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

A comprehensive European dataset on monthly atmospheric NHinline-formula₃, acid gases (HNOinline-formula₃, SOinline-formula₂, HCl), and aerosols (NHinline-formula $M8inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimga9b2fdba183dceff94210c316afa95ef acp-21-875-2021-ie00001.svg8pt15ptacp-21-875-2021-ie00001.png , NOinline-formula $M9inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg1933cd4f78557ae19e1c84fa4d0b5473 acp-21-875-2021-ie00002.svg9pt16ptacp-21-875-2021-ie00002.png , SOinline-formula $M10inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimg6051c44ba131ac206db43e824688e92d acp-21-875-2021-ie00003.svg13pt17ptacp-21-875-2021-ie00003.png , 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 $M15inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimg9def59c1763723bf85d4c029a1ebd14e acp-21-875-2021-ie00004.svg13pt17ptacp-21-875-2021-ie00004.png 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 $M19inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimg49798bc14746e7788afe38c7f4bc425f acp-21-875-2021-ie00005.svg13pt17ptacp-21-875-2021-ie00005.png 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 $M22inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimg51ca01690260423140b5b0de9583232a acp-21-875-2021-ie00006.svg8pt15ptacp-21-875-2021-ie00006.png , and NOinline-formula $M23inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimgd4917cb251612ae03efebb0a66479930 acp-21-875-2021-ie00007.svg9pt16ptacp-21-875-2021-ie00007.png 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 $M25inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimg97d6670f25963670a5296db4fdcc740f acp-21-875-2021-ie00008.svg8pt15ptacp-21-875-2021-ie00008.png and NOinline-formula $M26inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimgd96e0e0e6a6172a7d34ac185b1d0a8a7 acp-21-875-2021-ie00009.svg9pt16ptacp-21-875-2021-ie00009.png aerosol. In the aerosol phase, NHinline-formula $M27inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimg85fd114689db246c4a85801ec5a5cef2 acp-21-875-2021-ie00010.svg8pt15ptacp-21-875-2021-ie00010.png was highly correlated with both NOinline-formula $M28inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimga1391ede9a489b3338de96b652340830 acp-21-875-2021-ie00011.svg9pt16ptacp-21-875-2021-ie00011.png and SOinline-formula $M29inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimg29af680a2c2c3e13b3242191be5b1002 acp-21-875-2021-ie00012.svg13pt17ptacp-21-875-2021-ie00012.png , with a near-inline-formula1:1 relationship between the equivalent concentrations of NHinline-formula $M31inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimgc1c3b9106f16e5133fc0596b88c825a9 acp-21-875-2021-ie00013.svg8pt15ptacp-21-875-2021-ie00013.png and sum (NOinline-formula $M32inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}+$ 16pt16ptsvg-formulamathimgddbe66d730701bfadefff5769c4fc105 acp-21-875-2021-ie00014.svg16pt16ptacp-21-875-2021-ie00014.png SOinline-formula $M33inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-})$ 16pt17ptsvg-formulamathimgce48dadc7f162bbff7d21af668787813 acp-21-875-2021-ie00015.svg16pt17ptacp-21-875-2021-ie00015.png , 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 $M38inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimgaf927745ea04007aff9f618f883ce002 acp-21-875-2021-ie00016.svg13pt17ptacp-21-875-2021-ie00016.png 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 $M39inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimg43551dd6939ff027a946c13e8be02135 acp-21-875-2021-ie00017.svg13pt17ptacp-21-875-2021-ie00017.png 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 $M55inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimgb2f4688389b28d9f41ec2e2928fae4e0 acp-21-875-2021-ie00018.svg8pt15ptacp-21-875-2021-ie00018.png and NOinline-formula $M56inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg3a8cc437bdf71dca45b98349c6558e05 acp-21-875-2021-ie00019.svg9pt16ptacp-21-875-2021-ie00019.png . The seasonal profile of NOinline-formula $M57inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimga402a240ce5132e2a27ae10dba777396 acp-21-875-2021-ie00020.svg9pt16ptacp-21-875-2021-ie00020.png was mirrored by NHinline-formula $M58inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimg86d32ea7a5702b11ca110db304544d0d acp-21-875-2021-ie00021.svg8pt15ptacp-21-875-2021-ie00021.png , 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-formula_x, 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-formula_2.5.