NH3 Deposition Research Articles

Combining Top‐Down and Bottom‐Up Approaches to Evaluate Recent Trends and Seasonal Patterns in UK N2O Emissions

AbstractAtmospheric trace gas measurements can be used to independently assess national greenhouse gas inventories through inverse modeling. Atmospheric nitrous oxide (N2O) measurements made in the United Kingdom (UK) and Republic of Ireland are used to derive monthly N2O emissions for 2013–2022 using two different inverse methods. We find mean UK emissions of 90.5 ± 23.0 (1σ) and 111.7 ± 32.1 (1σ) Gg N2O yr−1 for 2013–2022, and corresponding trends of −0.68 ± 0.48 (1σ) Gg N2O yr−2 and −2.10 ± 0.72 (1σ) Gg N2O yr−2, respectively, for the two inverse methods. The UK National Atmospheric Emissions Inventory (NAEI) reported mean N2O emissions of 73.9 ± 1.7 (1σ) Gg N2O yr−1 across this period, which is 22%–51% smaller than the emissions derived from atmospheric data. We infer a pronounced seasonal cycle in N2O emissions, with a peak occurring in the spring and a second smaller peak in the late summer for certain years. The springtime peak has a long seasonal decline that contrasts with the sharp rise and fall of N2O emissions estimated from the bottom‐up UK Emissions Model (UKEM). Bayesian inference is used to minimize the seasonal cycle mismatch between the average top‐down (atmospheric data‐based) and bottom‐up (process model and inventory‐based) seasonal emissions at a sub‐sector level. Increasing agricultural manure management and decreasing synthetic fertilizer N2O emissions reduces some of the discrepancy between the average top‐down and bottom‐up seasonal cycles. Other possibilities could also explain these discrepancies, such as missing emissions from NH3 deposition, but these require further investigation.

Reactive nitrogen losses from Canadian agricultural soils over 36 years

Agriculture is a major source of reactive nitrogen (Nr) losses through ammonia (NH3) volatilization, nitrous oxide (N2O) emissions and nitrate (NO3−) leaching. A Canadian Agricultural Nitrogen Budget for Reactive N (CANBNr) model was developed to estimate the nitrogen (N) balance in soils, including N removals by harvested crops and Nr losses for the years 1981–2016 across Canada. Annual N inputs to farmland include commercial fertilizer N, livestock manure N, symbiotic and asymbiotic biological N fixation and atmospheric N deposition of NOx and NH3. The total annual N input for Canadian farmland was 5528 Gg N (103.2 kg N ha−1) in 2016 where N removal by crops and Nr accounted for 72.5 % (74.9 kg N ha−1) and 13.4 % (13.9 kg N ha−1), respectively. The Nr losses from N2O emissions, NH3 volatilization and NO3− leaching accounted for 64 Gg N (1.2 kg N ha−1), 330 Gg N (6.2 kg N ha−1) and 348 Gg N (6.5 kg N ha−1), which represents between1.2–6.3 % of total N input. A total of 777 Gg N (14.5 kg N ha−1) remained in the soil as surplus N (14.1 % of total N input), which could be available to subsequent crops in dryer regions but might be subject to N2O loss through nitrification or denitrification processes or NO3− leaching following heavy rains in humid regions. Nitrous oxide and ammonia emissions increased over a 36-year period due to increased fertilizer N inputs. The percentage of N inputs that was estimated to be lost as Nr increased from 17.9 % in 1981 to the peak level of 19.8 % in 2001 and then declined to 13.4 % in 2016. Total N removal by crops increased at a greater rate than N input during the 2001–2016 period resulting in an increased N uptake by crops over the last 15 years. The improved management of fertilizer N for agricultural systems represents a key opportunity for both farmers and policy makers to further reduce Nr losses from Canadian farmland without negatively impacting productivity.

