Nitrosat, a Satellite Mission Concept for Mapping Reactive Nitrogen at the Landscape Scale

Two key forms of reactive nitrogen (Nr) in the atmosphere are nitrogen oxides (NO+NO₂) and ammonia (NH₃). Both species are abundantly emitted from anthropogenic sources (fossil fuel combustion, agriculture) with devastating consequences on the environment, human health and climate. Complementing sparse ground-based measurements, current satellite sounders provide daily coverage of their global distribution. However, the spatial resolution of these instruments (>20 km² for NO₂ and >100km² for NH₃) is orders of magnitudes greater than the typical size of the main Nr sources (industries, farms, roads), which makes identification of the emitters, and corresponding quantification of their emission strengths particularly challenging.

To understand and address the impacts of a perturbed nitrogen cycle, and in response to the current observational gap, a dedicated satellite for the monitoring of NO₂ and NH₃ at high spatial resolution has been conceptualised. Nitrosat, as it is being called, is currently in Phase 0 of ESA’s Earth Explorer 11 call. Its main objective is to quantify simultaneously the emission sources of NH₃ and NO_x at the landscape scale (<0.25 km²) and to characterize seasonal patterns (<1 month) in their emissions. The two imaging spectrometers onboard Nitrosat will operate respectively in the infrared for NH₃ and the visible for NO₂, offering gapless coverage in a single swath.

Starting from representative examples of measurement techniques that are presently used to derive emission fluxes from NH₃ and NO₂ satellite observations, we discuss the limitations of current sounders. We introduce the Nitrosat measurement concept and, exploiting both model simulations and aircraft campaign data, provide examples from the Phase 0 studies of how Nitrosat will enable retrieval of emission fluxes from local and diffuse sources in a way that will not be possible with other current or planned missions.