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Wet Deposition, Reduction, and Bioaccumulation of Mercury in Freshwater Reservoirs in South Korea

2023-12-31T15:00:00Z

Title: Wet Deposition, Reduction, and Bioaccumulation of Mercury in Freshwater Reservoirs in South Korea Author(s): Jisook Yang Abstract: The biogeochemical processes within freshwater ecosystems involve a complex interplay of physical, chemical, and biological factors, influencing the movement and accumulation of mercury. Factors such as water flow rates, turbulence, evaporation, and precipitation patterns impact the distribution of mercury. Faster water flow can redistribute mercury particles, altering their concentrations. Chemical reactions can convert inorganic mercury into organic forms, making it accessible to organisms. Biological interactions contribute to bioaccumulation, with microorganisms and plants forming the foundation of the food web. Mercury accumulates along the food chain, posing potential risks to both freshwater ecosystems and human health. Understanding these processes is crucial for managing mercury pollution, conserving freshwater ecosystems, and preserving water quality. In chapter2, understanding the partitioning and associated factors of mercury in wet deposition is crucial for predicting the fate and transport of atmospheric mercury. In this study, we collected weekly rainfall samples in Gwangju, a suburban area in the southwest of South Korea, over a five-year period from 2017 to 2022. The volume-weighted mean concentration of total mercury (THg) in rainfall was 4.6 ng L-1, which was similar to or lower than levels in remote control areas or rural regions in China, and notably lower than urban concentrations. A significant negative correlation was observed between precipitation amount and THg (p < 0.0001), providing valuable insights into the effective removal processes of atmospheric THg during initial precipitation events. As known, Gaseous Elemental Mercury is primarily removed during wet precipitation. Therefore, theoretically, atmospheric THg concentrations should be higher during humid summer months. However, our research results showed that THg concentrations were highest during the winter season. Additionally, there were positive correlations between THg concentrations and atmospheric particulate matter, especially PM2.5, ammonium ion, and nitrogen dioxide. These findings underscore the importance of particulate-bound mercury in atmospheric THg removal, highlighting the close relationship between THg concentrations in precipitation and the formation of secondary fine particles, particularly during high-winter PM2.5 events. In the high PM2.5 winter and spring periods, THg concentrations were significantly correlated with lead, cadmium, and nickel, suggesting industrial emissions, metal smelting, and petroleum refining as potential sources. Gwangju, as a suburban area, is influenced by long-distance transport from external sources, impacting mercury's external input and long-distance transport. This study provides essential data to enhance understanding and management strategies regarding mercury's external influx and long-distance transport. In chapter 3, this study investigated the behavior and sources of atmospheric methylmercury (MeHg) through wet deposition in a suburban area of South Korea. Weekly samples of wet deposition were collected from January 2017 to December 2022 in Gwangju, and the volume-weighted mean (VWM) concentration of MeHg was determined to be 0.045 ng L-1. The annual wet deposition flux of MeHg was found to be 0.045 ng m-2, indicating a significant pathway for the transport of atmospheric pollutants into local ecosystems. Seasonal variations were observed, with the highest VWM concentrations of MeHg found in winter and the highest wet deposition fluxes in summer. The study identified a statistically significant positive correlation between PM2.5 and MeHg concentrations, with MeHg concentrations increasing as fine dust concentrations increased in spring and winter. The sources of MeHg in seasonal rainwater were investigated using cluster analysis of backward trajectory and concentration weighted trajectory analysis. Air masses inflow from the northwest were identified as a significant source of MeHg in spring rainwater, while photodemethylation of dimethylmercury and domestic fuel combustion emissions were found to contribute to MeHg in summer and winter, respectively. Additionally, the study found that high MeHg concentrations in spring rainwater were likely due to the migration of atmospheric PMeHg over long distances. To confirm in-situ methylation of mercury in rainwater, this study conducted methylation incubation on bulk rainwater samples collected for each season. The results showed a high methylation rate in summer, which was in contrast to the observed seasonal MeHg concentration. Moreover, the potential role of dissolved organic matter (DOM) as a methyl donor for mercury methylation in rainwater was tracked for understanding methylation in rainwater. This study discovered that in summer, a protein-like DOM of marine origin contributed to the methylation of mercury. Meanwhile, in winter, a humic-like DOM of terrestrial origin was found to contribute to the methylation process. These findings suggest that seasonal variations in the type of DOM present in rainwater may be responsible for the observed seasonal variations in mercury methylation rates. However, it's important to note that other factors such as temperature, pH, and microbial communities may also contribute to the methylation process. Further research is needed to better understand the mechanisms behind in-situ methylation of mercury in rainwater and its potential implications for environmental health. This study provides valuable insights into the behavior and sources of atmospheric MeHg in a suburban area of South Korea. The study emphasizes the potential risks of atmospheric pollutants to local ecosystems and human health, highlighting the importance of monitoring atmospheric mercury pollution. In chapter 4, dissolved gaseous mercury (DGM) formation in lake water primarily results from photochemical processes mediated by dissolved organic matter (DOM). In this research, we investigated the impact of various DOM components on the rate constant (kr) of Hg(II) photoreduction in lake surface water. We collected data on kr and the fluorescence properties of DOM from three Korean lakes exhibiting different trophic states. Through excitation-emission matrix fluorescence spectroscopy and parallel factor analysis, we identified three major fluorophores: plant-derived terrigenous humic-like DOM (C1), autochthonous DOM (C2), and soil fulvic-like DOM (C3). Principal component analysis (PCA) loading matrix revealed that kr increases when bulk DOM contains high flavin-like and soil-derived fulvic-like fractions. Pearson's correlation results corroborated the PCA analysis, showing a strong positive correlation between kr and the soil fulvic-like DOM component (r = 0.92) and the redox index (r = 0.92). These findings were further confirmed using a partial least squares-regression model that successfully predicted kr (r = 0.99) by incorporating multiple DOM components. Overall, our results indicate that kr can be modeled by considering the fluorescence intensities of diverse DOM components. This modeling approach has the potential to enhance Hg biogeochemical models, allowing for more accurate predictions of Hg redox rates in various lake systems. In chapter 5, in this study, we conducted a comprehensive analysis of total mercury (THg) and methylmercury (MeHg) concentrations, alongside various environmental factors, in two mesotrophic lakes and one oligotrophic lake. Our investigation focused on understanding the factors influencing mercury accumulation in freshwater ecosystems, particularly in fish. We examined these factors in water, particles, sediments, and biota (phytoplankton, zooplankton, and fish) to gain deeper insights into mercury dynamics within these environments. Our results showed significant variations in THg concentrations among the lakes, with higher levels in one of the mesotrophic lakes, likely due to pollution sources downstream. We identified positive correlations between MeHg and total phosphorus (TP) and sulfate ions in surface waters, indicating their influence on MeHg concentrations. Additionally, sediment-associated microorganisms and organic matter played a role in enhancing MeHg transformation and bioavailability. We also observed differences in mercury accumulation in phytoplankton, zooplankton, and fish across the lakes, highlighting the impact of trophic levels and dietary sources. This study sheds light on the complex interactions influencing mercury dynamics in freshwater ecosystems and provides valuable insights for environmental conservation and mercury management efforts. These findings can inform the development of strategies to control mercury pollution, monitor water quality, ensure fish safety, and protect aquatic ecosystems. Furthermore, they offer a basis for policymaking aimed at regulating and mitigating the impact of mercury contamination in freshwater environments.

