Critical Load Exceedances Research Articles

Land degradation neutrality: Testing the indicator in a temperate agricultural landscape

Land degradation directly affects around 25% of land globally, undermining progress on most of the UN Sustainable Development Goals (SDG), particularly target 15.3. To assess land degradation, SDG indicator 15.3.1 combines sub-indicators of productivity, soil carbon and land cover. Over 100 countries have set Land Degradation Neutrality (LDN) targets. Here, we demonstrate application of the indicator for a well-established agricultural landscape using the case study of Great Britain. We explore detection of degradation in such landscapes by: 1) transparently evaluating land cover transitions; 2) comparing assessments using global and national data; 3) identifying misleading trends; and 4) including extra sub-indicators for additional forms of degradation. Our results demonstrate significant impacts on the indicator both from the land cover transition evaluation and choice or availability of data. Critically, we identify a misleading improvement trend due to a trade-off between improvement detected by the productivity sub-indicator, and 30-year soil carbon loss trends in croplands (11% from 1978 to 2007). This carbon loss trend would not be identified without additional data from Countryside Survey (CS). Thus, without incorporating field survey data we risk overlooking the degradation of regulating and supporting ecosystem services (linked to soil carbon), in favour of signals from improving provisioning services (productivity sub-indicator). Relative importance of these services will vary between socioeconomic contexts. Including extra sub-indicators for erosion or critical load exceedance, as additional forms of degradation, produced a switch from net area improving (9%) to net area degraded (58%). CS data also identified additional degradation for soil health, including 44% arable soils exceeding bulk density thresholds and 35% of CS squares exceeding contamination thresholds for metals.

Global food loss and waste embodies unrecognized harms to air quality and biodiversity hotspots.

Global food loss and waste (FLW) undermines the resilience and sustainability of food systems and is closely tied to the United Nation's Sustainable Development Goals on climate, resource use and food security. Here we reveal strong yet under-discussed interconnections between FLW and two other Sustainable Development Goals of Human Health and Life on Land via the nitrogen cycle. We find that eliminating global FLW in 2015 would have reduced anthropogenic NH3 emissions associated with food production by 11.4 Tg (16%), decreased local PM2.5 concentrations by up to 5 μg m-3 and PM2.5-related years of life lost by 1.5 million years, and mitigated nitrogen critical load exceedances in global biodiversity hotspots by up to 19%. Halving FLW in 2030 will reduce years of life lost by 0.5-0.8 million years and nitrogen deposition by 4.7-6.0 Tg N per year (4%) (range for socioeconomic pathways). Complementary to near-term NH3 mitigation potential via technological measures, our study emphasizes incentivizing FLW reduction efforts from air quality and ecosystem health perspectives.

Assessment of Critical Loads of Nitrogen Deposition in Natural Ecosystems of China

Excessive nitrogen (N) deposition causes a series of environmental problems, including biodiversity loss. Therefore, assessing current N deposition thresholds of natural ecosystems is critical for regional N management and pollution control. In this study, the critical loads of N deposition in mainland China were estimated using the steady-state mass balance method, and the spatial distribution of ecosystems that exceeded the critical load was evaluated. The results showed that areas with critical loads of N deposition higher than 56, in the range of 14-56, and lower than 14 kg·(hm2·a)-1 accounted for 6%, 67%, and 27% of that in China, respectively. The areas with higher critical loads of N deposition were mainly distributed in the eastern Tibetan Plateau, northeastern Inner Mongolia, and parts of south China. Lower critical loads of N deposition were mainly distributed in the western Tibetan Plateau, northwest China, and parts of southeast China. Moreover, the areas where N deposition exceeded the critical loads accounted for 21% of that in mainland China, being mainly distributed in southeast and northeast China. The exceedances of critical loads of N deposition in northeast China, northwest China, and the Qinghai-Tibet Plateau were generally lower than 14 kg·(hm2·a)-1. Therefore, the management and control of N in these areas that exceeded the critical load of deposition is more worthy of future attention.

