Anthropogenic Nitrogen Emissions Research Articles

Spatial–temporal differentiation and control strategies of nitrogen environmental loss in China's coastal regions based on flow analysis

Nitrogen pollution emissions from human production and living activities in coastal regions are important topics in the management of environmental pollution in coastal waters. However, to date, there has been relatively little research systematically assessing the environmental loss of nitrogen (NEL) from human activities that negatively affect marine ecosystems. This study categorised emission sources into five subsystems, namely livestock, farming, aquatic, industrial, and residential. Through flow analysis, the anthropogenic emissions of nitrogen in the gas, liquid, and solid phases from 11 coastal provinces in China in 2011, 2015, and 2020 were determined. A nitrogen cost index was constructed by combining the social indicators of each province. The effectiveness of nitrogen emission control since the land-sea coordination and the future challenges for the coastal region were discussed from various perspectives. The results of the study showed that the total NEL that poses a potential threat to marine ecosystems in coastal areas of China has decreased from 18.93 TgN to 14.66 TgN since the proposal of land-sea coordination, with livestock systems and aquatic systems emitting the most. The Bohai and Yellow Seas area were most threatened by nitrogen pollution. Among the three oceanic pathways, liquid-phase nitrogen discharge from each subsystem was effectively controlled, and the control of gas-phase nitrogen emissions is still the most numerous NEL state, although it has had a significant effect. The results of the correlation analysis suggest that NEL flow can characterize the regional management of nutrient-based organic pollutants. Past management tools and environmental investments in China have been more effective in controlling emissions from point and line sources involving artificial facilities, but less direct effect on mariculture. How to control surface source pollution from livestock and aquaculture will be an important challenge to reduce reactive nitrogen emissions in the future.

Evaluating the anthropogenic nitrogen emissions to water using a hybrid approach in a city cluster: Insights into historical evolution, attribution, and mitigation potential

Anthropogenic reactive nitrogen (Nr) emissions from agricultural production and food consumption in city clusters have caused water quality degradation and scarcity. In this study, anthropogenic Nr emissions to the water environment were quantitatively evaluated in the Yangtze River Delta city cluster from 2011 to 2020 using coupling nitrogen (N) flow analysis and the grey water footprint (GWF) method. The spatiotemporal characteristics of the GWF and the relative contributions of natural and human factors to the water pollution level (WPL) were analyzed. The results showed that from 2011 to 2020, the total N-related GWF decreased by 12.1 %, mainly driven by reduced fertilizer application and livestock numbers. In 2020, the primary pollution source changed from livestock to humans; however, non-point sources still dominated the GWF. The spatial clustering trend of the GWF was significant: high and low GWF were mainly concentrated in the northeast and southwest regions, respectively. From 2011 to 2020, the mean center of the GWF moved west due to the decrease and increase in the eastern and western regions, respectively, supporting the pollution haven hypothesis. The WPL ranged from 2.67 to 5.03 and fluctuated due to variations in precipitation. The relative contributions of natural and human factors to the WPL evolution were 72.9 % and 27.1 %, respectively. According to the scenario analysis, increasing the N use efficiency to 50 %, manure recycling rate to 80 %, and sewage treatment rate in urban and rural regions to 98 % and 40 %, respectively, could decrease GWF by 39.6 %. The present study establishes an open framework to evaluate anthropogenic N emissions to water, and the outcomes provide valuable references for sustainable N management in city clusters.

