Decreasing trends of ammonia emissions over Europe seen from remote sensing and inverse modelling.

Ammonia (NH3), a significant precursor of particulate matter, is the most important alkaline gas in the atmosphere and directly affects biodiversity, ecosystems, soil acidification. It also indirectly affects climate and human health. In addition, its concentrations are constantly rising because of the increasing feeding needs of the global population accompanied by a larger use of fertilizers and animal farming. The combination of its increasing atmospheric levels with its environmental and human impact has led many countries to adopt abatement strategies in order to conform with respective regulations. While the significance of ammonia is pronounced, its emissions are often associated with large uncertainties, while its atmospheric abundance is difficult to measure. However, during the last decade, several satellite products have been developed that measure ammonia very effectively, with low uncertainty, and most importantly, with a global coverage. Here, we use satellite observations of column ammonia in combination with an inversion algorithm to derive ammonia emissions with a high resolution over Europe for the period 2013–2020. Ammonia emissions peak in Northern Europe due to agricultural application and livestock management and the local maxima are found over Western Europe (industrial activity) and over Spain (pig farming). Our calculations show that these emissions have decreased by −26 % since 2013 (from 5431 Gg in 2013 to 3994 Gg in 2020) showing that the abatement strategies adopted by the European Union have been very efficient. The slight increase (+4.4 %) reported in 2015 is also reproduced here and is attributed to some European countries exceeding annual emission targets. Ammonia emissions are low in winter (286 Gg) and peak in summer (563 Gg) and are dominated by the temperature dependent volatilization of ammonia from the soil. The largest emission decreases were observed in Central and Eastern Europe (−38 %) and in Western Europe (−37 %), while smaller decreases were recorded in Northern (−17 %) and Southern Europe (−7.6 %). Our results are associated with relatively low uncertainties reaching a maximum of 42 %; when complemented against independent ground-based observations, modelled concentrations using the posterior emissions showed improved statistics, also following the observed seasonal trends. The posterior emissions presented here also agree well with respective estimates reported in the literature and inferred from different methodologies. These results indicate that the posterior emissions of ammonia calculated with satellite measurements and combined with our adapted inverse modelling framework constitute a robust basis for European NH3 estimates and show the de facto evolution of ammonia emissions since 2013. [ABSTRACT FROM AUTHOR]