Assessing impacts of sulfur deposition on aquatic ecosystems: A decision support system for the Southern Appalachians

AbstractWith climate change and ongoing impacts from human development and resource extraction, US federal land management agencies are acutely concerned with managing for healthy aquatic ecosystems in the Southern Appalachian Mountain (SAM) Region. Here, we describe development of a spatial decision support application to assess the biological and ecological impacts of atmospheric S and N deposition on aquatic ecosystems of the region. We first summarize foundational published work to predict continuous maps of surface water acid neutralizing capacity (ANC) and soil base cation weathering (BCw). We use the predicted ANC and BCw maps to estimate steady‐state critical loads (CLs) of atmospheric S and N deposition. We included estimated CLs of atmospheric N to get a complete picture of CLs and potential exceedances. We then present a logic‐based decision support model for assessing effects of S and N deposition based on statistically modeled stream ANC and CL exceedance. The model is easily modified for continuous monitoring of CL exceedance patterns as new S and N deposition and ANC data become available. We present mapped model results for the SAM study area and an important subset of the region, the Great Smoky Mountains National Park. ANC modeling results revealed that predicted acid sensitivity was spatially variable, with areas of relatively low stream ANC (<50 μeq · L−1) and soil BCw (<50 meq · m−2 · year−1) predominantly found in certain critical areas. Within the Great Smoky Mountains National Park, evidence for S CL exceedance based on an ANC criterion of 50 μeq · L−1 was strong at locations where ambient S deposition was at least two times the CL. We also predicted likely impacts of CL exceedances on aquatic insect species richness and native fish abundance. Responses for insect species richness and fish impact showed variability similar to CL exceedance, with increasing impact positively correlated with elevation. Finally, we discuss ways that the decision support system can be used to prioritize management across the region.

Atmospheric deposition of reactive nitrogen to a deciduous forest in the southern Appalachian Mountains.

Assessing nutrient critical load exceedances requires complete and accurate atmospheric deposition budgets for reactive nitrogen (Nr). The exceedance is the total amount of Nr deposited to the ecosystem in excess of the critical load, which is the amount of Nr input below which harmful effects do not occur. Total deposition includes all forms of Nr (i.e., organic and inorganic) deposited to the ecosystem by wet and dry pathways. Here we present results from the Southern Appalachian Nitrogen Deposition Study (SANDS), in which a combination of measurements and field-scale modeling was used to develop a complete annual Nr deposition budget for a deciduous forest at the Coweeta Hydrologic Laboratory. Wet deposition of ammonium, nitrate, nitrite, and bulk organic N were measured directly. The dry deposited Nr fraction was estimated using a bidirectional resistance-based model driven with speciated measurements of Nr air concentrations (e.g., ammonia, ammonium aerosol, nitric acid, nitrate aerosol, bulk organic N in aerosol, total alkyl nitrates, and total peroxy nitrates), micrometeorology, canopy structure, and biogeochemistry. Total annual deposition was ~6.7 kg N ha-1 yr-1, which is on the upper end of Nr critical load estimates recently developed for similar ecosystems in the nearby Great Smoky Mountains National Park. Of the total (wet + dry) budget, 51.1% was contributed by reduced forms of Nr , with oxidized and organic forms contributing ~41.3% and 7.6%, respectively. Our results indicate that reductions in deposition would be needed to achieve the lowest estimates (~3.0 kg N ha-1 yr-1) of Nr critical loads in southern Appalachian forests.

Long-term declines in atmospheric nitrogen and sulfur deposition reduce critical loads exceedances at multiple Canadian rural sites, 2000–2018

