Pan-European rural monitoring network shows dominance of NH<sub>3</sub> gas and NH<sub>4</sub>NO<sub>3</sub> aerosol in inorganic atmospheric pollution load

Abstract. A comprehensive European dataset on monthly atmospheric NH3, acid gases (HNO3, SO2, HCl), and aerosols (NH4+, NO3-, SO42-, Cl?, Na+, Ca2+, Mg2+) is presented analysed. Speciated measurements were made with a low-volume denuder filter pack method (DEnuder for Long-Term Atmospheric sampling, DELTA®) as part of the EU NitroEurope (NEU) integrated project. Altogether, there 64 sites in 20 countries (2006–2010), coordinated between seven laboratories. Bulk wet-deposition carried out at 16 co-located (2008–2010). Inter-comparisons chemical analysis DELTA® allowed an assessment comparability The form concentrations different gas aerosol components measured varied individual grouped according to country, regions, four main ecosystem types (crops, grassland, forests, semi-natural). smallest (with exception SO42- Na+) northern Europe (Scandinavia), broad elevations all across other regions. SO2 highest central eastern Europe, larger emissions, but particulate more homogeneous Gas-phase NH3 was most abundant single component majority sites, largest variability network. NH4+, NO3- cropland intensively managed agricultural areas (e.g. Borgo Cioffi Italy), remote semi-natural forest Lompolojänkkä, Finland), highlighting potential drive formation both NH4+ aerosol. In phase, highly correlated near-1:1 relationship equivalent sum (NO3-+ SO42-), which around 60 % NH4NO3. Distinct seasonality also observed data, influenced by changes interactions, influence meteorology partitioning inorganic species. Springtime maxima attributed period manure spreading, while peak summer trough winter linked temperature rainfall deposition, gas–aerosol-phase equilibrium. Seasonality mainly driven emissions (combustion), peaking winter, except southern where occurred summer. Particulate showed large peaks contrasting much smaller occurring early spring coincided concentrations, stable (NH4)2SO4. HNO3 complex, related traffic industrial photochemistry, HNO3:NH4NO3 partitioning. While seen (increased photochemistry), absence regions may be explained depletion through reaction surplus semi-volatile Cooler, wetter conditions favour persistence NH4NO3 consistent higher springtime NO3-. seasonal profile mirrored illustrating gas–aerosol these components. dominant species total NEU With current projected trends NOx, can expected continue dominate pollution load over next decades, especially substantial exceedances ecological thresholds Europe. shift from (NH4)2SO4 atmosphere maintain fraction reactive N phase warm weather, continues contribute air quality limits PM2.5.