A Decadal Change in Atmospheric Nitrogen Deposition at a Rural Site in Southern China

Elevated atmospheric reactive nitrogen (Nr) emissions and the subsequent nitrogen (N) deposition have negatively impacted the global environment, particularly in China. In order to assess the long-term trends in atmospheric N deposition in the south of China, Taojiang County in Hunan Province was selected as a representative rural area for study. We analyzed interannual variation in atmospheric Nr, including gaseous ammonia (NH3), nitrogen dioxide (NO2), nitrate acid (HNO3) vapor, particulate ammonium (NH4+), and nitrate (NO3−) in air and NH4+-N and NO3−-N in precipitation from 2011 to 2020. The 10-year average atmospheric wet-plus-dry N deposition was 41.9 kg N ha−1 yr−1, which decreased by approximately 24% after 2012, indicating that NH3 and NOx emissions were effectively reduced by emission controls introduced in 2013. Wet deposition accounted for approximately 74% of the total N deposition and was significantly influenced by annual precipitation amount. Reduced N (NH3, pNH4+, and NH4+ in rainwater) was the dominant form, comprising approximately 58% of the total N deposition, while oxidized N (pNO3−, NO2, HNO3, and NO3− in rainwater) accounted for 42% of the total N deposition. Atmospheric HNO3, NO2, and NH3 concentrations and deposition declined by 30–80% over the decade, while particulate NH4+ and NO3− concentrations and deposition remained at relatively stable levels, which suggests that ongoing research and policy should focus on rural particulate pollution. Future strategies must concentrate on the integrated control of NH3 and NOx emissions to mitigate air pollution and protect human health, particularly in rural areas because current abatement efforts are primarily directed toward urban areas and the industrial sector, whereas non-point source NH3 pollution, influenced mainly by agricultural activities, dominates in rural regions.

Evidence and causes of recent decreases in nitrogen deposition in temperate forests in Northeast China

High reactive nitrogen (N) emissions due to anthropogenic activities in China have led to an increase in N deposition and ecosystem degradation. The Chinese government has strictly regulated reactive N emissions since 2010, however, determining whether N deposition has reduced requires long-term monitoring. Here, we report the patterns of N deposition at a rural forest site (Qingyuan) in northeastern China over the last decade. We collected 456 daily precipitation samples from 2014 to 2022 and analysed the temporal dynamics of N deposition. NH4+-N, NO3−-N, and total inorganic N (TIN) deposition ranged from 10.5 ± 3.5 (mean ± SD), 6.1 ± 1.6, and 16.6 ± 4.7 kg N ha−1 year−1, respectively. Over the measurement period, TIN deposition at Qingyuan decreased by 55 %, whereas that in comparable sites in East Asia declined by 14–34 %. We used a random forest model to determine factors influencing the deposition of NH4+-N, NO3−-N, and TIN during the study period. NH4+-N deposition decreased by 60 % because of decreased agricultural NH3 emissions. Furthermore, NO3−-N deposition decreased by 42 %, due to reduced NOx emissions from agricultural soil and fossil fuel combustion. The steep decline in N deposition in northeastern China was attributed to reduced coal consumption, improved emission controls on automobiles, and shifts in agricultural practices. Long-term monitoring is needed to assess regional air quality and the impact of N emission control regulations.

Fabrication of Ag3PO4/N-doped TiO2 nanotubes heterojunction photocatalysts for visible-light-driven photocatalysis

