Ammonia (NH3) is a short-lived atmospheric pollutant with significant environmental and health impacts. Monitoring NH3 remains challenging, as diurnal variability at local scales is still poorly documented. In this study, we analyze two years (July 2022–June 2024) of NH3 total columns from the Geostationary Interferometric Infrared Sounder onboard China's FengYun-4B (GIIRS-B) over East Asia. After applying quality and uncertainty filters, we find good agreement with Infrared Atmospheric Sounding Interferometer (IASI) morning observations (weighted Pearson R = 0.64) and identify relationships between NH3 and skin/land surface temperature over five major NH3 hotspots, suggesting contributions from agriculture (urea fertilizer use), livestock, and secondary urban sources. GIIRS-B's high temporal resolution reveals a clear bimodal diurnal pattern, with NH3 enhancements in the early morning and mid-afternoon in four regions. A dedicated analysis of GIIRS-B NH3 retrieval uncertainties provides a realistic physical benchmark for geostationary infrared observations. Using radiative transfer simulations (4A/OP) driven by atmospheric chemistry model outputs (CHIMERE), we evaluate the potential of the European InfraRed Sounder (IRS) onboard MTG-S to retrieve NH3 at sub-daily resolution. IRS uncertainties are generally larger and more variable than those of IASI, but under favorable thermal-contrast conditions they can become comparable. GIIRS-B and IRS exhibit consistent diurnal uncertainty patterns with nighttime maxima and daytime minima, confirming the realism of the IRS performance assessment. These results highlight the added value of geostationary sounders for improving NH3 emission monitoring, source attribution, and diurnal process understanding in support of future European air-quality regulations.