NHx Deposition Research Articles

Large Seasonal Variation in Nitrogen Isotopic Abundances of Ammonia Volatilized from a Cropland Ecosystem and Implications for Regional NH3 Source Partitioning.

Ammonia (NH3) volatilization from agricultural lands is a main source of atmospheric reduced nitrogen species (NHx). Accurately quantifying its contribution to regional atmospheric NHx deposition is critical for controlling regional air nitrogen pollution. The stable nitrogen isotope composition (expressed by δ15N) is a promising indicator to trace atmospheric NHx sources, presupposing a reliable nitrogen isotopic signature of NH3 emission sources. To obtain more specific seasonal δ15N values of soil NH3 volatilization for reliable regional seasonal NH3 source partitioning, we utilized an active dynamic sampling technique to measure the δ15N-NH3 values volatilized from maize cropping land in northeast China. These values varied from -38.0 to -0.2‰, with a significantly lower rate-weighted value observed in the early period (May-June, -30.5 ± 6.7‰) as compared with the late period (July-October, -8.5 ± 4.3‰). Seasonal δ15N-NH3 variations were related to the main NH3 production pathway, degree of soil ammonium consumption, and soil environment. Bayesian isotope mixing model analysis revealed that without considering the seasonal δ15N variation in soil-volatilized NH3 could result in an overestimate by up to absolute 38% for agricultural volatile NH3 to regional atmospheric bulk ammonium deposition during July-October, further demonstrating that it is essential to distinguish seasonal δ15N profile of agricultural volatile NH3 in regional source apportionment.

Contrasting change trends in dry and wet nitrogen depositions during 2011 to 2020: Evidence from an agricultural catchment in subtropical Central China

Over the past decade, China has experienced a decline in atmospheric reactive nitrogen (Nr) emissions. Given that China's subtropical region is a significant nitrogen (N) deposition hotspot, it is essential to accurately quantify the ten-year variations in dry and wet N depositions in the context of reductions in atmospheric Nr emissions. Here, we evaluated the spatiotemporal variation in N deposition on forest, paddy field and tea field ecosystems in a typical subtropical agricultural catchment from 2011 to 2020. Our findings indicated a significant decrease in total N deposition in both the tea field ecosystem (41.5–30.5 kg N ha−1) and the forest ecosystem (40.8–25.7 kg N ha−1) (P < 0.05), but no significant change in the paddy field ecosystem (29.3–32.9 kg N ha−1). Specifically, dry N deposition exhibited significant declines except in the paddy field ecosystem, whereas wet N deposition had no significant change. The reduction in total oxidized and reduced N depositions in forest and tea field ecosystems is attributed to the decrease in NOx and NH3 emissions. Additionally, The ratio of NHx deposition to total N deposition all exceeded 0.5 in three ecosystems and the NHx/NOy ratio had an increasing trend (P < 0.05) in the paddy field, indicating that reactive N emissions from agricultural sources were the primary contributor to overall N deposition. Our study emphasizes that despite the decreasing trend in N deposition, it still exceeds the critical loads of natural ecosystems and requires stringent N emissions control, particularly from agricultural sources, in the future.

Spatiotemporal variations of atmospheric nitrogen deposition in China during 2008–2020

In recent decades, China has been a hotspot for reactive nitrogen (N) deposition due to intensive fossil fuel burning and increased agricultural activities. The Chinese government has implemented active measures to protect air quality and reduce N deposition. In this study, we combined monthly measurements of monitoring sites and satellite data to construct random forest (RF) models for the estimation of monthly N deposition in China during 2008–2020. This provides N deposition estimation in latest years and has a finer temporal resolution. The RF models can address more complicated relationships between variables and performed well in estimating N deposition compared to the measurements with average correlation coefficients of 0.79 and 0.77 for dry and wet N deposition, respectively. We found that mean annual total N deposition was 22.0 ± 0.8 kg N ha−1 yr−1 during 2008–2020, which accounted for 64% (±4%) of reactive N emissions in China. Although the total N deposition was high, it has stabilized and decreased slightly over the past decade, with dry and wet N deposition contributing equally to the total N deposition. NHx deposition (FNHx) still slowly increased (0.09 kg N ha−1 yr−2) during 2015–2020, when N fertilizer application and livestock started to decrease. This is due to the rapid decline in NOx and SO2 emissions during the same period. Our results provide a new perspective on the spatiotemporal variations in the total N deposition in China. It is necessary to optimize other emission reduction strategies to mitigate air pollution and N deposition.