As an environmentally friendly and energy-efficient technology, photocatalysis holds considerable potential for eliminating organic pollutants. In this study, novel visible-light-driven Ag3PO4-decorated nitrogen-doped TiO2 nanotubes (Ag3PO4/N-TNTs) photocatalysts with advanced properties of heterostructures were successfully synthesized and used to degrade methylene blue (MB) dye. The fabrication of Ag3PO4/N-TNTs photocatalysts involved a two-step electrochemical anodization to obtain TiO2 nanotubes (TNTs) and the wet impregnation of the amorphous tubular structure in NH3 solution, followed by calcination in air to obtain crystallized nitrogen-doped TiO2 nanotubes (N-TNTs). Finally, the decoration of the N-TNTs with Ag3PO4 nanoparticles was conducted to enhance visible-light reactivity. Various heterojunction photocatalysts were obtained by changing the concentration of NH3 (0.5–2.5 M) and the dosage of Ag3PO4 (0.25–1.5 wt%) in the composites. Results of ultraviolet–visible (UV–Vis) absorption, photocurrent transient, and electrochemical impedance spectroscopy measurement revealed that Ag3PO4/N-TNTs possessed a significant response in the visible-light range and good photoelectronic properties. The superior photocatalytic activity of the Ag3PO4/N-TNTs catalyst was achieved under the optimal conditions of N-doping using 2-M NH3 and Ag3PO4 deposition at a dosage of 0.75 wt%. Based on the degradation efficiency (DE) of MB, the optimal Ag3PO4/N-TNTs exhibited rate constants of 4.5 and 2 times higher than those of the pristine TNTs and N-TNTs, respectively. The high stability of Ag3PO4/N-TNTs was confirmed through four cycles of reutilization, with a small decay of only 5.3% in the DE of MB dye for each run of photocatalysis. The scavenger tests of generated reactive oxygen species revealed that ·OH and ·O2− were the primary contributors to photocatalytic performance. The synthesized Ag3PO4/N-TNTs heterostructure photocatalysts were proven to possess efficient separation of photogenerated charge carriers, high reactivity, and stability in the visible-light region.

Severe photochemical pollution formation associated with strong HONO emissions from dew and guttation evaporation

The unknown daytime source of HONO has been extensively investigated due to unexplained atmospheric oxidation capacity and current modelling bias, especially during cold seasons. In this study, abrupt morning increases in atmospheric HONO at a rural site in the North China Plain (NCP) were observed almost on daily basis, which were closely linked to simultaneous rises in atmospheric water vapor content and NH3 concentrations. Dew and guttation water formation was frequently observed on wheat leaves, from which water samples were taken and chemically analyzed for the first time. Results confirmed that such natural processes likely governed the daily nighttime deposition and daytime release of HONO and NH3, which have not been considered in the numerous HONO budget studies investigating its large missing daytime source in the NCP. The dissolved HONO and NH3 in leaf surface water droplets reached 1.4 and 23 mg L−1 during the morning on average, resulting in averaged atmospheric HONO and NH3 increases of 0.89 ± 0.61 and 43.7 ± 29.3 ppb during morning hours, with relative increases of 186 ± 212 % and 233 ± 252 %, respectively. The high atmospheric oxidation capacity contained within HONO was stored in near surface liquid water (such as dew, guttation and soil surface water) during nighttime, which prevented its atmospheric dispersion after sunset and protected it from photodissociation during early morning hours. HONO was released in a blast during later hours with stronger solar radiation, which triggered and then accelerated daytime photochemistry through the rapid photolysis of HONO and subsequent OH production, especially under high RH conditions, forming severe secondary gaseous and particulate pollution. Results of this study demonstrate that global ecosystems might play significant roles in atmospheric photochemistry through nighttime dew formation and guttation processes.

Vertical profile and flux measurements of ammonia in a deciduous forest in Japan towards improvement of bi-directional exchange model

Ammonia (NH3) is a major component of reduced nitrogen. A bi-directional exchange of NH3 takes place between the atmosphere and surface. Although the study of reduced nitrogen is gaining prominence in recent years, quantification of NH3 deposition is technically challenging due to its complex properties; consequently, this generates significant uncertainty regarding the estimation of total nitrogen deposition. Based on past studies, it has been reported that Asia has numerous high-risk nitrogen deposition sites; however, only a few studies have investigated the NH3 exchange in this region. To clarify the exchange process and achieve improved accuracy regarding the estimation of nitrogen deposition in Asia, we conducted measurements of vertical profile and flux of NH3 in a forest in Japan during summer (July 21–August 1, 2015), winter (February 23–February 29, 2016), and autumn (September 27–October 11, 2016). We measured daytime and nighttime NH3 concentrations at four or five heights of an observation tower in the forest by using filter-pack holders, and determined NH3 fluxes by using the aerodynamic gradient method. During daytime, NH3 showed emission above the canopy in summer, and near the forest floor in winter. There was no clear emission trend in autumn. During nighttime, NH3 showed deposition from the top of the canopy to the forest floor for all observed periods. NH3 exchange in the forest possibly had seasonal and diurnal variations. The flux measurements showed large NH3 emissions at daytime in summer and small depositions at nighttime in summer and autumn. Mean flux during the observation periods was 0.026 μg m−2 s−1. The daytime flux during leafy periods was found to have a strong correlation with solar radiation. At daytime in summer, flux also had strong correlations with relative humidity and horizontal wind speed. We also estimated NH3 fluxes in summer by employing a bi-directional exchange model to verify the applicability of the model at the study site. Inferred fluxes obtained via the model showed large emissions during daytime and agreed with the observation results; however, the model could not reproduce the nighttime deposition. This discrepancy was improved by applying more suitable cuticular resistances.