Weather & Climate conditions affecting air quality over Korea

2020-12-31T15:00:00Z

Title: Weather & Climate conditions affecting air quality over Korea Author(s): Dasom Lee Abstract: Air quality directly affects human health and the ecosystem and the economically growing East Asia is one of the largest aerosol emitters. Emissions aside, weather condition leads to a favorable condition for high concentrations of aerosol occurrence and formation. The weather conditions of poor ventilation due to atmospheric stagnant, which is a potential factor that could worsen air quality. In Part1, we examined the long-term changes in the near-surface wind speed and its link with how the air in northeast Asia may or may not have become increasingly stagnant since 1958. To explain the long-term change, we used 59 years atmospheric data. We found a negative correlation between the near-surface wind speed and the static stability over the Yellow Sea and the western part of South Korea. Moreover, the near-surface wind speed has shown a declining trend with increased static stability, while their negative correlation appears to strengthen in recent years. Further investigations using historical global climate models single-forcing experiments suggested that long-term enhanced stabilization in the atmosphere due to global warming caused by anthropogenic greenhouse gases, implying future of atmospheric stagnant condition and its potential implication on regional air quality. In Part2, we investigated the relationship between synoptic weather patterns, which is relatively short-term weather change, and surface ozone concentration over South Korea. To determine a dominant synoptic weather type, we used Spatial Synoptic Classification (SSC) data. We found that dry moderate (DM), dry tropical (DT), and moist tropical (MT) are commonly associated with high ozone, especially DT type has a high efficiency of ozone production in high levels of concentration. The weather properties in DT type have shown the long-term increasing trend of 2m temperature and decreasing trend of relative humidity, implying long-term air quality change due to climate change. The efficiency of high ozone occurrence estimated that mean daily maximum eight-hour ozone concentrations (MDA8 O3) may increase by 3.5% (7.5%) with DT frequency increased by doubled (tripled). We concluded that synoptic weather and its long-term trends play important roles in recent increased surface ozone in South Korea. In Part3, we validate the relationship between synoptic weather patterns and regional air quality using PM10 concentration over South Korea. We found that DM and dry polar (DP) occurs frequently with high PM cases during winter. It is interesting to note that the DP type is also associated with the non-high PM range, because of its dominant pattern in the winter with prevailing northerly winds. On the other hand, DM and DT are found highly corelated with the high PM cases during spring. The circulation system characteristics with the high PM cases have shown a weak northwesterly wind field in the DM weather type during winter and strengthened static stability in the lower troposphere in the DM and DT weather types during spring. Furthermore, a major source of pollutants is originated from China with high values of mean PM10 concentrations during winter and spring seasons. We concluded that synoptic weather and its variation with lead time play important roles in regional high PM cases in South Korea. Finally, our results demonstrated that atmospheric stagnant due to weather and climate change are a likely connection to regional air quality over Korea.

