Formation Mechanisms and Source Apportionments of Airborne Nitrate Aerosols at a Himalayan-Tibetan Plateau Site: Insights from Nitrogen and Oxygen Isotopic Compositions

Impact of sea ice on the physicochemical characteristics of marine aerosols in the Arctic Ocean

Marine aerosols (MA) can be influenced by sea ice concentration, potentially playing a pivotal role in the formation of cloud condensation nuclei and exerting an impact on regional climate. In this study, a high-resolution aerosol observation system was employed to measure the concentration and size of aerosols in the floating ice region and seawater region of the Arctic Ocean during the 8th and 9th Chinese Arctic Expedition Research Cruise. The identification of aerosol sources was conducted using a modified positive definite matrix factorization method and a backward air mass trajectory model. Two types of MA including the sea-salt aerosol (SSA) and the marine biogenic aerosol (BA) were identified and their concentrations were calculated. Then the physical-chemical characteristics of MA in the floating ice region and seawater region were compared under normalized conditions (-2.5 °C < T < -0.1 °C; 5.80 m/s < WS < 10.95 m/s) to discern the impact of sea ice. A unimodal distribution was observed for MA number concentration with a dominant peak ranging from 0.5 μm to 1.0 μm in size range. The findings revealed that the presence of sea ice cover led to a significant reduction of 52.2 % in the number concentration of SSA, while exerting minimal influence on its composition. BA number concentration in the floating ice region was 33.3 % higher than that in the seawater region. Strong winds (wind speed >6.5 m/s) transported organic matter and nutrients entrapped in sea ice into the atmosphere, leading to an increase in BA concentration. However, the presence of sea ice cover hampered the exchange of biogenic gases between the ocean and air, resulting in a reduction of secondary BA formation. Our study elucidates the correlation between MA release and sea ice coverage in the Arctic Ocean, thereby establishing a theoretical foundation for climate prediction models.

Nitrogen isotope characteristics and importance of NOx from biomass burning in China

Biomass burning is a primary source of atmospheric nitrogen oxide (NOx), however, the lack of isotopic fingerprints from biomass burning limits their use in tracing atmospheric nitrate (NO3-) and NOx. A total of 25 biomass fuels from 10 provinces and regions in China were collected, and the δ15N values of biomass fuels (δ15N-biomass) and δ15N-NOx values of biomass burning (δ15N-NOx values of BB, open burning, and rural cooking stove burning) were determined. The δ15N-NOx values of open burning and rural cooking stove burning ranged from -0.8 ‰ to 11.6 ‰ and 0.8 ‰ to 9.5 ‰, respectively, indicating a significant linear relation with δ15N-biomass. Based on the measured δ15N-NOx values of BB and biomass burning emission inventory data, the δ15N-NOx values of BB in different provinces and regions of China were calculated using the δ15N-NOx model, with a mean value of 5.0 ± 1.8 ‰. The spatial variations in the estimated δ15N-NOx values of BB in China were mainly controlled by the differences in the δ15N-NOx values and the proportions of NOx emissions from various straw burning activities in provinces and regions of China. Furthermore, by using the combined local emissions of biomass burning with regional transportations of NOx based on air-mass backward trajectories, we established an improved δ15N-NOx model and obtained more accurate δ15N-NOx values of BB in regions (2.3 ‰ to 8.4 ‰). By utilising the reported δ15N-NOx values of precipitation and particulate matter from 21 cities in China and the more accurate δ15N-NOx values of BB, the NOx contributions from four sources (mobile sources, coal combustion, biomass burning, and microbial N cycle) at the national scale were estimated using a Bayesian model. The significant contributions of biomass burning (20.9 % to 44.3 %) to NOx emissions were revealed, which is vital for controlling NOx emissions in China.

Insights into anthropogenic impact on atmospheric inorganic aerosols in the largest city of the Tibetan Plateau through multidimensional isotope analysis

