Ten years of measurements (2012-2022) of the atmospheric composition at Saclay/SIRTA observatory in the Ile de France region as part of ICOS and ACTRIS

Abstract. CO2 is the main contributor to global warming, and cities now account for more than two-thirds of emissions of this gas. Atmospheric observatories located on the outskirts of cities are therefore important facilities for measuring the impact on atmospheric composition of the emission reductions planned by cities. The Saclay observatory, part of the ICOS and ACTRIS research infrastructures and located 20 km southwest of Paris, has been monitoring greenhouse gases (CO2, CH4), reactive gases (NOx, O3, CO), and carbonaceous aerosols (eBC) since 2012. This study presents 10 years of monitoring of these compounds, characterizing diurnal, seasonal cycles and decadal trends. In order to best characterize the impact of Parisian emissions, we defined two sets of data depending on whether the station is downwind of Paris or, conversely, in background conditions with westerly winds. This strategy allows us to characterize the urban offset in the Saclay measurement series. The results show a significant decrease in the urban offset of compounds mainly linked to traffic emissions: -35.6 %, -52.3 %, and -56.7 % for CO, NOx, and eBClf. There was also a 15 % decrease in urban offset of CO2 between the 2012–2017 and 2019–2022 periods, a figure consistent with the Airparif inventories' estimate of the decrease in emissions in Paris over the same period.