Abstract. Daily air concentrations of inorganic nitrogen (N) species, including gaseous HNO3 and particulate-bound (p)NH4+ and pNO3-, and sulfur (S) species, including SO2 and pSO42-, and precipitation concentrations of NO3-, NH4+, and SO42-, have been routinely monitored by the Canadian Air and Precipitation Monitoring Network (CAPMoN) since 1983. Data at 15 rural sites from 2000–2018 were used to estimate dry and wet N and S deposition fluxes, which were then used to explore their spatiotemporal trends and assess ecosystem damage through a retrospective analysis of critical loads (CLs) exceedances. Total (dry + wet) N deposition ranged from 1.7–9.5 kgNha-1yr-1 among the 15 sites, though dry deposition of NH3 and some oxidized N species were not included due to lack of monitoring data. Based on additional N measurements in 2010 at one of the sites, annual total N deposition may be underestimated by up to 32 %. Total N deposition was dominated by wet NO3- and wet NH4+ deposition, which together comprised 71 %–95 %. Contributions to dry N deposition were 40 %–74 % by HNO3, 11 %–40 % by pNH4+, and 5 %–25 % by pNO3-. Total S deposition ranged from 1.3–8.5 kgSha-1yr-1 and was dominated by wet deposition of SO42- and dry deposition of SO2. Relative percentages of wet and dry S deposition were 45 %–89 % and 11 %–55 %, respectively. Acidic ion fluxes were greatest in southeastern Canada and were comparable among the west coast, prairie, remote, and eastern Canadian sites. Oxidized N (dry HNO3, dry pNO3-, wet NO3-) deposition was greater than that of reduced N (dry pNH4+, wet NH4+) in the early 2000s. In 2014–2018, reduced N deposition surpassed that of oxidized N in southeastern Canada. Total N and S deposition decreased significantly at a rate of −0.03 to −0.25 kgNha-1yr-1 (−1.1 % yr−1 to −3.3 % yr−1) and −0.08 to −0.66 kgSha-1yr-1 (−3.5 % yr−1 to −6.6 % yr−1), respectively, among the sites. The weak declining trend in total N deposition at the west coast site was consistent with the slower decline in NOx emissions in western Canada. Reductions in total N deposition were driven by its oxidized form as trends in reduced N were non-significant. As a result, reduced N contributions to total N deposition increased on average from 42 % in 2000–2004 to 53 % in 2014–2018. Anthropogenic NOx and SO2 emissions reductions in both eastern Canada and eastern US were highly effective in reducing total oxidized N and total S deposition, respectively, in eastern Canada. Acidic deposition exceeded terrestrial CL at 5 of the 14 sites and aquatic CL at 2 of the 5 sites in the early 2000s. However, exceedances have been trending downwards and acidic deposition fluxes were mostly near or below CL after 2012 for the subset of sites assessed, which support recovery from historical acidification. Further assessments of CL exceedances are required in other Canadian regions susceptible to acidification and affected by elevated or increasing N and S emissions.

Assessing critical loads and exceedances for acidification and eutrophication in the forests of East and Southeast Asia: A comparison with EANET monitoring data

Spatial variations in sulfur (S) and nitrogen (N) deposition have changed in East and Southeast Asia in recent decades. Nevertheless, in this region, including the tropics, regional-scale assessments of the long-term risk of acidification and eutrophication (N saturation) for terrestrial ecosystems using a critical load approach have not been updated since 2001. To evaluate future risks, maps of critical loads and exceedances were updated using recently acquired spatial datasets of soil properties, soil minerals, climate, tree plantations, and the annual S and N depositions estimated using the Community Multiscale Air Quality (CMAQ) model. The resulting maps were verified using data on long-term trends in soil pH and nitrate concentration in surface water acquired by the Acid Deposition Monitoring Network in East Asia (EANET). It was found that N deposition exceeded the critical load for eutrophication not only in East Asia but also in some parts of the tropical monsoon and humid regions in Southeast Asia, whereas S deposition partly exceeded the critical load for soil acidification in China and small parts of the tropical monsoon region. The high-risk areas for eutrophication coincided well with the EANET sites, where the increase in nitrate concentration in the surface water was significant over the last 20 years. Hence, the estimated map of the critical load exceedance for eutrophication is more plausible for assessing the risk in East and Southeast Asia than that for acidification, although the critical load exceedance for acidification would be sufficiently significant as an updated risk map based on the latest input values. This update also suggests that increased N deposition around megacities, water discharge, and tree plantations may play an important role in the spatial variability of eutrophication risks in the tropics of Southeast Asia.

An exceedance score for the assessment of the impact of nitrogen deposition on habitats in the UK

Large areas of nitrogen-sensitive habitats are currently estimated to be in exceedance of their critical loads (CLs) as indicators for protection from nitrogen deposition. In the UK, deposition estimates from the semi-empirical Concentration Based Estimated Deposition (CBED) model are used for official reporting of current exceedances. The UK Integrated Assessment Model (UKIAM) framework is designed to provide future projections of concentrations and deposition due to projected changes in emissions. UKIAM has been extended to provide alternative deposition estimates aligned with those of CBED, and the results combined with the range in habitat CL values to create an exceedance score, leading to a probabilistic evaluation of CL exceedances. The utility of the method is demonstrated by analysing a series of hypothetical scenarios. It is shown that NH3 mitigation is likely to be four times more effective in reducing CL exceedances in the UK than the mitigation of NOx emissions.