Precipitation trend increases the contribution of dry reduced nitrogen deposition

Given the leveling off in oxidized nitrogen emissions around the world, the atmospheric deposition of reduced nitrogen (NHx = NH3 + NH4+) has become progressively critical, especially dry deposition, which presents great threats to plant growth. A combination of historical deposition data of measured wet NHx and modeled dry NHx in China suggests that dry NHx deposition has been increasing substantially (4.50% yr−1, p < 0.05) since 1980. Here, chemical transport model (WRF-EMEP) results indicate that variation in NH3 emissions is not a dominant factor resulting in the continually increasing trends of dry NHx deposition, while climate change-induced trends in precipitation patterns with less frequent light rain and more frequent consecutive rain events (with ≥2 consecutive rainy days) contribute to the increase in dry NHx deposition. This will continue to shift NHx deposition from wet to dry form at a rate of 0.12 and 0.23% yr−1 (p < 0.05) for the period of 2030–2100 in China under the RCP4.5 and RCP8.5 scenarios, respectively. Further analysis for North America and Europe demonstrates results similar to China, with a consistent increase in the contribution of dry NHx deposition driven by changing precipitation patterns from ~30% to ~35%. Our findings, therefore, uncover the change of precipitation patterns has an increasing influence on the shifting of NHx deposition from wet to dry form in the Northern Hemisphere and highlight the need to shift from total NHx deposition-based control strategies to more stringent NH3 emission controls targeting dry NHx deposition in order to mitigate the potential negative ecological impacts.

Transformation From NHx to NOy Deposition Aggravated China's Forest Soil Acidification

AbstractElevated nitrogen (N) deposition and changes in reduced or oxidized component contribution greatly affect soil acidification. China has experienced a significant transformation of N deposition components from NHx to NOy over the past 40 years, but the effects of component transformation on soil acidification are poorly understood. Therefore, long‐term monitoring data and literature on N deposition, combined with the results of isotope experiments, were used to explore the contributions of different N forms on soil acidification in China's forests. Here, all processes related to NHx and NOy, including the transformation to NH4+ and NO3− and subsequent N cycling in the soil, were considered. We found that N‐induced soil acidification in 80% area of China’s forests was dominated by NHx deposition, and the other areas (South China) were dominated by NOy deposition in 2010s. From 1980 to 2019, the average contribution of NHx was higher than that of NOy but the latter contribution continued to increase. Meanwhile, the results showed that soil acidification increased with the decrease of the ratio of NHx to NOy (RNHx/NOy), which is mainly because NOy is more easily leached in the form of NO3− than that of NHx under the influence of different plant preferences and soil retention rates, resulting in a higher net proton production of NOy. Our research has powerful implications for policymaking, provides a theoretical basis for formulating different N reduction policies in different regions, and points out that the synergistic effect of RNHx/NOy changes should be considered to alleviate soil acidification.

