A database of atmospheric inorganic nitrogen deposition fluxes in China from satellite monitoring

Over the past century, atmospheric inorganic nitrogen (IN) deposition to terrestrial ecosystems has significantly increased and caused various environmental issues. China has been one of the hotspot regions for IN deposition, yet limited data exist regarding IN deposition fluxes in China at the regional scale. In this study, based on NO2 and NH3 columns acquired by satellite sensors, coupled with atmospheric chemical transport model (CTM), mixed-effects model and site observations, we constructed regional-scale IN dry and wet deposition models respectively, and finally proposed a spatially explicit database of IN deposition fluxes in China. The database includes the dry, wet and total deposition fluxes in China during 2011-2020, and the data are presented in raster form with a resolution of 0.25° × 0.25°. Overall, the database is of great importance for monitoring and simulating the trends of IN deposition over a long time series in China.

Subsurface fertilization boosts crop yields and lowers greenhouse gas emissions: A global meta-analysis

The subsurface application (SA) of nitrogenous fertilizers is a potential solution to mitigate climate change and improve food security. However, the impacts of SA technology on greenhouse gas (GHG) emissions and agronomic yield are usually evaluated separately and their results are inconsistent. To address this gap, we conducted a meta-analysis synthesizing 40 peer-reviewed studies on the effects of SA technology on GHG and ammonia (NH₃) emissions, nitrogen uptake (NU), crop yield, and soil residual NO₃-N in rice paddies and upland cropping system. Compared to the surface application of N, SA technology significantly increased rice yields by 32 % and crop yield in upland systems by 62 %. The largest SA-induced increases in crop yield were found at low N input rates (<100 kg Nha^-1) in rice paddies and medium N input rates (100-200 kg Nha^-1) in upland systems, suggesting that soil moisture is a key factor determining the efficiency of SA technology. SA treatments increased yields by more at reduced fertilizer rates (~30 % less N), a shallow depth (<10 cm), and with urea in both cropping systems than at the full (recommended) N rate, a deeper depth (10-20 cm), and with ammonical fertilizer. SA treatments significantly increased NU in rice paddies (34 %) and upland systems (18 %), and NO₃-N (40 %) in paddyland; however, NO₃-N decreased (28 %) in upland conditions. Ammonia mitigation was greater in paddyland than in upland conditions. SA technology decreased the carbon footprint (CF) in paddyland by 29 % and upland systems by 36 %, and overall by 33 %. Compared with broadcasting, SA significantly reduced CH₄ emissions by 16 %, N₂O emissions by 30 %, and global warming potential (GWP) by 10 % in paddy cultivation. Given SA increased grain yield and NU while reducing NH₃, CF, and GWP, this practice provides dual benefits - mitigating climate change and ensuring food security.

Technical note: AQMEII4 Activity 1: evaluation of wet and dry deposition schemes as an integral part of regional-scale air quality models

We present in this technical note the research protocol for phase 4 of the Air Quality Model Evaluation International Initiative (AQMEII4). This research initiative is divided into two activities, collectively having three goals: (i) to define the current state of the science with respect to representations of wet and especially dry deposition in regional models, (ii) to quantify the extent to which different dry deposition parameterizations influence retrospective air pollutant concentration and flux predictions, and (iii) to identify, through the use of a common set of detailed diagnostics, sensitivity simulations, model evaluation, and reduction of input uncertainty, the specific causes for the current range of these predictions. Activity 1 is dedicated to the diagnostic evaluation of wet and dry deposition processes in regional air quality models (described in this paper), and Activity 2 to the evaluation of dry deposition point models against ozone flux measurements at multiple towers with multiyear observations (to be described in future submissions as part of the special issue on AQMEII4). The scope of this paper is to present the scientific protocols for Activity 1, as well as to summarize the technical information associated with the different dry deposition approaches used by the participating research groups of AQMEII4. In addition to describing all common aspects and data used for this multi-model evaluation activity, most importantly, we present the strategy devised to allow a common process-level comparison of dry deposition obtained from models using sometimes very different dry deposition schemes. The strategy is based on adding detailed diagnostics to the algorithms used in the dry deposition modules of existing regional air quality models, in particular archiving diagnostics specific to land use-land cover (LULC) and creating standardized LULC categories to facilitate cross-comparison of LULC-specific dry deposition parameters and processes, as well as archiving effective conductance and effective flux as means for comparing the relative influence of different pathways towards the net or total dry deposition. This new approach, along with an analysis of precipitation and wet deposition fields, will provide an unprecedented process-oriented comparison of deposition in regional air quality models. Examples of how specific dry deposition schemes used in participating models have been reduced to the common set of comparable diagnostics defined for AQMEII4 are also presented.

