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

Abstract

A comprehensive European dataset on monthly atmospheric NH3, acid gases (HNO3, SO2, HCl), and aerosols (NH+4, NO3, SO24 , Cl, NaC, Ca2C, Mg2C/ 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 SO24 and NaC) were in northern Europe (Scandinavia), with broad elevations of all components across other regions. SO2concentrations were highest in central and eastern Europe, with larger SO2emissions, but particulate SO24 concentrations were more homogeneous between regions. Gas-phase NH3 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 NH3, NHC4, and NO3were 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. Lompolojankka, Finland), highlighting the potential for NH3 to drive the formation of both NHC4and NO3aerosol. In the aerosol phase, NHC4was highly corre-lated with both NO3and SO24 , with a near-1 V 1 relationship between the equivalent concentrations of NHC4and sum (NO3C SO24/, of which around 60% was as NH4NO3. 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 NH3 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 SO2was mainly driven by emissions (combustion), with concentrations peaking in winter, except in southern Europe, where the peak occurred in summer. Particulate SO24 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 SO24 coincided with peaks in NH3 concentrations, attributed to the formation of the stable (NH4/2SO4. HNO3concentrations were more complex, related to traffic and industrial emissions, photochemistry, and HNO3:NH4NO3partitioning. While HNO3concentrations were seen to peak in the summer in eastern and southern Europe (increased photochemistry), the absence of a spring peak in HNO3in all regions may be explained by the depletion of HNO3through reaction with surplus NH3 to form the semi-volatile aerosol NH4NO3. Cooler, wetter conditions in early spring favour the formation and persistence of NH4NO3in the aerosol phase, consistent with the higher springtime concentrations of NHC4and NO3. The seasonal profile of NO3was mirrored by NHC4, illustrating the influence of gas-aerosol partitioning of NH4NO3in the seasonality of these components. Gas-phase NH3 and aerosol NH4NO3were the dominant species in the total inorganic gas and aerosol species measured in the NEU network. With the current and projected trends in SO2, NOx , and NH3 emissions, concentrations of NH3 and NH4NO3can be expected to continue to dominate the inorganic pollution load over the next decades, especially NH3, which is linked to substantial exceedances of ecological thresholds across Europe. The shift from (NH4/2SO4 to an atmosphere more abundant in NH4NO3is expected to maintain a larger fraction of reactive N in the gas phase by partitioning to NH3 and HNO3in warm weather, while NH4NO3continues to contribute to exceedances of air quality limits for PM2:5. © 2021 Royal Society of Chemistry. All rights reserved.