PM_2.5 composition at an atmospheric ammonia hotspot in Europe

Fine particulate matter (PM_2.5) pollution poses a significant environmental risk to human health, ecosystems, and climate, both across Europe and globally. Ammonia (NH₃), primarily emitted by agro-livestock activities, is a major precursor gas for secondary particulate formation; critically, however, it remains largely unregulated in European directives and is seldom monitored within conventional air quality networks. This study investigates the chemical composition, formation mechanisms, and potential sources of PM_2.5 within a critical European agricultural region (Central Ebro Valley, CEV), recognized as a major atmospheric NH₃ hotspot. NH₃ concentration and PM_2.5 chemical composition data were collected during two intensive campaigns spanning distinct winter and summer seasonal regimes. Results demonstrate that the study area is characterized by an NH₃-saturated environment, which fundamentally controls PM_2.5 formation dynamics and leads to severe regional pollution episodes. The high ammoniacal availability is identified as a key factor promoting the generation of high concentrations of Secondary Inorganic Aerosols (SIA), particularly during winter. Furthermore, the characteristic summer atmospheric dynamics, featuring high vertical dispersion driven by the intense development of the Planetary Boundary Layer (PBL), enhances the region's role as an atmospheric NH₃ reservoir for the wider Mediterranean Basin, establishing it as a major source area in Southern Europe. A preliminary source contribution analysis based on Positive Matrix Factorization (PMF) identified five factors with marked seasonality, confirming the unquestionable impact of agro-livestock NH₃ emissions, biomass burning, mineral resuspension from arid land surfaces, and African dust intrusion during summer. This study underscores the critical importance of policy decisions targeting emission controls within the agricultural and livestock sectors to effectively mitigate PM_2.5 levels locally and regionally.