campus
(a) Horizontal profiles of the atmospheric CO 2 mixing ratio . Comparison with in situ laboratory measurements   (b) Radial wind speed