Modelling changes in secondary inorganic aerosol formation and nitrogen deposition in Europe from 2005 to 2030

Abstract. Secondary inorganic PM2.5 particles are formed from SOx (SO2+SO42-), NOx (NO+NO2), and NH3 emissions, through the formation of either ammonium sulfate ((NH4)2SO4) or ammonium nitrate (NH4NO3). EU limits and WHO guidelines for PM2.5 levels are frequently exceeded in Europe, in particular in the winter months. In addition the critical loads for eutrophication are exceeded in most of the European continent. Further reductions in NH3 emissions and other PM precursors beyond the 2030 requirements could alleviate some of the health burden from fine particles and also reduce the deposition of nitrogen to vulnerable ecosystems. Using the regional-scale EMEP/MSC-W model, we have studied the effects of year 2030 NH3 emissions on PM2.5 concentrations and depositions of nitrogen in Europe in light of present (2017), past (2005), and future (2030) conditions. Our calculations show that in Europe the formation of PM2.5 from NH3 to a large extent is limited by the ratio between the emissions of NH3 on one hand and SOx plus NOx on the other hand. As the ratio of NH3 to SOx and NOx is increasing, the potential to further curb PM2.5 levels through reductions in NH3 emissions is decreasing. Here we show that per gram of NH3 emissions mitigated, the resulting reductions in PM2.5 levels simulated using 2030 emissions are about a factor of 2.6 lower than when 2005 emissions are used. However, this ratio is lower in winter. Thus further reductions in the NH3 emissions in winter may have similar potential to SOx and NOx in curbing PM2.5 levels in this season. Following the expected reductions of NH3 emission, depositions of reduced nitrogen (NH3+NH4+) should also decrease in Europe. However, as the reductions in NOx emission are larger than for NH3, the fraction of total nitrogen (reduced plus oxidised nitrogen) deposited as reduced nitrogen is increasing and may exceed 60 % in most of Europe by 2030. Thus the potential for future reductions in the exceedances of critical loads for eutrophication in Europe will mainly rely on the ability to reduce NH3 emissions.

A decadal synthesis of atmospheric emissions, ambient air quality, and deposition in the oil sands region.

This review is part of a series synthesizing peer‐reviewed literature from the past decade on environmental monitoring in the oil sands region (OSR) of northeastern Alberta. It focuses on atmospheric emissions, air quality, and deposition in and downwind of the OSR. Most published monitoring and research activities were concentrated in the surface‐mineable region in the Athabasca OSR. Substantial progress has been made in understanding oil sands (OS)‐related emission sources using multiple approaches: airborne measurements, satellite measurements, source emission testing, deterministic modeling, and source apportionment modeling. These approaches generally yield consistent results, indicating OS‐related sources are regional contributors to nearly all air pollutants. Most pollutants exhibit enhanced air concentrations within ~20 km of surface‐mining activities, with some enhanced >100 km downwind. Some pollutants (e.g., sulfur dioxide, nitrogen oxides) undergo transformations as they are transported through the atmosphere. Deposition rates of OS‐related substances primarily emitted as fugitive dust are enhanced within ~30 km of surface‐mining activities, whereas gaseous and fine particulate emissions have a more diffuse deposition enhancement pattern extending hundreds of kilometers downwind. In general, air quality guidelines are not exceeded, although these single‐pollutant thresholds are not comprehensive indicators of air quality. Odor events have occurred in communities near OS industrial activities, although it can be difficult to attribute events to specific pollutants or sources. Nitrogen, sulfur, polycyclic aromatic compounds (PACs), and base cations from OS sources occur in the environment, but explicit and deleterious responses of organisms to these pollutants are not as apparent across all study environments; details of biological monitoring are discussed further in other papers in this special series. However, modeling of critical load exceedances suggests that, at continued emission levels, ecological change may occur in future. Knowledge gaps and recommendations for future work to address these gaps are also presented. Integr Environ Assess Manag 2022;18:333–360. © 2021 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

The impact of nitrogen and sulfur emissions from shipping on the exceedance of critical loads in the Baltic Sea region