Ammonia Dry Deposition in an Alpine Ecosystem Traced to Agricultural Emission Hotpots

Elevated reactive nitrogen (N_r) deposition is a concern for alpine ecosystems, and dry NH₃ deposition is a key contributor. Understanding how emission hotspots impact downwind ecosystems through dry NH₃ deposition provides opportunities for effective mitigation. However, direct NH₃ flux measurements with sufficient temporal resolution to quantify such events are rare. Here, we measured NH₃ fluxes at Rocky Mountain National Park (RMNP) during two summers and analyzed transport events from upwind agricultural and urban sources in northeastern Colorado. We deployed open-path NH₃ sensors on a mobile laboratory and an eddy covariance tower to measure NH₃ concentrations and fluxes. Our spatial sampling illustrated an upslope event that transported NH₃ emissions from the hotspot to RMNP. Observed NH₃ deposition was significantly higher when backtrajectories passed through only the agricultural region (7.9 ng m^-2 s^-1) versus only the urban area (1.0 ng m^-2 s^-1) and both urban and agricultural areas (2.7 ng m^-2 s^-1). Cumulative NH₃ fluxes were calculated using observed, bidirectional modeled, and gap-filled fluxes. More than 40% of the total dry NH₃ deposition occurred when air masses were traced back to agricultural source regions. More generally, we identified that 10 (25) more national parks in the U.S. are within 100 (200) km of an NH₃ hotspot, and more observations are needed to quantify the impacts of these hotspots on dry NH₃ deposition in these regions.

Quantification of Atmospheric Ammonia Concentrations: A Review of Its Measurement and Modeling

Ammonia (NH3), the most prevalent alkaline gas in the atmosphere, plays a significant role in PM2.5 formation, atmospheric chemistry, and new particle formation. This paper reviews quantification of [NH3] through measurements, satellite-remote-sensing, and modeling reported in over 500 publications towards synthesizing the current knowledge of [NH3], focusing on spatiotemporal variations, controlling processes, and quantification issues. Most measurements are through regional passive sampler networks. [NH3] hotspots are typically over agricultural regions, such as the Midwest US and the North China Plain, with elevated concentrations reaching monthly averages of 20 and 74 ppbv, respectively. Topographical effects dramatically increase [NH3] over the Indo-Gangetic Plains, North India and San Joaquin Valley, US. Measurements are sparse over oceans, where [NH3] ≈ a few tens of pptv, variations of which can affect aerosol formation. Satellite remote-sensing (AIRS, CrIS, IASI, TANSO-FTS, TES) provides global [NH3] quantification in the column and at the surface since 2002. Modeling is crucial for improving understanding of NH3 chemistry and transport, its spatiotemporal variations, source apportionment, exploring physicochemical mechanisms, and predicting future scenarios. GEOS-Chem (global) and FRAME (UK) models are commonly applied for this. A synergistic approach of measurements↔satellite-inference↔modeling is needed towards improved understanding of atmospheric ammonia, which is of concern from the standpoint of human health and the ecosystem.

Towards a coupled paradigm of NH3 -CO2 biosphere-atmosphere exchange modelling

Stomatal conductance, one of the major plant physiological controls within NH₃ biosphere-atmosphere exchange models, is commonly estimated from semi-empirical multiplicative schemes or simple light- and temperature-response functions. However, due to their inherent parameterization on meteorological proxy variables, instead of a direct measure of stomatal opening, they are unfit for the use in climate change scenarios and of limited value for interpreting field-scale measurements. Alternatives based on H₂ O flux measurements suffer from uncertainties in the partitioning of evapotranspiration at humid sites, as well as a potential decoupling of transpiration from stomatal opening in the presence of hygroscopic particles on leaf surfaces. We argue that these problems may be avoided by directly deriving stomatal conductance from CO₂ fluxes instead. We reanalysed a data set of NH₃ flux measurements based on CO₂ -derived stomatal conductance, confirming the hypothesis that the increasing relevance of stomatal exchange with the onset of vegetation activity caused a rapid decrease of observed NH₃ deposition velocities. Finally, we argue that developing more mechanistic representations of NH₃ biosphere-atmosphere exchange can be of great benefit in many applications. These range from model-based flux partitioning, over deposition monitoring using low-cost samplers and inferential modelling, to a direct response of NH₃ exchange to climate change.

