This study investigates single pixel nadir viewing detection limit of atmospheric ammonia for a range of area flux mapping satellite infrared sensor spectral resolutions (0.05 cm-1 to 2.0 cm-1) and measurement noise levels. The detection level of ammonia is computed directly from simulated satellite ammonia spectral signatures, which has the advantage of being independent of retrieval methodologies. Information on the frequency of a given detection limit, and the cumulative probability of detection, are provided as a function of instrument spectral resolution and noise. For example, a Cross-track Infrared Sounder-like instrument with a modest spectral resolution of 0.625 cm-1 and excellent signal-to-noise ratio of ~1600 would be able to detect ammonia on average ~70% of the time; these same instrument specifications will have a detection limit of 0.2 ppbv (surface) or 1.6x1015 molec cm-2 (total column) that can be achieved at a detection rate of 10% as it requires favourable infrared remote sensing conditions (large thermal contrast). Under more typical atmospheric states a detection limit of 0.5 ppbv (3.5x1015 molec cm-2) is achieved at a 50% detection rate. This detection limit information is valuable for applications that incorporate remote sensing data in conditions when the atmospheric  ammonia amounts are below the detection limit of the satellite sensor (e.g. non-growing season in crop fertilizer source regions). As the simulations use real-world atmospheric state observations as inputs the results can be used to provide general guidance on the detection limits of past, current, and potential new environmental flux mapping instruments used for ammonia monitoring covering large geographical regions.

We conduct the first 4D-Var inversion of NH3 accounting for NH3 bidirectional flux, using CrIS satellite NH3 observations over Europe in 2016. We find posterior NH3 emissions peak more in springtime than prior emissions at continental to national scales, and annually they are generally smaller than the prior emissions over central Europe, but larger over most of the rest of Europe. Annual posterior anthropogenic NH3 emissions for 25 European Union members (EU25) are 25% higher than the prior emissions and very close(<2% difference) to other inventories. Our posterior annual anthropogenic emissions for EU25, the UK, the Netherlands, and Switzerland are generally 10-20% smaller than when treating NH3 fluxes as uni-directional emissions, while the monthly regional difference can be up to 34% (Switzerland in July). Compared to monthly mean in-situ observations, our posterior NH3 emissions from both schemes generally improve the magnitude and seasonality of simulated surface NH3 and bulk NHx wet deposition throughout most of Europe, whereas evaluation against hourly measurements at a background site shows the bi-directional scheme better captures observed diurnal variability of surface NH3. This contrast highlights the need for accurately simulating diurnal variability of NH3 in assimilation of sun-synchronous observations and also the potential value of future geostationary satellite observations. Overall, our top-down ammonia emissions can help to examine the effectiveness of air pollution control policies to facilitate future air pollution management, as well as helping us understand the uncertainty in top-downNH3emission estimates associated with treatment of NH3surface exchange.