The Response of Nitrogen Deposition in China to Recent and Future Changes in Anthropogenic Emissions

AbstractWe present a model analysis that quantifies the influence of anthropogenic emission variations on the recent (2005–2015) and future (2030) changes in national scale nitrogen deposition in China and the interacting relationships between their emissions and depositions. We find that the 0.39 Tg N yr−1 increase in the dry deposition of reduced nitrogen (NHx) during 2007–2015 is dominated by the decrease in sulfur dioxides (SO2) emissions, which reduces the formation of ammonium (NH4+) aerosols from gas‐phase ammonia (NH3). Although the decrease in SO2 emissions led to conversion from wet to dry deposition of NHx, the NH4+ wet deposition increased by 0.75 Tg N yr−1 with large interannual variations driven by precipitation. Different from NHx, the oxidized nitrogen (NOy) deposition increased by 1.6 Tg N yr−1 from 2005 to 2011 and decreased by 1.1 Tg N yr−1 from 2011 to 2015, following the changes in nitrogen oxides (NOx) emissions. Based on the national scale emission‐deposition relationship estimated from model sensitivity experiments, we find that future emission control would reduce the NOy deposition by 2.4 ± 0.08 to 3.4 ± 0.12 Tg N yr−1 (49%–73%), but only lead to small decreases of 0.93 ± 0.05 to 1.2 ± 0.06 Tg N yr−1 (9%–14%) in NHx deposition in China from 2015 to 2030. The decrease in NOx and SO2 emissions under future scenarios would enhance the NHx dry deposition over domestic China, diminishing the effectiveness of the NH3 emission control. The results emphasize that stricter strategies for controlling agricultural NH3 emissions should be taken to alleviate the adverse environmental impacts from atmospheric nitrogen deposition.

Quantifying the wet deposition of reactive nitrogen over China: Synthesis of observations and models

Accurate estimation on reaction nitrogen (Nr) deposition is highly demanded for assessing the impacts on the environment and human beings. This study investigated the wet deposition of inorganic nitrogen (IN) in mainland China by measurements from over 500 sites from five observational networks/databases and ensemble results of eleven chemical transport models (CTMs). Each data source has its focus and limitations and together formed a comprehensive view over China. But the inconsistency among different sources may hinder the appropriate usage of data. Model evaluation results demonstrated the models' deficiency in simulating the wet NO3− deposition over Southeast China (40 % underestimation) and showed an overall underestimation of wet NH4+ deposition over the hotspot regions (5–60 % underestimation). A synthesis of this study and twelve reference studies was conducted to quantify the national amount of wet IN deposition. The estimations by CTMs ranged 2.4–3.9 Tg(N) yr−1 for wet NOy deposition and 4–6.7 Tg(N) yr−1 for wet NHx deposition, after adjusting the results with 10–19 % underestimations in wet NOy deposition and 1–40 % underestimations in wet NHx deposition. The estimations by ground observations ranged 7.1–9 Tg(N) yr−1 for wet NOy deposition and 8–13.1 Tg(N) yr−1 for wet NHx deposition, which were 20–275 % higher than the estimation by CTMs, but the results were strongly influenced by the abundances and representative of measurements. Studies using statistical techniques to interpolate site observations predicted 3–5.5 Tg(N) yr−1 for wet NOy deposition and 3.9–7.2 Tg(N) yr−1 for wet NHx deposition. This approach benefited from high accuracy and good robustness of the statistical models, but the uncertainty in the interpolation methods could be a potential drawback.

The enhanced role of atmospheric reduced nitrogen deposition in future over East Asia–Northwest Pacific

The atmospheric nitrogen deposition plays a crucial role in natural ecosystem, and the changes in emissions substantially affect the amount of nitrogen deposition. Along with the decrease in NOx emissions and increase in NH3 emissions, the reduced nitrogen deposition may play a more important role in future. However, to what extent these changes may modify the reduced nitrogen deposition across East Asia, which is fulfilled with a large amount of nitrogen deposition, to the northwestern Pacific has not yet to be clear. Based on the results of multi-model ensemble of Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), the future changes of reduced nitrogen (NHx) deposition is firstly examined. Here we show the changes of NHx deposition flux is substantially modulated by both climate change and emissions, exhibiting an increasing trend over East Asia-Northwest Pacific in future under representative concentration pathways (RCP) 8.5 scenario, largely controlled by increase of NH3 emissions, contrasting to the oxidized nitrogen deposition which is projected to decrease. Specifically, the ratio of NHx to total nitrogen deposition in eastern China increases from 38% at present to 56% by the end of the century under RCP 8.5, indicative of a transition in the form of dominant nitrogen deposition from oxidized to reduced one. The increase is clearly discernable over the marginal seas and northwestern Pacific. Moreover, we identify a meridional shift of high wet NHx deposition from northern China in summer to southern China in the other seasons. Based on simulations from regional models Weather Research and Forecasting (WRF) and Community Multi-scale Air Quality (CMAQ), we find that the synergistically nonlinear modulation of NHx concentration and precipitation triggers the north-south shift of wet NHx deposition. The findings in this study indicate a potentially more important role of reduced nitrogen deposition on the natural ecosystem in future.