Global estimates of dry ammonia deposition inferred from space-measurements

Ammonia (NH₃), as an alkaline gas, contributes substantially to atmospheric nitrogen deposition, which can cause biodiversity loss, water eutrophication and soil acidification. Advances in the application of satellite observations allow us to gain deeper insights into atmospheric NH₃ concentrations at large spatial scales. A new satellite-based methodology is proposed for estimating dry NH₃ deposition with consideration of bi-directional NH₃ exchange. We estimate the global dry NH₃ deposition for nine years (2008-2016) by using the Infrared Atmospheric Sounding Interferometer Instrument (IASI) NH₃ retrievals. Satellite-based dry NH₃ deposition is in general consistent with measured dry NH₃ deposition over the monitoring sites (R² = 0.65). Global dry NH₃ deposition over 8 kg N ha^-1 is mainly distributed in the Eastern China, Northern and Central Pakistan, and Northern India. An annual increase rate of 0.27 and 0.13 kg N ha^-1 y^-1 in dry NH₃ deposition during 2008-2016 occurs in Eastern China and Sichuan Basin, which are the major Chinese agricultural regions. The NH₃ compensation point is high during warm months, and can be above 1 μg m^-3 such as in Eastern China, implying the importance of considering the NH₃ compensation points for estimating dry NH₃ deposition. We find, if the upward NH₃ flux was ignored, it will cause 11%, 17%, 5% and 3% overestimation in dry NH₃ deposition in Eastern China, Northern India, Eastern United States and Western Europe, respectively. This study presents the potential of using the satellite retrievals to estimate the large-scale dry NH₃ deposition, and the methodology is able to provide temporally continuous and spatially complete fine-resolution datasets.

A review of measurements of air-surface exchange of reactive nitrogen in natural ecosystems across North America

This review summarizes the state of the science of measurements of dry deposition of reactive nitrogen (Nr) compounds in North America, beginning with current understanding of the importance of dry deposition at the U.S. continental scale followed by a review of micrometeorological flux measurement methods. Measurements of Nr air-surface exchange in natural ecosystems of North America are then summarized, focusing on the U.S. and Canada. Drawing on this synthesis, research needed to address the incompleteness of dry deposition budgets, more fully characterize temporal and geographical variability of fluxes, and better understand air-surface exchange processes is identified. Our assessment points to several data and knowledge gaps that must be addressed to advance dry deposition budgets and air-surface exchange modeling for North American ecosystems. For example, recent studies of particulate (NO3-) and gaseous (NOx, HONO, peroxy nitrates) oxidized N fluxes challenge the fundamental framework of unidirectional flux from the atmosphere to the surface employed in most deposition models. Measurements in forest ecosystems document the importance of in-canopy chemical processes in regulating the net flux between the atmosphere and biosphere, which can result in net loss from the canopy. These results emphasize the need for studies to quantify within- and near-canopy sources and sinks of the full suite of components of the Nr chemical system under study (e.g., NOy or HNO3-NH3-NH4NO3). With respect to specific ecosystems and geographical locations, additional flux measurements are needed particularly in agricultural regions (NH3), coastal zones (NO3- and organic N), and arid ecosystems and along urban to rural gradients (NO2). Measurements that investigate non-stomatal exchange processes (e.g., deposition to wet surfaces) and the biogeochemical drivers of bidirectional exchange (e.g., NH3) are considered high priority. Establishment of long-term sites for process level measurements of reactive chemical fluxes should be viewed as a high priority long-term endeavor of the atmospheric chemistry and ecological communities.

Causes of Large Increases in Atmospheric Ammonia in the Last Decade across North America

Decadal trends of atmospheric ammonia (NH₃) and their potential causes were explored through the analysis of monitored data collected at 15 sites in the United States and 7 sites in Canada. Large percentage increases in the annual average concentration of atmospheric NH₃, for example, >100% at 6 sites and 40-100% at 10 sites, were observed over the most recent 8-13 year period. In contrast, a decrease or a narrow variation in NH₃ emissions was reported at the state or provincial level in both countries during the same period. Decreased emissions of SO₂ and NO _x across North America in the past decade would have reduced the chemical loss of atmospheric NH₃ to form particulate NH₄ ⁺. Such a chemical mechanism was verified through regression analysis at about half of the monitored sites, where the increasing trends in atmospheric NH₃ were partially explained by the reduced NH₄ ⁺. Excluding the reduced contribution from this chemical loss to generate the adjusted annual NH₃ concentration through two approaches, no decreasing trends can be obtained to align those in emissions at most sites, implying that other factors also contributed to the increase in the annual NH₃ concentration. Correlation analysis results implied that enhanced drought conditions and increased ambient temperatures also likely contributed to the increasing trend in the annual NH₃ concentration at some sites. The large percentage increases in the annual NH₃ concentration cannot be fully explained by all the identified causes, leading to oppugning the reality of the decrease in NH₃ emissions reported across North America in the recent decade.