Urban-scale analysis of nitrogen deposition in Japan: Validation of chemical transport modeling and the sensitivity of anthropogenic nitrogen emissions to dry and wet depositions

Nitrogen (N) deposition has been considered as one of the important factors contributing to the critical impacts on nature, including eutrophication and biodiversity loss. To understand the effect of anthropogenic N emissions on urban-scale dry and wet N depositions, an investigation using a chemical transport model coupled with a sensitivity analysis solver was conducted in the domains that hold large megacities in Japan, the Tokyo metropolitan area, and Osaka, in the whole year of 2017. The validation results from the comparison of the modeling results to the observed results of N depositions showed that the overall calculated results replicated the observed results in the same order of magnitude. The results of the whole year calculation showed that both dry and wet N depositions were higher in spring and summer, mainly due to the higher emission of ammonia (NH3) due to the higher temperatures in those seasons. The results of the sensitivity analysis showed that NH3 was the most critical anthropogenic emission to the urban-scale N depositions, revealing the importance of the strategy to mitigate NH3 reduction. The findings of this study provide a basic understanding of urban-scale N deposition in terms of atmospheric chemistry and anthropogenic emission of N compounds.

Soil Nitrogen and Sulfur Leaching in a Subtropical Forest at a Transition State under Decreasing Atmospheric Deposition

Anthropogenic emissions of nitrogen- (N) and sulfur (S)-containing pollutants have declined across China in recent years. However, the responses of N and S depositions and dynamics in soil remain unclear in subtropical forests. In this study, the wet and throughfall depositions of dissolved inorganic N (DIN) and SO42− were continuously monitored in a mildly polluted subtropical forest in Southeast China in 2017 and 2018. Moreover, these solutes in soil water along the soil profile were monitored in 2018. Throughfall deposition of DIN and S decreased by 59% and 53% in recent 3 years, respectively, which can be majorly attributed to the decreases in wet depositions of NO3− and SO42−. Meanwhile, NH4+ deposition remained relatively stable at this site. Even though N deposition in 2018 was below the N saturation threshold for subtropical forests, significant N leaching still occurred. Excess export of N occurred in the upper soil layer (0–15 cm), reaching 6.86 ± 1.54 kg N/ha/yr, while the deeper soil (15–30 cm) was net sink of N as 8.29 ± 1.71 kg N/ha/yr. Similarly, S was excessively exported from the upper soil with net flux of 14.7 ± 3.15 kg S/ha/yr, while up to 6.37 ± 3.18 kg S/ha/yr of S was retained in the deeper soil. The significant N and S leaching under declined depositions suggested that this site possibly underwent a transition state, recovering from historically high acid deposition. Furthermore, the rainfall intensity remarkably regulated leaching and retention of SO42− and DIN at this site. The impacts of climate changes on N and S dynamics require further long-term monitoring in subtropical forests.

Assessing critical load exceedances and ecosystem impacts of anthropogenic nitrogen and sulphur deposition at unmanaged forested catchments in Europe

Anthropogenic emissions of nitrogen (N) and sulphur (S) compounds and their long-range transport have caused widespread negative impacts on different ecosystems. Critical loads (CLs) are deposition thresholds used to describe the sensitivity of ecosystems to atmospheric deposition. The CL methodology has been a key science-based tool for assessing the environmental consequences of air pollution. We computed CLs for eutrophication and acidification using a European long-term dataset of intensively studied forested ecosystem sites (n = 17) in northern and central Europe. The sites belong to the ICP IM and eLTER networks. The link between the site-specific calculations and time-series of CL exceedances and measured site data was evaluated using long-term measurements (1990–2017) for bulk deposition, throughfall and runoff water chemistry. Novel techniques for presenting exceedances of CLs and their temporal development were also developed. Concentrations and fluxes of sulphate, total inorganic nitrogen (TIN) and acidity in deposition substantially decreased at the sites. Decreases in S deposition resulted in statistically significant decreased concentrations and fluxes of sulphate in runoff and decreasing trends of TIN in runoff were more common than increasing trends. The temporal developments of the exceedance of the CLs indicated the more effective reductions of S deposition compared to N at the sites. There was a relation between calculated exceedance of the CLs and measured runoff water concentrations and fluxes, and most sites with higher CL exceedances showed larger decreases in both TIN and H+ concentrations and fluxes. Sites with higher cumulative exceedance of eutrophication CLs (averaged over 3 and 30 years) generally showed higher TIN concentrations in runoff. The results provided evidence on the link between CL exceedances and empirical impacts, increasing confidence in the methodology used for the European-scale CL calculations. The results also confirm that emission abatement actions are having their intended effects on CL exceedances and ecosystem impacts.