Lichen nitrogen concentrations and isotopes for indicating nitrogen deposition levels and source changes

Anthropogenic nitrogen (N) and sulfur (S) emissions triggered acid deposition and affected the environmental quality and ecosystem functions. Lichen N and S concentrations have been found increasing with N and S deposition at relatively low levels, respectively. However, it remains unclear whether lichen N and S concentrations can respond to corresponding deposition fluxes under high pollution environments and how to use lichen N isotopes (δ15N) to evaluate source contributions of N deposition quantitatively.Along an urban-to-rural transect surrounding a polluted area in northern China, we investigated lichen N and S to examine their sensitivity and applicability in estimating N and S deposition. Moreover, we established a new method to reconstruct site-based δ15N values of ammonium N (NHx) and oxidized N (NOy) deposition using lichen δ15N and then to differentiate relatively contributions of major N emission sources.Lichen N (1.0–3.9%, 2.5 ± 0.6%) and S (0.09–0.33%, 0.21 ± 0.06%) decreased linearly with distances from the polluted center area. Wet inorganic N and sulfate deposition (29.0 ± 6.1 kg-N/ha/yr and 25.8 ± 7.9 kg-S/ha/yr, respectively) estimated by integrating relationships from previous literature data were comparable with levels based on direct deposition observations in the study area. Lichen δ15N varied between −12.1 to −4.1‰ and averaged −7.1 ± 2.0‰ among our study sites, and reconstructed δ15N values of NHx and NOy deposition averaged −12.8 ± 1.0‰ and −3.2 ± 0.5‰, respectively. Source contribution analyses revealed significant contributions of volatilization ammonia (66 ± 3%) from wastes and fertilizers to NHx deposition and non-fossil fuel N oxides (62 ± 7%) from biomass burnings and microbial N cycles to NOy deposition.These results indicate that lichen N and S are sensitive to deposition fluxes under high pollution and applicable to estimating deposition levels. This work improves the methodology of atmospheric deposition biomonitoring based on lichen element and isotope records.

Epiphytic macrolichen communities indicate climate and air quality in the U.S. Midwest

Epiphytic lichens directly exposed to atmospheric conditions can help detect how diffuse but pressing global changes may impact regional forest health. For 388 plots in the U.S. Midwest region, we developed indices for climate and air quality based on variation in tree-dwelling macrolichen community compositions (NMS ordination scores), species indicator values, and species environmental optima. Three lichen indices (NMS axis 1 scores, thermophile scores, and climate centroid scores) strongly covaried with thermal and evaporative-demand variables. By contrast, three other indices (NMS axis 2 scores, nitrophile scores and nitrogen centroid scores) were correlated with NHx (reduced N) deposition. Lichen communities had contrasting responses to different forms of N (NOx vs. NHx). Overall, thermal climate variables appeared more influential than air quality in structuring regional communities, based on greater explained variation of community compositions. Richness of species and of potentially adaptive secondary metabolites declined in hot, dry, or NHx-rich sites. With continued monitoring, significant changes in lichen-community based indices could signal directional shifts in forest vegetation. Changes in the thermophile or nitrophile indices more specifically would indicate the agent and rate of change for forest-altering trends.

Toward the improvement of total nitrogen deposition budgets in the United States.