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.

Aspects of uncertainty in total reactive nitrogen deposition estimates for North American critical load applications

Determination of the amount of reactive nitrogen (Nr) deposition in excess of the ecosystem critical load (CL) requires an estimate of total deposition. Because the CL exceedance is used to inform policy decisions, uncertainty in both the CL and the exceedance itself must be understood. In this paper we review the state of the science with respect to the sources of uncertainty in total Nr deposition budgets used for CL assessments in North America and put forth recommendations for research and monitoring to improve deposition measurements and models. In the absence of methods to rigorously quantify uncertainty in total Nr deposition, a simple weighted deposition uncertainty metric (WDUM) is introduced as a tool for scientists and decision makers to use in assessing CL exceedances. Maps of the WDUM applied to National Atmospheric Deposition Program (NADP) Total Deposition (TDep) estimates show greater uncertainty in areas of the U.S. where dry deposition makes a larger contribution to the deposition budget, particularly ammonia (NH3) in agricultural areas and oxidized nitrogen (NOx) in urban areas. Organic N deposition is an important source of uncertainty over much of the U.S. Our analysis illustrates how the WDUM can be used to assess spatial patterns of deposition uncertainty and inform actions to improve deposition budgets for CL assessments at the local scale.

New Bidirectional Ammonia Flux Model in an Air Quality Model Coupled With an Agricultural Model

Ammonia surface flux is bidirectional; that is, net flux can be either upward or downward. In fertilized agricultural croplands and grasslands there is usually more emission than deposition especially in midday during warmer seasons. In North America, most of the ammonia emissions are from agriculture with a significant fraction of that coming from fertilizer. A new bidirectional ammonia flux modeling system has been developed in the Community Multiscale Air Quality (CMAQ) model, which has close linkages with the Environmental Policy Integrated Climate (EPIC) agricultural ecosystem model. Daily inputs from EPIC are used to calculate soil ammonia concentrations that are combined with air concentrations in CMAQ to calculate bidirectional surface flux. The model is evaluated against surface measurements of NH₃ concentrations, NH₄ ⁺ and SO₄ ^2- aerosol concentrations, NH₄ ⁺ wet deposition measurements, and satellite retrievals of NH₃ concentrations. The evaluation shows significant improvement over the base model without bidirectional ammonia flux. Comparisons to monthly average satellite retrievals show similar spatial distribution with the highest ammonia concentrations in the Central Valley of California (CA), the Snake River valley in Idaho, and the western High Plains. In most areas the model underestimates, but in the Central Valley of CA, it generally overestimates ammonia concentration. Case study analyses indicate that modeled high fluxes of ammonia in CA are often caused by anomalous high soil ammonia loading from EPIC for particular crop types. While further improvements to parameterizations in EPIC and CMAQ are recommended, this system is a significant advance over previous ammonia bidirectional surface flux models.

Summertime Soil-Atmosphere Ammonia Exchange in the Colorado Rocky Mountain Front Range Pine Forest

Understanding the NH3 exchange between forest ecosystems and the atmosphere is important due to its role in the nitrogen cycle. However, NH3 exchange is dynamic and difficult to measure. The goal of this study was to characterize this exchange by measuring the atmosphere, soil, and vegetation. Compensation point modeling was used to evaluate the direction and magnitude of surface-atmosphere exchange. Measurements were performed at the Manitou Experimental Forest Observatory (MEFO) site in the Colorado Front Range by continuous online monitoring of gas and particle phase NH3-NH4+ with an ambient ion monitoring system coupled with ion chromatographs (AIM-IC), direct measurements of [NH4+] and pH in soil extracts to determine ground emission potential (Γg), and measurements of [NH4+]bulk in pine needles to derive leaf emission potential (Γst). Two different soil types were measured multiple times throughout the study, in which Γg ranged from 5 to 2122. Γst values ranged from 29 to 54. Inferred fluxes (Fg) from each soil type predicted intervals of emission and deposition. By accounting for the total [NH4+] pool in each compartment, the lifetime of NH3 with respect to the surface-atmosphere exchange in the soil is on the order of years compared to much faster naturally occurring processes, i.e., mineralization and nitrification.

