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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.

Himalayan forests are biodiverse and support the cultural and economic livelihoods of their human communities. They are bounded to the south by the Indo-Gangetic Plain, which has among the highest concentrations of atmospheric ammonia globally. This source of excess nitrogen pushes northwards into the Himalaya, generating concern that Himalayan forests will be impacted. To estimate the extent to which atmospheric nitrogen is impacting Himalayan forests we focussed on lichen epiphytes, which are a well-established bioindicator for atmospheric nitrogen pollution. First, we reviewed published literature describing nitrogen thresholds (critical levels and loads) at which lichen epiphytes are affected, identifying a mean and confidence intervals based on previous research conducted across a diverse set of biogeographic and ecological settings. Second, we used estimates from previously published atmospheric chemistry models (EMEP-WRF and UKCA-CLASSIC) projected to the Himalaya with contrasting spatial resolution and timescales to characterise model variability. Comparing the lichen epiphyte critical levels and loads with the atmospheric chemistry model projections, we created preliminary estimates of the extent to which Himalayan forests are impacted by excess nitrogen; this equated to c. 80–85% and c. 95–98% with respect to ammonia and total nitrogen deposition, respectively. Recognising that lichens are one of the most sensitive bioindicators for atmospheric nitrogen pollution, our new synthesis of previous studies on this topic generated concern that most Himalayan forests are at risk from excess nitrogen. This is a desk-based study that now requires verification through biological surveillance, for which we provide key recommendations.

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 analyzed. Speciated measurements were made with a low-volume denuder and filter pack method (DELTA®) as part of the EU NitroEurope (NEU) integrated project. Altogether, there were 64 sites in 20 countries (2006–2010), coordinated between 7 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 4 main ecosystem types (crops, grassland, forests and semi-natural). 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 smallest 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 were 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 were 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.

In this study the GAG model, a process-based ammonia (NH3) emission model for urine patches, was extended and applied for the field scale. The new model (GAG_field) was tested over two modelling periods, for which micrometeorological NH3 flux data were available. Acknowledging uncertainties in the measurements, the model was able to simulate the main features of the observed fluxes. The temporal evolution of the simulated NH3 exchange flux was found to be dominated by NH3 emission from the urine patches, offset by simultaneous NH3 deposition to areas of the field not affected by urine. The simulations show how NH3 fluxes over a grazed field in a given day can be affected by urine patches deposited several days earlier, linked to the interaction of volatilization processes with soil pH dynamics. Sensitivity analysis showed that GAG_field was more sensitive to soil buffering capacity (β), field capacity (θfc) and permanent wilting point (θpwp) than the patch-scale model. The reason for these different sensitivities is dual. Firstly, the difference originates from the different scales. Secondly, the difference can be explained by the different initial soil pH and physical properties, which determine the maximum volume of urine that can be stored in the NH3 source layer. It was found that in the case of urine patches with a higher initial soil pH and higher initial soil water content, the sensitivity of NH3 exchange to β was stronger. Also, in the case of a higher initial soil water content, NH3 exchange was more sensitive to the changes in θfc and θpwp. The sensitivity analysis showed that the nitrogen content of urine (cN) is associated with high uncertainty in the simulated fluxes. However, model experiments based on cN values randomized from an estimated statistical distribution indicated that this uncertainty is considerably smaller in practice. Finally, GAG_field was tested with a constant soil pH of 7.5. The variation of NH3 fluxes simulated in this way showed a good agreement with those from the simulations with the original approach, accounting for a dynamically changing soil pH. These results suggest a way for model simplification when GAG_field is applied later at regional scale.

Although it is generally accepted that δ15N in lichen reflects predominating N isotope sources in the environment, confirmation of the direct correlation between lichen δ15N and atmospheric δ15N is still missing, especially under field conditions with most confounding factors controlled. To fill this gap and investigate the response of lichens with different tolerance to atmospheric N deposition, thalli of the sensitive Evernia prunastri and the tolerant Xanthoria parietina were exposed for ten weeks to different forms and doses of N in a field manipulation experiment where confounding factors were minimized. During this period, several parameters, namely total N, δ15N and chlorophyll a fluorescence, were measured. Under the experimental conditions, δ15N in lichens quantitatively responded to the δ15N of released gaseous ammonia (NH3). Although a high correlation between the isotopic signatures in lichen tissue and supplied N was found both in tolerant and sensitive species, chlorophyll a fluorescence indicated that the sensitive species very soon lost its photosynthetic functionality with increasing N availability. The most damaging response to the different N chemical forms was observed with dry deposition of NH3, although wet deposition of ammonium ions had a significant observable physiological impact. Conversely, there was no significant effect of nitrate ions on chlorophyll a fluorescence, implying differential sensitivity to dry deposition versus wet deposition and to ammonium versus nitrate in wet deposition. Evernia prunastri was most sensitive to NH3, then NH4+, with lowest sensitivity to NO3−. Moreover, these results confirm that lichen δ15N can be used to indicate the δ15N of atmospheric ammonia, providing a suitable tool for the interpretation of the spatial distribution of NH3 sources in relation to their δ15N signal.

In this paper a new process-based, weather-driven model for ammonia (NH3) emission from a urine patch has been developed and its sensitivity to various factors assessed. The GAG model (Generation of Ammonia from Grazing) is capable of simulating the TAN (total ammoniacal nitrogen) and the water content of the soil under a urine patch and also soil pH dynamics. The model tests suggest that ammonia volatilization from a urine patch can be affected by the possible restart of urea hydrolysis after a rain event as well as CO2 emission from the soil. The vital role of temperature in NH3 exchange is supported by our model results; however, the GAG model provides only a modest overall temperature dependence in total NH3 emission compared with the literature. This, according to our findings, can be explained by the higher sensitivity to temperature close to urine application than in the later stages and may depend on interactions with other nitrogen cycling processes. In addition, we found that wind speed and relative humidity are also significant influencing factors. Considering that all the input parameters can be obtained for larger scales, GAG is potentially suitable for field and regional scale application, serving as a tool for further investigation of the effects of climate change on ammonia emissions and deposition.

Dissolved organic nitrogen (DON) contributes significantly to the overall nitrogen budget, but is not routinely measured in precipitation or stream water. In order to investigate the contribution of DON to the deposition and export of N, precipitation, stream and soil water samples were collected from an ombrotrophic peatland and analysed for DON over a 2-year period. In wet-only deposition DON contributed up to 10 % of the total dissolved nitrogen (TDN) and was the most dominant fraction in soil water (99 %) and stream water (75 %). NH4+ was the most dominate form of N in precipitation, with NO3− contributing the least to precipitation, soil water and stream water. Precipitation and stream DON were qualitatively analysed by a two-dimensional gas chromatograph coupled to a nitrogen chemiluminescence detector (GC × GC-NCD) after trapping onto C18 solid phase extraction (SPE) cartridges. Ten unique compounds were detected and five identified as pyrrole, benzonitrile, dodecylamine, N-nitrosodipropylamine and decylamine. Five compounds were present in both precipitation and stream samples: pyrrole, benzonitrile and three unidentified compounds. The SPE-extraction efficiency for DON was very low (11 %), but with improvements DON speciation could become a valuable tool to provide information on its sources and pathways and inform chemical transport models.

A computationally efficient atmospheric chemical transport model (FRAME) was used to generate source-receptor concentration and deposition data from a variety of spatially distributed and point sources of nitrogen and sulphur emissions. The model was evaluated by comparison with measurements of nitrogen compounds in the gaseous, particulate and aqueous phase and found to be fit for purpose as a policy tool for assessing the effects of future emissions controls. A scenario for the year 2030 predicted that 64 % of the area of natural ecosystems in the UK would be subject to deposition of nitrogen exceeding critical loads.

