Effects of long-range transport on carboxylic acids, chlorinated VOCs, and oxidative potential in air pollution events

In the context of air quality research, the collection and analysis of fine particulate matter (PM2.5, with a diameter less than 2.5 μm) and volatile organic compound (VOCs) play a pivotal role in understanding and addressing environmental issues across the Korean Peninsula. PM2.5 and VOCs were collected over 4-hr intervals from October 17 to November 26, 2021 during the 2021 Satellite Integrated Joint Monitoring of Air Quality campaign at Olympic Park in the Republic of Korea to understand the factors controlling air quality over the Seoul Metropolitan Area. Source apportionment was performed using the positive matrix factorization (PMF) model incorporating PM2.5 and VOCs. The factor identified by chlorinated VOCs as a major component was presumed to be due to transboundary influx and was referred to as the long-range transport factor. The long-range transport factor of PM2.5 was composed of NO3-, SO42-, NH4+, and di-carboxylic acids. Back trajectory analysis showed that the airflows originated from China and passed through the west coast of Korea to the Korean Peninsula. In the PMF results using PM2.5 and VOCs, long-range transport factors were identified in both analyses, and the high correlation observed between these factors confirms that they were transported from abroad. The dithiothreitol oxidation potential normalized to quinine showed the highest oxidation potential during the same period as the long-range transport factors increased. In conclusion, PM2.5 from external sources significantly contribute to elevated levels of dithiothreitol assay-oxidative potential (DTT-OP) in Korea. The toxic concentration, expressed as the mean ± standard deviation, was determined to be 0.29 ± 0.05 μM/m³, peaking at 0.39 μM/m³. This level is 1.8 times higher than that observed outside the event period. A notable increase in secondary pollutants was observed during these periods. These pollutants are known to enhance oxidative potential, thereby potentially impacting human health.

What controls aerosol δ15N-NO3-? NOx emission sources vs. nitrogen isotope fractionation

Atmospheric δ¹⁵N-NO₃^- has been used to reveal NO_x (NO + NO₂) sources as NO₃^- is the ultimate sink of NO_x. However, it remains questionable whether the nitrogen isotope fractionation among NO_y (NO, NO₂, NO₃, N₂O₅, HNO₃ and NO₃^-) engender the misjudgment of NO_x emission sources by affecting δ¹⁵N-NO_y. To explore this issue, we integrated the dataset of aerosol δ¹⁵N-NO₃^- values and ratios of f_NO2 (f_NO2 = NO₂/(NO₂ + NO)), calculated the nitrogen isotope fractionation factors (Δ_s) among NO_y, compared the total energy consumption in Beijing-Tianjin-Hebei region (BTH) from 2013 to 2018. Results showed that, although the total energy consumption structure changed from 2013 to 2018 in BTH, there were fewer interannual variances of aerosol δ¹⁵N-NO₃^- values. Nitrogen isotope fractionation factors between NO and NO₂ (Δ₀), NO₂ and NO₃ (Δ₂), NO₂ and N₂O₅ (Δ₃), NO₂ and ClONO₂ (Δ₄) also displayed less interannual variations from 2013 to 2018 in BTH. But both aerosol δ¹⁵N-NO₃^- and Δ_s displayed significant seasonal patterns, and there was significant relationship between monthly aerosol δ¹⁵N-NO₃^- and Δ_s, which suggested that Δ_s have important influence on shaping aerosol δ¹⁵N-NO₃^- and further discriminating NO_x emission sources. This study implies that we should refine the Δ_s when employing atmospheric δ¹⁵N-NO₃^- to quantify NO_x source allocation.

Elucidating sources of atmospheric NOX pollution in a heavily urbanized environment using multiple stable isotopes

Atmospheric deposition of nitrogen (N) from rain and aerosols can be a significant non-point source - particularly in urbanized coastal areas and contribute to coastal eutrophication and hypoxia. Here, we present geochemical and isotopic data from surface waters coupled with an 18-month time series of geochemical and isotopic data measured on wet and dry deposition over Hong Kong from June 2018. Dual stable isotopes of nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) of rain and total suspended particulates (TSP) were analyzed to trace the sources and understand seasonal pattern of atmospheric nitrate. The δ¹⁵N of TSP, δ¹⁵N-NO₃ in rain and TSP ranged from +0.94 to +17.6‰, -4.1 to +3.0‰ and -1.3 to +9.0‰ respectively. δ¹⁵N varied seasonally with higher values in winter and lower values in summer. This variation can be explained by a change in the sources of atmospheric NO_x driven by the East Asian Monsoon. It was found that most NO_x comes from coal burning in winter and a mix of vehicle emissions, fossil fuel combustion and lightning in summer. Moreover, the estimated dry and wet deposition of nitrate and ammonium in Hong Kong is around 18 kg N ha^-1 annually, which is of the same order of magnitude as N released by sewage effluents and groundwater. This implies that atmospheric N deposition over the N-limited waters of the eastern side of Hong Kong could contribute significantly to the N budget. Therefore, atmospheric N deposition may alter the local N marine cycling, thus monitoring its impact is crucial for water quality in Southern China.