Potential for mitigating global agricultural ammonia emission: A meta-analysis

Ammonia (NH₃) emission from agricultural sources has contributed significantly to air pollution, soil acidification, water eutrophication, biodiversity loss, and declining human health. Although there are numerous strategies for reducing NH₃ emission from agricultural systems, the effectiveness of these measures is highly variable. Furthermore, the integrated assessment of measures to reduce NH₃ emission both from livestock production and cropping systems based on animal and crop type is lacking. Therefore, we conducted a global meta-analysis and integrated assessment of measures to reduce NH₃ emission from agricultural systems. Most of the studied mitigation strategies were effective in reducing NH₃ emission. In the livestock production system, dietary additive, urease inhibitor (UI), manure acidification and deep manure placement have the highest mitigation potential relative to other mitigation strategies, with reduction ranges of 35.1-54.2%, 24.3-68.7%, 88.8-95.0%, and 93.8-99.7%, respectively, relative to the control, while manure storage management could significantly reduce NH₃ emission by 70.0-82.1%. In the cropping system, fertilizer source, use of enhanced efficiency fertilizers, and method of field application are most effective for reducingNH₃ emission. The use of ammonium nitrate, controlled release fertilizer (CRF), and deep placement of fertilizers could reduce NH₃ emission by 88.3, 56.8, and 48.0%, respectively. Choosing a proper fertilizer is critical for decreasing NH₃ emission from cropping systems. We conclude that carefully planned and adopted strategies suited for local conditions are promising for minimizing NH₃ emission from agricultural systems on a global scale, while possible effects of those mitigation measures on the emission of greenhouse gases should be studied in the future.

Mixed method approach to assess atmospheric nitrogen deposition in arid and semi-arid ecosystems

Arid and semi-arid ecosystems (aridlands) cover a third of Earth's terrestrial surface and contain organisms that are sensitive to low level atmospheric pollutants. Atmospheric nitrogen (N) inputs to aridlands are likely to cause changes in plant community composition, fire frequency, and carbon cycling and storage. However, few studies have documented long-term rates of atmospheric N inputs in aridlands because dry deposition is technically difficult to quantify, and extensive sampling is needed to capture fluxes with spatially and temporally heterogeneous rainfall patterns. Here, we quantified long-term spatial and temporal patterns of inorganic N deposition in protected aridland ecosystems across an extensive urban-rural gradient using multiple sampling methods. We compared long-term rates of N deposition from ion-exchange resin (IER) collectors (bulk and throughfall, 2006-2015), wet-dry bucket collectors (2006-2015), and dry deposition from the inferential method using passive samplers (2010-2012). From mixed approaches with IER collectors and inferential methods, we determined that 7.2 ± 0.4 kgNha^-1y^-1 is deposited to protected Sonoran Desert within metropolitan Phoenix, Arizona and 6.1 ± 0.3 kgNha^-1y^-1 in nearby desert ecosystems. Regional scale models overestimated deposition rates for our sampling period by 60% and misidentified hot spots of deposition across the airshed. By contrast, the easy-deployment IER throughfall collectors showed minimal spatial variation across the urban-rural gradient and underestimated deposition fluxes by 54%, largely because of underestimated dry deposition in throughfall. However, seasonal sampling of the IER collectors over 10 years allowed us to capture significant seasonal variation in N deposition and the importance of precipitation timing. These results, derived from the longest, spatially and temporally explicit dataset in drylands, highlight the need for long-term, mixed methods to estimate atmospheric nutrient enrichment to aridlands in a rapidly changing world.

Effects of elevated ozone concentration and nitrogen addition on ammonia stomatal compensation point in a poplar clone

The stomatal compensation point of ammonia (χ_s) is a key factor controlling plant-atmosphere NH₃ exchange, which is dependent on the nitrogen (N) supply and varies among plant species. However, knowledge gaps remain concerning the effects of elevated atmospheric N deposition and ozone (O₃) on χ_s for forest species, resulting in large uncertainties in the parameterizations of NH₃ incorporated into atmospheric chemistry and transport models (CTMs). Here, we present leaf-scale measurements of χ_s for hybrid poplar clone '546' (Populusdeltoides cv. 55/56 x P. deltoides cv. Imperial) growing in two N treatments (N0, no N added; N50, 50 kg N ha^-1 yr^-1 urea fertilizer added) and two O₃ treatments (CF, charcoal-filtered air; E-O₃, non-filtered air plus 40 ppb) for 105 days. Our results showed that χ_s was significantly reduced by E-O₃ (41%) and elevated N (19%). The interaction of N and O₃ was significant, and N can mitigate the negative effects of O₃ on χ_s. Elevated O₃ significantly reduced the light-saturated photosynthetic rate (A_sat) and chlorophyll (Chl) content and significantly increased intercellular CO₂ concentrations (Ci), but had no significant effect on stomatal conductance (g_s). By contrast, elevated N did not significantly affect all measured photosynthetic parameters. Overall, χ_s was significantly and positively correlated with A_sat, g_s and Chl, whereas a significant and negative relationship was observed between χ_s and Ci. Our results suggest that O₃-induced changes in A_sat, Ci and Chl may affect χ_s. Our findings provide a scientific basis for optimizing parameterizations of χ_s in CTMs in response to environmental change factors (i.e., elevated N deposition and/or O₃) in the future.