Reactive nitrogen (Nr) compounds have different fates in the atmosphere due to differences in governing processes of physical transport, deposition and chemical transformation. Nr compounds addressed here include reduced nitrogen (NHx: ammonia (NH3) and its reaction product ammonium (NH4+)), oxidized nitrogen (NOy: nitrogen monoxide (NO) + nitrogen dioxide (NO2) and their reaction products) as well as organic nitrogen compounds (organic N). Pollution abatement strategies need to take into account these differences in the governing processes of these compounds when assessing their impact on ecosystem services, biodiversity, human health and climate. NOx (NO + NO2) emitted from traffic affects human health in urban areas where the presence of buildings increases the residence time in streets. In urban areas this leads to enhanced exposure of the population to NOx concentrations. NOx emissions have little impact on nearby ecosystems because of the small dry deposition rates of NOx. These compounds need to be converted into nitric acid (HNO3) before removal through deposition is efficient. HNO3 sticks quickly to any surface and is thereby either dry deposited or incorporated into aerosols as nitrate (NO3−). In contrast to NOx compounds, NH3 has potentially high impacts on ecosystems near the main agricultural sources of NH3 because of its large ground-level concentrations along with large dry deposition rates. Aerosol phase NH4+ and NO3− contribute significantly to background PM2.5 and PM10 (mass of aerosols with a diameter of less than 2.5 and 10 μm, respectively) with an impact on radiation balance as well as potentially on human health. Little is known quantitatively and qualitatively about organic N in the atmosphere, other than that it contributes a significant fraction of wet-deposited N, and is present in both gaseous and particulate forms in the atmosphere. Further studies are needed to characterize the sources, air chemistry and removal rates of organic N emissions.

Manure application to managed grassland is a common agricultural practice. There are, however, limited studies looking at the fluxes and interactions of reactive N compounds and aerosols following fertilisation with manure. In this study, state-of-the-art chemical analysers (GRAEGOR, QCLAS, PTRMS) were used to investigate concentrations, fluxes and chemical interactions of reactive nitrogen containing trace gases (NH3, HNO3, HONO) and aerosols (NO3−) above a grassland fertilised with 164 kg N ha−1 of cattle slurry. Emissions of NH3 peaked at >67 μg m−2 s−1, based on a 30 min average. The estimated overall loss of total ammoniacal nitrogen (TAN) from the applied slurry through NH3 emissions in the first 5 days was 33.5%. The average trimethylamine flux in the first 31 h following the first slurry application was 40 ng m−2 s−1 and amounted to 0.38% of the NH3-N emissions. Apparent nitrate aerosol emissions were observed following the slurry application peaking at 13.0 ng m−2 s−1. This suggests formation of NH4NO3 from reaction of the emitted NH3 with atmospheric HNO3, consistent with the observation of gaseous concentration products exceeding the dissociation constants of ammonium nitrate. Fluxes of total nitrate (HNO3 + NO3−) were bi-directional and positive during the mid-day period after fertilisation, suggesting that the slurry acted as a net source for these compounds. There is evidence of small HONO emission following fertilisation (up to 1 ng m−2 s−1), although the production process is currently not identified. By contrast, all compounds showed deposition to the adjacent unfertilised grassland.

Although the effects of atmospheric nitrogen deposition on species composition are relatively well known, the roles of the different forms of nitrogen, in particular gaseous ammonia (NH3), have not been tested in the field. Since 2002, we have manipulated the form of N deposition to an ombrotrophic bog, Whim, on deep peat in southern Scotland, with low ambient N (wet + dry = 8 kg N ha 1 yr 1) and S (4 kg S ha 1 yr 1) deposition. A gradient of ammonia (NH3, dry N), from 70 kg N ha 1 yr 1 down to background, 3–4 kg N ha 1 yr 1 was generated by free air release. Wet ammonium (NH4 +, wet N) was provided to replicate plots in a fine rainwater spray (NH4Cl at +8, +24, +56 kg N ha 1 yr 1). Automated treatments are coupled to meteorological conditions, in a globally unique, field experiment. Ammonia concentrations were converted to NH3-N deposition (kg N ha 1) using a site/vegetation specific parameterization. Within 3 years, exposure to relatively modest deposition of NH3, 20–56 kg NH3-N ha 1 yr 1 led to dramatic reductions in species cover, with almost total loss of Calluna vulgaris, Sphagnum capillifolium and Cladonia portentosa. These effects appear to result from direct foliar uptake and interaction with abiotic and biotic stresses, rather than via effects on the soil. Additional wet N by contrast, significantly increased Calluna cover after 5 years at the 56 kg N dose, but reduced cover of Sphagnum and Cladonia. Cover reductions caused by wet N were significantly different from and much smaller than those caused by equivalent dry N doses. The effects of gaseous NH3 described here, highlight the potential for ammonia to destroy acid heathland and peat bog ecosystems. Separating the effects of gaseous ammonia and wet ammonium deposition, for a peat bog, has significant implications for regulatory bodies and conservation agencies

This paper compares data on N fluxes compiled by Schulze and colleagues, with information available in the literature and publicly available open databases, and finds important discrepancies for a number of such fluxes for Europe (emissions, deposition, aerosol formation of compounds containing N) -- exceeding a factor of two in several cases. A qualitative assessment of the uncertainties of the respective approaches indicates that these differences are beyond the uncertainty margins that can be reasonably attributed to the respective data. We conclude that the results should be used with caution, that agricultural application of N should still be considered to be the largest source of N released to the environment, and that this agricultural N affects soils more strongly than atmospheric deposition, at the European scale.

Agricultural livestock represents the main source of ammonia (NH3) in Europe. In recent years, reduction policies have been applied to reduce NH3 emissions. In order to estimate the impacts of these policies, robust estimates of the emissions from the main sources, i.e. livestock farms are needed. In this paper, the NH3 emissions were estimated from a naturally ventilated livestock farm in Braunschweig, Germany during a joint field experiment of the GRAMINAE European project. An inference method was used with a Gaussian-3-D plume model and a local-scale dispersion and deposition model (FIDES-2-D). NH3 concentrations downwind of the source were used together with micrometeorological data to estimate the source strength over time. Mobile NH3 concentration measurements provided information on the spatial distribution of source strength. The estimated emission strength ranged between 6.0±0.17 kg NH3 d−1 (FIDES-2-D model) and 9.2±0.7 kg NH3 d−1 (Gaussian model). These estimates were 94% and 63% of what was obtained using emission factors from the German national inventory (9.6 kg d−1 NH3. However, the FIDES-2-D approach was shown to be very sensitive to the source size, the roughness height and to whether deposition was taken into account downwind of the source. Accounting for deposition in FIDES-2-D gives a potential emission estimate of 11.7 kg NH3 d−1, showing that deposition can explain the observed difference. The daily pattern of the source was correlated with net radiation and with the temperature inside the animal houses. The daily pattern resulted from a combination of a temperature effect on the source concentration together with an effect of variations in free and forced convection of the building ventilation rate. Further development of the plume technique is especially relevant for naturally ventilated farms, since the variable ventilation rate makes other emission measurements difficult.

The dry deposition of ammonia from the atmosphere to the surface can lead to eutrophication of sensitive ecosystems and acidification of the soil. A large proportion of the ammonia emitted from agricultural sources can be deposited within a few kilometres and, therefore, impacts of ammonia dry deposition often occur near to the source. To assess these impacts, short-range atmospheric dispersion models are often applied to simulate the emission, dispersion and deposition of ammonia. However, these models can be time-consuming to run and often require detailed input data and, therefore, for multiple assessments it is useful to have a method of screening to discard scenarios where impacts are expected to be negligible. The SCAIL model (Simple Calculation of Ammonia Impact Limits) has been developed for this purpose. SCAIL estimates the atmospheric concentration and dry deposition at the nearest edge of a sensitive ecosystem (receptor) downwind of an ammonia source. These estimates are calculated based on simple meteorological data, the emission rate of the source, land cover type and distance to the receptor. Analysis of the model predictions showed that uncertainty in the model input data leads to an uncertainty in concentration and dry deposition estimates of 25-30% and 40-45% respectively. Detailed atmospheric dispersion models will also have similar uncertainties since they use similar types of input data. Comparison of the concentration predictions with previous measurements made around eight farms showed that the model significantly underestimated concentrations although the model performance was similar to existing screening techniques. The measurement dataset was used to calibrate the SCAIL model which subsequently performed better, using independent verification data, than existing models calibrated in a similar way. The benefits of the SCAIL model are already being seen in the UK, where it is used to screen farms for potential impacts on statutory nature conservation areas.

