Pan-European rural monitoring network shows dominance of NH3 gas and NH4NO3 aerosol in inorganic atmospheric pollution load.

A comprehensive European dataset on monthly atmospheric NH 3 , acid gases (HNO 3 , SO 2 , HCl), and aerosols (NH 4+ , NO 3- , SO 42- , Cl - , Na + , Ca 2+ , Mg 2+) is presented and analysed. Speciated measurements were made with a low-volume denuder and filter pack method (DEnuder for Long-Term Atmospheric sampling, DELTA^®) as part of the EU NitroEurope (NEU) integrated project. Altogether, there were 64 sites in 20 countries (2006–2010), coordinated between seven European laboratories. Bulk wet-deposition measurements were carried out at 16 co-located sites (2008–2010). Inter-comparisons of chemical analysis and DELTA^® measurements allowed an assessment of comparability between laboratories. The form and concentrations of the different gas and aerosol components measured varied between individual sites and grouped sites according to country, European regions, and four main ecosystem types (crops, grassland, forests, and semi-natural). The smallest concentrations (with the exception of SO 42- and Na +) were in northern Europe (Scandinavia), with broad elevations of all components across other regions. SO 2 concentrations were highest in central and eastern Europe, with larger SO 2 emissions, but particulate SO 42- concentrations were more homogeneous between regions. Gas-phase NH 3 was the most abundant single measured component at the majority of sites, with the largest variability in concentrations across the network. The largest concentrations of NH 3 , NH 4+ , and NO 3- were at cropland sites in intensively managed agricultural areas (e.g. Borgo Cioffi in Italy), and the smallest were at remote semi-natural and forest sites (e.g. Lompolojänkkä, Finland), highlighting the potential for NH 3 to drive the formation of both NH 4+ and NO 3- aerosol. In the aerosol phase, NH 4+ was highly correlated with both NO 3- and SO 42- , with a near- 1:1 relationship between the equivalent concentrations of NH 4+ and sum (NO 3-+ SO 42-) , of which around 60 % was as NH 4 NO 3. Distinct seasonality was also observed in the data, influenced by changes in emissions, chemical interactions, and the influence of meteorology on partitioning between the main inorganic gases and aerosol species. Springtime maxima in NH 3 were attributed to the main period of manure spreading, while the peak in summer and trough in winter were linked to the influence of temperature and rainfall on emissions, deposition, and gas–aerosol-phase equilibrium. Seasonality in SO 2 was mainly driven by emissions (combustion), with concentrations peaking in winter, except in southern Europe, where the peak occurred in summer. Particulate SO 42- showed large peaks in concentrations in summer in southern and eastern Europe, contrasting with much smaller peaks occurring in early spring in other regions. The peaks in particulate SO 42- coincided with peaks in NH 3 concentrations, attributed to the formation of the stable (NH 4)2 SO 4. HNO 3 concentrations were more complex, related to traffic and industrial emissions, photochemistry, and HNO 3 :NH 4 NO 3 partitioning. While HNO 3 concentrations were seen to peak in the summer in eastern and southern Europe (increased photochemistry), the absence of a spring peak in HNO 3 in all regions may be explained by the depletion of HNO 3 through reaction with surplus NH 3 to form the semi-volatile aerosol NH 4 NO 3. Cooler, wetter conditions in early spring favour the formation and persistence of NH 4 NO 3 in the aerosol phase, consistent with the higher springtime concentrations of NH 4+ and NO 3-. The seasonal profile of NO 3- was mirrored by NH 4+ , illustrating the influence of gas–aerosol partitioning of NH 4 NO 3 in the seasonality of these components. Gas-phase NH 3 and aerosol NH 4 NO 3 were the dominant species in the total inorganic gas and aerosol species measured in the NEU network. With the current and projected trends in SO 2 , NO x , and NH 3 emissions, concentrations of NH 3 and NH 4 NO 3 can be expected to continue to dominate the inorganic pollution load over the next decades, especially NH 3 , which is linked to substantial exceedances of ecological thresholds across Europe. The shift from (NH 4)2 SO 4 to an atmosphere more abundant in NH 4 NO 3 is expected to maintain a larger fraction of reactive N in the gas phase by partitioning to NH 3 and HNO 3 in warm weather, while NH 4 NO 3 continues to contribute to exceedances of air quality limits for PM 2.5. [ABSTRACT FROM AUTHOR]