The hidden cost of using low-resolution concentration data in the estimation of NH3 dry deposition fluxes

Long-term monitoring stations for atmospheric pollutants are often equipped with low-resolution concentration samplers. In this study, we analyse the errors associated with using monthly average ammonia concentrations as input variables for bidirectional biosphere-atmosphere exchange models, which are commonly used to estimate dry deposition fluxes. Previous studies often failed to account for a potential correlation between ammonia exchange velocities and ambient concentrations. We formally derive the exact magnitude of these errors from statistical considerations and propose a correction scheme based on parallel measurements using high-frequency analysers. In case studies using both modelled and measured ammonia concentrations and micrometeorological drivers from sites with varying pollution levels, we were able to substantially reduce bias in the predicted ammonia fluxes. Neglecting to account for these errors can, in some cases, lead to significantly biased deposition estimates compared to using high-frequency instrumentation or corrected averaging strategies. Our study presents a first step towards a unified correction scheme for data from nation-wide air pollutant monitoring networks to be used in chemical transport and air quality models.

Temporal variability of ammonia emission potentials for six plant species in an evergreen subtropical forest in southwest China

The temporal variability of leaf ammonia (NH₃) emission potentials (the ratio of leaf tissue ammonium to proton concentration) and nitrogen (N) pools of six dominant plant species were investigated at the Tieshanping (TSP) forested catchment, southwest China. The results showed that the NH₃ emission potentials and N pools presented small variations among seasons, which were mainly controlled by plant species and the leaf age. Also, high emission potential in one species did not correspond to high tissue N content. Specifically, the Chinese fir (Cunninghamia lanceolata) had higher NH₃ emission potential (mean: 46.2) but lower N content (mean: 1.6% of Dw). The leaf privet (Ligustrum quihoui Carr.) was with the moderate emission potential (15) and the highest N content (2.7% of Dw) on average, which for the Masson pine (Pinus massoniana) were both low. Overall, the emission potentials of the six species were too low (<200) to build up a sufficiently high NH₃ partial pressure in the leaves. Therefore, the Masson pine dominant subtropical forest at TSP acts as a sink for the atmospheric NH₃, indicating that using the N flux in throughfall only may significantly underestimate the N income of the ecosystem. The results are informative for future modeling of plant-atmosphere NH₃ exchange and estimating N budget in local or regional scales.

Ambient concentration and dry deposition of major inorganic nitrogen species at two urban sites in Sichuan Basin, China

To assess pollution levels of major inorganic nitrogen species and their atmospheric deposition input to sensitive ecosystems in Sichuan Basin, southwest China, ambient concentrations of oxidized (NO_y ∼ NO₂, HNO₃, NO₃^-) and reduced (NH_x = NH₃, NH₄⁺) nitrogen species were collected at two urban sites during four one-month periods, each in a different season from July 2014 to April 2015. Estimated annual mean concentration of NO_y was 20.3 and 13.5 μg N m^-3 in Chengdu and Wanzhou, respectively, and NH_x was 16.9 and 13.6 μg N m^-3, respectively. Back trajectory cluster analysis indicated that high levels of NO_y and NH_x in Chengdu were mainly caused by local emissions while those in Wanzhou were caused by both the local emissions and long-range transport of pollutants. On annual basis, NO₂ contributed the most to NO_y, followed by NO₃^- and HNO₃, accounting for 87.5%, 10.5% and 2.0%, respectively, of NO_y in Chengdu, and 91.4%, 6.9% and 1.7%, respectively, in Wanzhou. NH₃ was the predominant contributor to NH_x, contributing 65.6% and 72.2% in Chengdu and Wanzhou, respectively. Dry deposition fluxes were estimated using the inferential method with measured ambient concentrations and modelled dry deposition velocities. The total inorganic nitrogen dry deposition flux was estimated to be 21.4 and 8.5 kg N ha^-1 yr^-1, with 44.3% and 41.4% from NO_y in Chengdu and Wanzhou, respectively. NO₂ and NH₃ each contributed about 80% of NO_y and NH_x dry deposition, respectively. Wet deposition was only collected in Wanzhou, where the annual wet deposition of NO₃^- and NH₄⁺ was 4.5 and 15.7 kg N ha^-1 yr^-1, respectively. The total wet plus dry deposition was 28.7 kg N ha^-1 yr^-1 in Wanzhou with 72.2% from reduced nitrogen. Therefore, controlling NH₃ emissions from agricultural, traffic, waste containers and sewage system sources would be effective to reduce the total nitrogen deposition in the Sichuan Basin area.