In a recent study, Magnani et al. report how atmospheric nitrogen deposition drives stand-lifetime net ecosystem productivity (NEPav) for midlatitude forests, with an extremely high C to N response (725kgCkg � 1 wet-deposited N for their European sites). We present here a re-analysis of these data, which suggests a much smaller C:N response for total N inputs. Accounting for dry, as well as wet N deposition reduces the C:N response to 177:1. However, if covariance with intersite climatological differences is accounted for, the actual C:N response in this dataset may be o70:1. We then use a model analysis of 22 European forest stands to simulate the findings of Magnani et al. Multisite regression of simulated NEPav vs. total N deposition reproduces a high C:N response (149:1). However, once the effects of intersite climatological differences are accounted for, the value is again found to be much smaller, pointing to a real C:N response of about 50‐75:1.

Dry deposition modelling typically assumes that canopy resistance (Rc) is independent of ammonia (NH3) concentration. An innovative flux chamber system was used to provide accurate continuous measurements of NH3 deposition to a moorland composed of a mixture of Calluna vulgaris (L.) Hull, Eriophorum vaginatum L. and Sphagnum spp. Ammonia was applied at a wide range of concentrations (1–100 μg m−3). The physical and environmental properties and the testing of the chamber are described, as well as results for the moorland vegetation using the ‘canopy resistance’ and ‘canopy compensation point’ interpretations of the data. Results for moorland plant species demonstrate that NH3 concentration directly affects the rate of NH3 deposition to the vegetation canopy, with Rc and cuticular resistance (Rw) increasing with increasing NH3 concentrations. Differences in Rc were found between night and day: during the night Rc increases from 17 s m−1 at 10 μg m−3 to 95 s m−1 at 80 μg m−3, whereas during the day Rc increases from 17 s m−1 at 10 μg m−3 to 48 s m−1 at 80 μg m−3. The lower resistance during the day is caused by the stomata being open and available as a deposition route to the plant. Rw increased with increasing NH3 concentrations and was not significantly different between day and night (at 80 μg m−3 NH3 day Rw=88 s m−1 and night Rw=95 s m−1). The results demonstrate that assessments using fixed Rc will over-estimate NH3 deposition at high concentrations (over ∼15 μg m−3).

Emissions of sulphur and oxidized nitrogen compounds in Europe have been reduced following a series of control measures during the last two decades. These changes have taken place during a period in which the primary gases and the wet deposition throughout Europe were extensively monitored. Since the end of the 1970s, for example land based sulphur emissions declined by between 90 and 70% depending on the region. Over the same period the total deposition of sulphur and its partitioning into wet and dry deposition have declined, but the spatial pattern in the reduction in deposition differs from that of emission and has changed with time. Such non-linearities in the emission-deposition relationship are important to understand as they complicate the process of assessing the effects of emission reduction strategies. Observed non-linearities in terrestrial sulphur emission-deposition patterns have been identified in north west Europe due to increases in marine emissions, and are currently slowing the recovery of freshwater ecosystems. Changes in the relative amounts of SO2 and NH3 in air over the last two decades have also changed the affinity of terrestrial surfaces for SO2 and have therefore changed the deposition velocity of SO2 over substantial areas. The consequence of this effect has been the very rapid reduction in ambient SO2 concentration in some of the major source areas of Europe, where NH3 did not change much. Interactions between the different pollutants, generating non-linearities are now being incorporated in long-range transport models to simulate the effects of historical emission trends and to provide projections into the future. This paper identifies non-linearities in emission deposition relationships for sulphur and nitrogen compounds in Europe using data from the EMEP long-rang transport model and measured concentration fields of the major ions in precipitation and of SO2 and NO2 in surface air.

An evaluation has been made of a number of contrasting atmospheric chemical transport models, of varying complexity, applied to estimate sulphur and nitrogen deposition in the UK. The models were evaluated by comparison with annually averaged measurements of gas, aerosol and precipitation concentrations from the national monitoring networks. The models were evaluated in relation to performance criteria. They were generally able to satisfy a criterion of ‘fitness for purpose’ that at least 50% of modelled concentrations should be within a factor of two of measured values. The second criterion, that the magnitude of the normalised mean bias should be less than 20%, was not always satisfied. Considering known uncertainties in measurement techniques, this criterion may be too strict. Overall, simpler models were able to give a good representation of measured gas concentrations whilst the use of dynamic meteorology, and complex photo-chemical reactions resulted in a generally better representation of measured aerosol and precipitation concentrations by more complex models. The models were compared graphically by plotting maps and cross-country transects of wet and dry deposition as well as calculating budgets of total wet and dry deposition to the UK for sulphur, oxidised nitrogen and reduced nitrogen. The total deposition to the UK varied by ±22–36% amongst the different models depending on the deposition component. At a local scale estimates of both dry and wet deposition for individual 5 km × 5 km model grid squares were found to vary between the different models by up to a factor of 4.

Atmospheric ammonia (NH3) dominates global emissions of total reactive nitrogen (Nr), while emissions from agricultural production systems contribute about two-thirds of global NH3 emissions; the remaining third emanates from oceans, natural vegetation, humans, wild animals and biomass burning. On land, NH3 emitted from the various sources eventually returns to the biosphere by dry deposition to sink areas, predominantly semi-natural vegetation, and by wet and dry deposition as ammonium (NH4+) to all surfaces. However, the land/atmosphere exchange of gaseous NH3 is in fact bi-directional over unfertilized as well as fertilized ecosystems, with periods and areas of emission and deposition alternating in time (diurnal, seasonal) and space (patchwork landscapes). The exchange is controlled by a range of environmental factors, including meteorology, surface layer turbulence, thermodynamics, air and surface heterogeneous-phase chemistry, canopy geometry, plant development stage, leaf age, organic matter decomposition, soil microbial turnover, and, in agricultural systems, by fertilizer application rate, fertilizer type, soil type, crop type, and agricultural management practices. We review the range of processes controlling NH3 emission and uptake in the different parts of the soil-canopy-atmosphere continuum, with NH3 emission potentials defined at the substrate and leaf levels by different [NH4+] / [H+] ratios (Γ). Surface/atmosphere exchange models for NH3 are necessary to compute the temporal and spatial patterns of emissions and deposition at the soil, plant, field, landscape, regional and global scales, in order to assess the multiple environmental impacts of airborne and deposited NH3 and NH4+. Models of soil/vegetation/atmosphere NH3 exchange are reviewed from the substrate and leaf scales to the global scale. They range from simple steady-state, "big leaf" canopy resistance models, to dynamic, multi-layer, multi-process, multi-chemical species schemes. Their level of complexity depends on their purpose, the spatial scale at which they are applied, the current level of parameterization, and the availability of the input data they require. State-of-the-art solutions for determining the emission/sink Γ potentials through the soil/canopy system include coupled, interactive chemical transport models (CTM) and soil/ecosystem modelling at the regional scale. However, it remains a matter for debate to what extent realistic options for future regional and global models should be based on process-based mechanistic versus empirical and regression-type models. Further discussion is needed on the extent and timescale by which new approaches can be used, such as integration with ecosystem models and satellite observations.