Biogenic emissions and land–atmosphere interactions as drivers of the daytime evolution of secondary organic aerosol in the southeastern US

Abstract. The interactions between biogenic volatile organic compounds (BVOCs), like isoprene and monoterpenes, and anthropogenic emissions of nitrogen and sulfur oxides lead to high concentrations of secondary organic aerosol (SOA) in the southeastern United States. To improve our understanding of SOA formation, we study the diurnal evolution of SOA in a land–atmosphere coupling context based on comprehensive surface and upper air observations from a characteristic day during the 2013 Southern Oxidant and Aerosol Study (SOAS) campaign. We use a mixed layer model (MXLCH-SOA) that is updated with new chemical pathways and an interactive land surface scheme that describes both biogeochemical and biogeophysical couplings between the land surface and the atmospheric boundary layer (ABL) to gain insight into the drivers of the daytime evolution of biogenic SOA. MXLCH-SOA reproduces observed BVOC and surface heat fluxes, gas-phase chemistry, and ABL dynamics well, with the exception of isoprene and monoterpene mixing ratios measured close to the land surface. This is likely due to the fact that these species do not have uniform profiles throughout the atmospheric surface layer due to their fast reaction with OH and incomplete mixing near the surface. The flat daytime evolution of the SOA concentration is caused by the dampening of the increase due to locally formed SOA by entrainment of SOA-depleted air from the residual layer. SOA formation from isoprene through the intermediate species isoprene epoxydiols (IEPOXs) and isoprene hydroxyhydroperoxides (ISOPOOHs) is in good agreement with the observations, with a mean isoprene SOA yield of 1.8 %. However, SOA from monoterpenes, oxidised by OH and O3, dominates the locally produced SOA (69 %), with a mean monoterpene SOA yield of 10.7 %. Isoprene SOA is produced primarily through OH oxidation via ISOPOOH and IEPOX (31 %). Entrainment of aged SOA from the residual layer likely contributes to the observed more oxidised oxygenated organic aerosol (MO-OOA) factor. A sensitivity analysis of the coupled land surface–boundary layer–SOA formation system to changing temperatures reveals that SOA concentrations are buffered under increasing temperatures: a rise in BVOC emissions is offset by decreases in OH concentrations and the efficiency with which SVOCs partition into the aerosol phase.

Natural forcing of the North Atlantic nitrogen cycle in the Anthropocene

Human alteration of the global nitrogen cycle intensified over the 1900s. Model simulations suggest that large swaths of the open ocean, including the North Atlantic and the western Pacific, have already been affected by anthropogenic nitrogen through atmospheric transport and deposition. Here we report an ∼130-year-long record of the 15N/14N of skeleton-bound organic matter in a coral from the outer reef of Bermuda, which provides a test of the hypothesis that anthropogenic atmospheric nitrogen has significantly augmented the nitrogen supply to the open North Atlantic surface ocean. The Bermuda 15N/14N record does not show a long-term decline in the Anthropocene of the amplitude predicted by model simulations or observed in a western Pacific coral 15N/14N record. Rather, the decadal variations in the Bermuda 15N/14N record appear to be driven by the North Atlantic Oscillation, most likely through changes in the formation rate of Subtropical Mode Water. Given that anthropogenic nitrogen emissions have been decreasing in North America since the 1990s, this study suggests that in the coming decades, the open North Atlantic will remain minimally affected by anthropogenic nitrogen deposition.

Nitrogen Emissions-Based Assessment of Anthropogenic Regional Ecological Risk: An Example of Taiwanese Urbanization, 1990-2015.