WATER TRANSPORT MECHANISM IN STACKED-GRAPHENE NANOSHEETS: ROLE OF STRONG COHESIVE INTERACTION AMONG WATER MOLECULES

2018-12-31T15:00:00Z

Title: WATER TRANSPORT MECHANISM IN STACKED-GRAPHENE NANOSHEETS: ROLE OF STRONG COHESIVE INTERACTION AMONG WATER MOLECULES Author(s): Chang-Min Kim Abstract: Graphene, a carbon-based 2-dimensional substance, has been considered as an ideal membrane material for purifying water because of its atomic thickness and extraordinary mechanical strength. Based on this outlook, two concepts for graphene-based membranes (nanoporous graphene and stacked graphene membranes), have recently been suggested. To date, computational simulations and actual experiments have proven the excellent performance and potential for graphene-based membranes. However, due to several technological and economic challenges, it has been also shown that the practical application of graphene-based membranes is not straightforward. This review provides highlights of the current status of graphene-based membranes, and then identifies a number of key challenges that need to be resolved prior to the practical implementation of graphene into water treatment systems. Stacked-graphene nanosheets have attracted a great attention as new type of membrane due to its outstanding performance with distinct physicochemical characteristics. Many studies have indicated that size exclusion dominates mass transport in stacked-graphene membrane, but ultrafast water transport behavior has not been completely explained. In this work, diffusive and size-exclusive transport in graphene oxide (GO) membrane were confirmed by testing water and several solvents by hindered diffusion mechanism. More importantly, by observing water transport at varying thickness of GO membrane, we show that collective water transport resulting from the exceptionally high cohesive interaction amongst water molecules is essential for the ultrafast transport of water. The results indicate that, in addition to slip on the smooth graphitic surfaces, physicochemical properties of the permeant molecules that enable strong cohesion are significantly influential in governing the superior performance of stacked-graphene membranes. Nanostructured graphene oxide (GO) membranes offer outstanding mass transport performances such as ultrahigh water flux and precise molecular sieving and thus have a great potential as a novel filtration platform for energy efficient molecular or ionic separation. However, the structural stability of the GO membranes in aqueous environments remains a challenging problem, which limits its practical application perspective. We, here, report novel GO composite membrane composed of GO and asymmetrically functionalized Janus GO (JGO) sheets. Single-side functionalized JGO was made by amidation of dodecylamine. Incorporation of JGO in GO film exhibited extraordinary stability in water at broad pH values even under agitation. Moreover, JGO-doped GO composite membrane showed robust integrity in acid or base solutions over months. It exhibits high molecular retention above 95 % for uncharged and charged dye molecules, rhodamine B and brilliant blue G while maintaining water permeability comparable with previously reported GO-based membranes under osmotic pressure. This work significantly expand possibilities for new way to enhance structural stability in graphene-based or 2D-material membrane and surface-selective functionalization in membrane fabrication for many applications.

Warm Arctic-Cold Continents trajectory under global warming and impacts of aerosol-induced atmospheric circulation changes on sea ice

2024-12-31T15:00:00Z

Title: Warm Arctic-Cold Continents trajectory under global warming and impacts of aerosol-induced atmospheric circulation changes on sea ice Author(s): Yungi Hong Abstract: The Arctic region is experiencing unprecedented warming at nearly four times the global average rate, which drives significant changes in global atmospheric circulation patterns and extreme weather events, profoundly impacting human societies and ecosystems worldwide. This dissertation aims to investigates the complex interplay between Arctic warming, anthropogenic forcings, and their impacts on regional weather patterns, employing statistical techniques and climate model ensemble analysis. The research reveals three key findings. First, it is demonstrated that while the Warm Arctic-Cold Continents (WACC) pattern persists under 1.5°C and 2.0°C warming scenarios, increased variability in East Asian temperature responses complicates prediction accuracy. Second, CESM1 large ensemble simulations reveal that WACC frequency peaks around 2020 before declining—beginning around 2024 for East Asia and 2039 for North America—coinciding with a northward shift of Arctic cold air boundaries as global warming restricts polar air intrusion. Third, this study shows that anthropogenic aerosols paradoxically contribute to sea ice reduction in the western Chukchi Sea by modifying North Pacific circulation patterns, enhancing northward surface stress in the Bering Strait. This mechanism, acting in concert with greenhouse gas forcing, creates a dipole circulation pattern that accelerates sea ice loss beyond what would be expected from greenhouse gases alone. These findings advance our understanding of Arctic climate dynamics and their global teleconnections, providing insights into the evolution of the WACC pattern and unexpected influence of anthropogenic aerosols on Arctic-midlatitude teleconnection.