Analysis of temporal and spatial variability of atmospheric CO 2 concentration within Paris from the GreenLITE TM laser imaging experiment

Abstract. In 2015, the Greenhouse gas Laser Imaging Tomography Experiment (GreenLITE TM ) measurement system was deployed for a long-duration experiment in the center of Paris, France. The system measures near-surface atmospheric CO 2 concentrations integrated along 30 horizontal chords ranging in length from 2.3 km to 5.2 km and covering an area of 25 km 2 over the complex urban environment. In this study, we use this observing system together with six conventional in-situ point measurements and the WRF-Chem model coupled with two urban canopy schemes (UCM, BEP) at a horizontal resolution of 1 km to analyze the temporal and spatial variations of CO 2 concentrations within the Paris city and its vicinity for the 1-year period spanning December 2015 to November 2016. Such an analysis aims at supporting the development of CO 2 atmospheric inversion systems at the city scale. Results show that both urban canopy schemes in the WRF-Chem model are capable of reproducing the seasonal cycle and most of the synoptic variations in the atmospheric CO 2 point measurements over the suburban areas, as well as the general corresponding spatial differences in CO 2 concentration that span the urban area. However, within the city, there are larger discrepancies between the observations and the model results with very distinct features during winter and summer. During winter, the GreenLITE TM measurements clearly demonstrate that one urban canopy scheme (BEP) provides a much better description of temporal variations and horizontal differences in CO 2 concentrations than the other (UCM) does. During summer, much larger CO 2 horizontal differences are indicated by the GreenLITE TM system than both the in-situ measurements and the model results, with systematic east-west variations.