Multiple Sulfur Isotope Constraints on Sources and Formation Processes of Sulfate in Beijing PM2.5 Aerosol

Procedures from samples to sulfur isotopic data: A review

RATIONALE

Sulfur isotopes have been widely used to solve some key scientific questions, especially in the last two decades with advanced instruments and analytical schemes. Different sulfur speciation and multiple isotopes analyzed in laboratories worldwide and in situ microanalysis have also been reported in many articles. However, methods of sampling to measurements are multifarious, and occasionally some inaccuracies are present in published papers. Vague methods may mislead newcomers to the field, puzzle readers, or lead to incorrect data-based correlations.

METHODS

We have reviewed multiple methods on sulfur isotopic analyses from the perspectives of sampling, laboratory work, and instrumental analysis in order to help reduce operational inhomogeneity and ensure the fidelity of sulfur isotopic data. We do not deem our proposed solutions as the ultimate standard methods but as a lead-in to the overall introduction and summary of the current methods used.

RESULTS

It has been shown that external contamination and transformation of different sulfur species should be avoided during the sampling, pretreatment, storage, and chemical treatment processes. Conversion rates and sulfur isotopic fractionations during sulfur extraction, purification, and conversion processes must be verified by researchers using standard or known samples. The unification of absence of isotopic fractionation is needed during all steps, and long-term monitoring of standard samples is recommended.

CONCLUSION

This review compiles more details on different methods in sampling, laboratory operation, and measurement of sulfur isotopes, which is beneficial for researchers' better practice in laboratories. Microanalyses and molecular studies are the frontier techniques that compare the bulk sample with the elemental analysis/continuous flow-gas source stable isotope ratio mass spectrometry method, but the latter is widely used. The development of sulfur isotopic measurements will lead to the innovation in scientific issues with sulfur proxies.

Sulfur isotope-based source apportionment and control mechanisms of PM2.5 sulfate in Seoul, South Korea during winter and early spring (2017-2020)

High level of particulate matter (PM) concentrations are a major environmental concern in Seoul, South Korea, especially during winter and early spring. Sulfate is a major component of PM and induces severe environmental pollution, such as acid precipitation. Previous studies have used numerical models to constrain the relative contributions of domestic and trans-boundary sources to PM2.5 sulfate concentration in South Korea. Because of the scarce measurement result of δ34S for PM2.5 sulfate in South Korea, poorly defined δ34S value of domestic sulfur sources, and no application of sulfur isotope fractionation during sulfate formation in previous observation-based studies, source apportionment results conducted by model studies have not been corroborated from independent chemical observations. Here, we examined the δ34S of PM2.5 in Seoul and domestic sulfur sources, and considered the sulfur isotope fractionation for accurate source apportionment constraint. Accordingly, domestic and trans-boundary sulfur sources accounted for approximately (16-32) % and (68-84) % of the sulfate aerosols in Seoul, respectively, throughout the winter and early spring of 2017-2020. Air masses passing through north-eastern China had relatively low sulfate concentrations, enriched δ34S, and a low domestic source contribution. Those passing through south-eastern China had relatively a high sulfate concentrations, depleted δ34S, and high domestic source contribution. Furthermore, elevated PM2.5 sulfate concentrations (>10 μg m-3) were exclusively associated with a weak westerly wind speed of <3 m s-1. From December 2019 to March 2020, Seoul experienced relatively low levels of PM2.5 sulfate, which might be attributed to favorable weather conditions rather than the effects of COVID-19 containment measures. Our results demonstrate the potential use of δ34S for accurate source apportionment and for identifying the crucial role of regional air mass transport and meteorological conditions in PM2.5 sulfate concentration. Furthermore, the data provided can be essential for relevant studies and policy-making in East Asia.