This study proposes a framework for evaluating anthropogenic nitrogen emissions and local vulnerability in order to assess regional ecological risk of human activity during a stable urbanization process. Taiwan, an isolated island with a unique environment, intensive agriculture, concentrated industries, and stable urbanization, was an ideal location for testing this framework. Local vulnerability is influenced by social characteristics, economic development, environmental protection, and other indicators related to these. Within the context of urbanization, therefore, and using official statistical data, human metabolism, agricultural and industrial production, and transportation were evaluated. The results indicate that the rate of anthropogenic nitrogen emissions decreased as the process of urbanization in Taiwan stabilized. While nitrogen emissions from agricultural production, household and industrial wastewater gradually decreased due to a reduction in the area of arable farmland, a reduction in the use of fertilizers and increased sewage treatment, nitrogen emissions from transportation increased due to higher energy consumption from vehicle use. Taiwan exhibited a higher degree of regional vulnerability in 1998 because motor vehicle density increased significantly, while rates of per capita green area and resource recovery remained relatively low. The study found that if Taiwan maintains its current conditions with respect to standard of living, agriculture, industry, and transportation, nitrogen emissions from human metabolism and agricultural and industrial production will not increase regional ecological risk, while nitrogen emissions from transportation will likely increase this risk. Therefore, this paper suggests that future environmental planning in Taiwan should prioritize low-emissions sustainable transportation.

Sawdust as ignition intensifier of coal water slurries containing petrochemicals

The low reactivity of coal processing wastes (filter cakes), petroleum derivatives, oil sludge, oil-water emulsions, coal-water suspensions, as well as used fuel and industrial oils is the main barrier to their common use as main fuel on thermal power stations and a boiler plants. It leads to high energy consumption for heating the combustion chambers and subsequent firing. In this work, we suggest using sawdust as an additive to intensify the heating and ignition of coal water slurries containing petrochemicals (CWSP) prepared from coal and oil wastes. We show that adding even 10% sawdust lowers the threshold temperature of sustainable ignition by as much as 70–80 K. The more volatiles are in coal processing wastes, the more noticeable the contribution of sawdust to the intensity of CWSP ignition. The role of sawdust as an ignition intensifier is found to increase significantly with the rising CWSP droplet size and air temperature. Replacing the coal component (e.g., filter cake) of the CWSP composition by sawdust of the same mass leads to a negligible change in the calorific value of the fuel. At the same time, the anthropogenic emissions of nitrogen and sulfur oxides are reduced considerably.

The Impact of Anthropogenic Emission of Nitrogen on the Functioning of Forests in the European Part of Russia

In recent decades the problem of air pollution with nitrogen compounds is in the spotlight. Due to the fact that nitrogen is actively involved in the natural cycle of substances, even a relatively small input of nitrogen can lead to ecological disturbances of different kind. The estimation of changes that will occur in natural ecosystems while maintaining the current level of anthropogenic emissions of nitrogen is presented on the case of forests in the European part of Russia. The study revealed that the most resistant to the effects of nitrogen emission are mixed-deciduous and coniferous-deciduous forests with a predominance of the meadow-forest edge species, and the most vulnerable are pine forests with a predominance of oligotrophic and mesotrophic species of vegetation.

Variation characteristics of nitrogen concentrations through forest hydrologic subcycles in various forests across mainland China