Unraveling the sources of atmospheric organic aerosols over the Arabian Sea: Insights from the stable carbon and nitrogen isotopic composition

The isotopic composition of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) in marine aerosols influenced by the continental outflows are useful proxies for understanding the aging and secondary formation processes. Every winter, the haze pollutants transported from South Asia significantly affect the chemical composition of marine atmospheric boundary layer of the Arabian Sea. Here, we assessed the δ¹³C of total carbon (TC) and δ¹⁵N of total nitrogen (TN) in marine aerosols collected over the Arabian Sea during a winter cruise (6-24 December 2018). TC (2.1-13.4 μg m^-3) is strongly correlated with TN (0.9-5.0 μg m^-3), likely because of their common source-emissions, biomass burning and fossil-fuel combustion in the Indo-Gangetic Plain and South Asia (corroborated by backward-air mass trajectories and satellite fire counts). Besides, the linear relationship between the mass ratios of water-soluble organic carbon (WSOC) to TC (0.04-0.65) and δ¹³C_TC (-25.1‰ to -22.9‰) underscores the importance of aging process. This means oxidation of organic aerosols during transport not only influences the WSOC levels but also affects their δ¹³C_TC. Likewise, the prevalent inverse linear relationship between the equivalent mass ratio of (NH₄⁺/non-sea-salt- or nss-SO₄^2-) and δ¹⁵N_TN (+15.3‰ to +25.1‰) emphasizes the overall significance of neutralization reactions between major acidic ([nss-SO₄^2-] ≫ [NO₃^-]) and alkaline species (NH₄⁺) in aerosols. Higher δ¹⁵N_TN values in winter than the spring inter-monsoon clearly emphasizes the significance of the anthropogenic combustion sources (i.e., biomass burning) in the South Asian outflow. A comparison of δ¹³C_TC and δ¹⁵N_TN with the source emissions revealed that crop-residue burning emissions followed by the coal fired power plants mostly dictate the atmospheric abundance of organic aerosols in the wider South Asian outflow.

Robust Evidence of 14C, 13C, and 15N Analyses Indicating Fossil Fuel Sources for Total Carbon and Ammonium in Fine Aerosols in Seoul Megacity

Carbon- and nitrogen-containing aerosols are ubiquitous in urban atmospheres and play important roles in air quality and climate change. We determined the ¹⁴C fraction modern (f_M) and δ¹³C of total carbon (TC) and δ¹⁵N of NH₄⁺ in the PM_2.5 collected in Seoul megacity during April 2018 to December 2019. The seasonal mean δ¹³C values were similar to -25.1‰ ± 2.0‰ in warm and -24.2‰ ± 0.82‰ in cold seasons. Mean δ¹⁵N values were higher in warm (16.4‰ ± 2.8‰) than in cold seasons (4.0‰ ± 6.1‰), highlighting the temperature effects on atmospheric NH₃ levels and phase-equilibrium isotopic exchange during the conversion of NH₃ to NH₄⁺. While 37% ± 10% of TC was apportioned to fossil-fuel sources on the basis of f_M values, δ¹⁵N indicated a higher contribution of emissions from vehicle exhausts and electricity generating units (power-plant NH₃ slip) to NH₃: 60% ± 26% in warm season and 66% ± 22% in cold season, based on a Bayesian isotope-mixing model. The collective evidence of multiple isotope analysis reasonably supports the major contribution of fossil-fuel-combustion sources to NH₄⁺, in conjunction with TC, and an increased contribution from vehicle emissions during the severe PM_2.5 pollution episodes. These findings demonstrate the efficacy of a multiple-isotope approach in providing better insight into the major sources of PM_2.5 in the urban atmosphere.

Pollution characterization and source identification of nitrogen-containing species in fine particulates: A case study in Hefei city, East China

To identify the nitrogen sources in atmospheric particulate matter, the stable isotope technique has been proven as an effective method. In this study, PM_2.5 samples at different pollution levels were collected from March 2018 to February 2019 in Hefei to analyze and compare the chemical composition. The results showed that the concentrations of PM_2.5, total nitrogen (TN) and nitrogenous species, as well as the total nitrogen isotopic composition (δ¹⁵N) increased with the aggravation of pollution. Ammonium nitrogen (NH₄⁺-N, 54%) was the dominant nitrogen-containing specie during the whole campaign, followed by nitrate nitrogen (NO₃^--N, 34%) and organic nitrogen (ON, 12%). The δ¹⁵N was positively correlated with NH₄⁺-N/TN but negatively correlated with NO₃^--N/TN. NH₄NO₃ and NH₄HSO₄ were the dominant forms of the secondary inorganic aerosols. In addition, a significant positive correlation was observed between the temperature and δ¹⁵N. Nitrogen source identification of PM_2.5 was conducted using Positive Matrix Factorization (PMF) model, δ¹⁵N values and Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. The results indicated that the contributions of the four main nitrogen sources were obtained and shown in descending order: combustion and industrial emission (42.06%) > secondary aerosols (24.04%) > vehicle exhaust (23.57%) > re-suspended dust (10.33%). The nitrogen aerosols might be mainly influenced by local emissions on normal and slight pollution days, while by both local emissions and transport from other areas on moderate and serious pollution days. Furthermore, nitrogen-containing species in PM_2.5 primarily originated from long/medium-distance transportation in two serious pollution events during the entire campaign.

