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Most of the global fossil fuel CO 2 emissions arise out of 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, has been operated with the aim of monitoring the regional atmospheric impact of the emissions out 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 (TRN and MON), two are peri-urban (GON and 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 CO2 measurements over nearly one 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 centre 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 traffic, residential and commercial heating), the more the CO 2 concentration is elevated, as is the associated variability which reflects the variability of the urban sources. Furthermore, two elevated sites (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 are driven by the biospheric and anthropogenic CO 2 flux seasonal cycles, by the ABLH seasonal cycle and also by synoptic variations. Gradients of several ppm are observed between the rural and peri-urban stations, mostly from the influence of urban emissions that are in the footprint of the 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 can be 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 atmospheric impact of urban CO 2 emissions. For wind speeds less than 3 m s −1 , the accumulation of the local CO 2 emissions in the urban atmosphere forms a dome of several tens of ppm at the peri-urban stations, mostly under the influence of relatively local emissions including those from the Charles-De-Gaulle (CDG) airport facility and from aircrafts in flight. When wind speed increases, ventilation transforms the CO 2 dome into a plume. Higher CO 2 background concentrations of several ppm are advected from the remote Benelux-Ruhr and London regions, impacting concentrations at the five stations of the network even at wind speeds higher than 9 m s −1 . For wind speeds ranging between 3 and 8 m s −1 , the impact of Paris emissions can be detected in the peri-urban stations when they are downwind of the city, while the rural stations often seem disconnected from the city emission plume. As a conclusion, our study highlights a high sensitivity of the stations to wind speed and direction, to their distance from the city, but also to the ABLH cycle depending on their elevation. We learn some lessons regarding the design of an urban CO 2 network: 1/ careful attention should be paid to properly setting background sites that will be representative of the different wind sectors; 2/ the downwind stations should as much as possible be positioned symmetrically in relation to the city centre, at the peri-urban/rural border; 3/ the stations should be installed at ventilated sites (away from strong local sources) and the air inlet set-up above the building or biospheric canopy layer, whichever is the greatest; and 4/ high resolution wind information should be available with the CO 2 measurements.

Diurnal, synoptic and seasonal variability of atmospheric CO&lt;sub&gt;2&lt;/sub&gt; in the Paris megacity area

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