Diurnal, synoptic and seasonal variability of atmospheric CO<sub>2</sub> in the Paris megacity area

Abstract

Abstract. Most of the global fossil fuel CO 2 emissions arise from urbanized and industrialized areas. Bottom-up inventories quantify them but with large uncertainties. In 2010-2011, the first atmospheric in situ CO 2 measurement network for Paris, the capital of France, began operating with the aim of monitoring the regional atmospheric impact of the emissions coming from this megacity. Five stations sampled air along a northeast-southwest axis that corresponds to the direction of the dominant winds. Two stations are classified as rural (Traînou -TRN; Montgé-en-Goële -MON), two are peri-urban (Gonesse -GON; Gif-sur-Yvette -GIF) and one is urban (EIF, located on top of the Eiffel Tower). In this study, we analyze the diurnal, synoptic and seasonal variability of the in situ CO 2 measurements over nearly 1 year (8 August 2010-13 July 2011). We compare these datasets with remote CO 2 measurements made at Mace Head (MHD) on the Atlantic coast of Ireland and support our analysis with atmospheric boundary layer height (ABLH) observations made in the center of Paris and with both modeled and observed meteorological fields. The average hourly CO 2 diurnal cycles observed at the regional stations are mostly driven by the CO 2 biospheric cycle, the ABLH cycle and the proximity to urban CO 2 emissions. Differences of several µmol mol −1 (ppm) can be observed from one regional site to the other. The more the site is surrounded by urban sources (mostly residential and commercial heating, and traffic), the more the CO 2 concentration is elevated, as is the associated variability which reflects the variability of the urban sources. Furthermore, two sites with inlets high above ground level (EIF and TRN) show a phase shift of the CO 2 diurnal cycle of a few hours compared to lower sites due to a strong coupling with the boundary layer diurnal cycle. As a consequence, the existence of a CO 2 vertical gradient above Paris can be inferred, whose amplitude depends on the time of the day and on the season, ranging from a few tenths of ppm during daytime to several ppm during nighttime. The CO 2 seasonal cycle inferred from monthly means at our regional sites is driven by the biospheric and anthropogenic CO 2 flux seasonal cycles, the ABLH seasonal cycle and also synoptic variations. Enhancements of several ppm are observed at peri-urban stations compared to rural ones, mostly from the influence of urban emissions that are in the footprint of the Published by Copernicus Publications on behalf of the European Geosciences Union. 3336 I. Xueref-Remy et al.: Diurnal, synoptic and seasonal variability of atmospheric CO 2 peri-urban station. The seasonal cycle observed at the urban station (EIF) is specific and very sensitive to the ABLH cycle. At both the diurnal and the seasonal scales, noticeable differences of several ppm are observed between the measurements made at regional rural stations and the remote measurements made at MHD, that are shown not to define background concentrations appropriately for quantifying the regional (∼ 100 ...

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