Discerning the Concentration and Bi‐Directional Flux of Ammonia in an Urban Estuary Using the Relaxed Eddy Accumulation Method

AbstractNarragansett Bay, the largest estuary in New England, is a heavily urbanized watershed impacted by deposition and runoff. Nutrient budgets and local policy rely on deposition data from a 1990 study that did not include any direct observations of dry deposition of gaseous ammonia (NH3(g)) and particulate ammonium (p‐NH4+) due to uncertainties in their flux direction and measurement difficulty. Recent work has shown that wet deposition of ammonium (NH4+) to the Bay has increased by a factor of 6 over the past three decades, leading to a 2.5‐fold increase in wet nitrogen (N) deposition. This documented increase in wet deposition of NH4+ is concurrent with managed nutrient reductions in urbanized estuaries but has potentially increased the impacts of atmospheric N deposition, including the dry deposition of NH3(g) and p‐NH4+. However, the lack of NH3(g) and p‐NH4+ measurements hinders our interpretation of this important N source. For the first time over Narragansett Bay, and to our knowledge over open water, dry (particulate and gas phase) total ammonia (NHx = NH3(g) + p‐NH4+) and the bidirectional NH3(g) flux were quantified using a relaxed eddy accumulation sampling technique. We find that dry deposition of NHx comprises 9.6% of total (wet + dry) N deposition. During the fall season, the dominant flux direction for NH3(g) is upward, which also has implications for urban air quality. We estimate that NH3(g) emitted from the Bay to the atmosphere makes up to 10% of the local NH3(g) emission budget for fall.

Atmospheric Reactive Nitrogen Deposition from 2010 to 2021 in Lake Taihu and the Effects on Phytoplankton

The effects of nitrogen deposition reduction on nutrient loading in freshwaters have been widely studied, especially in remote regions. However, understanding of the ecological effects is still rather limited. Herein, we re-estimated nitrogen deposition, both of wet and dry deposition, in Lake Taihu with monthly monitoring data from 2010 to 2021. Our results showed that the atmospheric deposition of reactive nitrogen (namely NH4+ and NO3-) in Lake Taihu was 4.94-11.49 kton/yr, which equaled 13.9%-27.3% of the riverine loading. Dry deposition of NH4+ and NO3- contributed 53.1% of the bulk deposition in Lake Taihu. Ammonium was the main component of both wet and dry deposition, which may have been due to the strong agriculture-related activities around Lake Taihu. Nitrogen deposition explained 24.9% of the variation in phytoplankton community succession from 2010 to 2021 and was the highest among all the environmental factors. Atmospheric deposition offset the effects of external nitrogen reduction during the early years and delayed the emergence of nitrogen-fixing cyanobacterial dominance in Lake Taihu. Our results implied that a decrease in nitrogen deposition due to a reduction in fertilizer use, especially a decrease in NH4+ deposition, could limit diatoms and promote non-nitrogen-fixing cyanobacterial dominance, followed by nitrogen-fixing taxa. This result was also applied to other shallow eutrophic lakes around the middle and lower reaches of the Yangtze River, where significant reduction of fertilizer use recorded during the last decades.