Particulate inorganic nitrogen aerosols (PIN) significantly influence air pollution and pose health risks worldwide. Despite extensive observations on ammonium (pNH4+) and nitrate (pNO3-) aerosols in various regions, their key sources and mechanisms in the Tibetan Plateau remain poorly understood. To bridge this gap, this study conducted a sampling campaign in Lhasa, the Tibetan Plateau's largest city, with a focus on analyzing the multiple isotopic signatures (δ15N, ∆17O). These isotopes were integrated into a Bayesian mixing model to quantify the source contributions and oxidation pathways for pNH4+ and pNO3-. Our results showed that traffic was the largest contributor to pNH4+ (31.8 %), followed by livestock (25.4 %), waste (21.8 %), and fertilizer (21.0 %), underscoring the impact of vehicular emissions on urban NH3 levels in Lhasa. For pNO3-, coal combustion emerged as the largest contributor (27.3 %), succeeded by biomass burning (26.3 %), traffic emission (25.3 %), and soil emission (21.1 %). In addition, the ∆17O-based model indicated a dominant role of NO2 + OH (52.9 %) in pNO3- production in Lhasa, which was similar to previous observations. However, it should be noted that the NO3 + volatile organic component (VOC) contributed up to 18.5 % to pNO3- production, which was four times higher than the Tibetan Plateau's background regions. Taken together, the multidimensional isotope analysis performed in this study elucidates the pronounced influence of anthropogenic activities on PIN in the atmospheric environment of Lhasa.

Nitrogen and oxygen isotope characteristics, formation mechanism, and source apportionment of nitrate aerosols in Wuhan, Central China

Understanding the sources and formation mechanisms of nitrate in PM2.5 is important for effective and precise prevention and control of particulate matter pollution. In this study, we detected stable nitrogen and oxygen isotope signatures of NO- 3 (expressed as δ15N-NO- 3 and δ18O-NO3-) in PM2.5 samples in Wuhan, the largest city in central China. The sources and formation pathways of NO3- were quantitatively analyzed using the modified version of the Bayesian isotope mixing (MixSIR) model, and the regional transport characteristics of NO3- were analyzed using the hybrid single-particle Lagrangian integrated trajectory (HYSPLIT) model and concentration-weighted trajectory (CWT) method. The results showed that NO3- significantly contributed to the ambient PM2.5 pollution and its driving effect increased with the gradient of pollution level. The average δ15N-NO3- and δ18O-NO3- values were 4.7 ± 0.9 ‰ and 79.7 ± 2.9 ‰, respectively. δ15N-NO3- and δ18O-NO3- were more enriched in winter and increased dramatically in heavily polluted days. The reaction pathway of NO2 + OH dominated nitrate formation in summer, while the reaction pathway of N2O5+ H2O dominated in other seasons and contributed more in polluted days than clean days. The contributions of vehicle emission, coal combustion, biomass burning, biogenic soil emission, and ship emission sources to NO3- were 26.4 %, 23.4 %, 22.8 %, 15.3 %, and 12.1 %, respectively. In addition to local emissions, air mass transport from the northern China had a significant impact on particulate NO3- in Wuhan. Overall, we should pay special attention to vehicle and ship emissions and winter coal combustion emissions in future policymaking.

Isotopic Constraints on Sources and Transformations of Nitrate in the Mount Everest Proglacial Water

Glacier melting exports a large amount of nitrate to downstream aquatic ecosystems. Glacial lakes and glacier-fed rivers in proglacial environments serve as primary recipients and distributors of glacier-derived nitrate (NO3-), yet little is known regarding the sources and cycling of nitrate in these water bodies. To address this knowledge gap, we conducted a comprehensive analysis of nitrate isotopes (δ15NNO3, δ18ONO3, and Δ17ONO3) in waters from the glacial lake and river of the Rongbuk Glacier-fed Basin (RGB) in the mountain Everest region. The concentrations of NO3- were low (0.43 ± 0.10 mg/L), similar to or even lower than those observed in glacial lakes and glacier-fed rivers in other high mountain regions, suggesting minimal anthropogenic influence. The NO3- concentration decreases upon entering the glacial lake due to sedimentation, and it increases gradually from upstream to downstream in the river as a soil source is introduced. The analysis of Δ17ONO3 revealed a substantial contribution of unprocessed atmospheric nitrate, ranging from 34.29 to 56.43%. Denitrification and nitrification processes were found to be insignificant in the proglacial water of RGB. Our study highlights the critical role of glacial lakes in capturing and redistributing glacier-derived NO3- and emphasizes the need for further investigations on NO3- transformation in the fast-changing proglacial environment over the Tibetan Plateau and other high mountain regions.

