New quantitative measurements and spectroscopic line parameters of ammonia in the 685–1250 cm^-1 spectral region for atmospheric remote sensing

Abstract. Ammonia (NH₃) is a toxic pollutant, generally linked to agricultural emissions, and plays a major role in the formation of fine aerosols which have a significant and detrimental effect on human health. NH₃ is one of the key pollutants that can be monitored by satellite instruments orbiting the Earth, including the Infrared Atmospheric Sounding Interferometer (IASI) and the Cross-track Infrared Sounder (CrIS). The interpretation of these measured atmospheric spectra requires accurate radiative transfer modelling, which relies on the quality of the input spectroscopic line parameters.

In this work we present new high quality high-resolution infrared spectra of self- and air-broadened NH₃ at 296 K using a Bruker IFS 125HR spectrometer and a 24.45 cm pathlength sample cell with silver chloride windows. Using a multispectrum fitting approach, we then determine new spectroscopic line parameters over the range 685 cm^-1 to 1250 cm^-1 for the NH₃ 0100 00 0 s ← 0000 00 0 a and 0100 00 0 a ← 0000 00 0 s transitions associated with the ν₂ mode; the Q branches of these transitions are the strongest NH₃ features observed in atmospheric spectra. Our analysis utilises the Voigt lineshape, with speed-dependent Voigt and Rosenkranz line mixing for the strongest lines. To date this is the most complete experimental and multispectrum analysis of air-broadened NH₃ over this spectral region. Our derived spectroscopic line parameters reproduce the new measurements substantially better than line parameters from the HITRAN 2020 database, which were derived from a mixture of ab initio calculations and previous laboratory measurements. We have revised values for parameters such as line intensities and air-broadened Lorentz halfwidths, in some cases by almost 10 %. We have substantially lowered the uncertainties of key parameters, such as line intensities. In addition to the measured speed dependence and Rosenkranz line mixing parameters, which we believe are the first reported for the ν₂ band of NH₃ in air, we also determine a range of parameters for the ν₂ band that are not currently in HITRAN, for example self- and air-pressure-induced shifts. We expect these new parameters to provide a more accurate basis for incorporation into atmospheric radiative transfer models to measure NH₃ concentrations from satellite.