Atmospheric wet deposition in urban and suburban sites across the United States

Compared to rural areas, there is a much poorer understanding of trends in wet atmospheric deposition across urban and suburban areas of the U.S. and how they compare to rural sites over time. The goal of this study was to investigate long-term trends in atmospheric wet deposition of nitrogen (ammonium and nitrate; NH4+ and NO3−), sulfate (SO42−), cations, and chloride (Cl−) for urban and suburban sites in the U.S. using data from the National Atmospheric Deposition Program (NADP) and to determine how long-term deposition trends relate to urbanization. For urban and suburban sites, we found that from the late 1970s to 2020, atmospheric deposition of NO3−, dissolved inorganic N (DIN), SO42−, hydrogen (H+), and magnesium (Mg2+) decreased over time, while deposition of NH4+ and potassium (K+) significantly increased over this period. When sites were categorized based on population density, we found no significant differences in rates of wet deposition between urban and background rural values, but the difference between urban and background rural values increased significantly over time for NO3− and DIN. When sites were categorized based on impervious surface area (ISA), we found that rates of NH4+, DIN, SO42, and calcium (Ca2+) deposition were significantly higher in urban sites (that had greater than 25% ISA) compared to background rural values. There were statistically significant positive relationships between population density and deposition of Mg2+, K+, sodium (Na+), and Cl− and significant positive relationships between % ISA and deposition of NO3− and Mg2+. Together, the results of this study demonstrate the urban enhancement of many forms of atmospheric deposition and the need for increased long-term monitoring of atmospheric deposition in cities and suburban sites throughout the U.S., especially in the western U.S.

Atmospheric ammonia deposition and its role in a cool-temperate fragmented deciduous broad-leaved forest

Forest fragmentation can increase dry deposition of atmospheric ammonia (NH3) volatilized from agricultural areas, and consequently increase spatial variability of Nr deposition within the forest (“elevated reactive nitrogen (Nr) deposition”). However, little is known about this effect and its impact on forest production in a deciduous broad-leaved forest in Asian countries. Here, we performed the field observations of atmospheric concentration and deposition of inorganic Nr gases and particles in a Japanese fragmented forest from May 2018 to April 2019. Leaf traits and growth of Japanese oak tree and understory vegetation were also measured. Atmospheric concentration and deposition of dissolved inorganic Nr (DIN) to tree canopies were measured via filter sampling and throughfall methods, respectively. Canopy retention/exchange was also estimated as a part of dry deposition using the canopy budget model. The results demonstrated that annual dry deposition of NH3 to tree canopies (4.4 kgN ha−1 y−1) was dominant in the annual total DIN deposition (7.1 kgN ha−1 y−1) at the forest edge, including the elevated Nr deposition of approximately 2 kgN ha−1 y−1. Additionally, agricultural activities such as fertilization in the area surrounding the forest likely enhanced the potential of canopy retention of NH4+, readily absorbed by tree canopy. The growth rates of tree and understory vegetation (dwarf bamboo) were higher at the edge than that at the interior, emphasizing a need for further investigation to quantify the effect of low Nr deposition (<3–5 kgN ha−1 y−1) on forest production, along with the conventional experimental studies under a high Nr deposition.

Ammonia Cycling and Emerging Secondary Aerosols from Arable Agriculture: A European and Irish Perspective

Ammonia (NH3) is a naturally occurring, highly reactive and soluble alkaline trace gas, originating from both natural and anthropogenic sources. It is present throughout the biosphere, yet plays a complicated role in atmospheric acid–base reactions resulting in the formation of inorganic secondary inorganic aerosols (SIAs). While the general mechanisms are recognised, factors controlling the reactions leading to SIA formation are less explored. This review summarises the current knowledge of NH3 sources, emission and deposition processes and atmospheric reactions leading to the formation of SIA. Brief summaries of NH3 and SIA long-range transport and trans-boundary pollution, a discussion of precursor species to SIAs (other than NH3), abiotic and biotic controls and state-of-the-art methods of measurement and modelling of pollutants are also included. In Ireland, NH3 concentrations remained below National and European Union limits, until 2016 when a rise in emissions was seen due to agricultural expansion. However, due to a lack of continuous monitoring, source and receptor relationships are difficult to establish, including the appointment of precursor gases and aerosols to source regions and industries. Additionally, the lack of continuous monitoring leads to over- and underestimations of precursor gases present, resulting in inaccuracies of the estimated importance of NH3 as a precursor gas for SIA. These gaps in data can hinder the accuracy and precision of forecasting models. Deposition measurements and the modelling of NH3 present another challenge. Direct source measurements are required for the parameterization of bi-directional fluxes; however, high-quality data inputs can be limited by local micrometeorological conditions, or the types of instrumentation used. Long-term measurements remain challenging for both aerosols and precursor gases over larger areas or arduous terrains.