Chemical characterization and stable nitrogen isotope composition of nitrogenous component of ambient aerosols from Kanpur in the Indo-Gangetic Plains

Measurements of water-soluble total nitrogen (WSTN), water-soluble inorganic nitrogen (WSIN), water-soluble organic nitrogen (WSON) and ẟ¹⁵N_TN (total N) was carried out on PM_2.5 aerosol samples during wintertime to understand the major sources of ambient nitrogenous species at a heavily polluted location of Kanpur in north India. During the nighttime sampling campaign, WSON and NH₄⁺_N contributed dominantly to the WSTN. Ammonium-rich condition persisted during sampling (NH₄⁺/SO₄^2- average equivalent mass ratio = 3.1 ± 0.7), suggesting complete neutralization of SO₄^2- and formation of NH₄NO₃, which is stable in winter due to low temperature and high relative humidity (RH). Stagnant atmospheric conditions during wintertime enhanced concentrations of ionic species (SO₄^2-, NH₄⁺, and NO₃^-) at this location. Good correlations between NO₃^-_N, NH₄⁺_N and biomass burning tracer K⁺_BB (and also between NO₃^-_N, NH₄⁺_N and SO₄^2-) suggests a strong impact of biomass burning activities. Multi-linear regression (MLR) analysis shows a strong dependence of ẟ¹⁵N on NO₃^-_N, SO₄^2- and WSON in night-1 (10:00 pm to 2:00 am) and on NO₃^-_N and SO₄^2- in night-2 (2:00 am to 6:00 am) depicting different formation and removal mechanism of aerosols during both the time-periods. ẟ¹⁵N_TN in PM_2.5 varied from +8.8 to +15.5‰ (10.8 ± 1.3), similar to the variability observed for many urban locations in India and elsewhere. NH₄⁺_N and WSON control the final ẟ¹⁵N value of nitrogenous aerosols. High relative humidity during nighttime enhanced the secondary organic aerosols formation due to aqueous-phase formation and gas to particle-phase partitioning. Isotopic fractionations associated with multi-phase reactions during gas to particle conversion of NH₃ would result in an increase in ẟ¹⁵N by ~48‰ to 51‰ (at T of 5.4 °C to 15.4 °C) than that of the emission source(s), which indicates the most likely N-emission sources at Kanpur to be from agriculture activities and waste generation.

Ion-exchange resin and denitrification pretreatment for determining δ15 N-NH4 + , δ15 N-NO3 - , and δ18 O-NO3 - values

RATIONALE

There has never been a highly sensitive method for simultaneously measuring the δ¹⁵ N and δ¹⁸ O values of nitrate ions (NO₃ ^- ) and the δ¹⁵ N values of ammonium ions (NH₄ ⁺ ) in particulate matter using denitrifying bacteria. In this study, we explored a method that combines use of an anion-exchange resin and denitrifying bacteria to make such measurements.

METHODS

The δ¹⁵ N-NH₄ ⁺ values of samples obtained using the hypobromite and denitrifying bacteria method were measured by isotope ratio mass spectrometry. Tests (effect of flow rate, breakthrough, and acid concentration) were conducted to verify the removal of NO₃ ^- using an AG1-X8 anion-exchange resin for NH₄ ⁺ measurements and the enrichment of NO₃ ^- . For aerosol samples, the optimized method was used to measure the δ¹⁵ N-NO₃ ^- , δ¹⁸ O-NO₃ ^- , and δ¹⁵ N-NH₄ ⁺ values of atmospheric particulate matter (PM_2.5 , aerodynamic diameter < 2.5 μm).

RESULTS

The δ¹⁵ N-NO₃ ^- and δ¹⁸ O-NO₃ ^- values measured following extraction with 1-6 mol/L HCl, at sample flow rates of 1-2 mL/min, with total anion amounts of less than 2.2 mmol, and in concentration tests were found to be in very close agreement with reagent values. The precisions and the accuracies of the δ¹⁵ N-NH₄ ⁺ and δ¹⁵ N-NO₃ ^- values were in all cases less than 1‰. In addition, the accuracies for the δ¹⁸ O-NO₃ ^- values were less than 1.4‰ and generally acceptable. The δ¹⁵ N-NH₄ ⁺ , δ¹⁵ N-NO₃ ^- , and δ¹⁸ O-NO₃ ^- values in six PM_2.5 samples were similar to those reported in previous studies.