Multiple Sulfur Isotopic Evidence for Sulfate Formation in Haze Pollution

The mechanism of sulfate formation during winter haze events in North China remains largely elusive. In this study, the multiple sulfur isotopic composition of sulfate in different grain-size aerosol fractions collected seasonally from sampling sites in rural, suburban, urban, industrial, and coastal areas of North China are used to constrain the mechanism of SO2 oxidation at different levels of air pollution. The Δ33S values of sulfate in aerosols show an obvious seasonal variation, except for those samples collected in the rural area. The positive Δ33S signatures (0‰ < Δ33S < 0.439‰) observed on clean days are mainly influenced by tropospheric SO2 oxidation and stratospheric SO2 photolysis. The negative Δ33S signatures (-0.236‰ < Δ33S < ∼0‰) observed during winter haze events (PM2.5 > 200 μg/m3) are mainly attributed to SO2 oxidation by H2O2 and transition metal ion catalysis (TMI) in the troposphere. These results reveal that both the H2O2 and TMI pathways play critical roles in sulfate formation during haze events in North China. Additionally, these new data provide evidence that the tropospheric oxidation of SO2 can produce significant negative Δ33S values in sulfate aerosols.

Health risk assessment of PM2.5 and PM2.5-bound trace elements in Pretoria, South Africa

Exposure to outdoor air pollutants poses a risk for both non-carcinogenic and carcinogenic respiratory disease outcomes. A standardized health risk assessment (US EPA) utilizes air quality data, body mass and breathing rates to determine potential risk. This health risk assessment study assesses the hazard quotient (HQ) for total PM_2.5 and trace elemental constituents (Br, Cl, K, Ni, S, Si, Ti and U) exposure in Pretoria, South Africa. The World Health Organization (WHO) air quality guideline (5 µg m^-3) and the yearly South African National Ambient Air Quality Standard (NAAQS) (20 µg m^-3) were the references dosages for total PM_2.5. A total of 350 days was sampled in Pretoria, South Africa. The mean total PM_2.5 concentration during the 34-month study period was 23.2 µg m^-3 (0.7-139 µg m^-3). The HQ for total PM_2.5 was 1.17, 3.47 and 3.78 for adults, children and infants. Non-carcinogenic risks for trace elements K, Cl, S and Si were above 1 for adults. Seasonally, Si was the highest during autumn for adults (1.9) and during spring for S (5.5). The HQ values for K and Cl were highest during winter. The exposure to Ni posed a risk for cancer throughout the year and for As during winters.

Sulfur-isotope anomalies recorded in Antarctic ice cores as a potential proxy for tracing past ozone layer depletion events

Changes in the cosmic-ray background of the Earth can impact the ozone layer. High-energy cosmic events [e.g. supernova (SN)] or rapid changes in the Earth's magnetic field [e.g. geomagnetic Excursion (GE)] can lead to a cascade of cosmic rays. Ensuing chemical reactions can then cause thinning/destruction of the ozone layer-leading to enhanced penetration of harmful ultraviolet (UV) radiation toward the Earth's surface. However, observational evidence for such UV "windows" is still lacking. Here, we conduct a pilot study and investigate this notion during two well-known events: the multiple SN event (≈10 kBP) and the Laschamp GE event (≈41 kBP). We hypothesize that ice-core-Δ³³S records-originally used as volcanic fingerprints-can reveal UV-induced background-tropospheric-photochemical imprints during such events. Indeed, we find nonvolcanic S-isotopic anomalies (Δ³³S ≠ 0‰) in background Antarctic ice-core sulfate during GE/SN periods, thereby confirming our hypothesis. This suggests that ice-core-Δ³³S records can serve as a proxy for past ozone-layer-depletion events.

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.

Anthropogenic Emission Sources of Sulfate Aerosols in Hangzhou, East China: Insights from Isotope Techniques with Consideration of Fractionation Effects between Gas-to-Particle Transformations

Sulfate (SO₄^2-) is a major species in atmospheric fine particles (PM_2.5), inducing haze formation and influencing Earth's climate. In this study, the δ³⁴S values in PM_2.5 sulfate (δ³⁴S-SO₄^2-) were measured in Hangzhou, east China, from 2015 September to 2016 October. The result showed that the δ³⁴S-SO₄^2- values varied from 1.6 to 6.4‰ with the higher values in the winter. The estimated fractionation factor (α³⁴S_g→p) from SO₂ to SO₄^2- averaged at 3.9 ± 1.6‰. The higher α³⁴S_g→p values in the winter were mainly attributed to the decrease of ambient temperature. We further compared the quantified source apportionments of sulfate by isotope techniques with and without the consideration of fractionation factors. The result revealed that the partitioned emission sources to sulfate with the consideration of the fractionation effects were more logical, highlighting that fractionation effects should be considered in partitioning emission sources to sulfate using sulfur isotope techniques. With considering the fractionation effects, coal burning was the dominant source to sulfate (85.5%), followed by traffic emissions (12.8%) and oil combustion (1.7%). However, the coal combustion for residential heating contributed only 0.9% to sulfate on an annual basis in this megacity.