Frameworks for limiting ecosystem exposure to excess nutrients and acidity require accurate and complete deposition budgets of reactive nitrogen (Nr). While much progress has been made in developing total Nr deposition budgets for the U.S., current budgets remain limited by key data and knowledge gaps. Analysis of National Atmospheric Deposition Program Total Deposition (NADP/TDep) data illustrates several aspects of current Nr deposition that motivate additional research. Averaged across the continental U.S., dry deposition contributes slightly more (55%) to total deposition than wet deposition and is the dominant process (>90%) over broad areas of the Southwest and other arid regions of the West. Lack of dry deposition measurements imposes a reliance on models, resulting in a much higher degree of uncertainty relative to wet deposition which is routinely measured. As nitrogen oxide (NOx) emissions continue to decline, reduced forms of inorganic nitrogen (NHx = NH3 + NH4+) now contribute >50% of total Nr deposition over large areas of the U.S. Expanded monitoring and additional process-level research are needed to better understand NHx deposition, its contribution to total Nr deposition budgets, and the processes by which reduced N deposits to ecosystems. Urban and suburban areas are hotspots where routine monitoring of oxidized and reduced Nr deposition is needed. Finally, deposition budgets have incomplete information about the speciation of atmospheric nitrogen; monitoring networks do not capture important forms of Nr such as organic nitrogen. Building on these themes, we detail the state of the science of Nr deposition budgets in the U.S. and highlight research priorities to improve deposition budgets in terms of monitoring and flux measurements, leaf- to regional-scale modeling, source apportionment, and characterization of deposition trends and patterns.

Summertime ambient ammonia and its effects on ammonium aerosol in urban Beijing, China.

To improve the understanding of NH3 variation in urban Beijing, high temporal resolution (1-min averaged) NH3 data derived during the summer of 2009 were analyzed. The results indicated that after rain there was generally an increase in NH3 concentration. The analysis of the 1-min data revealed that a weak but significant linear correlation existed between NH3 and NOx in August, indicating a connection between NH3 concentrations and traffic levels during summer. This was further supported by peak NH3 levels approximately coinciding with morning rush hours. The daily NH3 concentrations were weakly correlated with NH4+ concentrations, suggesting that NH3 played an important precursor role in NH4+ in PM2.5 formation. The mean mass ratio of NH3/NHx was 0.76±0.13. This revealed that NHx (NH3+NH4+) was influenced by local sources during the summer period and that NH3 dry deposition could contribute substantially to NHx deposition. A high temperature, relative humidity, and degree of oxidization could increase photochemical reactions and aqueous processing, having the important effect of SO2 to SO42- and NH3 to NH4+ conversion in summer. The back trajectory analysis indicates that the transport of air masses from the North China Plain region contributed to the atmospheric NH3 and NH4+ aerosol variations in Beijing.

Isotopic evidence for the occurrence of biological nitrification and nitrogen deposition processing in forest canopies.

This study examines the role of tree canopies in processing atmospheric nitrogen (Ndep ) for four forests in the United Kingdom subjected to different Ndep : Scots pine and beech stands under high Ndep (HN, 13-19kgNha(-1) yr(-1) ), compared to Scots pine and beech stands under low Ndep (LN, 9kgNha(-1) yr(-1) ). Changes of NO3 -N and NH4 -N concentrations in rainfall (RF) and throughfall (TF) together with a quadruple isotope approach, which combines δ(18) O, Δ(17) O and δ(15) N in NO3 (-) and δ(15) N in NH4 (+) , were used to assess N transformations by the canopies. Generally, HN sites showed higher NH4 -N and NO3 -N concentrations in RF compared to the LN sites. Similar values of δ(15) N-NO3 (-) and δ(18) O in RF suggested similar source of atmospheric NO3 (-) (i.e. local traffic), while more positive values for δ(15) N-NH4 (+) at HN compared to LN likely reflected the contribution of dry NHx deposition from intensive local farming. The isotopic signatures of the N-forms changed after interacting with tree canopies. Indeed, (15) N-enriched NH4 (+) in TF compared to RF at all sites suggested that canopies played an important role in buffering dry Ndep also at the low Ndep site. Using two independent methods, based on δ(18) O and Δ(17) O, we quantified for the first time the proportion of NO3 (-) in TF, which derived from nitrification occurring in tree canopies at the HN site. Specifically, for Scots pine, all the considered isotope approaches detected biological nitrification. By contrast for the beech, only using the mixing model with Δ(17) O, we were able to depict the occurrence of nitrification within canopies. Our study suggests that tree canopies play an active role in the N cycling within forest ecosystems. Processing of Ndep within canopies should not be neglected and needs further exploration, with the combination of multiple isotope tracers, with particular reference to Δ(17) O.