Increased anthropogenic nitrogen emissions and more severe environmental issues (e.g. air pollution, soil acidification, and plant nutrient imbalances) are striking forest ecosystems. Data on concentrations in throughfall and stemflow were collected to estimate variation characteristics of nitrogen concentrations through forest hydrological processes across China. A typical study was carried out in the three forest types in the Jinyun Mountain region of Chongqing, from May to October 2012. Nitrogen concentrations in throughfall and stemflow are higher than those in atmospheric precipitation. DIN concentrations in atmospheric precipitation, throughfall, and stemflow, across China and in the Jinyun Mountain region, were 2.18 and 1.51, 3.19 and 3.88, and 5.14 and 3.92 mg N L−1, respectively. concentration was higher than concentration, suggesting is the dominant nitrogen component in China. Additionally, across China, a linear relationship existed between DIN and , and between DIN and in atmospheric precipitation. DIN concentrations in throughfall and stemflow changed with the observed changes in precipitation, and DIN concentrations in precipitation positively correlated with those in throughfall and in stemflow were also observed. Moreover, average DIN concentrations in throughfall and stemflow varied in different forest types, resulting from differences in forest canopy structures and tree species characteristics. In the Jinyun Mountain region, both throughfall and stemflow DIN concentrations were the highest in the mixed broadleaved/coniferous forest, followed by evergreen broadleaved forest, and the lowest in moso bamboo forest. Monthly variations of concentrations, in throughfall and stemflow, were observed in the Jinyun Mountain region.

Grand challenges in marine biogeochemistry

The ocean plays a central role in our earth's climate system and also provides a range of important ecosystem services, including food, energy, transport, and nutrient cycling. Marine biogeochemistry focuses on the study of complex biological, chemical, and physical processes involved in the cycling of key chemical elements within the ocean, and between the ocean and the seafloor, land and atmosphere. The ocean is increasingly perturbed by human induced alterations to our planet, including anthropogenic emissions of nitrogen, phosphorus, carbon and trace elements, and climate change. The establishment of a detailed understanding of biogeochemical processes, including their rates, is essential to the identification and assessment of climatic and chemical feedbacks associated with changes in the chemical and physical environment that are mediated through ocean biology, chemistry and physics. Important research areas in marine biogeochemistry involve the cycling of organic and inorganic forms of carbon, nitrogen and phosphorus, the cycling and biological roles of essential trace elements, and the fate and climatic impact of marine produced trace gases.

Nitrogen isotopes in ice core nitrate linked to anthropogenic atmospheric acidity change

Nitrogen stable isotope ratio (δ(15)N) in Greenland snow nitrate and in North American remote lake sediments has decreased gradually beginning as early as ∼1850 Christian Era. This decrease was attributed to increasing atmospheric deposition of anthropogenic nitrate, reflecting an anthropogenic impact on the global nitrogen cycle, and the impact was thought to be amplified ∼1970. However, our subannually resolved ice core records of δ(15)N and major ions (e.g., NO3(-), SO4(2-)) over the last ∼200 y show that the decrease in δ(15)N is not always associated with increasing NO3(-) concentrations, and the decreasing trend actually leveled off ∼1970. Correlation of δ(15)N with H(+), NO3(-), and HNO3 concentrations, combined with nitrogen isotope fractionation models, suggests that the δ(15)N decrease from ∼1850-1970 was mainly caused by an anthropogenic-driven increase in atmospheric acidity through alteration of the gas-particle partitioning of atmospheric nitrate. The concentrations of NO3(-) and SO4(2-) also leveled off ∼1970, reflecting the effect of air pollution mitigation strategies in North America on anthropogenic NO(x) and SO2 emissions. The consequent atmospheric acidity change, as reflected in the ice core record of H(+) concentrations, is likely responsible for the leveling off of δ(15)N ∼1970, which, together with the leveling off of NO3(-) concentrations, suggests a regional mitigation of anthropogenic impact on the nitrogen cycle. Our results highlight the importance of atmospheric processes in controlling δ(15)N of nitrate and should be considered when using δ(15)N as a source indicator to study atmospheric flux of nitrate to land surface/ecosystems.