The global nitrogen (N) cycle at the beginning of the 21st century has been shown to be strongly influenced by the inputs of reactive nitrogen (Nr) from human activities, including combustion-related NOx, industrial and agricultural N fixation, estimated to be 220 Tg N yr−1 in 2010, which is approximately equal to the sum of biological N fixation in unmanaged terrestrial and marine ecosystems. According to current projections, changes in climate and land use during the 21st century will increase both biological and anthropogenic fixation, bringing the total to approximately 600 Tg N yr−1 by around 2100. The fraction contributed directly by human activities is unlikely to increase substantially if increases in nitrogen use efficiency in agriculture are achieved and control measures on combustion-related emissions implemented. Some N-cycling processes emerge as particularly sensitive to climate change. One of the largest responses to climate in the processing of Nr is the emission to the atmosphere of NH3, which is estimated to increase from 65 Tg N yr−1 in 2008 to 93 Tg N yr−1 in 2100 assuming a change in global surface temperature of 5 °C in the absence of increased anthropogenic activity. With changes in emissions in response to increased demand for animal products the combined effect would be to increase NH3 emissions to 135 Tg N yr−1. Another major change is the effect of climate changes on aerosol composition and specifically the increased sublimation of NH4NO3 close to the ground to form HNO3 and NH3 in a warmer climate, which deposit more rapidly to terrestrial surfaces than aerosols. Inorganic aerosols over the polluted regions especially in Europe and North America were dominated by (NH4)2SO4 in the 1970s to 1980s, and large reductions in emissions of SO2 have removed most of the SO42− from the atmosphere in these regions. Inorganic aerosols from anthropogenic emissions are now dominated by NH4NO3, a volatile aerosol which contributes substantially to PM10 and human health effects globally as well as eutrophication and climate effects. The volatility of NH4NO3 and rapid dry deposition of the vapour phase dissociation products, HNO3 and NH3, is estimated to be reducing the transport distances, deposition footprints and inter-country exchange of Nr in these regions. There have been important policy initiatives on components of the global N cycle. These have been regional or country-based and have delivered substantial reductions of inputs of Nr to sensitive soils, waters and the atmosphere. To date there have been no attempts to develop a global strategy to regulate human inputs to the nitrogen cycle. However, considering the magnitude of global Nr use, potential future increases, and the very large leakage of Nr in many forms to soils, waters and the atmosphere, international action is required. Current legislation will not deliver the scale of reductions globally for recovery from the effects of Nr deposition on sensitive ecosystems, or a decline in N2O emissions to the global atmosphere. Such changes would require substantial improvements in nitrogen use efficiency across the global economy combined with optimization of transport and food consumption patterns. This would allow reductions in Nr use, inputs to the atmosphere and deposition to sensitive ecosystems. Such changes would offer substantial economic and environmental co-benefits which could help motivate the necessary actions.

1. Atlantic oak woods are of high conservation value and contain many rare lichens and bryophytes. The effects of nitrogen pollution on these epiphytic communities have not been previously studied. We investigated the composition of Atlantic oak wood epiphytic communities in relation to atmospheric N deposition in order to identify N indicator species and propose a critical load for such communities. 2. The epiphytic communities of seven Atlantic oak woods receiving estimated total nitrogen deposition in the range 10-53 kg N ha-1 year-1 were surveyed. Exposure of epiphytes to atmospheric N over 7 months was measured in terms of stemflow flux and concentration and airborne NH3. 3. Redundancy analysis (RDA) related the species to stemflow chemistry and bark pH. Different groups of species were found at (i) a coastal low N deposition site (Graphina ruiziana, Lecania cyrtella, Lobaria pulmonaria, Opegrapha atra, Orthotrichum affine, Melanelia fuliginosa ssp. glabratula and Pertusaria hymenea), (ii) inland low N deposition sites (I. myosuroides, F. tamarisci, Plagiochila atlantica, Cladonia chlorophaea, C. squamosa, Hypotrachyna laevigata and Thelotrema lepadinum) and (iii) high N deposition sites (Hypnun andoi, Hypnum cupressiforme. Calicium viride, Chrysothrix candelaris, C. coniocraea, Hypogymnia physodes, Parmelia saxatilis, Phyllospora rosei and Usnea subfloridana). 4. RDA analysis showed that bark pH and NH 4+ concentration in the stemflow explained the greatest amount of variation in the species composition among the sites. The sites separated into two groups (low and high NH4+ concentration) leading to an estimate for the critical load for N deposition for epiphytes in Atlantic oak woods of 11-18 kg N ha-1 year -1. 5. Analysis of the occurrence of species against N levels suggested that I. myosuroides and F. tamarisci are indicators of small N inputs, whereas H. andoi, C. coniocraea, P. saxatilis and H. physodes are tolerant of increased N loads. 6. Different forms of N (NO3-, NH 4+, NH3) were found to significantly affect the occurrence of different species. Some species were sensitive to total flux while others were sensitive to concentration.

Atmospheric budgets of reduced nitrogen for the major political regions of the British Isles are investigated with a multi-layer atmospheric transport model. The model is validated against measurements of NH3 concentration and is developed to provide atmospheric budgets for defined subdomains of the British Isles. The study shows that NH3 emissions, transport and deposition in a specific region can have profound effects on the composition of the atmosphere over other regions. An important feature is the high export of reduced nitrogen from Republic of Ireland and Northern Ireland to England and Scotland. England is also a net exporter of reduced nitrogen. By contrast, Scotland and Wales are approximately "ammonia neutral" as import of reduced nitrogen roughly balances export. Overall, only 7% of reduced nitrogen in the British Isles is derived from non-British Isles sources (29 kt N yr-1), compared with emissions of 387 kt N yr-1. Most of the reduced nitrogen is deposited in the British Isles (64%), while 36% is exported (148 kt N yr-1). © Springer 2005.

Monitoring and modelling of deposition of air pollutants is essential to develop and evaluate policies to abate the effects related to air pollution and to determine the losses of pollutants from the atmosphere. Techniques for monitoring wet deposition fluxes are widely applied. A recent intercomparison experiment, however, showed that the uncertainty in wet deposition is relatively high, up to 40%, apart from the fact that most samplers are biased because of a dry deposition contribution. Wet deposition amounts to about 80% of the total deposition in Europe with a range of 10–90% and uncertainty should therefore be decreased. During recent years the monitoring of dry deposition has become possible. Three sites have been operational for 5 years. The data are useful for model development, but also for model evaluation and monitoring of progress in policy. Data show a decline in SO2 dry deposition, whereas nitrogen deposition remained constant. Furthermore, surface affinities for pollutants changed leading to changes in deposition. Deposition models have been further developed and tested with dry deposition measurements and total deposition measurements on forests as derived from throughfall data. The comparison is reasonable given the measurement uncertainties. Progress in ozone surface exchange modelling and monitoring shows that stomatal uptake can be quantified with reasonable accuracy, but external surface uptake yields highest uncertainty. 2004 Elsevier Ltd. All rights reserved.

Simple bioenergetics models were used to derive annual nitrogen excretion rates of each seabird species occurring at colonies in the UK. These were combined with population distribution data and an estimated fraction of nitrogen volatilized to estimate the spatial distribution of NH3 emissions from seabird colonies at a 1 km resolution. The effect of these emissions on atmospheric NH3 concentrations and nitrogen deposition in the UK was assessed using the FRAME atmospheric chemistry and transport model. The total emission of NH3 from the UK seabird colonies is estimated at 2.7 kt yr−1. Emissions from seabirds are largely concentrated in remote parts of Britain, where agricultural and other anthropogenic emissions are minimal. Although seabirds account for less than 1% of total UK NH3 emissions (∼370 kt yr−1), their occurrence in remote areas and frequently large colony sizes results in seabirds providing a major fraction of the atmospheric nitrogen deposition for many remote ecosystems.