CONCLUSIONS

Our proposed method for removing anions using AG1-X8 resin, for isotopic analysis using denitrifying bacteria, and for concentrating samples containing low concentrations of NO₃ ^- will make it possible to perform high-precision and accurate analyses easily and inexpensively. These methods are applicable not only to aerosols, but also to samples from diverse locations such as rivers, oceans, and Antarctica.

Isotopic source analysis of nitrogen-containing aerosol: A study of PM2.5 in Guiyang (SW, China)

The source of fine particulate matter (PM_2.5) has been a longstanding subject of debate, the nitrogen-15 isotope (δ¹⁵N) has been used to identify the major sources of atmospheric nitrogen. In this study, PM_2.5 samples (n = 361) were collected from September 2017 to August 2018 in the urban area of Guiyang (SW, China), to investigate the chemical composition and potential sources of PM_2.5. The results showed an average PM_2.5 of 33.0 μg m^-3 ± 20.0 μg m^-3. The concentration of PM_2.5 was higher in Winter, lower in Summer. The major water resolved inorganic ions (WSIIs) were Ca²⁺, NH₄⁺, Na⁺, SO₄^2-, NO₃^-, Cl^-. Nitrogen-containing aerosols (i.e., NO₃^- and NH₄⁺) suddenly strengthened during the winter, when NO₃^- became the dominant contributor. Over the sampling period, the molar ratio of NH₄⁺/(NO₃^- + 2 × SO₄^2-) ranged from 0.1 to 0.9, thus indicating the full fixation of NH₄⁺ by existing NO₃^- and SO₄^2- in PM_2.5. The annual value of NOR was 0.1 while rised to 0.5 in Winter. The variations of NOR (Nitrogen oxidation ratio) (0.1-0.5) values suggest that the secondary formation of NO₃^- occurred every season and was most influential during the winter. The total particulate nitrogen (TN) δ¹⁵N value of PM_2.5 ranged from -5.9‰ to 25.3‰ over the year with annual mean of +11.8‰ ± 4.7‰, whereas it was between -5.9‰ and 14.3‰ during the winter with mean of 7.0‰ ± 3.8‰. A Bayesian isotope mixing model (Stable Isotope Analysis in R; SIAR) was applied to analyze the nitrogen sources. The modeling results showed that 29%, 21%, and 40% of TN in PM_2.5 during the winter in Guiyang was due to nitrogen-emissions from coal combustion, vehicle exhausts, and biomass burning, respectively. Our results demonstrate that biomass burning was the main contributor to PM during the winter, 80% of the air mass comes from rural areas of Guizhou border, this transport process can increase the risk of particulate pollution in Guiyang.

Fossil-driven secondary inorganic PM2.5 enhancement in the North China Plain: Evidence from carbon and nitrogen isotopes

Measuring isotopic ratios in aerosol particles is a powerful tool for identifying major sources, particularly in separating fossil from non-fossil sources and investigating aerosol formation processes. We measured the radiocarbon, stable carbon, and stable nitrogen isotopic composition of PM_2.5 in Beijing (BJ) and Changdao (CD) in the North China Plain (NCP) from May to mid-June 2016. The mean PM_2.5 concentrations were 48.6 ± 28.2 μg m^-3 and 71.2 ± 29.0 μg m^-3 in BJ and CD, respectively, with a high contribution (∼66%) from secondary inorganic aerosol (SIA; NO₃^-, NH₄⁺, and SO₄^2-). The mean δ¹³C of total carbon (TC) and δ¹⁵N of total nitrogen (TN) values differed significantly between the two sites (p-value of <0.001): -25.1 ± 0.3‰ in BJ and -24.5 ± 0.4‰ in CD and 10.6 ± 1.8‰ in BJ and 5.0 ± 3.1‰ in CD, respectively. In BJ, the average δ¹⁵N (NH₄⁺) and δ¹⁵N (NO₃^-) values were 12.9 ± 2.3‰ and 5.2 ± 3.5‰, respectively. The ionic molar ratios and isotopic ratios suggest that NO₃^- in BJ was formed through the phase-equilibrium reaction of NH₄NO₃ under sufficient NH_{3 (g)} conditions, promoted by fossil-derived NH_{3 (g)} transported with southerly winds. In BJ, fossil fuel sources comprised 52 ± 7% of TC and 45 ± 28% of NH₄⁺ on average, estimated from radiocarbon (¹⁴C) analysis and the δ¹⁵N and isotope mixing model, respectively. These multiple-isotopic composition results emphasize that PM_2.5 enhancement is derived from fossil sources, in which vehicle emissions are a key contributor. The impact of the coal source was sporadically noticeable. Under regional influences, the fossil fuel-driven SIA led to the PM_2.5 enhancements. Our findings demonstrate that the multiple-isotope approach is highly advantageous to elucidate the key sources and limiting factors of secondary inorganic PM_2.5 aerosols.