A multi-isotopic evaluation of groundwater in a rapidly developing area and implications for water management in hyper-arid regions

Management of water resources in hyper-arid areas faces vital challenges in a global climate change context. Consequently, understanding the effects on groundwater sources can help mitigating the problem of water scarcity and the negative impact of human intervention on the environment. A case study area in the hyper-arid climate of the United Arab Emirates, was tackled here with the focus on applying stable isotopes as tools for evaluating groundwater sources and quality assessment. The results of major ions indicate variable increase in groundwater salinity moving away from Al Hajar Mountains recharge areas to the discharge areas (Arabian Gulf coast). The data of stable isotopes (δ¹⁸O_H2O, δ²H_H2O, δ¹⁸O_NO3, δ¹⁵N_NO3, δ¹⁸O_SO4, δ³⁴S_SO4, δ¹¹B) suggest impact of paleo-groundwater in the abstractions of the wells nearest to the coast. Nitrate isotopes indicate farming activities sources that can be masked due to the contribution from the nitrate-poor paleo-groundwater. Nitrate reduction processes are expected near to the recharge front. Sulphate and boron isotopes further suggest that influence of ancient evaporite dissolution in salinization. Management efforts should be focused on the diffuse sources of quality mitigations that can be vital in fingerprinting local and regional (transboundary) effects.

The isotopic patterns and source apportionment of nitrate and ammonium in atmospheric aerosol

Nitrate (NO₃^-) and ammonium (NH₄⁺) are the major components in inorganic aerosol. However, their sources and formation processes remain unclear. This study conducted a year-round field measurement of TSP, PM_2.5 and PM_1.0 in five different sites in the Beijing-Tianjin-Hebei (BTH) region to determine the concentrations of water-soluble inorganic ions (WSIIs) and the isotopic compositions of inorganic nitrogen (δ¹⁵N-NH₄⁺, δ¹⁵N-NO₃^-, and δ¹⁸O-NO₃^-). The results showed the highest concentration of WSIIs in winter and lowest in summer. δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, and δ¹⁵N-NH₄⁺ were in the range of -6.1-18.2, 52.2-103.8, and -28.7-36.2‰, respectively. The seasonal variations of δ¹⁵N-NO₃^- and δ¹⁵N-NH₄⁺ were an indication of relative contributions of the main sources and effects of meteorological conditions. The source apportionment identified fossil fuel combustion (38.2-50.6%), agricultural emissions (18-24.7%), biomass burning (16.3-22.7%), and road dust/soil (8.7-23.4%) were the main sources of inorganic aerosols. The local sources and regional migration contributed to the level of inorganic aerosol pollution. In winter, the aerosol in the BTH region was affected by the air mass from the northwest. While in spring and summer, the air mass was mainly from the South China. The low temperature and high relative humidity favored to the formation of inorganic nitrogen aerosol, and solar radiation affected the formation processes of inorganic aerosols by changing the oxidation pathway of NO₃^- and accelerating the volatilization and dissociation of ammonium nitrate (NH₄NO₃). This study discovered the main source contributions of inorganic nitrogen aerosol using N and O isotopes composition, and the obtained information has a great importance in understanding the effects of meteorological conditions on formation and the contribution of regional transport.