Measurements of the concentrations of nitrogen compounds in air and precipitation in the UK have been made since the mid-19th century, but no networks operating to common protocols and having traceable analytical procedures were established until the 1950s. From 1986 onwards, a high-quality network of sampling stations for precipitation chemistry was established across the UK. In the following decade, monitoring networks provided measurement of NO2, NH3, HNO3 and a satisfactory understanding of the dry deposition process for these gases allowed dry deposition to be quantified. Maps of N deposition for oxidized and reduced compounds at a spatial scale of 5 km × 5 km are available from 1986 to 2000. Between 1950 and 1985, the more limited measurements, beginning with those of the European Air Chemistry Network (EACN) provide a reasonable basis to estimate wet deposition of NO−3−N and NH+4−N. For the first half of the century, estimates of deposition were scaled with emissions assuming a constant relationship between emission and deposition for each of the components of the wet and dry deposition budget at the country scale. Emissions of oxidized N, which dominated total nitrogen emissions throughout the century, increased from 312 kt N annually in 1900 to a peak of 787 kt for the decade 1980–1990 and then declined to 460 kt in 2000. Emissions of reduced N, largely from coal combustion were about 168 kt N in 1900, increasing to a peak of 263 kt N in 2000 and by now dominated by agricultural sources. Reduced N dominated the deposition budget at the country scale, increasing from 163 kt N in 1900 to 211 kt N in 2000, while deposition of oxidized N was 66 kt N in 1900 and 191 kt N in 2000. Over the century, 68 Mt (Tg) of fixed N was emitted within the UK, 78% as NO x , while 29 Mt of nitrogen was deposited (43% of emissions), equivalent to 1.2 t N ha−1, on average, with 60% in the reduced form. Deposition to semi-natural ecosystems is approximately 15 Tg N, equivalent to between 1 and 5 t N ha−1, over the century and appears to be accumulating in soil. The N deposition over the century is similar in magnitude to the total soil N inventory in surface horizons.

Integrated assessment modelling aims to bring together information on emissions, atmospheric transport between sources and exposed areas or populations, criteria for environmental protection, and potential emission control measures and their costs, in order to explore effective abatement strategies. We describe the development of a new UK scale Integrated Assessment Model which can be used to investigate strategies for the attainment of national emission ceilings. The model optimises abatement strategies in relation to acidification, eutrophication, and/or human-exposure to particulate PM10, with reference to the deposition of sulphur and nitrogen (oxidised and reduced), and concentrations of primary and secondary particles. The model combines sector specific emissions, atmospheric transport and deposition, ecosystem specific critical load exceedances, and pollution abatement costs to determine optimised abatement strategies using benefit and, where applicable, recovery functions.

There is substantial uncertainty in the effectiveness of measures to reduce emissions of agricultural trace gases, including ammonia (NH3), methane (CH4) and nitrous oxide (N2O). The only way to test whether emission abatement programmes are successful is through monitoring of air concentrations and deposition. However, where NH3 emissions have been reduced in Europe, either through abatement policies or reductions in agricultural activity, it was difficult to demonstrate the link with reduced concentrations and deposition. The outbreak of Foot and Mouth Disease (FMD) in Great Britain in 2001 provides a major case study to test the link between agricultural emissions and air concentrations. This paper examines the spatial distribution of anticipated change in emissions and concentrations of NH3, CH4 and N2O as a result of the FMD outbreak. It then assesses the extent to which atmospheric monitoring could be used to detect the changes and attribute them to the effect of FMD. Two of the areas worst affected by FMD were selected for detailed analysis: in Cumbria (N.W. England) and in Devon (S.W. England). Compared with values prior to FMD, average agricultural emissions were reduced by as much as 50–100%, with estimated reductions in atmospheric mixing ratios of 0.7–3.3 ppb for NH3, 10–60 ppb for CH4 and 0.1–0.7 ppb for N2O. For NH3 and CH4, modelled changes are larger than the precision of analytical techniques and, where sampling is made at replicate sites, the changes are also larger than the inter-annual variability of existing monitoring data. By contrast, for N2O only the largest changes occurring in Cumbria are expected to be detectable and distinguishable from inter-annual variability. Based on the results, a measurement strategy has been established to follow NH3 and CH4 concentrations during and after the period of restocking. By comparing NH3 (a reactive gas) with CH4 (an inert tracer), the measurements aim to distinguish constraints on the link between changing emissions and air concentrations. Improving this linkage is essential to demonstrate that the economic costs of emission abatement translate into environmental benefits.

Summary 1Atlantic bryophytes are of European conservation importance, yet the effect on them of excess atmospheric nitrogen is relatively unknown. This study assesses the effects of increased atmospheric N deposition on the growth and tissue N of epiphytic Atlantic bryophytes, and their potential to recover following a decline in N deposition. 2The N received in stemflow by bryophytes at two sites was measured and compared to model predictions. 3Four species of epiphytic bryophytes (Isothecium myosuroides, Dicranum scoparium, Frullania tamarisci and Ulota crispa), typical of Atlantic Oak woods, were studied in a 12-month reciprocal transplant experiment between a pristine Oak woodland receiving a modelled atmospheric deposition of 12 kg N ha−1 year−1 and a polluted one receiving 54 kg N ha−1 year−1. 4Tissue N concentration increased and growth declined following an increase in atmospheric N deposition in all species except Ulota crispa. Conversely, tissue N concentration decreased and growth increased in Frullania tamarisci following a decrease in atmospheric N deposition, with similar non-significant patterns in the other species. 5The reciprocal transplants indicate a detrimental effect of increased N deposition on the bryophyte species studied. The study indicated recovery following a decrease in atmospheric N deposition, but the responses caused by decreased N deposition were smaller than those due to increased N deposition. This suggests that the time-scale for recovery of bryophytes from excess N deposition is longer than the timescale of nitrogen impacts.

Deposition of sulphur and oxidised nitrogen have been estimated for the United Kingdom (UK) for 1996 using an atmospheric transport model (FRAME, Fine Resolution Atmospheric Multi-pollutant Exchange). Firstly, the modelled deposition is compared with measurement based estimates of NEGTAP (National Expert Group on Transboundary Air Pollution). Secondly, the modelled wet deposition and surface air concentrations are compared with measurements from the UK's monitoring networks using linear regression. FRAME gives 65 and 102 ktonnes(kt) for oxidised nitrogen dry and wet UK deposition, respectively, compared with the NEGTAP estimation of 87 and 91 kt . The modelled contributions of sulphur dry and wet deposition are 138 and 200 kt , respectively, compared with the NEGTAP estimation of 125 and 171 kt . FRAME is found to perform well for nitrogen dioxide concentrations and nitrate wet deposition, but the concentration and subsequent dry deposition of nitric acid is underestimated. Deposition of sulphur and sulphur dioxide surface concentrations are coherent with the NEGTAP estimation but they are slightly overestimated by FRAME. The paper also emphasises the uncertainties associated with the model when used to assess the effects of abatement policies.

In the context of international efforts to reduce the impactsof atmospheric NH3 and NH4 + (collectively, NHx), it is important to establish the link between NH3emissions and monitoring of NHx concentrations and deposition. This is equally relevant to situations where NH3emissions changes are certain (e.g. due to changed source sectoractivity), as to cases where NH3 abatement technologies havebeen implemented. Correct interpretation of adequate atmosphericmeasurements is essential, since monitoring data provide the onlymeans to evaluate trends in regional NH3 emissions.These issues have been reviewed using available measurements and modelling from nine countries. In addition to historic datasets,the analysis here considers countries where NH3 source sector activity changed (both increases and decreases) and countries where NH3 abatement policies have been implemented.In The Netherlands an `ammonia gap' was identified between the expected reduction and results of monitoring, and was attributedinitially to ineffectiveness of the abatement measures. The analysis here for a range of countries shows that atmospheric interactions complicate the expected changes, particularly sinceSO2 emissions have decreased at the same time, while at manysites the few years of available data show substantial inter-annual variation. It is concluded that networks need to beestablished that speciate between NH3 and aerosol NH4 +, in addition to providing wet deposition, and sample at sufficient sites for robust regional estimates to be established. Such measurements will be essential to monitor compliance of the international agreements on NH3 emission abatement.