Regional haze formation enhanced the atmospheric pollution levels in the Yangtze River Delta region, China: Implications for anthropogenic sources and secondary aerosol formation

High-time-resolution (3-hour) PM_2.5 samples were collected simultaneously from the rural and urban areas in the Yangtze River Delta region during winter. The aerosol samples were analyzed for carbonaceous components, organic tracers, water-soluble inorganic ions and stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic compositions of total carbon and total nitrogen. The values of PM_2.5 and secondary organic carbon (SOC) for both sampling sites were observed 2 times higher in haze events compare to those in clear days, implying severe pollution occurred by photochemical oxidation during haze periods. The PM mass of rural samples showed similar temporal trend and significant correlation with the urban PM, reflecting pollution sources or their formation process are most likely identical. Diurnal variations of PM_2.5 and carbonaceous components revealed that pollution levels increased at daytime due to the photochemical oxidation. In addition, SOC and OC were influenced by the relative humidity (RH%) and temperature (T °C), indicating that such meteorological factors play important roles in the occurrence of regional air pollution. The concentrations of levoglucosan, polycyclic aromatic hydrocarbons, hopanes, and n-alkanes were 625 ± 456 and 519 ± 301 ng m^-3, 32.6 ± 24.7 and 28.7 ± 20.1 ng m^-3, 1.83 ± 1.51 and 1.26 ± 1.34 ng m^-3, and 302 ± 206 and 169 ± 131 ng m^-3 for rural and urban samples, respectively. Levoglucosan is the most abundant organic compounds, exhibited 2-3 times higher in haze than clear days, suggesting biomass burning (BB) emission substantially affects the haze pollution in winter. Furthermore, NO₃^- was the dominant ionic species followed by SO₄^2-, NH₄⁺, Cl^- and other minor species for both sites. The δ¹³C and δ¹⁵N values demonstrate that anthropogenic activities such as fossil fuel combustion and BB are the major sources for carbonaceous and nitrogenous aerosols. This study implies that both the regional anthropogenic emissions and meteorological conditions influenced the regional haze formation, leading enhancement of pollution levels in eastern China during winter.

Fossil fuel-related emissions were the major source of NH3 pollution in urban cities of northern China in the autumn of 2017

As the most important gas-phase alkaline species, atmospheric ammonia (NH₃) contributes considerably to the formation and development of fine-mode particles (PM_2.5), which affect air quality and environmental health. Recent satellite-based observations suggest that the North China Plain is the largest agricultural NH₃ emission source in China. However, our isotopic approach shows that the surface NH₃ in the intraregional urban environment of Beijing-Tianjin-Shijiazhuang is contributed primarily by combustion-related processes (i.e., coal combustion, NH₃ slip, and vehicle exhaust). Specifically, the Batch fractionation model was used to describe the partitioning of gaseous NH₃ into particles and to trace the near-ground atmospheric NH₃ sources. With the development of haze pollution, the dynamics of δ¹⁵N-NH₄⁺ were generally consistent with the fractionation model. The simulated initial δ¹⁵N-NH₃ values ranged from -22.6‰ to -2.1‰, suggesting the dominance of combustion-related sources for urban NH₃. These emission sources contributed significantly (92% on hazy days and 67% on clean days) to the total ambient NH₃ in urban cities, as indicated by a Bayesian mixing model. Based on the Batch fractionation model, we concluded the following: 1) δ¹⁵N-NH₄⁺ can be used to model the evolution of fine-mode aerosols and 2) combustion-related sources dominate the near-ground atmospheric NH₃ in urban cities. These findings highlight the need for regulatory controls on gaseous NH₃ emissions transported from local and surrounding industrial sources.

Nitrogen isotope composition of ammonium in PM2.5 in the Xiamen, China: impact of non-agricultural ammonia

Since NH₃ is a significant precursor to ammonium in PM_2.5 and contributes significantly to atmospheric nitrogen deposition but largely remains unregulated in China, the insight into the source of NH₃ emissions by the isotopic investigation is important in controlling NH₃ emissions. In this study, atmospheric concentrations of NH₃ and water-soluble ion composition in PM_2.5 as well as nitrogen isotope ratios in NH₄⁺ (δ¹⁵N-NH₄⁺) in Xiamen, China, were measured. Results showed that average NH₃ concentration for the five sites in Xiamen was 7.9 μg m^-3 with distinct higher values in the warm season and lower values in the cold season, and PM_2.5 concentration for the two sites (urban and suburban) was 59.2 μg m^-3 with lowest values in summer. In the PM_2.5, NH₄⁺ concentrations were much lower than NH₃ and showed a stronger positive correlation with NO₃^- than that with SO₄^2- suggesting the formation of NH₄NO₃ and equilibrium between NH₃ and NH₄⁺. Although the concentrations of NH₃ at the urban site were significantly higher than those at the suburban site, no significant spatial difference in NH₄⁺ and δ¹⁵N-NH₄⁺ was obtained. The distinct heavier δ¹⁵N-NH₄⁺ values in summer than in other seasons correlated well with the equilibrium isotopic effects between NH₃ and NH₄⁺ which depend on temperature. The initial δ¹⁵N-NH₃ values were in the range of waste treatment (- 25.42‰) and fossil fuel combustion (- 2.5‰) after accounting for the isotope fractionation. The stable isotope mixing model showed that fossil fuel-related NH₃ emissions (fossil fuel combustion and NH₃ slip) contributed more than 70% to aerosol NH₄⁺. This finding suggested that the reduction of NH₃ emissions from urban transportation and coal combustion should be a priority in the abatement of PM_2.5 pollution in Xiamen.