Multi isotope systematics of precipitation to trace the sources of air pollutants in Seoul, Korea

Multiple sulfur (δ³⁴S_sulfate, Δ³³S_sulfate, & Δ³⁶S_sulfate), nitrogen and oxygen (δ¹⁵N_nitrate & δ¹⁸O_nitrate) and strontium (⁸⁷Sr/⁸⁶Sr) isotope compositions of precipitation collected from Seoul, South Korea were analyzed to study the sources, transportation and deposition of air pollutants in East Asia. The δ³⁴S_sulfate values (from 1.9 to 14.6‰ with a median of 4.7‰) and the δ¹⁵N_nitrate values (from -2.0 to 13.3‰ with a median of 1.0‰) suggest that fossil fuel use (emission from coal combustion and vehicle exhaust) is a predominant source for sulfur and nitrogen, but other natural sources including biogenic contributions of DMS also add to their total budget. The seasonal variations are observed in δ³⁴S_sulfate and δ¹⁵N_nitrate values (both higher in winter season), which is likely to result from the increase of coal use for domestic heating in China. The δ¹⁸O_nitrate values also varied seasonally depending on the NO_x oxidation pathway, being about 20‰ higher in winter than in summer season. The Δ³³S_sulfate and Δ³⁶S_sulfate values are not anomalous, showing the absence of MIF signals in precipitation of Seoul. The ⁸⁷Sr/⁸⁶Sr ratio of the precipitation samples range from 0.70988 to 0.71487 with a median of 0.71073, indicating the influence of at least three end-member (silicate dust, carbonate dust and anthropogenic emission). Ionic ratios such as (K⁺+NH₄⁺)/(Ca²⁺+Mg²⁺) and Cl^-/Na⁺ suggest the overwhelming effect of anthropogenic input rather than carbonate dust on the end-member with lower ⁸⁷Sr/⁸⁶Sr ratios.

Atmospheric lead pollution in a typical megacity: Evidence from lead isotopes

Atmospheric lead (Pb) pollution has adverse health effects on humans, while the sources and atmospheric process of Pb are key scientific problems. In this study, the concentrations and isotopic composition of Pb in fine particulate matter (PM_2.5), coal and street dust samples collected from a typical megacity Beijing were analyzed to identify the sources of atmospheric Pb. Results showed that the Pb concentrations in PM_2.5 were high in winter (168.1 ± 32.0 ng/m³) and low in summer (27.7 ± 9.1 ng/m³), whereas Pb isotopic values presented opposite variation trends. The abnormally elevated Pb concentrations in winter were probably related to coal combustion, while declined Pb concentration in summer may be attributed to favorable meteorological parameters such as high temperature, high wind speed, and frequent rain events. Pb isotopic ratios indicated that anthropogenic sources (coal combustion and vehicle exhaust) and natural sources were the main contributors to Pb in PM_2.5. Combined with the binary model, the anthropogenic sources predominantly contributed to Pb in the Beijing atmosphere by approximately 85% annually, while the natural sources accounted for the rest of 15%. More specifically, the contribution of natural sources was about 9.4% in spring, 29.7% in summer, 16.0% in autumn and 6.1% in winter, suggesting that natural sources might contribute more lead into the atmosphere during clear days. Furthermore, the contribution of the vehicle exhaust to atmospheric Pb was nonnegligible in megacity, highlighting that the ownership of motor vehicles in megacity should be regulated and more efforts should be paid to strengthen vehicle emission standard. This study may enrich the reservoir of Pb isotopic composition in nature and provides a new method to investigate the Pb migration and transformation in the environment, and also serve as a theoretical reference for pollution control measures.

Simultaneous Compound-Specific Analysis of δ33S and δ34S in Organic Compounds by GC-MC-ICPMS Using Medium- and Low-Mass-Resolution Modes

Compound-specific isotope analysis of sulfur (δ³⁴S-CSIA) in organic compounds was established in the last decade employing gas chromatography connected to multiple-collector inductively coupled plasma mass spectrometry (GC-MC-ICPMS). However, δ³³S-CSIA has not yet been reported so far. In this study, we present a method for the simultaneous determination of δ³³S and δ³⁴S in organic compounds by GC-MC-ICPMS applying medium- and also low-mass-resolution modes. The method was validated using the international isotope reference materials IAEA-S-1, IAEA-S-2, and IAEA-S-3. Overall analytical uncertainty including normalization and reproducibility for δ³³S and δ³⁴S was usually better than ±0.2 mUr (σ) for analytes containing at least 100 pmol of S. Further, it is demonstrated that, despite small isobaric interferences, results obtained at low mass resolution are indistinguishable from medium mass resolution offering the benefit of increased sensitivity and versatility of this method. Additionally, the method was applied for the δ³³S and δ³⁴S isotope analysis of industrially produced organic compounds to investigate potential mass-independent fractionation (MIF). The relation between δ³⁴S and δ³³S in these compounds followed a mass-dependent fractionation trend (MDF; Δ³³S ≤ ±0.2 mUr). Degradation of dimethyl disulfide by direct photolysis caused a small but significant MIF (Δ³³S = 0.55 ± 0.04 mUr, n = 3), demonstrating sufficient sensitivity of the method for these types of studies.