Increasing importance of deposition of reduced nitrogen in the United States

Rapid development of agriculture and fossil fuel combustion greatly increased US reactive nitrogen emissions to the atmosphere in the second half of the 20th century, resulting in excess nitrogen deposition to natural ecosystems. Recent efforts to lower nitrogen oxides emissions have substantially decreased nitrate wet deposition. Levels of wet ammonium deposition, by contrast, have increased in many regions. Together these changes have altered the balance between oxidized and reduced nitrogen deposition. Across most of the United States, wet deposition has transitioned from being nitrate-dominated in the 1980s to ammonium-dominated in recent years. Ammonia has historically not been routinely measured because there are no specific regulatory requirements for its measurement. Recent expansion in ammonia observations, however, along with ongoing measurements of nitric acid and fine particle ammonium and nitrate, permit new insight into the balance of oxidized and reduced nitrogen in the total (wet + dry) US nitrogen deposition budget. Observations from 37 sites reveal that reduced nitrogen contributes, on average, ∼65% of the total inorganic nitrogen deposition budget. Dry deposition of ammonia plays an especially key role in nitrogen deposition, contributing from 19% to 65% in different regions. Future progress toward reducing US nitrogen deposition will be increasingly difficult without a reduction in ammonia emissions.

Global inorganic nitrogen dry deposition inferred from ground- and space-based measurements

Atmospheric nitrogen (N) dry deposition is an important component in total N deposition. However, uncertainty exists in the assessment of global dry deposition. Here, we develop empirical models for estimating ground N concentrations using NO₂ satellite measurements from the Ozone Monitoring Instrument (OMI) and ground measurements from 555 monitoring sites. Global patterns and trends in the fluxes of NO₂, HNO₃, NH₄⁺, and NO₃⁻ were assessed for 2005–2014. Moreover, we estimated global NH₃ dry deposition directly using data from 267 monitoring sites. Our results showed that East Asia, the United States, and Europe were important regions of N deposition, and the total annual amount of global inorganic N deposition was 34.26 Tg N. The dry deposition fluxes were low in Africa and South America, but because of their large area, the total amounts in these regions were comparable to those in Europe and North America. In the past decade, the western United States and Eurasia, particularly eastern China, experienced the largest increases in dry deposition, whereas the eastern United States, Western Europe, and Japan experienced clear decreases through control of NO_x and NH₃ emissions. These findings provide a scientific background for policy-makers and future research into global changes.

Land Use Specific Ammonia Deposition Velocities: a Review of Recent Studies (2004-2013)

Land use specific deposition velocities of atmospheric trace gases and aerosols-particularly of reactive nitrogen compounds-are a fundamental input variable for a variety of deposition models. Although the concept is known to have shortcomings-especially with regard to bi-directional exchange-the often limited availability of concentration data and meteorological input variables make it a valuable simplification for regional modeling of deposition fluxes. In order to meet the demand for an up-to-date overview of recent publications on measurements and modeling studies, we compiled a database of ammonia (NH₃) deposition velocities published from 2004 to 2013. Observations from a total of 42 individual studies were averaged using an objective weighing scheme and classified into seven land use categories. Weighted average and median deposition velocities are 2.2 and 2.1 cm s^-1 for coniferous forests, 1.5 and 1.2 cm s^-1 for mixed forests, 1.1 and 0.9 cm s^-1 for deciduous forests, 0.9 and 0.7 cm s^-1 for semi-natural sites, 0.7 and 0.8 cm s^-1 for urban sites, 0.7 and 0.6 cm s^-1 for water surfaces, and 1.0 and 0.4 cm s^-1 for agricultural sites, respectively. Thus, values presented in this compilation were considerably lower than those found in former studies (e.g., VDI 2006). Reasons for the mismatch were likely due to different land use classification, different averaging methods, choices of measurement locations, and improvements in measurement and in modeling techniques. Both data and code used for processing are made available as supplementary material to this article.

