Atmospheric ammonia variability and link with particulate matter formation: a case study over the Paris area

International audienceThe Paris megacity experiences frequent particu- late matter (i.e.PM2.5, particulate matter with a diameter less than 2.5 μm) pollution episodes in spring (March–April). At this time of the year, large numbers of the particles consist of ammonium sulfate and nitrate which are formed from am- monia (NH3) released during fertilizer spreading practices and transported from the surrounding areas to Paris. There is still limited knowledge of the emission sources around Paris, their magnitude, and their seasonality.Using space-borne NH3 observation records of 10 years (2008–2017) and 5 years (2013–2017) provided by the In- frared Atmospheric Sounding Interferometer (IASI) and the Cross-Track Infrared Sounder (CrIS) instrument, regional patterns of NH3 variabilities (seasonal and interannual) are derived. Observations reveal identical high seasonal variabil- ity with three major NH3 hotspots found from March to Au- gust. The high interannual variability is discussed with re- spect to atmospheric total precipitation and temperature.A detailed analysis of the seasonal cycle is performed us- ing both IASI and CrIS instrument data, together with out- puts from the CHIMERE atmospheric model. For 2014 and 2015, the CHIMERE model shows coefficients of determina- tion of 0.58 and 0.18 when compared to IASI and CrIS, re- spectively. With respect to spatial variability, the CHIMERE monthly NH3 concentrations in spring show a slight un- derrepresentation over Belgium and the United Kingdom and an overrepresentation in agricultural areas in the FrenchBrittany–Pays de la Loire and Plateau du Jura region, as well as in northern Switzerland. In addition, PM2.5 concentra- tions derived from the CHIMERE model have been evalu- ated against surface measurements from the Airparif network over Paris, with which agreement was found (r2 = 0.56) with however an underestimation during spring pollution events.Using HYSPLIT cluster analysis of back trajectories, we show that NH3 total columns measured in spring over Paris are enhanced when air masses originate from the north-east (e.g. the Netherlands and Belgium), highlighting the impor- tance of long-range transport in the NH3 budget over Paris. Variability in NH3 in the north-east region is likely to impact NH3 concentrations in the Parisian region since the cross- correlation function is above 0.3 (at lag = 0 and 1 d).Finally, we quantify the key meteorological parameters driving the specific conditions important for the formation of PM2.5 from NH3 in the Île-de-France region in spring. Data- driven results based on surface PM2.5 measurements from the Airparif network and IASI NH3 measurements show that a combination of the factors such as a low boundary layer of ∼ 500 m, a relatively low temperature of 5 ◦C, a high relative humidity of 70 %, and wind from the north-east contributes to a positive PM2.5 and NH3 correlation