Article révisé par les pairs
Résumé : Abstract. The Paris megacity experiences frequent particulate matter (i.e.PM2.5, particulate matterwith a diameter less than 2.5 µm) pollution episodes in spring(March–April). At this time of the year, large numbers of the particlesconsist of ammonium sulfate and nitrate which are formed from ammonia(NH3) released during fertilizer spreading practices and transportedfrom the surrounding areas to Paris. There is still limited knowledge of theemission sources around Paris, their magnitude, and their seasonality. Using space-borne NH3 observation records of 10 years (2008–2017) and5 years (2013–2017) provided by the Infrared Atmospheric SoundingInterferometer (IASI) and the Cross-Track Infrared Sounder (CrIS)instrument, regional patterns of NH3 variabilities (seasonal andinterannual) are derived. Observations reveal identical high seasonalvariability with three major NH3 hotspots found from March to August.The high interannual variability is discussed with respect to atmospherictotal precipitation and temperature. A detailed analysis of the seasonal cycle is performed using both IASI andCrIS instrument data, together with outputs from the CHIMERE atmosphericmodel. For 2014 and 2015, the CHIMERE model shows coefficients ofdetermination of 0.58 and 0.18 when compared to IASI and CrIS,respectively. With respect to spatial variability, the CHIMERE monthlyNH3 concentrations in spring show a slight underrepresentation overBelgium and the United Kingdom and an overrepresentation in agricultural areasin the French Brittany–Pays de la Loire and Plateau du Jura region, as wellas in northern Switzerland. In addition, PM2.5 concentrationsderived from the CHIMERE model have been evaluated against surfacemeasurements from the Airparif network over Paris, with which agreement wasfound (r2 = 0.56) with however an underestimation during springpollution events. Using HYSPLIT cluster analysis of back trajectories, we show that NH3total columns measured in spring over Paris are enhanced when air massesoriginate from the north-east (e.g. the Netherlands and Belgium), highlightingthe importance of long-range transport in the NH3 budget over Paris.Variability in NH3 in the north-east region is likely to impact NH3concentrations in the Parisian region since the cross-correlation functionis above 0.3 (at lag = 0 and 1 d). Finally, we quantify the key meteorological parameters driving the specificconditions important for the formation of PM2.5 from NH3 in theÎle-de-France region in spring. Data-driven results based on surfacePM2.5 measurements from the Airparif network and IASI NH3measurements show that a combination of the factors such as a low boundarylayer of ∼500 m, a relatively low temperature of 5 ∘C, a high relative humidity of 70 %, and wind from the north-eastcontributes to a positive PM2.5 and NH3 correlation.

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