Abstract. The emissions of nitrogen (N) and sulfur (S) species to the atmosphere from shipping significantly contribute to S and N deposition near the coast and to acidification and/or eutrophication of soils and freshwater. In the countries around the Baltic Sea, the shipping volume and its relative importance as a source of emissions are expected to increase if no efficient regulations are implemented. To assess the extent of environmental damage due to ship emissions for the Baltic Sea area, the exceedance of critical loads (CLs) for N and S has been calculated for the years 2012 and 2040. The paper evaluates the effects of several future scenarios, including the implementation of NECA and SECA (Nitrogen And Sulfur Emission Control Areas). The implementation of NECA and SECA caused a significant decrease in the exceedance of CLs for N as a nutrient while the impact on the – already much lower – exceedance of CLs for acidification was less pronounced. The relative contribution from Baltic shipping to the total deposition decreased from 2012 in the 2040 scenario for both S and N. In contrast to exceedances of CLs for acidification, shipping still has an impact on exceedances for eutrophication in 2040. Geographically, the impact of shipping emissions is unevenly distributed even within each country. This is illustrated by calculating CL exceedances for 21 Swedish counties. The impact, on a national level, is driven by a few coastal counties, where the impact of shipping is much higher than the national summary suggests.

Critical loads of headwater streams in China using SSWC model modified by comprehensive F-factor

In order to evaluate the potential risk of surface water acidification in regions with historically-elevated acid deposition and to measure the recovery of such ecosystems after policy changes, critical loads and their exceedances were estimated for 349 headwater streams across China using a modified SSWC model. Such a model considered the acid-neutralizing capacity derived from high base cation deposition and the robust retention of sulfate and nitrate. Results indicated that China's streams had higher critical loads (averaged at 4.7 keq·ha−1·yr−1) and were less sensitive to acid deposition as compared to Europe and North America. The proportion of surveyed streams with acid deposition exceeded critical load decreased from 40.4% in 2005 to 29.5% in 2018, indicating a significant decrease in risk of surface water acidification, and thus a benefit from the emission abatement in recent years. Nonetheless, a relatively high risk of acidification still existed in southeast China with lower critical loads and most critical load exceedances. More efforts should be put into implementing emission control policies in the future.

Long-term variability in base cation, sulfur and nitrogen deposition and critical load exceedance of terrestrial ecosystems in China

The rapid development of China's industrial economy and implementation of air pollution controls have led to great changes in sulfur (S), nitrogen (N) and base cation (BC) deposition in the past three decades. We estimated China's anthropogenic BC emissions and simulated BC deposition from 1985 to 2015 with a five-year interval using a multilayer Eulerian model. Deposition of S and N from 2000 to 2015 with a five-year interval was simulated with the EMEP MSC-W model and the Multi-resolution Emission Inventory of China (MEIC). The critical load (CL) and its exceedance were then calculated to evaluate the potential long-term acidification risks. From 1985 to 2005, the BC deposition in China was estimated to have increased by 16 % and then decreased by 33 % till 2015. S deposition was simulated to increase by 49 % from 2000 to 2005 and then decrease by 44 % in 2015, while N deposition increased by 32 % from 2000 to 2010 with a limited reduction afterward. The maximum CL of S was found to increase in 67 % of mainland China areas from 1985 to 2005 and to decline in 55 % of the areas from 2005 to 2015, attributed largely to the changed BC deposition. Consistent with the progress of national controls on SO2 and NOX emissions, the CL exceedance of S increased from 2.9 to 4.6 Mt during 2000–2005 and then decreased to 2.5 Mt in 2015, while that of N increased from 0.4 in 2000 to 1.2 Mt in 2010 and then decreased to 1.1 Mt in 2015. The reduced BC deposition due to particle emission controls partially offset the benefit of SO2 control on acidification risk reduction in the past decade. It demonstrates the need for a comprehensive strategy for multi-pollutant control against soil acidification.

Assessing the costs and environmental benefits of IMO regulations of ship-originated SOx and NOx emissions in the Baltic Sea

To assess the value of the environmental benefits of the Sulphur Emission regulation (SECA) that came into force in 2015, changes in depositions of SOx and NOx from ship exhaust gas emissions were modelled and monetized for the Baltic Sea region for the years 2014 and 2016. During this period, the total deposition of SOx in the study area decreased by 7.3%. The decrease in ship-originated SOx deposition from 38 kt to 3.4 kt (by over 88%) was translated into a monetary value for the ecosystem impacts of nearly 130 million USD, according to the EcoValue08 model. This is less than the modelled health benefits, but it is not insignificant. For NOx, there was no decreasing trend. The exceedance of the critical loads of SOx and NOx was also estimated. The effect of Baltic shipping on the exceedance of critical loads of acidification after SECA is very small, but Baltic shipping still has a considerable effect on the exceedance of critical loads for eutrophication.