Interactions of particulate matter and pulmonary surfactant: Implications for human health

Particulate matter (PM), which is the primary contributor to air pollution, has become a pervasive global health threat. When PM enters into a respiratory tract, the first body tissues to be directly exposed are the cells of respiratory tissues and pulmonary surfactant. Pulmonary surfactant is a pivotal component to modulate surface tension of alveoli during respiration. Many studies have proved that PM would interact with pulmonary surfactant to affect the alveolar activity, and meanwhile, pulmonary surfactant would be adsorbed to the surface of PM to change the toxic effect of PM. This review focuses on recent studies of the interactions between micro/nanoparticles (synthesized and environmental particles) and pulmonary surfactant (natural surfactant and its models), as well as the health effects caused by PM through a few significant aspects, such as surface properties of PM, including size, surface charge, hydrophobicity, shape, chemical nature, etc. Moreover, in vitro and in vivo studies have shown that PM leads to oxidative stress, inflammatory response, fibrosis, and cancerization in living bodies. By providing a comprehensive picture of PM-surfactant interaction, this review will benefit both researchers for further studies and policy-makers for setting up more appropriate regulations to reduce the adverse effects of PM on public health.

Nrf2 mitigates prolonged PM2.5 exposure-triggered liver inflammation by positively regulating SIKE activity: Protection by Juglanin

Air pollution containing particulate matter (PM) less than 2.5 μm (PM2.5) plays an essential role in regulating hepatic disease. However, its molecular mechanism is not yet clear, lacking effective therapeutic strategies. In this study, we attempted to investigate the effects and mechanisms of PM2.5 exposure on hepatic injury by the in vitro and in vivo experiments. At first, we found that PM2.5 incubation led to a significant reduction of nuclear factor erythroid-derived 2-related factor 2 (Nrf2), along with markedly reduced expression of different anti-oxidants. Notably, suppressor of IKKε (SIKE), known as a negative regulator of the interferon pathway, was decreased in PM2.5-incubated cells, accompanied with increased activation of TANK-binding kinase 1 (TBK1) and nuclear factor-κB (NF-κB). The in vitro studies showed that Nrf2 positively regulated SIKE expression under the conditions with or without PM2.5. After PM2.5 treatment, Nrf2 knockdown further accelerated SIEK decrease and TBK1/NF-κB activation, and opposite results were observed in cells with Nrf2 over-expression. Subsequently, the gene loss- and gain-function analysis demonstrated that SIKE deficiency further aggravated inflammation and TBK1/NF-κB activation caused by PM2.5, which could be abrogated by SIKE over-expression. Importantly, SIKE-alleviated inflammation was mainly dependent on TBK1 activation. The in vivo studies confirmed that SIKE- and Nrf2-knockout mice showed significantly accelerated hepatic injury after long-term PM2.5 exposure through reducing inflammatory response and oxidative stress. Juglanin (Jug), mainly isolated from Polygonum aviculare, exhibits anti-inflammatory and anti-oxidant effects. We found that Jug could increase Nrf2 activation, and then up-regulated SIKE in cells and liver tissues, mitigating PM2.5-induced liver injury. Together, all these data demonstrated that Nrf2 might positively meditate SIKE to inhibit inflammatory and oxidative damage, ameliorating PM2.5-induced liver injury. Jug could be considered as an effective therapeutic strategy against this disease by improving Nrf2/SIKE signaling pathway.

