A comprehensive European dataset on monthly atmospheric NHinline-formula3, acid gases (HNOinline-formula3, SOinline-formula2, HCl), and aerosols (NHinline-formula
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, NOinline-formula
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, SOinline-formula
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, Clinline-formula−, Nainline-formula+, Cainline-formula2+, Mginline-formula2+) 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.
page876The 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 SOinline-formula
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and Nainline-formula+) were in northern Europe (Scandinavia), with broad elevations of all components across other regions. SOinline-formula2 concentrations were highest in central and eastern Europe, with larger SOinline-formula2 emissions, but particulate SOinline-formula
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concentrations were more homogeneous between regions. Gas-phase NHinline-formula3 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 NHinline-formula3, NHinline-formula
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, and NOinline-formula
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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 NHinline-formula3 to drive the formation of both NHinline-formula
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and NOinline-formula
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aerosol. In the aerosol phase, NHinline-formula
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was highly correlated with both NOinline-formula
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and SOinline-formula
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, with a near-inline-formula1:1 relationship between the equivalent concentrations of NHinline-formula
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and sum (NOinline-formula
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SOinline-formula
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, of which around 60 % was as NHinline-formula4NOinline-formula3.
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 NHinline-formula3 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 SOinline-formula2 was mainly driven by emissions (combustion), with concentrations peaking in winter, except in southern Europe, where the peak occurred in summer. Particulate SOinline-formula
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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 SOinline-formula
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coincided with peaks in NHinline-formula3 concentrations, attributed to the formation of the stable (NHinline-formula4)2SOinline-formula4. HNOinline-formula3 concentrations were more complex, related to traffic and industrial emissions, photochemistry, and HNOinline-formula3:NHinline-formula4NOinline-formula3 partitioning. While HNOinline-formula3 concentrations were seen to peak in the summer in eastern and southern Europe (increased photochemistry), the absence of a spring peak in HNOinline-formula3 in all regions may be explained by the depletion of HNOinline-formula3 through reaction with surplus NHinline-formula3 to form the semi-volatile aerosol NHinline-formula4NOinline-formula3. Cooler, wetter conditions in early spring favour the formation and persistence of NHinline-formula4NOinline-formula3 in the aerosol phase, consistent with the higher springtime concentrations of NHinline-formula
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and NOinline-formula
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. The seasonal profile of NOinline-formula
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was mirrored by NHinline-formula
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, illustrating the influence of gas–aerosol partitioning of NHinline-formula4NOinline-formula3 in the seasonality of these components.
Gas-phase NHinline-formula3 and aerosol NHinline-formula4NOinline-formula3 were the dominant species in the total inorganic gas and aerosol species measured in the NEU network. With the current and projected trends in SOinline-formula2, NOinline-formulax, and NHinline-formula3 emissions, concentrations of NHinline-formula3 and NHinline-formula4NOinline-formula3 can be expected to continue to dominate the inorganic pollution load over the next decades, especially NHinline-formula3, which is linked to substantial exceedances of ecological thresholds across Europe. The shift from (NHinline-formula4)2SOinline-formula4 to an atmosphere more abundant in NHinline-formula4NOinline-formula3 is expected to maintain a larger fraction of reactive N in the gas phase by partitioning to NHinline-formula3 and HNOinline-formula3 in warm weather, while NHinline-formula4NOinline-formula3 continues to contribute to exceedances of air quality limits for PMinline-formula2.5.