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.

Effects of contact metamorphism on δ13C, δ15N, and XPS nitrogen functional forms of inertinite-rich Witbank coals (South Africa): Indications for possible alteration by hydrothermal fluids

In this study, δ13C, δ15N, and nitrogen functional forms were used to evaluate the effects of contact metamorphism on inertinite-rich Main Karoo Basin coals (South Africa). Eighteen samples were collected at 0.25 m intervals from the western and eastern sides of a ∼ 2.6 m thick dolerite dyke emplaced into the No. 2 Seam; an unaltered coal from the same seam was included as a control (Witbank Coalfield). Inertinite (semifusinite and inertodetrinite) dominated the organic composition of the samples. δ13C and δ15N for the unaltered coal were − 21.1‰ VPDB and 1.5‰ Air, respectively. For the intruded coals from the western side, δ13C ranged between −22.4 and −21.4‰ VPDB, and δ15N between 1.2 and 2.6‰ Air. The δ13C for samples from the eastern side ranged between −21.8 and −20.7‰ VPDB, and between 1.8 and 3.0‰ Air for δ15N. Differences in δ15N between the unaltered and the intruded coals were generally more significant than in δ13C, especially for samples from the eastern side. The positive shifts in δ15N may reflect reactions between organic nitrogen in coal and mobile nitrogen transported by hydrothermal fluids. The absence of a relationship between δ15N and distance from the dyke, may reflect secondary, localized contact metamorphism due to hydrothermal fluid migration within the intruded seam. The circulation of a fluid transporting 15N-rich moieties, apparently more active on the eastern side, was supported by the higher ash yields, δ15N, and Ntotal/Corganic ratios. The correlation between Ntotal/Corganic and ash yields suggests deposition of NH4+ − bearing inorganic components preferentially into the eastern side. Similar to the unaltered coal (N-5 = 47.24 at.%), N-pyrrolic was the dominant nitrogen functional form of the intruded coals, reflecting the developmental pathways for the inertinite macerals as opposed to the contact metamorphism. The low proportion of graphitic (quaternary) nitrogen, irrespective of sample distance from the dyke, suggests metamorphic temperatures lower than 900 °C.

Spatial heterogeneity of ammonia fluxes in a deciduous forest and adjacent grassland

Gas-phase ammonia (NH3), emitted primarily from agriculture, contributes significantly to reactive nitrogen (Nr) deposition. Excess deposition of Nr to the environment causes acidification, eutrophication, and loss of biodiversity. The exchange of NH3 between land and atmosphere is bidirectional and can be highly heterogenous when underlying vegetation and soil characteristics differ. Direct measurements that assess the spatial heterogeneity of NH3 fluxes are lacking. To this end, we developed and deployed two fast-response, quantum cascade laser-based open-path NH3 sensors to quantify NH3 fluxes at a deciduous forest and an adjacent grassland separated by 700 m in North Carolina, United States from August to November, 2017. The sensors achieved 10 Hz precisions of 0.17 ppbv and 0.23 ppbv in the field, respectively. Eddy covariance calculations showed net deposition of NH3 (-7.3 ng NH3-N m−2 s−1) to the forest canopy and emission (3.2 ng NH3-N m−2 s−1) from the grassland. NH3 fluxes at both locations displayed diurnal patterns with absolute magnitudes largest midday and with smaller peaks in the afternoons. Concurrent biogeochemistry data showed over an order of magnitude higher NH3 emission potentials from green vegetation at the grassland compared to the forest, suggesting a possible explanation for the observed flux differences. Back trajectories originating from the site identified the upwind urban area as the main source region of NH3. Our work highlights the fact that adjacent natural ecosystems sharing the same airshed but different vegetation and biogeochemical conditions may differ remarkably in NH3 exchange. Such heterogeneities should be considered when upscaling point measurements, downscaling modeled fluxes, and evaluating Nr deposition for different natural land use types in the same landscape. Additional in-situ flux measurements accompanied by comprehensive biogeochemical and micrometeorological records over longer periods are needed to fully characterize the temporal variabilities and trends of NH3 fluxes and identify the underlying driving factors.