Evidence for differential effects of reduced and oxidised nitrogen deposition on vegetation independent of nitrogen load

Nitrogen (N) deposition impacts natural and semi-natural ecosystems globally. The responses of vegetation to N deposition may, however, differ strongly between habitats and may be mediated by the form of N. Although much attention has been focused on the impact of total N deposition, the effects of reduced and oxidised N, independent of the total N deposition, have received less attention. In this paper, we present new analyses of national monitoring data in the UK to provide an extensive evaluation of whether there are differences in the effects of reduced and oxidised N deposition across eight habitat types (acid, calcareous and mesotrophic grasslands, upland and lowland heaths, bogs and mires, base-rich mires, woodlands). We analysed data from 6860 plots in the British Countryside Survey 2007 for effects of total N deposition and N form on species richness, Ellenberg N values and grass:forb ratio. Our results provide clear evidence that N deposition affects species richness in all habitats except base-rich mires, after factoring out correlated explanatory variables (climate and sulphur deposition). In addition, the form of N in deposition appears important for the biodiversity of grasslands and woodlands but not mires and heaths. Ellenberg N increased more in relation to NHx deposition than NOy deposition in all but one habitat type. Relationships between species richness and N form were habitat-specific: acid and mesotrophic grasslands appear more sensitive to NHx deposition while calcareous grasslands and woodlands appeared more responsive to NOy deposition. These relationships are likely driven by the preferences of the component plant species for oxidised or reduced forms of N, rather than by soil acidification.

Seasonal Variation of Ammonia and Ammonium Aerosol at a Background Station in the Yangtze River Delta Region, China

The measurement of atmospheric NH3 was conducted by mean of passive samplers from September 2009 to December 2010 at Lin’an regional background station located in the economically blooming Yangtze River Delta (YRD) region in eastern China. NH4 + in fine particles was also measured in 2010 at this site. The NH3 concentration ranged from 0.1 to 41.8 ppb, with the annual average of 16.5 ± 11.2 ppb in 2010. The daily NH4 + concentrations ranged from 0.02 to 19.2 μg/m 3 , with an annual average of 4.3 ± 3.5 μg/m 3 . NH3 concentrations were highest in summer and lowest in winter, showing positive correlations with agricultural activities and temperature. The highest concentrations of NH4 + were in autumn coinciding with the period of active open burning of agricultural residues. The mean mass ratio of NH3/NHx is estimated to be 0.8 ± 0.1 during 2010, indicating that NHx was mainly influenced by local sources around Lin’an. The air mass back trajectory analysis suggests that both local sources and long-distance transport played important roles in the observed ammonium aerosol at Lin'an. High NHx deposition in this regional background station suggests the urgency of reducing NH3 emission in the YRD region.