Seasonal and long-term trends of sulfate, nitrate, and ammonium in PM2.5 in Beijing: implication for air pollution control

Particulate matter pollution in Beijing is a serious environmental problem. In response to this, the Beijing government has implemented comprehensive emission reduction measures in recent years. To assess the effectiveness of these measures, the seasonal and long-term trends in chemical compositions of PM_2.5 in Beijing have been studied based on daily samples collected from July 2015 to April 2016 and literature investigation (2000-2014). Results show that the concentrations of sulfate, nitrate, and ammonium in PM_2.5 have significant seasonal variations, which are related to the changes in meteorological conditions and emission intensities. In addition, the long-term data display that the concentrations of sulfate, nitrate, and ammonium have significantly decreased between 2013 and 2016, which are consistent with the reduction in PM_2.5 levels (~ 11.2 μg/m³ per year). The declines could not be interpreted by the meteorological factors. It suggests that the air pollution control measures in Beijing (2013-2016), especially the decreasing consumption of coal, can effectively decrease the mass concentration of fine particles. To further improve the air quality, similar measures should be adopted in the areas around Beijing. These air pollution control measures taken in Beijing can provide invaluable guidance for mega-cities in China and other developing countries to decrease their PM_2.5 concentration and reduce health risk from particulate pollution.

Method development for on-line species-specific sulfur isotopic analysis by means of capillary electrophoresis/multicollector ICP-mass spectrometry

In this work, a method for species-specific isotopic analysis of sulfur via capillary electrophoresis hyphenated on-line with multicollector ICP-MS (CE/MC-ICP-MS) was developed. Correction for the mass bias caused by instrumental mass discrimination was realized via external correction with multiple-injection sample-standard bracketing. By comparing the isotope ratio measurement results obtained using the newly developed on-line CE/MC-ICP-MS method with those obtained via traditional MC-ICP-MS measurement after analyte/matrix separation by anion exchange chromatography for isotopic reference materials and an in-house bracketing standard, the most suitable data evaluation method could be identified. The repeatability for the sulfate-δ³⁴S value (calculated from 18 measurements of a standard conducted over seven measurement sessions) was 0.57‰ (2SD) and thereby only twice that obtained with off-line measurements (0.30‰, n = 68). As a proof of concept for analysis of samples with a real matrix, the determination of the sulfur isotopic composition of naturally present sulfate was performed for different river systems. The CE/MC-ICP-MS results thus obtained agreed with the corresponding off-line MC-ICP-MS results within the 2SD ranges, and the repeatability of consecutive δ³⁴S measurements (n = 3) was between 0.3‰ and 1.3‰ (2SD). Finally, the isotopic analysis of two different S-species in a river water sample spiked with 2-pyridinesulfonic acid (PSA) was also accomplished. Graphical abstract The CE/MC-ICP-MS method developed allows for species-specific S isotopic analysis in samples containing multiple species. Mass bias is corrected for by multiple-injection sample-standard bracketing, while the repeatability (2SD) of the resulting ³⁴δ-values is <1‰.

Bioaccessibility of metals/metalloids in willow catkins collected in urban parks of Beijing and their health risks to human beings

Air pollution and its resulting health risks in Beijing City have been widely investigated by scientists and administrators. However, the health risks caused by willow and poplar catkins in April and May (known as "spring snow") have been rarely reported. Poplar and willow are the two common trees in Beijing City that generate many whirling catkins in the air. The chemical composition of catkins remains unknown. In this study, catkins and dust samples were collected in several parks in Beijing. The total concentrations of metals/metalloids in catkins measured through inductively coupled plasma mass spectrometry were generally lower than those of the corresponding dust samples, and they were lower than the risk control standard for soil contamination of development land. The simulated rain and lung fluid extraction rates of catkin samples were significantly higher than those of the dust samples. The concentration of extracted Pb and Zn using simulated rainwater exceeded the environmental quality standards for surface water (0.1 and 2.0 mg/L for Pb and Zn, respectively), indicating the possibility of runoff pollution. Scanning electron microscopy images showed that fine particles (<10 μm) are attached to the surface of catkins. Therefore, the metals/metalloids in fine particles adsorbed by the catkin samples possess higher bioaccessibility than that in the dust samples based on different sizes of particles. A significant correlation is found between Pb in catkin and Pb in dust. Therefore, attention should be paid to the possible increase in metal/metalloid concentrations in catkins planted in contaminated areas.