A novel method to synthesize pure-phase Si2N2O powders in a fluidized bed reactor

Si2N2O ceramic, an emerging functional and structural material, has a wide range of applications. However, the preparation of pure-phase Si2N2O powder remains challenging due to the mass transfer resistance and undesirable side reactions in the conventional methods. Herein, a novel molecular approach combined with the decomposition process has been developed to synthesize pure-phase Si2N2O powders. The hydrated Si(NH)2 precursors were synthesized through the chemical vapor deposition (CVD) of SiCl4, NH3, and humidified N2 in a fluidized bed reactor (FBR) in two steps. Then, the hydrated Si(NH)2 precursors were decomposed into amorphous and subsequently transformed into crystalline powders under different temperatures and time. It was found that the molar ratio of N/O of the hydrolyzed Si(NH)2 can be controlled by N2 ventilation time and played an important role in synthesizing high pure Si2N2O powder. When it varied from 2.5:1 to 2:1, pure-phase Si2N2O powder was obtained after heat treatment at 1300–1500 °C, which features a big tolerance for N/O ratios. This newly developed method offered a chance for the preparation of high-quality Si2N2O powder with high efficiency and low cost.

Assessing the representativity of NH&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; measurements influenced by boundary-layer dynamics and the turbulent dispersion of a nearby emission source

Abstract. This study presents a fine-scale simulation approach to assess the representativity of ammonia (NH3) measurements in the proximity of an emission source. Close proximity to emission sources (&lt; 5 km) can introduce a bias in regionally representative measurements of the NH3 molar fraction and flux. Measurement sites should, therefore, be located a significant distance away from emission sources, but these requirements are poorly defined and can be difficult to meet in densely agricultural regions. This study presents a consistent criterion to assess the regional representativity of NH3 measurements in proximity to an emission source, calculating variables that quantify the NH3 plume dispersion using a series of numerical experiments at a fine resolution (20 m). Our fine-scale simulation framework with explicitly resolved turbulence enables us to distinguish between the background NH3 and the emission plume, including realistic representations of NH3 deposition and chemical gas–aerosol transformations. We introduce the concept of blending distance based on the calculation of turbulent fluctuations to systematically analyze the impact of the emission plume on simulated measurements, relative to this background NH3. We perform a suite of systematic numerical experiments for flat homogeneous grasslands, centered around the CESAR Observatory at Cabauw, to analyze the sensitivity of the blending distance, varying meteorological factors, emission/deposition and NH3 dependences. Considering these sensitivities, we find that NH3 measurements at this measurement site should be located at a minimum distance of 0.5–3.0 and 0.75–4.5 km from an emission source for NH3 molar fraction and flux measurements, respectively. The simulation framework presented here can easily be adapted to local conditions, and paves the way for future ammonia research to integrate simulations at high spatio-temporal resolutions with observations of NH3 concentrations and fluxes.