Nitrogen deposition to the United States: distribution, sources, and processes

Abstract. We simulate nitrogen deposition over the US in 2006–2008 by using the GEOS-Chem global chemical transport model at 1/2°×2/3° horizontal resolution over North America and adjacent oceans. US emissions of NOx and NH3 in the model are 6.7 and 2.9 Tg N a−1 respectively, including a 20% natural contribution for each. Ammonia emissions are a factor of 3 lower in winter than summer, providing a good match to US network observations of NHx (≡NH3 gas + ammonium aerosol) and ammonium wet deposition fluxes. Model comparisons to observed deposition fluxes and surface air concentrations of oxidized nitrogen species (NOy) show overall good agreement but excessive wintertime HNO3 production over the US Midwest and Northeast. This suggests a model overestimate N2O5 hydrolysis in aerosols, and a possible factor is inhibition by aerosol nitrate. Model results indicate a total nitrogen deposition flux of 6.5 Tg N a−1 over the contiguous US, including 4.2 as NOy and 2.3 as NHx. Domestic anthropogenic, foreign anthropogenic, and natural sources contribute respectively 78%, 6%, and 16% of total nitrogen deposition over the contiguous US in the model. The domestic anthropogenic contribution generally exceeds 70% in the east and in populated areas of the west, and is typically 50–70% in remote areas of the west. Total nitrogen deposition in the model exceeds 10 kg N ha−1 a−1 over 35% of the contiguous US.

Riverine nitrogen export in Swedish catchments dominated by atmospheric inputs

We present the first estimates of net anthropogenic nitrogen input (NANI) in European boreal catchments. In Swedish catchments, nitrogen (N) deposition is a major N input (31–94%). Hence, we used two different N deposition inputs to calculate NANI for 36 major Swedish catchments. The relationship between riverine N export and NANI was strongest when using only oxidized deposition (NOy) as atmospheric input (r2 = 0.70) rather than total deposition (i.e., both oxidized and reduced nitrogen, NOy + NHx deposition, r2 = 0.62). The y-intercept (NANI = 0) for the NANI calculated with NOy is significantly different from zero (p = 0.0042*) and indicates a background flux from the catchment of some 100 kg N km−2 year−1 in addition to anthropogenic inputs. This agrees with similar results from North American boreal catchments. The slope of the linear regressions was 0.25 for both N deposition inputs (NOy and NOy + NHx), suggesting that on average, 25% of the anthropogenic N inputs is exported by rivers to the Baltic Sea. Agricultural catchments in central and southern Sweden have increased their riverine N export up to tenfold compared to the inferred background flux. Although the relatively unperturbed northernmost catchments receive significant N loads from atmospheric deposition, these catchments do not show significantly elevated riverine N export. The fact that nitrogen export in Swedish catchments appears to be higher in proportion to NANI at higher loads suggests that N retention may be saturating as loading rates increase. In northern and western Sweden the export of nitrogen is largely controlled by the hydraulic load, i.e., the riverine discharge normalized by water surface area, which has units of distance time−1. Besides hydraulic load the percent total forest cover also affects the nitrogen export primarily in the northern and western catchments.

Atmospheric ammonia and particulate ammonium from agricultural sources in the North China Plain

Abstract Atmospheric ammonia (NH3) and particulate ammonium (NH4+) were measured using passive samplers and particulate samplers at four rural and two suburban sites in the North China Plain (NCP) from August 2006 to September 2009. The annual mean concentrations of NH3 and NH4+ in PM10 across the sites were 15.6 and 12.4 μg m−3, respectively. Higher concentrations of NH3 and NH4+ were found at the rural sites compared with the suburban sites. NH3 concentrations were highest in summer and lowest in winter, showing positive correlations with N fertilizer application rates and temperature in each season. Particulate NH4+ showed significant positive correlations with particulate SO42− and NO3− at the six sites. The annual mean concentrations of secondary inorganic particles (sum of NH4+, SO42− and NO3−) accounted for 26.1–41.3% of the PM10 concentration across all sites. The formation of inorganic NH4+ particles was limited mainly by the availability of H2SO4 and HNO3 and partly by NH3. The total NHx deposition in the NCP averaged 43.5 kg N ha−1 yr−1, accounting for 84.1% of the mean NH3 emission intensity. The serious particulate NH4+ pollution and high NHx deposition suggest the urgency of reducing NH3 emission from agricultural sources in the NCP.

