Pan-European rural monitoring network shows dominance of NH3 gas and NH4NO3 aerosol in inorganic atmospheric pollution load
Authors: Tang Y. Sim , Flechard Chris R. , Dämmgen Ulrich , Vidic Sonja , Djuricic Vesna , Mitosinkova Marta , Uggerud Hilde T. , Sanz Maria J. , Simmons Ivan , Dragosits Ulrike , Nemitz Eiko , Twigg Marsailidh , van Dijk Netty , Fauvel Yannick , Sanz Francisco , Ferm Martin , Perrino Cinzia , Catrambone Maria , Leaver David , Braban Christine F. , Cape J. Neil , Heal Mathew R. , Sutton Mark A. 
Published: January 2021

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.