Challenges characterizing N deposition to high elevation protected areas: A case study integrating instrument, simulated, and lichen inventory datasets for the Devils Postpile National Monument and surrounding region, USA

Excess N deposition is a common stressor of ecological health. Sensitive biota, including many lichens, respond to small differences in atmospheric N deposition. For lands with a conservation mandate, obtaining sufficient N data to evaluate risks is a challenge, especially in Mediterranean or arid landscapes. Managers in the Western U.S. commonly use epiphytic (“tree-dwelling”) lichens to supplement N assessments, although options for higher elevations lacking epiphytes are poorly developed. Managers instead rely on broad-scale air quality simulations like the Total Deposition Model (TDEP). The Sierra Nevada Range is an example where anthropogenic N reaches mid-to-high elevation protected areas but managers lack tools for monitoring at ecologically relevant spatial-scales. Our main goals were to demonstrate how well-studied epiphytic bioindicators can supplement N assessments and recommend saxicolous (“rock-dwelling”) candidates for similar development to improve coverage in higher elevations. As a case study, we characterized N deposition at a small (323 ha) protected area, Devils Postpile National Monument (DEPO), by integrating data from TDEP, air instruments, and assays of %N in the epiphyte Letharia vulpina. We used the regional N threshold associated with detrimental effects to epiphytic lichen communities, 2.9 kg N ha−1 yr−1 in throughfall (i.e. under a tree canopy), as a provisional critical load (CL) for indicating areas at risk of ecological impacts. Results clearly showed the need for empirical, diverse, and finer-scaled information, even for an area as small as DEPO. Importantly, simulations from TDEP overestimated dry deposition of oxidized N, making total N estimates nearly twice measured values (6.0 vs 3.03 – 3.66 kg N ha−1 yr−1, respectively). While small, our L. vulpina dataset (n = 5) from the Monument indicated highly variable N in throughfall (1.7 – 4.44 kg N ha−1 yr−1), with small-scale drivers, like proximity to the river canyon and the Central Valley, tipping the scale towards CL exceedance. Regional L. vulpina assays (n = 355) showed a distinct north-to-south gradient in California where N deposition at DEPO was highly similar to the nearby protected area, Yosemite National Park, where ecological impacts of excess N are well-documented. To expand N bio-monitoring into higher elevation areas, we recommend vetting five widespread saxicolous species, Rhizoplaca melanophthalma, Umbilicaria phaea, U. polaris, Xanthoparmelia coloradoënsis, and X. cumberlandia.

Inconsistencies in sulfur dioxide emissions from the Canadian oil sands and potential implications

Satellite-derived and reported sulfur dioxide (SO2) emissions from the Canadian oil sands are shown to have been consistent up to 2013. Post-2013, these sources of emissions data diverged, with reported emissions dropping by a factor of two, while satellite-derived emissions for the region remained relatively constant, with the discrepancy (satellite-derived emissions minus reported emissions) peaking at 50 kt(SO2) yr−1 around 2016. The 2013–2014 period corresponds to when new flue-gas desulfurization units came on-line. Previous work has established a high level of consistency between at-stack SO2 emissions observations and satellite estimates, and surface monitoring network SO2 concentrations over the same multi-year period show similar trends as the satellite data, with a slight increase in concentrations post-2013. No clear explanation for this discrepancy currently exists. The implications of the discrepancy towards estimated total sulfur deposition to downwind ecosystems were estimated relative to 2013 emissions levels, with the satellite-derived values leaving the area of regional critical load exceedances of aquatic ecosystems largely unchanged from 2013 values, 335 000 km2, and reported values potentially decreasing this area to 185 000 km2.