The exchange of ammonia (NH3) between the atmosphere and the land surface is controlled by both atmospheric and land surface processes and can thus be bi-directional. Whether emission or deposition occurs depends on the nitrogen (N) status of the ecosystem. Resistance models have been developed to represent this bi-directional pattern of NH3 exchange. Major pathways include exchange with leaf cuticles, plant tissues (via stomata) and with the soil surface. However, the parameters that quantify the emission potential of the foliage and ground surface, the NH3 stomatal compensation point, χs, and the soil surface NH3 concentration, χsoil, respectively, are entirely empirical in these models and do not consider the influence of the plant and soil N status. On the other side, grassland ecosystem models simulate the ecosystem N dynamics, but until now NH3 biosphere–atmosphere exchange was only treated in a very simple manner. A two-layer resistance model for NH3 exchange has thus been combined with the grassland ecosystem model PaSim in order to link NH3 exchange with the ecosystem N dynamics. For this purpose, the plant substrate N pool in previous versions of PaSim has been divided between apoplastic and symplastic compartments, and the apoplastic substrate N pool has been linked to the stomatal NH3 exchange. In addition, soil ammoniacal N (NHx) has been partitioned between the soil surface and several soil layers, and the soil surface NH3 exchange has been linked to the soil surface ammonium (NH4+). The new combined model has been parameterised and applied to an intensively managed grassland site in Southern Scotland. The comparison with micrometeorological measurements of NH3 fluxes has shown that the model can qualitatively reproduce the effects of cutting and fertilisation on the net NH3 exchange above the canopy. In particular, the model reproduces the expected strong coupling of the NH3 exchange with the dynamics of the apoplastic substrate N pool. However, peak NH3 emissions are underestimated, and it is postulated that this could be related to the representation of leaf litter emissions from the soil surface in the model, and to the simulated soil NHx dynamics. Nevertheless, this new version of PaSim is a valuable tool for investigating the influence of different management scenarios or of climate change on NH3 exchange.

The chapter reviews progress in monitoring and modelling of atmospheric nitrogen (N) deposition at regional and global scales. The Working Group expressed confidence in the inorganic N wet deposition estimates in U.S., eastern Canada, Europe and parts of East Asia. But, long-term wet or dry N deposition information in large parts of Asia, South America, parts of Africa, Australia/Oceania, and oceans and coastal areas is lacking. Presently, robust estimates are only available for inorganic N as existing monitoring generally does not measure the complete suite of N species, impeding the closing of the atmospheric N budget. The most important species not routinely measured are nitrogen dioxide (NO2), ammonia (NH3), organic N and nitric acid (HNO3). Uncertainty is much higher in dry deposition than in wet deposition estimates. Inferential modelling (combining air concentrations with exchange rates) and direct flux measurements are good tools to estimate dry deposition; however, they are not widely applied. There is a lack of appropriate parameterizations for different land uses and compounds for input into inferential models. There is also a lack of direct dry deposition flux measurements to test inferential models and atmospheric model estimates.

Atmospheric nitrogen (N) and sulphur (S) deposition from industrial, transport and agricultural sources may exert a range of different types of impacts on Natura 2000 sites in Europe. The FRAME (Fine Resolution Atmospheric Multi-pollutant Exchange) model, incorporating emission point sources and sectors, was used to provide footprints of N and S deposition across the UK from 160 sources or groups of sources. The resulting matrix of source attribution by sector, and exceedance statistics for each site, provide a means of impact assessment for the whole UK’s Natura 2000 network. For 2005 80 % of Special Areas of Conservation (SACs) in the UK have at least one feature exceeding their minimum N critical load, while the figure for acidity exceedance is 75 %. By 2020, the values are estimated at 74 and 62 %, respectively, indicating that current policies are insufficient to avoid exceedance of the critical loads. Although NOx emissions are projected to decrease substantially, the modest reduction in exceedance is a consequence of the contribution of NH3 from agricultural sources, which is projected to decrease only slightly between 2005 and 2020. Future analysis should address the spatial importance of source location and site proximity to a source, and examine the relationship between dry and wet deposition by source sector.

International audience; Ammonia emissions from livestock production can have negative impacts on nearby protected sites and ecosystems that are sensitive to eutrophication and acidification. Trees are effective scavengers of both gaseous and particulate pollutants from the atmosphere making tree belts potentially effective landscape features to support strategies aiming to reduce ammonia impacts. This research used the MODDAS-THETIS a coupled turbulence and deposition turbulence model, to examine the relationships between tree canopy structure and ammonia capture for three source types—animal housing, slurry lagoon, and livestock under a tree canopy. By altering the canopy length, leaf area index, leaf area density, and height of the canopy in the model the capture efficiencies varied substantially. A maximum of 27% of the emitted ammonia was captured by tree canopy for the animal housing source, for the slurry lagoon the maximum was 19%, while the livestock under trees attained a maximum of 60% recapture. Using agro-forestry systems of differing tree structures near 'hot spots' of ammonia in the landscape could provide an effective abatement option for the livestock industry that complements existing source reduction measures.

Intensive field measurements of NH3 and CO2 exchange were made over a wet heathland in the vicinity ( 10%). Corrections were applied using a local-scale dispersion-exchange model. The analysis highlights how advection modifies the classical one-dimensional inferential resistance approach. It is concluded that ecosystems in the vicinity of agricultural land receive more dry deposition than would be estimated using NH3 concentration monitoring and standard inferential models. In the present study, this effect represented an overall increase in total NH3 deposition of 32%.

A new resistance model is described to interpret the biosphere-atmosphere exchange fluxes of ammonia (NH 3 ) with vegetation, and compared with previous modelling approaches for NH 3 . The new model constitutes an extension of an existing one-layer canopy compensation point model: in addition to bi-directional foliar stomatal exchange and deposition to leaf cuticles, the model treats NH 3 emission from a ground layer. This may originate from fertilizer evaporation, the soil or decomposing plant parts. The emission potentials of the foliage and ground Surface are given by the NH 3 gas concentrations at equilibrium with the ammonium (NH + 4 ) concentration in the apoplastic fluid or soil solution. From these concentrations, as well as the transfer resistances of the different exchange pathways, the net compensation point of the canopy (X c ) may be derived. The net flux is determined by the relative magnitude of X c and the NH 3 air concentration above the vegetation. The two-layer canopy compensation point is applied to: (i) an oilseed rape canopy, in which NH 3 emission from decomposing leaf litter at the ground surface presents a second major source; and (ii) a wheat stubble field, in which emission from the soil contributes significantly to the net exchange. For both canopies, the model performance is contrasted with the single-layer X c model, which is not able to reproduce the temporal patterns of exchange. The two-layer model is proposed as the optimum compromise between simplicity and accuracy, capable of describing bi-directional NH 3 exchange in atmospheric transport models over a very wide range of vegetation types.

The Hull Acid Rain Model (HARM) is extensively used in the UK to help in formulating strategies for reducing the emissions of acidifying pollutants. Here we describe the development of a 10 km version of HARM, which incorporates both software development and a new representation of the dry deposition of reduced N. Output from the new model for the UK is compared with S and N deposition data available from the UK's monitoring networks for 1992–1994 using linear regression. The model is able to reproduce the overall patterns of deposition for S, oxidised and reduced N. Although improved from earlier model versions, HARM still appears to underestimate dry deposition of reduced N. We describe the implications of the changes to HARM for assessing current policy commitments to reduce emissions of acidifying pollutants through changes in critical loads (CL) exceedance.

Continuous micrometorological measurements of ammonia (NH3)exchange were made for a period of 19 months (May 1998–November 1999) over intensively managed grassland in southern Scotland. This study focused on the influence of management activities, such as cutting and fertilising, on vegetation-atmosphere exchange of NH3. Measurements were conducted within the European project GRAMINAE (GRassland AMmonia INteractions Across Europe) within which the Scottish site forms one of 6 sites in an E–W transect across Europe. NH3 emissions were enhanced (up to 300 ng m-2 s-1) after cutting followed by larger emissions after fertilising (up to 1400 ng m-2 s-1). Annual budget calculations show the intensive grassland acted as a net source (1.8 kg N ha-1 yr1) although fluxes were bi-directional with deposition dominating in the winter and emission in the summer. Initial modelling of the NH3 exchange using a `canopy compensation point' model has been conducted for key periods. The dynamics of the fluxes during these key periods, such as before and after cutting and fertilising, may be reproduced by introducing different values of the apoplastic ratio, Γ = [NH4+]/[H+].