Atmospheric Dry Deposition of Water-Soluble Nitrogen to the Subarctic Western North Pacific Ocean during Summer

To estimate dry deposition flux of atmospheric water-soluble nitrogen (N), including ammonium (NH4+), nitrate (NO3−), and water-soluble organic nitrogen (WSON), aerosol samples were collected over the subarctic western North Pacific Ocean in the summer of 2016 aboard the Korean icebreaker IBR/V Araon. During the cruise, concentrations of NH4+, NO3−, and WSON in bulk (fine + coarse) aerosols ranged from 0.768 to 25.3, 0.199 to 5.94, and 0.116 to 14.7 nmol m−3, respectively. Contributions of NH4+, NO3−, and WSON to total water-soluble N represented ~74%, ~17%, and ~9%, respectively. Water-soluble N concentrations showed a strong gradient from the East Asian continent to the subarctic western North Pacific Ocean, indicating that water-soluble N species were mainly derived from anthropogenic or terrestrial sources. During sea fog events, coarse mode NO3− was likely to be scavenged more efficiently by fog droplets than fine mode NO3−; besides, WSON was detected only in fine mode, suggesting that there may have been a significant influence of sea fog on WSON, such as the photochemical conversion of WSON into inorganic N. Mean dry deposition flux for water-soluble total N (6.3 ± 9.4 µmol m−2 d−1) over the subarctic western North Pacific Ocean was estimated to support a minimum carbon uptake of 42 ± 62 µmol C m−2d−1 by using the Redfield C/N ratio of 6.625.

Traffic-related dustfall and NOx, but not NH3, seriously affect nitrogen isotopic compositions in soil and plant tissues near the roadside

Ammonia (NH₃) emissions from traffic have received particular attention in recent years because of their important contributions to the growth of secondary aerosols and the negative effects on urban air quality. However, few studies have been performed on the impacts of traffic NH₃ emissions on adjacent soil and plants. Moreover, doubt remains over whether dry nitrogen (N) deposition still contributes a minor proportion of plant N nutrition compared with wet N deposition in urban road environments. This study investigated the δ¹⁵N values of road dustfall, soil, moss, camphor leaf and camphor bark samples collected along a distance gradient from the road, suggesting that samples collected near the road have significantly more positive δ¹⁵N values than those of remote sites. According to the SIAR model (Stable Isotope Analysis in R) applied to dustfall and moss samples from the roadside, it was found that NH₃ from traffic exhaust (8.8 ± 7.1%) contributed much less than traffic-derived NO₂ (52.2 ± 10.0%) and soil N (39.0 ± 13.8%) to dustfall bulk N; additionally, 68.6% and 31.4% of N in mosses near the roadside could be explained by dry N deposition (only 20.4 ± 12.5% for traffic-derived NH₃) and wet N deposition, respectively. A two-member mixing model was used to analyse the δ¹⁵N in continuously collected mature camphor leaf and camphor bark samples, which revealed a similarity of the δ¹⁵N values of plant-available deposited N to ¹⁵N-enriched traffic-derived NO_x-N. We concluded that a relatively high proportion of N inputs in urban road environments was contributed by traffic-related dustfall and NO_x rather than NH₃. These information provide useful insights into reducing the impacts of traffic exhaust on adjacent ecosystems and can assist policy makers in determining the reconstruction of a monitoring network for N deposition that reaches the road level.

Seasonal pattern of ammonium 15N natural abundance in precipitation at a rural forested site and implications for NH3 source partitioning

Excess ammonia (NH₃) emissions and deposition can have negative effects on air quality and terrestrial ecosystems. Identifying NH₃ sources is a critical step for effectively reducing NH₃ emissions, which are generally unregulated around the world. Stable nitrogen isotopes (δ¹⁵N) of ammonium (NH₄⁺) in precipitation have been directly used to partition NH₃ sources. However, nitrogen isotope fractionation during atmospheric processes from NH₃ sources to sinks has been previously overlooked. Here we measured δ¹⁵NNH₄⁺ in precipitation on a daily basis at a rural forested site in Northeast China over three years to examine its seasonal pattern and attempt to constrain the NH₃ sources. We found that the NH₄⁺ concentrations in precipitation ranged from 5 to 1265 μM, and NH₄⁺ accounted for 65% of the inorganic nitrogen deposition (20.0 kg N ha^-1 yr^-1) over the study period. The δ¹⁵N values of NH₄⁺ fluctuated from -24.6 to +16.2‰ (average -6.5‰) and showed a repeatable seasonal pattern with higher values in summer (average -2.3‰) than in winter (average -16.4‰), which could not be explained by only the seasonal changes in the NH₃ sources. Our results suggest that in addition to the NH₃ sources, isotope equilibrium fractionation contributed to the seasonal pattern of δ¹⁵NNH₄⁺ in precipitation, and thus, nitrogen isotope fractionation should be considered when partitioning NH₃ sources based on δ¹⁵NNH₄⁺ in precipitation.