Reactive nitrogen emissions from crop and livestock farming in India

The rapid increase in anthropogenic nitrogen emissions to the atmosphere is matter of concern for the environment, as these may lead to photochemical air pollution, reduced visibility, eutrophication of surface waters, changes in biodiversity, acid rain, stratospheric ozone depletion, and global warming. In this study, ambient emissions of reactive nitrogen (ammonia and nitrous oxide) from animal and crop farming are analyzed for the base year 2003. This objective was achieved by the systematic development of a spatially resolved emissions inventory on a Geographic Information System (GIS) platform. Emissions of ammonia (NH3) and nitrous oxide (N2O) were estimated: (i) from livestock; 1705 Gg/yr and 214 Gg yr � 1 and (ii) fertilizer applications; 2697 Gg yr � 1 and 326 Gg yr � 1 . These estimated emissions were compared and contrasted with global, U.S., and European emissions of reactive nitrogen; emissions from India were second only to China. From the spatially resolved emission inventory, it was observed that the state of Uttar Pradesh has the highest NH3 emission (522 Gg yr

Anthropogenic nitrogen emissions during the Holocene and their possible effects on remote ecosystems

[1] Reactive nitrogen (Nr = NH3-N + NOx-N) is an important atmospheric pollutant, contributing to acidification, eutrophication and biodiversity changes in ecosystems. This study estimates Nr emissions from anthropogenic sources on a global scale since the advent of agriculture ∼8000 B.C., using a simple model based on the development of human population and per capita factors of Nr emissions originating from livestock production, biomass burning (biofuel use and forest and savannah burning), and other anthropogenic sources (humans and pets, N-fertilizer use, and fossil fuel combustion). The estimated global cumulative anthropogenic emissions of Nr to the atmosphere are ∼17.4 Pg N (8.6 Pg NH3-N and 8.8 Pg NOx-N) for 8000 B.C. through the year 2000 A.D., with 28% of this amount emitted during 1850–2000 A.D., 42% during 1–1850 A.D., and 30% during the previous 8000 years. Forest and savannah burning represent the major cumulative flux of both NH3-N and NOx-N (3.5 and 5.8 Pg, respectively). Livestock production and biofuel burning are responsible for emissions of 3.3 and 1.2 Pg NH3-N, respectively, while the application of synthetic fertilizers contributes 0.26 Pg NH3-N. The different duration of biofuel and fossil fuel use (10,000 versus ∼150 years) causes the higher cumulative NOx-N emissions from biofuel than from fossil fuel use (1.9 versus 1.1 Pg). The cumulative Nr emissions on a land area basis are 1.3 and 3.0 Mg N ha−1 globally and in Europe, respectively. Since an estimated 60% of Nr emitted in Europe is also deposited there, the average cumulative anthropogenic Nr deposition would be ∼1.8 Mg N ha−1, representing ∼30% of the current N pools in forest and alpine meadow soils of European glaciated areas (i.e., soils of similar age as the emissions). Despite large uncertainties in the model (13.7–30.5 Pg N over the last 10,000 years), the relative temporal distributions of total cumulative Nr emissions vary within relatively narrow ranges for different assumptions, with 70%–84% of the emissions occurring prior to 1850 A.D. We conclude that the majority of the total cumulative Nr flux from anthropogenic sources over the last 10,000 years occurred in the preindustrial period and could have increased soil N pools of some remote ecosystems much earlier than is currently assumed.

Atmospheric deposition of nutrients and excess N formation in the North Atlantic

Abstract. Anthropogenic emissions of nitrogen (N) to the atmosphere have been strongly increasing during the last century, leading to greater atmospheric N deposition to the oceans. The North Atlantic subtropical gyre (NASTG) is particularly impacted. Here, upwind sources of anthropogenic N from North American and European sources have raised atmospheric N deposition to rates comparable with N2 fixation in the gyre. However, the biogeochemical fate of the deposited N is unclear because there is no detectable accumulation in the surface waters. Most likely, deposited N accumulates in the main thermocline instead, where there is a globally unique pool of N in excess of the canonical Redfield ratio of 16N:1 phosphorus (P). To investigate this depth zone as a sink for atmospheric N, we used a biogeochemical ocean transport model and year 2000 nutrient deposition data. We examined the maximum effects of three mechanisms that may transport excess N from the ocean surface to the main thermocline: physical transport, preferential P remineralization of sinking particles, and nutrient uptake and export by phytoplankton at higher than Redfield N:P ratios. Our results indicate that atmospheric deposition may contribute 13–19% of the annual excess N input to the main thermocline. Modeled nutrient distributions in the NASTG were comparable to observations only when non-Redfield dynamics were invoked. Preferential P remineralization could not produce realistic results on its own; if it is an important contributor to ocean biogeochemistry, it must co-occur with N2 fixation. The results suggest that: 1) the main thermocline is an important sink for anthropogenic N deposition, 2) non-Redfield surface dynamics determine the biogeochemical fate of atmospherically deposited nutrients, and 3) atmospheric N accumulation in the main thermocline has long term impacts on surface ocean biology.

