Atmospheric ammonia (NH₃) over the Paris megacity: 9 years of total column observations from ground-based infrared remote sensing

International audienceIn this paper, we present the first multi-year time series of atmospheric NH3 ground-based measurements in the Paris region (Créteil, 48.79° N, 2.44° E, France) retrieved with the mid-resolution “Observations of the Atmosphere by Solar absorption Infrared Spectroscopy” (OASIS) ground-based Fourier Transform infrared solar observatory. Located in an urban region, OASIS has previously been used for monitoring air quality (tropospheric ozone and carbon monoxide), thanks to its specific column sensitivity across the whole troposphere including the planetary boundary layer. A total of 4920 measurements of atmospheric total columns of ammonia have been obtained from 2009 to 2017, with uncertainties ranging from 20 % to 35 %, and are compared with NH3 concentrations derived from the Infrared Atmospheric Sounding Interferometer (IASI). OASIS ground-based measurements show significant interannual, and seasonal variabilities of atmospheric ammonia. NH3 total columns over the Paris megacity (12 million people) vary seasonally by 2 orders of magnitude, from approximately 1015 molecules cm−2 in winter to 1017 molecules cm−2 for spring peaks, probably due to springtime spreading of fertilizers on surrounding croplands. Also, we observe a correlation of daily NH3-OASIS total columns with daily PM2.5 in situ measurements from the closest Airparif surface station in springtime