Source signatures from combined isotopic analyses of PM2.5 carbonaceous and nitrogen aerosols at the peri-urban Taehwa Research Forest, South Korea in summer and fall

Isotopes are essential tools to apportion major sources of aerosols. We measured the radiocarbon, stable carbon, and stable nitrogen isotopic composition of PM_2.5 at Taehwa Research Forest (TRF) near Seoul Metropolitan Area (SMA) during August-October 2014. PM_2.5, TC, and TN concentrations were 19.4 ± 10.1 μg m^-3, 2.6 ± 0.8 μg C m^-3, and 1.4 ± 1.4 μg N m^-3, respectively. The δ¹³C of TC and the δ¹⁵N of TN were - 25.4 ± 0.7‰ and 14.6 ± 3.8‰, respectively. EC was dominated by fossil-fuel sources with F_ff (EC) of 78 ± 7%. In contrast, contemporary sources were dominant for TC with F_c (TC) of 76 ± 7%, revealing the significant contribution of contemporary sources to OC during the growing season. The isotopic signature carries more detailed information on sources depending on air mass trajectories. The urban influence was dominant under stagnant condition, which was in reasonable agreement with the estimated δ¹⁵N of NH₄⁺. The low δ¹⁵N (7.0 ± 0.2‰) with high TN concentration was apparent in air masses from Shandong province, indicating fossil fuel combustion as major emission source. In contrast, the high δ¹⁵N (16.1 ± 3.2‰) with enhanced TC/TN ratio reveals the impact of biomass burning in the air transported from the far eastern border region of China and Russia. Our findings highlight that the multi-isotopic composition is a useful tool to identify emission sources and to trace regional sources of carbonaceous and nitrogen aerosols.

Stable carbon and nitrogen isotopic composition of PM10 over Indo-Gangetic Plains (IGP), adjoining regions and Indo-Himalayan Range (IHR) during a winter 2014 campaign

For source identification, a field campaign involving simultaneous sampling of particulate matter (PM₁₀) was conducted at eight sampling sites in the Indian mainland during winter 2014. The sampling sites include Delhi (upper IGP), Lucknow (middle IGP), and Kolkata (lower IGP) in the Indo-Gangetic Plains (IGP); Mohal-Kullu and Darjeeling in the Indo-Himalayan Range (IHR). In addition, Ajmer, located upwind of the IGP in NW-India and Giridih and Bhubaneswar, in the downwind to the IGP has also been chosen. To characterize the sources of the ambient PM₁₀, stable isotope ratios of carbon (δ¹³C_TC) and nitrogen (δ¹⁵N_TN) for the total carbon (TC) and total nitrogen (TN) fractions have been considered. Ancillary chemical parameters, such as organic carbon (OC), elemental carbon (EC), and water-soluble ionic components (WSIC) mass concentrations are also presented in this paper. There was very small variation in the daily average δ¹³C_TC ratios (- 24.8 to - 25.9‰) among the sites. Comparison with end-member stable C isotopic signatures of major typical sources suggests that the PM₁₀ at the sites was mainly from fossil fuel and biofuel and biomass combustion. Daily average δ¹⁵N_TN ratios were not observed to vary much between sites either (8.3 to 11.0‰), and the low δ¹⁵N_TN levels also indicate substantial contributions from biofuel and biomass burning of primarily C3 andC4 plant matter. Graphical abstract Scatter plot of the average (± 1 standard deviation (SD)) δ¹³C_TC (‰) compared to δ¹⁵N_TN (‰) at the sampling sites.

Nitrogen isotope variations of ammonium across rain events: Implications for different scavenging between ammonia and particulate ammonium

Enhanced ammonia (NH₃) emissions and deposition caused negative effects on air quality and ecosystems. Precipitation is an efficient pathway to remove NH₃ and particulate ammonium (p-NH₄⁺) from the atmosphere into ecosystems. However, precipitation scavenging of p-NH₄⁺ in chemical transport models has often considered fine p-NH₄⁺, with inadequate constraints on NH₃ and coarse p-NH₄⁺. Based on distinct δ¹⁵N values between NH₃ and NH₄⁺ in PM_2.5 (particulate matters with aerodynamic diameters ≤ 2.5 μm) or TSP (total suspended particulates), this paper interpreted intra-event variations of precipitation NH₄⁺ concentrations and δ¹⁵N values (δ¹⁵N-NH₄⁺ values) at Guiyang (Xiao et al., 2015). Generally decreased NH₄⁺ concentrations across rain events reflected decreasing scavenging of NH₃ and p-NH₄⁺. Using a Bayesian isotope mixing model, we found that differing contributions between ¹⁵N-depleted NH₃ and ¹⁵N-enriched p-NH₄⁺ were responsible for the three-stage variations of intra-event δ¹⁵N-NH₄⁺ values. The decreases of δ¹⁵N-NH₄⁺ values across the first and third stages indicated more decreases in scavenging p-NH₄⁺ than NH₃, while the increases of δ¹⁵N-NH₄⁺ values across the second stage were resulted primarily from more increases in scavenging p-NH₄⁺ (particularly fine p-NH₄⁺) than NH₃. These results stressed influences of differing scavenging between NH₃ and p-NH₄⁺ on precipitation δ¹⁵N-NH₄⁺ values, which should be considered in modeling precipitation scavenging of atmospheric p-NH₄⁺.