Greater Contribution From Agricultural Sources to Future Reactive Nitrogen Deposition in the United States

AbstractMany sensitive ecosystems in areas protected for biodiversity conservation in the United States suffer from exposure to excess reactive nitrogen (Nr) released by fossil fuel combustion and agricultural practices and deposited onto the land surface and water bodies. The Community Multiscale Air Quality (CMAQ) model was applied over the contiguous United States to link emissions and climate change to reactive nitrogen deposition by simulating both present‐day and future speciated Nr deposition to protected areas. Future conditions included examining the Representative Concentration Pathway 8.5 climate and the Shared Socio‐Economic Pathway 5 emission scenarios. We further identify protected areas that would benefit most from better Nr management strategies by comparing the simulated deposition with multiple critical loads (CLs) for both biodiversity and acidification in terrestrial and aquatic ecosystems. Achieved by further NOx emission reductions from the mobile and power generation sectors, future Nr deposition is expected to decrease. However, in regions with intensive fertilizer application or hosting concentrated animal feeding operations, the reduction may be offset by rising agricultural NH3 emissions. The protected areas having CL exceedances in 2050 are expected to increase by 5.5% for empirical lichen‐based CL, and by 11% and 22% for surface water and forest soil acidity, respectively, because of the agricultural NH3 emission increase. By linking the deposition simulations with a water quality model, we identified that atmospheric deposition is the dominant source of nitrogen for several remote watersheds, including several lakes in National Parks and National Wilderness areas in Colorado, Montana, and Minnesota.

Changes in spatial patterns of ammonia dry deposition flux and deposition threshold exceedance according to dispersion model formalism and horizontal resolution.

Ammonia (NH3) emitted into the atmosphere from agricultural sources may affect nearby sensitive ecosystems due to high dry deposition fluxes on vegetation and soil surfaces, contributing to critical load exceedances. Ammonia fluxes near sources are simulated by either short-range atmospheric models or regional models using large grid cell sizes. However, studies are missing on the comparison of the results simulated by these two types of models. This paper presents the effect of model formalism, input factors, especially grid cell size and wind speed and the choice of deposition threshold on the spatial patterns of NH3 dry deposition fluxes and deposition threshold exceedances. We used the Eulerian chemistry-transport model CHIMERE and the Gaussian plume model OPS-ST on two study domains characterised by contrasting land use. We showed that the average annual NH3 dry deposition fluxes over each whole domain are similar for both models. By contrast, NH3 dry deposition fluxes near sources are higher when simulated with OPS-ST that provides analytical solutions that can be sampled with small grid cell sizes (i.e., from 25 to 1600m in this study), than with CHIMERE, which uses large grid cell sizes (i.e., 800 and 1600m). As a result, the spatial patterns of deposition threshold exceedance were very different between both models. These patterns depend mainly on grid cell size, the input factors and the choice of the deposition threshold value. We show that the model formalism has a relatively small effect on the results and that the differences result mainly from the spatial resolutions to which they can be applied. Simulation results must therefore be interpreted carefully, taking into account the simulation conditions.

Acid wet-deposition modeling sensitivity to WRF-CMAQ planetary boundary layer schemes and exceedance of critical loads over an industrializing coastal valley in northwestern British Columbia, Canada

Computational tools used to implement the critical-load approach of atmospheric deposition impact often do not elucidate modeling uncertainty making it difficult for environmental policy-makers to know how much confidence to put in its results, also hampering aspects that may need improving. This study evaluated acid deposition modeling for various parameterizations of the planetary boundary-layer (PBL) over the Terrace-Kitimat valley, a physiographically complex region of northwestern British Columbia. Of five schemes, simulations with the Mellor-Yamada-Nakanishi-Niino Level 3 and Mellor-Yamada-Janjic PBL schemes best captured weekly wet deposition fluxes of acidifying ions (SO42−, NO3−, NH4+) within a factor of 2 of observations at an industrial fence line station. Alongside the Yonsei University PBL scheme, these two schemes slightly overestimated the chemical species at a station that was distant from major anthropogenic precursor sources in the valley, hence useful for worst-scenario projections of atmospheric deposition on the natural environment. Forest soils in the vicinity of a large aluminum smelter in Kitimat was estimated to be in exceedance of critical load of acidity by 30.1–53.5 kg S ha−1 yr−1. Exceedance of critical load of nutrient nitrogen restricted to the Terrace area (≤ 7 km2), ranged between none and 0.71 kg ha−1 yr−1. This work provides guidance for using PBL schemes in the Weather Research and Forecasting model that is coupled to a deposition model when calculating critical-load exceedance over temperate, rugged, coastal geographies.

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.