Review of stable mercury isotopes in ecology and biogeochemistry

Due to the advent of cold vapor-multicollector-inductively coupled plasma mass spectrometry (CV-MC-ICP-MS) in the past two decades, many research groups studying mercury (Hg) biogeochemistry have integrated stable Hg isotopes into their research. Currently, >200 studies using this technique have been published and this has greatly enhanced our understanding of the Hg biogeochemical cycle beyond what Hg concentration and speciation analyses alone can provide. These studies are largely divided into two groups: (i) controlled experiments investigating fractionation of Hg isotopes and refining tools of isotopic analyses, and (ii) studies of natural variations of Hg isotopes. It is now known that Hg isotopes undergo both mass dependent fractionation (MDF; reported as the ratio of mass ²⁰²Hg to ¹⁹⁸Hg) and mass independent fractionation (MIF), with MIF occurring at odd masses (¹⁹⁹Hg, ²⁰¹Hg) to a larger magnitude and at even masses (²⁰⁰Hg, ²⁰⁴Hg) to a much smaller magnitude. The two types of MIF are controlled by different photochemical processes. The range of isotopic variations of MDF, odd-MIF, and even-MIF are now well documented in a diverse set of environmental samples, and researchers are continuing to explore how the field of Hg isotope biogeochemistry can be further developed and taken to the next level of understanding. One application that has received considerable attention is the use of Hg isotopes to examine the environmental controls on the production and degradation of methylmercury (MeHg), the most toxic and bioaccumulative form of Hg. Since MeHg is efficiently assimilated and biomagnified along food chains, MeHg has the potential to be a robust ecological tracer. In this review, we give an updated overview of the field of Hg isotopes and focus on how Hg isotopes of MeHg can be used to address fundamental ecological questions, including energy transfer across ecosystem interfaces and as a tracer for animal movements.

Stable Sulfur Isotopes Revealed a Major Role of Transition-Metal Ion-Catalyzed SO2 Oxidation in Haze Episodes

Secondary sulfate aerosols played an important role in aerosol formation and aging processes, especially during haze episodes in China. Secondary sulfate was formed via atmospheric oxidation of SO₂ by OH, O₃, H₂O₂, and transition-metal-catalyzed (TMI) O₂. However, the relative importance of these oxidants in haze episodes was strongly debated. Here, we use stable sulfur isotopes (δ³⁴S) of sulfate aerosols and a Rayleigh distillation model to quantify the contributions of each oxidant during a haze episode in Nanjing, a megacity in China. The observed δ³⁴S values of sulfate aerosols showed a negative correlation with sulfur oxidation ratios, which was attributed to the sulfur isotopic fractionations during the sulfate formation processes. Using the average fractionation factor calculated from our observations and zero-dimensional (0-D) atmospheric chemistry modeling estimations, we suggest that OH oxidation was trivial during the haze episode, while the TMI pathway contributed 49 ± 10% of the total sulfate production and O₃/H₂O₂ oxidations accounted for the rest. Our results displayed good agreement with several atmospheric chemistry models that carry aqueous and heterogeneous TMI oxidation pathways, suggesting the role of the TMI pathway was significant during haze episodes.

Contamination patterns in river water from rural Beijing: A hydrochemical and multiple stable isotope study

The pollution of urban river has reached a critical level. In the present study, the hydrochemical composition of local surface water collected during two seasons from the rural area around urban Beijing, China, was examined. Concentrations of selected cations and anions reveal the temporal and spatial distribution of anthropogenic and natural pollution. Multiple stable isotopes (δ¹⁵N_nitrate, δ¹⁸O_nitrate, δ³⁴S_sulfate, δ¹⁸O_sulfate, Δ³³S_sulfate) analyses were applied for detecting of specific hydrochemical processes and identifying the major sources of contamination. Twenty-eight percent of the river water samples from the wet season and 34% from the dry season exhibit a minor water quality of class IV (poor) and below due to high pH values as well as high nitrate, sulfate and fluoride concentrations. This water is hazardous for human health and the aquatic ecology. The seemingly better river water quality in the wet season is caused by higher precipitation and, hence, dilution of the water constituents. Multiple isotopes identify urban wastewater as the prime pollution source. The higher fraction of deteriorated river water in the southern rural area (42%) compared to the north (24%) points to a higher contribution of urban effluents to the downstream rivers in the rural south. It can, thus, be concluded that less discharge of wastewater to the local surface and subsurface environments by better controls of public and private sewage disposal combined with the renewal of leaking sewer pipes would reduce the risk of anthropogenic contamination of river water.