Towards a climate-dependent paradigm of ammonia emission and deposition

Existing descriptions of bi-directional ammonia (NH3) land-atmosphere exchange incorporate temperature and moisture controls, and are beginning to be used in regional chemical transport models. However, such models have typically applied simpler emission factors to upscale the main NH3 emission terms. While this approach has successfully simulated the main spatial patterns on local to global scales, it fails to address the environment- and climate-dependence of emissions. To handle these issues, we outline the basis for a new modelling paradigm where both NH3 emissions and deposition are calculated online according to diurnal, seasonal and spatial differences in meteorology. We show how measurements reveal a strong, but complex pattern of climatic dependence, which is increasingly being characterized using ground-based NH3 monitoring and satellite observations, while advances in process-based modelling are illustrated for agricultural and natural sources, including a global application for seabird colonies. A future architecture for NH3 emission-deposition modelling is proposed that integrates the spatio-temporal interactions, and provides the necessary foundation to assess the consequences of climate change. Based on available measurements, a first empirical estimate suggests that 5°C warming would increase emissions by 42 per cent (28-67%). Together with increased anthropogenic activity, global NH3 emissions may increase from 65 (45-85) Tg N in 2008 to reach 132 (89-179) Tg by 2100.

Sensitivity of continental United States atmospheric budgets of oxidized and reduced nitrogen to dry deposition parametrizations

Reactive nitrogen (Nr) is removed by surface fluxes (air-surface exchange) and wet deposition. The chemistry and physics of the atmosphere result in a complicated system in which competing chemical sources and sinks exist and impact that removal. Therefore, uncertainties are best examined with complete regional chemical transport models that simulate these feedbacks. We analysed several uncertainties in regional air quality model resistance analogue representations of air-surface exchange for unidirectional and bi-directional fluxes and their effect on the continental Nr budget. Model sensitivity tests of key parameters in dry deposition formulations showed that uncertainty estimates of continental total nitrogen deposition are surprisingly small, 5 per cent or less, owing to feedbacks in the chemistry and rebalancing among removal pathways. The largest uncertainties (5%) occur with the change from a unidirectional to a bi-directional NH3 formulation followed by uncertainties in bi-directional compensation points (1-4%) and unidirectional aerodynamic resistance (2%). Uncertainties have a greater effect at the local scale. Between unidirectional and bi-directional formulations, single grid cell changes can be up to 50 per cent, whereas 84 per cent of the cells have changes less than 30 per cent. For uncertainties within either formulation, single grid cell change can be up to 20 per cent, but for 90 per cent of the cells changes are less than 10 per cent.

Elevated atmospheric CO2 decreases the ammonia compensation point of barley plants

The ammonia compensation point ( ) controls the direction and magnitude of NH3 exchange between plant leaves and the atmosphere. Very limited information is currently available on how responds to anticipated climate changes. Young barley plants were grown for 2 weeks at ambient (400 μmol mol(-1)) or elevated (800 μmol mol(-1)) CO2 concentration with or NH4NO3 as the nitrogen source. The concentrations of and H(+) in the leaf apoplastic solution were measured along with different foliar N pools and enzymes involved in N metabolism. Elevated CO2 caused a threefold decrease in the concentration in the apoplastic solution and slightly acidified it. This resulted in a decline of the from 2.25 and 2.95 nmol mol(-1) under ambient CO2 to 0.37 and 0.89 nmol mol(-1) at elevated CO2 in the and NH4NO3 treatments, respectively. The decrease in at elevated CO2 reflected a lower N concentration (-25%) in the shoot dry matter. The activity of nitrate reductase also declined (-45 to -60%), while that of glutamine synthetase was unaffected by elevated CO2. It is concluded that elevated CO2 increases the likelihood of plants being a sink for atmospheric NH3 and reduces episodes of NH3 emission from plants.

Measurements and modeling of reactive nitrogen deposition in southeast Brazil

Increased reactive nitrogen (Nr) deposition due to expansion of agro-industry was investigated considering emission sources, atmospheric transport and chemical reactions. Measurements of the main inorganic nitrogen species (NO2, NH3, HNO3, and aerosol nitrate and ammonium) were made over a period of one year at six sites distributed across an area of ∼130,000 km2 in southeast Brazil. Oxidized species were estimated to account for ∼90% of dry deposited Nr, due to the region's large emissions of nitrogen oxides from biomass burning and road transport. NO2-N was important closer to urban areas, however overall HNO3-N represented the largest component of dry deposited Nr. A simple mathematical modeling procedure was developed to enable estimates of total Nr dry deposition to be made from knowledge of NO2 concentrations. The technique, whose accuracy here ranged from <1% to 29%, provides a useful new tool for the mapping of reactive nitrogen deposition.