Nitrogen emissions into freshwater ecosystems: is there a need for nitrate elimination in all wastewater treatment plants?

AbstractExcessive inputs of phosphorus and nitrogen are the main reasons of eutrophication of inland waters and coastal areas. Large efforts have been made to control phosphorus, but the measures to reduce nitrogen emissions failed at least partly. While it was possible to reduce nitrogen emissions from industry and municipal wastewater treatment plants, diffuse sources are showing only very minor decline. Examples of limnetic, marine and coastal systems are given to review the current knowledge about nitrogen transformation and the effects of nitrate in the environment. When N is the limiting nutrient in a particular water body, this does not necessarily mean that phytoplankton is controllable by NO3– removal. In systems with problems due to a high redox‐sensitive internal phosphorus load and under certain constraints, nitrate may be used as an ecotechnological measure to prevent anaerobic phosphorus release from sediments. Model simulations are used to demonstrate this. A schematic model of redox‐mediated temporal phosphorus storage in riverine lake sediments with short retention time is proposed. We conclude that while anthropogenic nitrogen emissions are a global problem, no fast and simple single solution exists. Additional nitrogen removal in wastewater treatment will have no effect, as long as diffuse sources and nitrate concentrations in groundwater remain at a high level. Emission reductions should be achieved in an integrated way, taking direct and indirect effects into account. In this sense, case by case decisions and a new definition of “sensitive areas” are required.

Modelling Nitrogen Deposition on a Local Scale—A Review of the Current State of the Art

Abstract. Local ammonia emissions from agricultural activities are often associated with high nitrogen deposition in the close vicinity of the sources. High nitrogen (N) inputs may significantly affect the local ecosystems. Over a longer term, high loads may change the composition of the ecosystems, leading to a general decrease in local biodiversity. In Europe there is currently a significant focus on the impact of atmospheric N load on local ecosystems among environmental managers and policy makers. Model tools designed for application in N deposition assessment and aimed for use in the regulation of anthropogenic nitrogen emissions are, therefore, under development in many European countries. The aim of this paper is to present a review of the current understanding and modelling parameterizations of atmospheric N deposition. A special focus is on the development of operational tools for use in environmental assessment and regulation related to agricultural ammonia emissions. For the often large number of environmental impact assessments needed to be carried out by local environmental managers there is, furthermore, a need for simple and fast model systems. These systems must capture the most important aspects of dispersion and deposition of N in the nearby environment of farms with animal production. The paper includes a discussion on the demands on the models applied in environmental assessment and regulation and how these demands are fulfilled in current state-of-the-art models.