Five-S-isotope evidence of two distinct mass-independent sulfur isotope effects and implications for the modern and Archean atmospheres

Anomalous sulfur isotopic compositions preserved in sedimentary rocks older than ∼2.5 billion years have been widely interpreted as the products of UV photolysis of sulfur dioxide in an anoxic atmosphere and used to track the history of primitive Earth and evolution of early life. In this study, we present strong observational evidence that there is an additional process that produces similar anomalous sulfur isotope signatures. This previously unknown origin not only offers a tool for quantifying the present-day atmospheric sulfur budget and evaluating its influences on climate and public health but also implies that anomalous sulfur isotopic compositions in some of the oldest rocks on Earth might have been produced in a way different from that previously thought.

The signature of mass-independent fractionation of quadruple sulfur stable isotopes (S-MIF) in Archean rocks, ice cores, and Martian meteorites provides a unique probe of the oxygen and sulfur cycles in the terrestrial and Martian paleoatmospheres. Its mechanistic origin, however, contains some uncertainties. Even for the modern atmosphere, the primary mechanism responsible for the S-MIF observed in nearly all tropospheric sulfates has not been identified. Here we present high-sensitivity measurements of a fifth sulfur isotope, stratospherically produced radiosulfur, along with all four stable sulfur isotopes in the same sulfate aerosols and a suite of chemical species to define sources and mechanisms on a field observational basis. The five-sulfur-isotope and multiple chemical species analysis approach provides strong evidence that S-MIF signatures in tropospheric sulfates are concomitantly affected by two distinct processes: an altitude-dependent positive ³³S anomaly, likely linked to stratospheric SO₂ photolysis, and a negative ³⁶S anomaly mainly associated with combustion. Our quadruple stable sulfur isotopic measurements in varying coal samples (formed in the Carboniferous, Permian, and Triassic periods) and in SO₂ emitted from combustion display normal ³³S and ³⁶S, indicating that the observed negative ³⁶S anomalies originate from a previously unknown S-MIF mechanism during combustion (likely recombination reactions) instead of coal itself. The basic chemical physics of S-MIF in both photolytic and thermal reactions and their interplay, which were not explored together in the past, may be another ingredient for providing deeper understanding of the evolution of Earth’s atmosphere and life’s origin.

Atmospheric sulfur isotopic anomalies recorded at Mt. Everest across the Anthropocene

Increased anthropogenic-induced aerosol concentrations over the Himalayas and Tibetan Plateau have affected regional climate, accelerated snow/glacier melting, and influenced water supply and quality in Asia. Although sulfate is a predominant chemical component in aerosols and the hydrosphere, the contributions from different sources remain contentious. Here, we report multiple sulfur isotope composition of sedimentary sulfates from a remote freshwater alpine lake near Mount Everest to reconstruct a two-century record of the atmospheric sulfur cycle. The sulfur isotopic anomaly is utilized as a probe for sulfur source apportionment and chemical transformation history. The nineteenth-century record displays a distinct sulfur isotopic signature compared with the twentieth-century record when sulfate concentrations increased. Along with other elemental measurements, the isotopic proxy suggests that the increased trend of sulfate is mainly attributed to enhancements of dust-associated sulfate aerosols and climate-induced weathering/erosion, which overprinted sulfur isotopic anomalies originating from other sources (e.g., sulfates produced in the stratosphere by photolytic oxidation processes and/or emitted from combustion) as observed in most modern tropospheric aerosols. The changes in sulfur cycling reported in this study have implications for better quantification of radiative forcing and snow/glacier melting at this climatically sensitive region and potentially other temperate glacial hydrological systems. Additionally, the unique Δ³³S-δ³⁴S pattern in the nineteenth century, a period with extensive global biomass burning, is similar to the Paleoarchean (3.6-3.2 Ga) barite record, potentially providing a deeper insight into sulfur photochemical/thermal reactions and possible volcanic influences on the Earth's earliest sulfur cycle.