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

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 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 SO42- and Na+) were in northern Europe (Scandinavia), with broad elevations of all components across other regions. SO2 concentrations were highest in central and eastern Europe, with larger SO2 emissions, but particulate SO42- 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, NH4+, and NO3- 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 NH3 to drive the formation of both NH4+ and NO3- aerosol. In the aerosol phase, NH4+ was highly correlated with both NO3- and SO42-, with a near-1:1 relationship between the equivalent concentrations of NH4+ and sum (NO3-+ SO42-), 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 SO2 was mainly driven by emissions (combustion), with concentrations peaking in winter, except in southern Europe, where the peak occurred in summer. Particulate SO42- 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 SO42- coincided with peaks in NH3 concentrations, attributed to the formation of the stable (NH4)2SO4. HNO3 concentrations were more complex, related to traffic and industrial emissions, photochemistry, and HNO3:NH4NO3 partitioning. While HNO3 concentrations were seen to peak in the summer in eastern and southern Europe (increased photochemistry), the absence of a spring peak in HNO3 in all regions may be explained by the depletion of HNO3 through reaction with surplus NH3 to form the semi-volatile aerosol NH4NO3. Cooler, wetter conditions in early spring favour the formation and persistence of NH4NO3 in the aerosol phase, consistent with the higher springtime concentrations of NH4+ and NO3-. The seasonal profile of NO3- was mirrored by NH4+, illustrating the influence of gas–aerosol partitioning of NH4NO3 in the seasonality of these components. Gas-phase NH3 and aerosol NH4NO3 were 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 NH4NO3 can 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 NH4NO3 is expected to maintain a larger fraction of reactive N in the gas phase by partitioning to NH3 and HNO3 in warm weather, while NH4NO3 continues to contribute to exceedances of air quality limits for PM2.5.

Evaluation of global EMEP MSC-W (rv4.34) WRF (v3.9.1.1) model surface concentrations and wet deposition of reactive N and S with measurements

“…In view of increasing growth in global anthropogenic emissions, the physical and chemical behaviour of reactive nitrogen (N r ) species, especially those that contain reduced N (i.e. gaseous NH 3 and particulate NH 4 + ) have been explored in both experimental and modelling studies (Liu et al, 2019;Wagner et al, 2020;Ciarelli et al, 2019;Tang et al, 2021). As the predominant alkaline gas, NH 3 exerts significant control on the formation of ambient particles and the acidity of deposition.…”

“…Speciated gas-and particlephase sampling and analysis is challenging and expensive (Tang et al, 2018b). Consequently, measurements are generally sparsely located and often not very well temporally resolved, even in regions of the world with well-developed air pollution monitoring networks (Tang et al, 2021), which again limits the interpretation of atmospheric chemical and meteorological processes. Moreover, different world regions have monitoring networks that are subject to different analytical and data handling protocols, potentially leading to systematic differences.…”

Canopy Exchange and Modification of Nitrogen Fluxes in Forest Ecosystems

“…A number of networks have been established to monitor gaseous reactive nitrogen, with particular focus on ammonia-see e.g., Ammonia Monitoring Network (AMoN) in the USA (in addition to CASTNET Estimating dry nitrogen deposition is more complex than what we described already for wet deposition and involves measurements and modeling approaches. At the monitoring sites where concentrations of gaseous nitrogen compounds are measured (commonly with passive samplers, filter packs, denuders [41,47,50]), dry deposition can be estimated by using inferential modeling approach, which consider the deposition velocities of a given compound in relation to the landuse and the vegetation type, but also canopy conductance [46,51]. Dry deposition can also be estimated by considering the differences in ion concentrations between bulk vs. throughfall water fluxes in the so-called canopy budget model ( [52], ref.…”

4D‐Var Inversion of European NH3 Emissions Using CrIS NH3 Measurements and GEOS‐Chem Adjoint With Bi‐Directional and Uni‐Directional Flux Schemes

“…NH 3 emissions have been reported to pose severe air pollution problems and contribute to premature death across the world (Lelieveld et al., 2015). Surface measurements of ambient and precipitation concentrations across Europe and the United States also show that NH x is becoming the dominant contributor to Nr pollution given the substantial reduction of SO x and NO x emissions over the past decades (Du et al., 2014; Elguindi et al., 2020; Ellis et al., 2013; Li et al., 2016; Sutton et al., 2020; Tang et al., 2021). With sustained decreasing trends in SO x and NO x emissions projected alongside increasing trends in NH 3 emissions, NH x pollution is expected to become worse during the next few years.…”