Isotopic evidence for enhanced fossil fuel sources of aerosol ammonium in the urban atmosphere

The sources of aerosol ammonium (NH₄⁺) are of interest because of the potential of NH₄⁺ to impact the Earth's radiative balance, as well as human health and biological diversity. Isotopic source apportionment of aerosol NH₄⁺ is challenging in the urban atmosphere, which has excess ammonia (NH₃) and where nitrogen isotopic fractionation commonly occurs. Based on year-round isotopic measurements in urban Beijing, we show the source dependence of the isotopic abundance of aerosol NH₄⁺, with isotopically light (-33.8‰) and heavy (0 to +12.0‰) NH₄⁺ associated with strong northerly winds and sustained southerly winds, respectively. On an annual basis, 37-52% of the initial NH₃ concentrations in urban Beijing arises from fossil fuel emissions, which are episodically enhanced by air mass stagnation preceding the passage of cold fronts. These results provide strong evidence for the contribution of non-agricultural sources to NH₃ in urban regions and suggest that priority should be given to controlling these emissions for haze regulation. This study presents a carefully executed application of existing stable nitrogen isotope measurement and mass-balance techniques to a very important problem: understanding source contributions to atmospheric NH₃ in Beijing. This question is crucial to informing environmental policy on reducing particulate matter concentrations, which are some of the highest in the world. However, the isotopic source attribution results presented here still involve a number of uncertain assumptions and they are limited by the incomplete set of chemical and isotopic measurements of gas NH₃ and aerosol NH₄⁺. Further field work and lab experiments are required to adequately characterize endmember isotopic signatures and the subsequent isotopic fractionation process under different air pollution and meteorological conditions.

Characterizing isotopic compositions of TC-C, NO3--N, and NH4+-N in PM2.5 in South Korea: Impact of China's winter heating

The origin of PM_2.5 has long been the subject of debate and stable isotopic tools have been applied to decipher. In this study, weekly PM_2.5 samples were simultaneously collected at an urban (Seoul) and rural (Baengnyeong Island) site in Korea from January 2014 through February 2016. The seasonal variation of isotopic species showed significant seasonal differences with sinusoidal variation. The isotopic results implied that isotope species from Baengnyeong were mostly originated from coal combustion during China's winter heating seasons, whereas in summer, the isotopic patterns observed that were more likely to be from marine. In Seoul, coal combustion related isotopic patterns increased during China's winter heating period while vehicle related isotopic patterns were dominated whole seasons by default. Therefore, aerosol formation was originated from long-range transported coal combustion-related NO_x by vehicle-related NH₃ in Seoul. δN-NH₄⁺ in Seoul showed highly enriched ¹⁵N compositions in all seasons, indicating that NH₃ from vehicle emission is the important source of NH₄⁺ in PM_2.5 in Seoul. In addition, Baengnyeong should be consistently considered as a key region for observing the changes of isotopic features depend on the contribution of individual emissions to the atmospheric as a result of the reduction of coal consumption in China.

Fossil Fuel Combustion-Related Emissions Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from (15)N-Stable Isotope in Size-Resolved Aerosol Ammonium

The reduction of ammonia (NH3) emissions is urgently needed due to its role in aerosol nucleation and growth causing haze formation during its conversion into ammonium (NH4(+)). However, the relative contributions of individual NH3 sources are unclear, and debate remains over whether agricultural emissions dominate atmospheric NH3 in urban areas. Based on the chemical and isotopic measurements of size-resolved aerosols in urban Beijing, China, we find that the natural abundance of (15)N (expressed using δ(15)N values) of NH4(+) in fine particles varies with the development of haze episodes, ranging from -37.1‰ to -21.7‰ during clean/dusty days (relative humidity: ∼ 40%), to -13.1‰ to +5.8‰ during hazy days (relative humidity: 70-90%). After accounting for the isotope exchange between NH3 gas and aerosol NH4(+), the δ(15)N value of the initial NH3 during hazy days is found to be -14.5‰ to -1.6‰, which indicates fossil fuel-based emissions. These emissions contribute 90% of the total NH3 during hazy days in urban Beijing. This work demonstrates the analysis of δ(15)N values of aerosol NH4(+) to be a promising new tool for partitioning atmospheric NH3 sources, providing policy makers with insights into NH3 emissions and secondary aerosols for regulation in urban environments.