Particle Dry Deposition to Water Surfaces: Processes and Consequences

Algal blooms (increased abundance of phytoplankton) are an increasingly common phenomenon which has been causally linked to increased fluxes of nutrient (particularly nitrogenous) compounds to aquatic ecosystems. These blooms have implications for water quality and human health in addition to ecosystem productivity, health and ecological diversity. Anthropogenic emissions of nitrogen to the atmosphere are estimated to be comparable to, or greater than, biogenic emissions but are considerably more concentrated in space. Although fluvial pathways typically dominate the annually averaged nitrogen flux to coastal waters, atmosphere–surface exchange represents a significant component of the total flux and may be particularly critical during the summertime when both the riverine input and ambient nutrient concentrations are often at a minimum. In this chapter, we present an overview of the physical and chemical processes which dictate the quantity (and direction) of atmosphere–surface fluxes of trace chemicals to (and above) water surfaces with particular emphasis on the role of particles. Dry deposition (transfer to the surface in the absence of precipitation) of particles is determined by meteorological conditions, atmospheric concentrations, surface type/condition and the specific chemical and physical properties of the particle. Dry deposition can be conceptualized as a three-step process: (1) the gas or particle is moved toward the surface by thermally or mechanically driven eddies; (2) it is transferred by diffusion across a thin layer close to the surface where turbulence is absent; and (3) the gas or particle is captured by the surface. In the case of larger particles a second parallel pathway exists; particles are drawn towards the surface by gravity. Atmospheric particles determine dry deposition fluxes not only by serving as a conduit for transfer but also because of their action as sources or sinks of trace gases. The example given here is the transfer of nitric acid to sea salt particles as a result of heterogeneous chemistry acting as a competing sink to surface removal. To illustrate the importance of current uncertainties in our understanding of dry deposition processes and to highlight the role of some of the key parameters in determining the transfer rate (the deposition velocity) a simple model of particle dry deposition is presented. The model describes the calculation of the rate at which a particle of a given size and chemical composition will be moved towards the surface under given environmental conditions. Observational and experimental techniques for measuring dry deposition fluxes are also reviewed. The techniques used for gases are largely reliant on use of highly temporally resolved sampling (e.g., concentrations sampled 10 times per second) or highly accurate and precise measurements of concentrations, either in the vertical to resolve the gradient to or from the surface or conditionally sampled by the direction of transfer (to or from the surface). These stringent measurement requirements represent significant barriers to application to measurement of particle dry deposition fluxes although, as discussed, innovative solutions are now becoming available. In the final section, we examine meteorological controls on deposition to the coastal zone. This region of the world’s oceans and seas is most significantly impacted by human activities. More than half of the world’s population lives within 100 km of a coast and hence the overwhelming majority of anthropogenic fluxes to aquatic systems occur in the coastal zone. We discuss the particular challenges that arise from efforts to simulate and measure fluxes close to the coastline. These arise in part from the complexity of atmospheric flow in this region where energy and chemical fluxes are highly inhomogeneous in space and time and thermally generated atmospheric circulations are commonplace.

INTERANNUAL VARIABILITY AT THREE INLAND WATER SITES: IMPLICATIONS FOR SENTINEL ECOSYSTEMS

In the sentinel-site approach to monitoring ecological resources, relatively small numbers of locations are chosen for intensive study, each location being selected to represent a certain, preferably large, class of ecosystems. This paper examines long-term studies at three inland aquatic ecosystems in California that, although never intended as sentinel sites, nevertheless illustrate the challenges posed by sentinel-site networks. The main mechanisms of interannual variability in primary productivity or producers are described for Castle Lake, Lake Tahoe, and northern San Francisco Bay. The external forces behind the variability include climate (in the form of both snowfall and total precipitation), fishing intensity, anthropogenic nitrogen emissions, spring weather systems, intrinsic population cycles, forest fires, water diversions, and invasions of exotics. Each mechanism is associated with certain critical ecosystem features (such as hydraulic residence time) that condition a responsiveness to one or a combination of these external forces (such as precipitation); these critical features are identified for each mechanism. Three demanding conditions must be fulfilled in order for a sentinel site to function with regard to a given stressor: (1) some subset of the network must encounter the stressor; (2) at least some sites in the subset must have the critical features that cause responsiveness to that stressor; and (3) the background variability at those sites must not disguise the response to the stressor of interest. In practice, reliable extrapolation from sentinel-site networks to regional trends appears to be beyond our ecological understanding at the present time.