Greenhouse gases total column measurements from ground-based FTIR spectrometers in the Paris' region for emission inventory optimization.

The ICOS-Cities project aims at evaluating different observational approaches to determine CO2 emissions from large cities, such as Paris (France). A developing approach consists in building a network of three (half-)autonomous FTIR spectrometers used for CO2 total column retrieval: two EM27/SUN placed up and down the prevailing wind axis (Gonesse and Saclay) and one IFS 125 HR placed in Paris Center (Jussieu). These measurements are complementary to the surface measurements carried out in Paris for a decade in the framework of the ICOS European research infrastructure. 

When the wind blows in the instrument location axis, the spatial gradient between stations is considered to be representative of the enhancement due to the surface fluxes in the area in between. It is therefore the signal assimilated by inverse modelling for surface fluxes calculations. The columns are driven not only by the geophysical variations, but also by measurement-specific effects such as solar zenith angle) and measurement noise. Part of our work consists in characterizing these effects so as to be able to make the best use of gradients in calculating fluxes. The expected XCO2 gradients (Saclay-Gonesse) are derived from WRF-GHG modelling that accounts for prior inventory of the Paris area emissions, large scale transport of CO2 constrained by a global inversion, and atmospheric transport constrained by ECMWF meteorological parameters.  The ratio of the measured and modelled gradients is used to estimate a correction to the prior inventory leading to new estimates of the Paris area emissions.  The aim is to analyse the potential contribution of total column estimates, with respect to surface measurements, to estimate urban emissions.