Increasing Nonfossil Fuel Contributions to Atmospheric Nitrate in Urban China from Observation to Prediction

China's nitrogen oxide (NOx) emissions have undergone significant changes over the past few decades. However, nonfossil fuel NOx emissions are not yet well constrained in urban environments, resulting in a substantial underestimation of their importance relative to the known fossil fuel NOx emissions. We developed an approach using machine learning that is accurate enough to generate a long time series of the nitrogen isotopic composition (δ15N) of atmospheric nitrate using high-level accuracies of air pollutants and meteorology data. Air temperature was found to be the critical driver of the variation of nitrate δ15N at daily resolution based on this approach, while significant reductions of aerosol and its precursor emissions played a key role in the change of nitrate δ15N on the yearly scale. Predictions from this model found a significant decrease in nitrate δ15N in Chinese megacities (Beijing and Guangzhou as representative cities in the north and south, respectively) since 2013, implying an enhanced contribution of nonfossil fuel NOx emissions to nitrate aerosols (up to 22%-26% in 2021 from 18%-22% in 2013 quantified by an isotope mixing model), as confirmed by the Weather Research and Forecasting model coupled with online chemistry (WRF-Chem) simulation. Meanwhile, the declining contribution in coal combustion (34%-39% in 2013 to 31%-34% in 2021) and increasing contribution of natural gas combustion (11%-14% in 2013 to 14%-17% in 2021) demonstrated the transformation of China's energy structure from coal to natural gas. This approach provides missing records for exploring long-term variability in the nitrogen isotope system and may contribute to the study of the global reactive nitrogen biogeochemical cycle.

Wildfire-Derived Nitrogen Aerosols Threaten the Fragile Ecosystem in Himalayas and Tibetan Plateau

Himalayas and Tibetan Plateau (HTP) is important for global biodiversity and regional sustainable development. While numerous studies have revealed that the ecosystem in this unique and pristine region is changing, their exact causes are still poorly understood. Here, we present a year-round (23 March 2017 to 19 March 2018) ground- and satellite-based atmospheric observation at the Qomolangma monitoring station (QOMS, 4276 m a.s.l.). Based on a comprehensive chemical and stable isotope (¹⁵N) analysis of nitrogen compounds and satellite observations, we provide unequivocal evidence that wildfire emissions in South Asia can come across the Himalayas and threaten the HTP's ecosystem. Such wildfire episodes, mostly occurring in spring (March-April), not only substantially enhanced the aerosol nitrogen concentration but also altered its composition (i.e., rendering it more bioavailable). We estimated a nitrogen deposition flux at QOMS of ∼10 kg N ha^-1 yr^-1, which is approximately twice the lower value of the critical load range reported for the Alpine ecosystem. Such adverse impact is particularly concerning, given the anticipated increase of wildfire activities in the future under climate change.

Enhanced aerosols over the southeastern Tibetan Plateau induced by open biomass burning in spring 2020

The Tibetan Plateau is the third pole of the world, with an essential role in regulating Northern Hemisphere climate. Previous studies showed that atmospheric aerosols over the Tibetan Plateau are influenced by biomass burning (BB) products from South and Southeast Asia. In fact, open biomass burning (OBB) is also an important form of BB in Southeast Asian countries, causing serious springtime air pollution yearly. However, there are still scientific gaps in the contribution of OBB to surrounding regional aerosols, especially on the Tibetan Plateau. In order to quantify this contribution, we collected samples of fine particulate matter and derived the concentrations of major water soluble ion, water soluble organic carbon (WSOC), and total carbon (TC) and total nitrogen (TN) as well as the dual isotopic compositions of carbon and nitrogen (δ¹³C and δ¹⁵N) during March-June on the southeastern Tibetan Plateau. δ¹³C and δ¹⁵N showed no significant difference (p > 0.05) between the OBB and non-OBB periods. Furthermore, both δ¹³C and δ¹⁵N (-25.7 ± 0.7 ‰ and 8.0 ± 3.6 ‰) values calculated during the whole sampling period were similar to the BB value, indicating that the primary source of TC and TN in aerosols was BB, whether OBB or non-OBB burning periods. TC and TN concentrations during the OBB period (6.5 ± 2.9 μg m^-3 and 1.2 ± 0.4 μg m^-3, respectively) were significantly higher than during the non-OBB period (4.1 ± 1.7 μg m^-3, with p = 0.014, and 0.7 ± 0.3 μg m^-3, with p = 0.013, respectively). Active fire data and surface smoke concentrations further indicated that BB emissions from Southeast Asia were higher during the OBB period. This suggests that OBB-related high BB emissions significantly enhanced atmospheric aerosols concentrations on the southeastern Tibetan Plateau.

