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

In this paper, we present the first multiyear time series of atmospheric ammonia (NH$_{3}$) ground-based measurements in the Paris region (Créteil, 48.79° N, 2.44° E, France) retrieved with the midresolution “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 down to the atmospheric 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 have been compared with NH$_{3}$ concentrations derived from the Infrared Atmospheric Sounding Interferometer (IASI). OASIS ground-based measurements show significant interannual and seasonal variabilities of atmospheric ammonia. NH$_{3}$ total columns over the Paris megacity (12 million people) vary seasonally by 2 orders of magnitude from approximately 0.1×10$^{16}$ molec. cm$^{-2}$ in winter to 10×10$^{16}$ molec. cm$^{-2}$ for spring peaks, probably due to springtime spreading of fertilizers on surrounding croplands.