Towards a low-resolution infrared sounder for monitoring atmospheric ammonia (NH₃) at high spatial resolution

Abstract. Over the past decade, hyperspectral infrared sounders on satellites have offered global measurements of atmospheric ammonia (NH₃), providing valuable insights into its sources. However, due to their coarse spatial resolution and gaps in spatial coverage, inferring emissions from smaller sources or utilizing data from single overpasses remains very challenging. While a high spatial resolution imaging-sounder would greatly enhance monitoring capabilities, developing an instrument that combines high spatial and spectral resolution is technologically difficult and expensive. Here, we analyze the feasibility of measuring NH₃ with instruments having a largely reduced spectral coverage and resolution compared to current operational sounders. We explore the performance trade-offs using simulated spectra, measurements from the Infrared Atmospheric Sounding Interferometer (IASI) satellite sounder, and spectra obtained from aircraft. The measured spectra are degraded spectrally, and their performance is evaluated using metrics such as NH₃ measurement uncertainty, signal-to-noise ratio, and false alarm rate. Instruments that measure across a continuous spectral interval and instruments covering specific well-chosen spectral bands are both examined. We demonstrate that a future dedicated NH₃ sounder with as few as three spectral bands of 1–5 cm^-1 is feasible and would enable the detection of NH₃ at both high spatial resolution and across continental scales. The advantage of choosing well-defined spectral bands is demonstrated, e.g. by showing that an instrument with five specific bands of 5 cm^-1 performs similarly to one with 20 contiguous channels across 900–1000 cm^-1. Additionally, we show that at high spectral resolutions (below 5 cm^-1), the NH₃ measurement capability is primarily driven by the instrumental noise. As the spectral resolution or number of measurement bands decreases, spectral interferences from other atmospheric constituents and the surface start to dominate the NH₃ retrieval uncertainty budget, fundamentally limiting the unambiguous identification of NH₃.