Source oxygen contributions of primary nitrate emitted from biomass burning

Atmospheric nitrate (NO3-) produced by photochemical oxidation in the atmosphere has high oxygen isotope ratios (δ18O values). Recently, the primary NO3- emitted from combustion sources was found to have much lower δ18O values. However, it is unclear how and to what extents the low δ18O signatures were controlled by major O sources during the primary NO3- formation of combustion processes. Here, we first measured concentrations and δ18O values of NO3- from burning five biomass materials (bb-NO3- and δ18Obb-NO3-, respectively) in China. Distinctly higher concentration levels of the bb-NO3- emissions (42.1 ± 8.1 μmol m-3) than ambient NO3- suggest it is a potential source of atmospheric NO3- pollution. Much lower δ18Obb-NO3- signatures (27.6 ± 2.7 ‰) than ambient NO3- support it as a primary emission source with different O sources and formation mechanism from secondary NO3-. Isotope mass-balance modeling revealed that atmospheric O2 and the biomass O dominated the O of bb-NO3- (53 ± 7 % and 40 ± 4 %, respectively) over the aqueous vapor (7 ± 3 %). Besides, we found increasing δ18Obb-NO3- values with the biomass N contents and relatively lower δ18Obb-NO3- values for biomasses with higher carbon (C) and lower O contents, indicating that biomass C, N, and O contents may influence the source O contributions of the bb-NO3-. This work provides a novel isotope analysis on the O source contribution of the bb-NO3-, which is useful for understanding the formation mechanism of combustion-related NO3- sources and evaluating the primary NO3- emissions.

Quantifying the source and formation of nitrate in PM2.5 using dual isotopes combined with Bayesian mixing model: A case study in an inland city of southeast China

Atmospheric nitrate has been attracting increasing attention because it is one of the important components of PM_2.5. Understanding the sources and formation mechanism of nitrate in PM_2.5 is essential to take effective measures to prevent and control the emission of nitrogen oxides in the atmosphere and reduce the formation of haze. In this study, PM_2.5 samples were collected from Ganzhou, an inland city in southeast China, during summer and winter. The concentrations of PM_2.5 and NO₃^- were determined, and the isotopic compositions of NO₃^- (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) were analyzed in order to quantify the relative contributions of different emission sources and formation pathways of nitrate in PM_2.5. The results showed that PM_2.5 and NO₃^- concentrations were lower in summer (39.80 ± 11.10 μg·m^-3 and 1.79 ± 0.70 μg·m^-3) while higher in winter (69.85 ± 29.58 μg·m^-3 and 10.83 ± 9.89 μg·m^-3). The values of δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- ranged from 42.84‰ to 56.80‰ and from -11.17‰ to -2.08‰ in summer, while from 55.86‰ to 78.66‰ and from -10.63‰ to -1.88‰ in winter, respectively. The results of δ¹⁵N-NO₃^- combined with Bayesian isotope mixing model showed that the relative contributions of vehicle exhaust, soil microbial activity, biomass combustion and coal fired power plants were 59.3%, 28.5%, 8.7% and 3.4% in summer, while 65.1%, 20.1%, 10.6% and 4.1% in winter, respectively. The results of δ¹⁸O-NO₃^- combined with Bayesian isotope mixing model showed that the possible relative contributions of pathway 1 (P1) (NO₂ + ·OH), pathway 2 (P2) (NO₃ + HC) and pathway 3 (P3) (N₂O₅ + H₂O) were 73.5%, 12.4% and 14.1% in summer, while 41.6%, 28.9% and 29.5% in winter, respectively. Moreover, P2 and P3 contributed more when NO₃^- concentrations were higher, suggesting that P2 and P3 were of significance to the formation of nitrate in PM_2.5, especially during winter.

Moisture movement, soil salt migration, and nitrogen transformation under different irrigation conditions: Field experimental research

Irrigation and fertilizer application can lead to significant changes in groundwater quality. In this study, a field irrigation experiment was carried out from April 9 to 23, 2021 under irrigation and fertigation conditions to understand the mechanisms of moisture movement, soil salt migration, and nitrogen transformation in the soil profile. Continuous in-situ monitoring and sampling of soil and irrigation water, as well as stable isotopes, chemical parameters, and soluble salt analyses, were performed in this research. The results showed that the time cost by the irrigation water in the vadose zone was about 5 h. The infiltrated irrigation water was accompanied by high concentrations of soluble salts, leached from the soil layers of 20-80 cm and 100-150 cm, which is associated with the leaching of Na⁺, Cl^-, SO₄^2-, and Ca²⁺ and the dissolution of minerals such as gypsum and halite. Furthermore, the variations in nitrogen concentrations (NH₄⁺ and NO₃^-) in the soil profile suggested that fertilizer application was the main source of NO₃^- in the soil and groundwater, while irrigation was the biggest driving force for nitrogen transport and transformation in soil. The application of urea fertilizer can increase the content of ammonium nitrogen at the soil layer of 0-80 cm. This nitrogen form can be subsequently transformed to nitrate nitrogen during the water transport to the groundwater. The current study provides a strong scientific basis for the protection and management of groundwater and soil quality in agricultural areas.