Modelling deposition and air concentration of reduced nitrogen in Poland and sensitivity to variability in annual meteorology

The relative contribution of reduced nitrogen to acid and eutrophic deposition in Europe has increased recently as a result of European policies which have been successful in reducing SO2 and NOx emissions but have had smaller impacts on ammonia (NH3) emissions. In this paper the Fine Resolution Atmospheric Multi-pollutant Exchange (FRAME) model was used to calculate the spatial patterns of annual average ammonia and ammonium (NH4+) air concentrations and reduced nitrogen (NHx) dry and wet deposition with a 5 km × 5 km grid for years 2002–2005. The modelled air concentrations of NH3 and dry deposition of NHx show similar spatial patterns for all years considered. The largest year to year changes were found for wet deposition, which vary considerably with precipitation amount. The FRAME modelled air concentrations and wet deposition are in reasonable agreement with available measurements (Pearson’s correlation coefficients above 0.6 for years 2002–2005), and with spatial patterns of concentrations and deposition of NHx reported with the EMEP results, but show larger spatial gradients. The error statistics show that the FRAME model results are in better agreement with measurements if compared with EMEP estimates. The differences in deposition budgets calculated with FRAME and EMEP do not exceed 17% for wet and 6% for dry deposition, with FRAME estimates higher than for EMEP wet deposition for modelled period and lower or equal for dry deposition. The FRAME estimates of wet deposition budget are lower than the measurement-based values reported by the Chief Inspectorate of Environmental Protection of Poland, with the differences by approximately 3%.Up to 93% of dry and 53% of wet deposition of NHx in Poland originates from national sources. Over the western part of Poland and mountainous areas in the south, transboundary transport can contribute over 80% of total (dry + wet) NHx deposition. The spatial pattern of the relative contribution of national sources to total deposition of NHx may change significantly due to the general circulation of air.

Nitrogen and sulfur deposition on regional and global scales: A multimodel evaluation

We use 23 atmospheric chemistry transport models to calculate current and future (2030) deposition of reactive nitrogen (NOy, NHx) and sulfate (SOx) to land and ocean surfaces. The models are driven by three emission scenarios: (1) current air quality legislation (CLE); (2) an optimistic case of the maximum emissions reductions currently technologically feasible (MFR); and (3) the contrasting pessimistic IPCC SRES A2 scenario. An extensive evaluation of the present‐day deposition using nearly all information on wet deposition available worldwide shows a good agreement with observations in Europe and North America, where 60–70% of the model‐calculated wet deposition rates agree to within ±50% with quality‐controlled measurements. Models systematically overestimate NHx deposition in South Asia, and underestimate NOy deposition in East Asia. We show that there are substantial differences among models for the removal mechanisms of NOy, NHx, and SOx, leading to ±1 σ variance in total deposition fluxes of about 30% in the anthropogenic emissions regions, and up to a factor of 2 outside. In all cases the mean model constructed from the ensemble calculations is among the best when comparing to measurements. Currently, 36–51% of all NOy, NHx, and SOx is deposited over the ocean, and 50–80% of the fraction of deposition on land falls on natural (nonagricultural) vegetation. Currently, 11% of the world's natural vegetation receives nitrogen deposition in excess of the “critical load” threshold of 1000 mg(N) m−2 yr−1. The regions most affected are the United States (20% of vegetation), western Europe (30%), eastern Europe (80%), South Asia (60%), East Asia (40%), southeast Asia (30%), and Japan (50%). Future deposition fluxes are mainly driven by changes in emissions, and less importantly by changes in atmospheric chemistry and climate. The global fraction of vegetation exposed to nitrogen loads in excess of 1000 mg(N) m−2 yr−1 increases globally to 17% for CLE and 25% for A2. In MFR, the reductions in NOy are offset by further increases for NHx deposition. The regions most affected by exceedingly high nitrogen loads for CLE and A2 are Europe and Asia, but also parts of Africa.