Seasonal Aerosol Acidity, Liquid Water Content and Their Impact on Fine Urban Aerosol in SE Canada

This study explores the drivers of aerosol pH and their impact on the inorganic fraction and mass of aerosol in the S.E. Canadian urban environments of Hamilton and Toronto, Ontario. We find that inter-seasonal pH variability is mostly driven by temperature changes, which cause variations of up to one pH unit. Wintertime acidity is reduced, compared to summertime values. Because of this, the response of aerosol to precursors fundamentally changes between seasons, with a strong sensitivity of aerosol mass to levels of HNO3 in the wintertime. Liquid water content (LWC) fundamentally influences the aerosol sensitivity to NH3 and HNO3 levels. In the summertime, organic aerosol is mostly responsible for the LWC at Toronto, and ammonium sulfate for Hamilton; in the winter, LWC was mostly associated with ammonium nitrate at both sites. The combination of pH and LWC in the two sites also affects N dry deposition flux; NO3− fluxes were comparable between the two sites, but NH3 deposition flux at Toronto is almost twice what was seen in Hamilton; from November to March N deposition flux slows down leading to an accumulation of N as NO3− in the particle phase and an increase in PM2.5 levels. Given the higher aerosol pH in Toronto, aerosol masses at this site are more sensitive to the emission of HNO3 precursors compared to Hamilton. For both sites, NOx emissions should be better regulated to improve air quality during winter; this is specifically important for the Toronto site as it is thermodynamically more sensitive to the emissions of HNO3 precursors.

Agricultural ammonia dry deposition and total nitrogen deposition to a Swiss mire

Mires are among the ecosystems most affected by eutrophication caused by excessive nitrogen (N) inputs via N deposition of ammonium (NH4+), nitrate (NO3–), nitric acid (HNO3), nitrogen dioxide (NO2), and ammonia (NH3). As NH3 emissions are mostly generated by agriculture, a particular conflict of interest exists in areas where agricultural production systems are located nearby sensitive ecosystems, as often is the case in Switzerland. Therefore, this study aimed at quantifying and evaluating total N inputs to a mire in the Swiss Alpine foothills during 2007–2019, focusing on NH3 dry deposition. Three surface–vegetation–atmosphere transfer models were used to estimate dry deposition of NH3 based on micrometeorological data in combination with monthly passive sampler NH3 concentration measurements. NH3-N dry deposition summed up to 1.5–5.2 kg N ha–1 a–1, while total N loads were 11.5–14.2 kg N ha–1 a–1, well beyond the critical N loads for raised bogs (5–10 kg N ha–1 a–1), and also higher than the lower limits for oligotrophic fens and mountain hay meadows (10 kg N ha–1 a–1). Hence, the mire studied is most likely negatively affected by eutrophication, which could ultimately lead to biodiversity loss and ecosystem instability. This is particularly remarkable as the monthly NH3 concentrations at the study site (0.4–4.7 µg m–3) are comparatively low for Switzerland.

Ammonia fluxes over an agricultural field in growing and fallow periods using relaxed eddy accumulation

Asia has a considerable risk of nitrogen deposition effects on ecosystems due to the high deposition amounts. Ammonia (NH3) emissions, primarily from agricultural production, have increased in East Asia, and the increasing contribution of NH3 deposition to the total nitrogen deposition is concerning. However, NH3 deposition in this region is uncertain due to lacking flux measurements and the complex bidirectional nature of NH3 atmosphere−surface exchange. To better understand the NH3 deposition in this region, we investigated NH3 fluxes over an agricultural field in Tokyo, Japan. Fluxes were determined using the relaxed eddy accumulation method incorporating the denuder/filter-pack sampling technique and continuously conducting daytime and nighttime measurements in soybean-growing (July 28–August 5, 2020) and fallow (March 4–March 10, 2021) periods. NH3 fluxes were mostly downward in both periods, indicating that the agricultural field was a sink for NH3 during the periods (mean flux = −0.091 μg m−2 s−1). The deposition velocities in the growing period were lower than in the fallow period possibly due to the smaller turbulence. The deposition velocities correlated positively with friction velocity, indicating that the turbulence is the dominant factor driving NH3 deposition. We compared inferred fluxes using a compensation point model with the measured fluxes. The model did not reproduce the observation results when using default inputs, but showed a mean NH3 emission of 0.054 μg m−2 s−1. Revised inputs considering on-site soil and foliage information improved the model's results; however, the inferred fluxes were still smaller than the measured fluxes (mean flux = −0.017 μg m−2 s−1). These results indicate that there were possibly other processes enhancing NH3 deposition in the agricultural field.