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Atmospheric ammonia (NH₃) is a major precursor of secondary fine particulate matter, significantly affecting air quality and public health. Emissions are predominantly agricultural, making their mitigation critical, particularly in France, one of the Europe’s largest NH₃ emitter. In line with European targets requiring a 13% reduction in NH₃ emissions by 2030 relative to 2005, a directive adopted in December 2024 mandates NH₃ concentration monitoring at rural sites and urban supersites.Evaluating ammonia emission trends and regulatory compliance is hindered by substantial uncertainties in current emission inventories. While chemistry–transport models and satellite observations offer valuable information on atmospheric NH₃, their reliability depends on validation against robust reference measurements. However, ground-based NH₃ observations in France remain limited and are subject to uncertainties related to measurement artefacts and spatial representativeness.In this context, the ROSAS project (funded by The French Agency for Ecological Transition ADEME) investigates the representativity of surface NH₃ measurements in support of satellite data analysis. A one-year measurement campaign (June 2024–June 2025) was conducted across three regions of interest—Brittany, Grand Est, and Auvergne–Rhône-Alpes—which together account for approximately 44% of French NH₃ emissions. In each region, seven NH₃ instruments (Radiello passive samplers and Picarro analyzers) were deployed. Spatial differences in NH₃ concentrations are examined in relation to local emission sources and meteorological conditions to characterize ground-based observational footprints. The representativity of surface measurements relative to satellite observations is further evaluated by analyzing correlations between IASI (Infrared Atmospheric Sounding Interferometer) NH₃ total columns and ground-based data under varying spatiotemporal coincidence criteria. The results are expected to inform the deployment of atmospheric ammonia measurements network and to improve the effective use of satellite observations.