The application of micrometeorology for flux measurements of nitrogen species between terrestrial ecosystems and the atmosphere and some of their main limitations are reviewed. New methods which are gaining rapid acceptance such as relaxed eddy accumulation are also described. A new development to provide long term average fluxes by time averaged gradients is shown to yield long-term average NH3 fluxes over moorland within 10% of values obtained using continuous wet denuder methods and at less than 10% of the cost. The use of mass balance methods to quantify fluxes at the plot, landscape and regional scale are described, and show that in suitable conditions and for some countries, methods to check national inventories of radiatively active gases are now available.

Two years of continuous measurements of SO2deposition fluxes to moorland vegetation are reported. The mean flux of 2.8 ng SO2 m-2 s-1 is regulated predominantly by surface resistance (r c) which, even for wet surfaces, was seldom smaller than 100 s m-1. The control of surface resistance is shown to be regulated by the ratio of NH3SO2 concentrations with an excess of NH3 generating the small surface resistances for SO2. A dynamic surface chemistry model is used to simulate the effects of NH3 on SO2 deposition flux and is able to capture responses to short-term changes in ambient concentrations of SO2, NH3 and meteorological conditions. The coupling between surface resistance and NH3/SO2 concentration ratios shows that the deposition velocity for SO2 is regulated by the regional pollution climate. Recent long-term SO2 flux measurements in a transect over Europe demonstrate the close link between NH3/SO2 concentrations and rc (SO2). The deposition velocity for SO2 is predicted to have increased with time since the 1970s and imply a 40% increase in v d at a site at which the annual mean ambient SO2 concentrations declined from 47 to 3 μg m-3 between 1973 and 1998.

An integrated field experiment was organized as part of the EC EXAMINE project to quantify surface–atmosphere fluxes of ammonia (NH3). Fluxes of NH3, NH4+ aerosol, HNO3 and HCl were measured using the gradient method with several continuous and batch sampling systems. Within-canopy NH3 was determined to quantify the contribution of leaf cuticles, sub-stomatal apoplastic (intercellular) fluids and other sources and sinks to net fluxes. The campaign included the first field measurements of apoplastic pH and [NH4+], providing independent estimates of the stomatal compensation point (χs) for comparison with micrometeorological results. The latter also compared fluxes before and after cutting. Under the clean conditions of the experiment, [NH3][HNO3] and [NH3][HCl] were much less than values required for aerosol formation, while the NH4+ aerosol size distribution indicated that aerosol evaporation would be much slower than the time-scale of turbulent exchange. Nevertheless, the measurements suggest that aerosol production/growth as well as formation of HCl may have occurred within the canopy. Apoplastic [NH4+] and pH of leaves showed no diurnal patterns, with the main control on χs being the temperature dependence of the solubility equilibria. Fluxes of NH3 were bi-directional (−200 to 620 ng m−2 s−1), with deposition generally occurring when the canopy was wet and emission when it was dry, particularly during the day. Nocturnal emissions indicated a non-stomatal NH3 source, while daytime emissions were larger than indicated by the apoplastic estimates of χs. The within-canopy data, together with an inverse Lagrangian source–sink analysis, showed decomposing litter to be a significant NH3 source, explaining nocturnal NH3 emissions and larger emissions following cutting. Daytime net fluxes were controlled by the top part of the canopy, due to χs for siliques apparently being larger than for leaves. The measurements have been used to develop multi-layer resistance models of NH3 exchange. A 3-layer ‘foliage–litter–silique model’ provides a detailed mechanistic treatment of the component fluxes, while a 2-layer ‘foliage–litter model’ is better suited to generalization in atmospheric transport models. Application of the new models should help improve estimates of regional atmospheric ammonia budgets.

A detailed literature review has been undertaken of the magnitude of non-agricultural sources of ammonia (NH 3 ) in the United Kingdom. Key elements of the work included estimation of nitrogen (N) excreted by different sources (birds, animals, babies, human sweat), review of miscellaneous combustion sources, as well as identification of industrial sources and use of NH 3 as a solvent. Overall the total non-agricultural emission of NH 3 from the UK in 1996 is estimated here as 54 (27–106) kt NH 3 –N yr −1 , although this includes 11 (6–23) kt yr −1 from agriculture related sources (sewage sludge spreading, biomass burning and agro-industry). Compared with previous estimates for 1990, component source magnitudes have changed both because of revised average emissions per source unit (emission factors) and changes in the source activity between 1990 and 1996. Sources with larger average emission factors than before include horses, wild animals and sea bird colonies, industry, sugar beet processing, household products and non-agricultural fertilizer use, with the last three sources being included for the first time. Sources with smaller emission factors than before include: land spreading of sewage sludge, direct human emissions (sweat, breath, smoking, infants), pets (cats and dogs) and fertilizer manufacture. Between 1990 and 1996 source activities increased for sewage spreading (due to reduced dumping at sea) and transport (due to increased use of catalytic converters), but decreased for coal combustion. Combined with the current UK estimates of agricultural NH 3 emissions of 229 kt N yr −1 (1996), total UK NH 3 emissions are estimated at 283 kt N yr −1 . Allowing for an import of reduced nitrogen (NH x ) of 30 kt N yr −1 and deposition of 230 kt N yr −1 , these figures imply an export of 83 kt NH 3 –N yr −1 . Although export is larger than previously estimated, due to the larger contribution of non-agricultural NH 3 emissions, it is still insufficient to balance the UK budget, for which around 150 kt NH 3 –N are estimated to be exported. The shortfall in the budget is, nevertheless, well within the range of uncertainty of the total emissions.

Arising from: F. Magnani et al. , 849–851 (2007)10.1038/nature05847 ; Magnani et al. reply Magnani et al.1 present a very strong correlation between mean lifetime net ecosystem production (NEP, defined as the net rate of carbon (C) accumulation in ecosystems2) and wet nitrogen (N) deposition. For their data in the range 4.9–9.8 kg N ha-1 yr-1, on which the correlation largely depends, the response is approximately 725 kg C per kg N in wet deposition. According to the authors, the maximum N wet deposition level of 9.8 kg N ha-1 yr-1 is equivalent to a total deposition of 15 kg N ha-1yr-1, implying a net sequestration near 470 kg C per kg N of total deposition. We question the ecological plausibility of the relationship and show, from a multi-factor analysis of European forest measurements, how interactions with site productivity and environment imply a much smaller NEP response to N deposition.

The global emission of ammonia (NH 3 ) is about 54 Mt N. The major global sources are excreta from domestic animals and fertilizers, but oceans, biomass burning and crops are also important. About 60% of the global NH 2 emission is estimated to come from anthropogenic sources. NH 3 -N emissions are of the same order as the NO x -N emissions on both global and European scales. Emitted NH 3 returns to the surface mainly in the form of dry deposition of NH 3 and wet deposition of ammonium (NH 4 + ). In countries with high NH 3 emission densities, dry deposition of NH 3 from local sources and wet deposition of NH 4 from remote sources dominate the deposition. In countries with low NH 3 emission densities only wet deposition of NH 4 + from remote sources dominates the deposition. Surface exchange of NH 3 is essentially bi-directional, depending on the NH 3 compensation point concentration of the vegetation and the airborne concentration. In general, the compensation point is larger for agricultural than semi-natural plants, and varies with plant growth stage. According to basic thermodynamics the leaf tissue or stomatal compensation point of NH 3 doubles for each increase of 5 °C. However, exchange of NH 3 does not only occur through the stomata, but it can also be deposited to leaf surfaces, as well as emitted back to the atmosphere from drying leaf surfaces. Atmospheric transport and deposition models can be used to interpolate NH 3 concentrations and depositions in space and time, to calculate import/export balances and to estimate past or future situations. Adverse effects on sensitive ecosystems caused by high N deposition can be reduced by lowering the emissions and, to a limited extent, also by removing sources close to the ecosystem to be protected.

