Atmospheric ammonia (NH₃) is a major precursor of secondary fine particulate matter and is predominantly emitted by agricultural activities. However, the representativeness and complementarity of satellite and surface NH₃ observations remain insufficiently characterized. This study evaluates the consistency between IASI satellite observations and surface NH₃ measurements across the three largest NH₃ emitting regions of France (Brittany, Grand Est, and Auvergne–Rhône-Alpes), representing 44 % of national emissions. A network of 24 monitoring sites, equipped with Radiello passive samplers and Picarro analyzers, operates from June 2024 to June 2025. Surface measurements show higher and more variable NH₃ concentrations at agricultural sites than at background sites, whereas traffic-influenced urban locations exhibit intermediate levels. IASI NH₃ columns reproduce surface regional patterns and seasonal variability, with spring and summer maxima consistent with agricultural emissions. Satellite–surface agreement is assessed across multiple spatial and temporal scales. Strong correlations (R ≥ 0.7) are obtained at biweekly to seasonal timescales when averaging IASI observations within 20–40 km of the monitoring sites, demonstrating its capability to characterize regional NH₃ variability. In contrast, correlations remain weaker at daily and sub-daily timescales, highlighting the limited ability of polar-orbiting satellites to capture localized variability. These findings demonstrate that satellite observations are well suited for long-term NH₃ monitoring and trend analysis, whereas dense surface networks remain essential for resolving fine-scale variability. The forthcoming IRS geostationary mission should improve NH₃ monitoring through enhanced temporal and spatial sampling.