The large spatial variability of ammonia (NH 3 ) emissions and deposition makes it difficult to estimate the input of reduced nitrogen to different ecosystems from measurements, since to quantify accurately the spatial variation in NH 3 concentration would require a very large number of monitoring stations. Such quantification is an important requirement for assessing the impacts of acidifying and nitrogen entrophicating deposition. These problems have been addressed in the current study by applying an existing multi-layer model of ammonia transport (TERN) (ApSimon et al., Atmospheric Environment 1994, 28, 665–678), together with a detailed NH 3 emission field, to develop a long-term statistical trajectory model (FRAME) over Great Britain at a 5 km×5 km grid resolution. The model treats the vertical concentration gradient explicitly with 33 layers, as well as provides a description of the land-use dependence of dry deposition using a resistance formulation dependent on major surface landcover classes. By treating vertical diffusion explicitly, the model provides the capability for examining the spatial variability at a finer scale than models assuming instantaneous mixing of emissions, and also avoids the need for air concentration and deposition correction factors. Model results are presented in the form of maps of annual mean NH 3 concentration and net dry deposition, and a comparison is made with NH 3 concentrations measured by a national monitoring network. A total annual budget for reduced nitrogen is given which shows the directional dependence of both total deposition and export of reduced nitrogen.

Ammonia fluxes over vegetated land are bi-directional, with differences between emission and deposition occurring in relation to environmental conditions (temperature and wetness), plant community (e.g. arable land, unfertilized ecosystems), as well as other factors such as plant phenology. In the present paper, measurements of net NH 3 fluxes over arable cropland are reported in the range −50 to 40 ng m -2 s -1 . These are used as examples to highlight the factors affecting the exchange process. Net fluxes are expected to depend on the competition between deposition to leaf surfaces and bi-directional exchange with a “stomatal compensation point” for leaf tissues ( χ s ). Current models describe this effect poorly, and two new resistance models are described here. These estimate a “canopy compensation point” ( χ c ), as the net potential for NH 3 emission from the canopy. In the simpler of the two models, leaf surface uptake is parametrized using a resistance ( R w ). This model is able to reproduce bi-directional fluxes, though there is also evidence that both cuticular NH 3 adsorption and desorption may occur, dependent on previous fluxes. A more complex dynamic approach is therefore developed, treating cuticular uptake as a capacitance ( C d ). The dynamic model is able to reproduce the pattern of desorption, but further development of both models is required to provide descriptions valid over longer periods and for a range of ecosystem types.

Measurements of atmospheric ammonia concentration along a gradient of decreasing concentration, species composition and tissue nitrogen content of a range of plant species were made in woodland in the vicinity of four intensive animal units in Scotland. Ammonia concentrations were large at woodland edges close to the livestock buildings (annual means 20–60 μg m−3) and exceed critical levels for NH3 (8 μg m−3 annual mean). Surveys of species composition of ground flora along an 0.5 km transect from livestock buildings show marked changes within 300 m downwind of the buildings. Species such as Deschampsia flexuosa, Holcus lanatus, Rubus idaeus and Urtica dioica were abundant close to livestock units and their percentage cover decreased rapidly with distance from source, while the more N-sensitive species such as Oxalis acetosella, Galium odoratum, mosses and ferns which are found upwind and outside the influence of the NH3 source, were scarce at all sites receiving >25 kg ha−1 N year−1. Visible injury to pine and spruce needles was observed immediately downwind of the buildings. Foliar nitrogen concentration of a number of species was large close to the buildings and declined with distance. Total nitrogen deposition at the woodland boundaries is estimated to range from 40 to 80 kg N ha−1 year−1 at the 4 sites and exceeds critical loads for acidic coniferous forest, i.e. 15–20 kg N ha−1 year−1 to protect ground flora, and is also often in excess of that (11–50 kg N ha−1 year−1) proposed to protect tree health. Foliar nitrogen content of mosses, (LN, % dry weight) is related to nitrogen deposition (FN, kg N ha−1 year−1) according to LN = 3.81(1-e−0.04FN).

Wet deposition of nitrogen is reasonably well monitored throughout Europe, whereas the dry deposition inputs are provided largely by models. Recent long-term measurements of NO2 and NH3 fluxes to semi-natural vegetation have shown that rates of NH3 deposition exceed those of NO2, typically by an order of magnitude. Incorporating the results of these dry deposition measurements in regional deposition budgets shows that the inputs of reduced nitrogen contribute the dominant fraction of the total nitrogen inputs in most regions of the UK. The results are illustrated by comparing the atmospheric mass-budget for oxidized nitrogen over the UK. Of the annual UK emissions of NOx, amounting to 780 kt N (Salway et al., 1997), only 5% is dry deposited to terrestrial surfaces within the country while 15% is wet deposited, whereas for the reduced nitrogen, 42% of emissions (of the 260 kt N, Salway et al., 1997) are dry deposited and 46% are wet deposited. Even more striking are the relative contributions of oxidized and reduced nitrogen to semi-natural vegetation, which is a particularly efficient sink for NH3. The species composition of semi-natural vegetation is also regarded as very sensitive to nitrogen inputs. The distribution of nitrogen deposition among different land uses shows that the average input to forest in the UK is 33 kg N ha−1 annually of which 78% is reduced nitrogen. The other land uses receive about 15 kg N ha−1 of nitrogen of which between 55% and 65% is NHx. Critical loads for nutrient nitrogen are exceeded primarily in forested and moorland areas as a consequence of NH3 dry deposition and wet NH4+ deposition. For forests the area in exceedance of 20 kg N ha−1 year−1 critical load represents 70% of the forest area (1.4 × 106 ha−1) while for moorland the area in exceedance is 13% of the moorland area and occupies 1.04 × 106 ha−1).

Ammonia is a reactive pollutant emitted primarily by agricultural sources near ground level in the rural environment. The consequence of these factors is that, in addition to the effects of long-range pollutant transport, ammonia has major effects at a local scale, with emission and receptor areas often closely located in the rural landscape. There is a substantial local spatial variability that needs to be considered in effects assessments, while variations in local deposition may affect the amount of ammonia available for impacts further afield. The wide-ranging UK programme ADEPT (Ammonia Distribution and Effects ProjecT) has addressed these issues through a combination of measurement and modelling activities concerning the distribution of emissions, atmospheric transport, deposition and effects assessment. The results are illustrated here by summarizing the findings of a joint experiment at Burrington Moor, Devon, and wider modelling contrasting the variability at a field scale with 5 km resolution estimates for the UK. The fraction of emitted NH3 deposited locally is shown to depend critically on the downwind land-cover, with fluxes being dependent on interactions with the ammonia compensation point. This will restrict deposition back to agricultural land, but may mean that non-conservation woodlands could be of benefit to recapture a significant fraction of emissions. The generalized models demonstrate the high spatial variability of ammonia impacts, with a case study being used to show the consequences at a field scale. In source regions substantial variability occurs at sub-1 km levels and this will have major consequences for the emission reduction targets needed to protect ecosystems.

Accurate models of the spatial distribution of ammonia (NH3) emissions are an essential input to models of atmospheric transport and deposition. This is especially important when resulting deposition maps are used to calculate patterns of critical loads exceedance or to determine suitable abatement measures. A new methodology has been developed to model the distribution of agricultural ammonia emissions and is applied here for the UK. The model employs a specific spatial weighted redistribution of NH3 emission sources onto suitable landcover types at a 1-km grid level. Key input data to the model are agricultural census data, a satellite-based landcover map and estimates of NH3 emission source strength. The model provides more realistic spatial NH3 emission estimates than previous models, especially for semi-natural/natural areas by relocating emission sources from extensively used upland areas to the more intensively farmed lowland areas within each parish. At present the model results are summarised as maps at a 5-km grid resolution to reduce uncertainty in the spatial location of NH3 sources. Compared with coarser resolution estimates this also provides a more accurate link to critical load exceedances. The more accurate redistribution also reduces the apparent critical loads exceedance on upland areas. Results are presented and compared for 1988 and 1996. These show broadly similar patterns between the years, although substantial local changes have occurred, particularly for intensive livestock farming. The model has been used to generate initial spatially resolved abatement scenarios and provides a general tool for locating NH3 emissions that could be applied to other regions.