Upward Diffusion Research Articles

Examining the role of density-driven transport on chlorinated vapor intrusion

This study investigates the influence of density-driven transport on chlorinated vapor intrusion through modeling and experiments. Density-driven transport involves downward vapor advection, potentially reducing vapor intrusion into buildings. A 1-D steady-state numerical model was developed using COMSOL Multiphysics, considering upward diffusion and downward density-driven advection in the subsoil and in the granular fill layer beneath building's foundations. Source vapor concentration and granular fill layer permeability emerged as the key factors affecting density-driven transport. Regardless of building characteristics, for permeabilities in the granular fill layer exceeding 10−7 m2, density-driven transport is expected to become relevant at vapor concentrations of 1 mg m−3, while for lower soil permeabilities (10−8-10−10 m2), density-driven transport impact is expected for vapor concentrations exceeding 1 g m−3. The results of laboratory column trichloroethylene (TCE) diffusion tests through sand and gravel supported these findings, showing a vertical stratification of TCE vapor concentrations consistent with the model. The trends expected by modeling also align with the findings of different field studies, where the source to building attenuation factors (AF) were found to decrease with increasing source vapor concentration. These outcomes highlight that the common approach adopted for vapor intrusion screening from soil gas data based on default AF values independent of source vapor concentration, may potentially lead to an overestimation of indoor concentrations in the presence of high vapor concentrations and highly permeable soils. Given the use of permeable granular fill layers in building construction, this study underscores the importance of accounting for density-driven transport to improve the accuracy of vapor intrusion risk assessment.

Enhanced Buried Interface Engineering for Efficient Inverted Perovskite Solar Cells Fabricated via Vapor-Solid Reaction.

Vapor-deposited inverted perovskite solar cells utilizing self-assembled monolayer (SAM) as hole transport material have gained significant attention for their high efficiencies and compatibility with silicon/perovskite monolithic tandem devices. However, as a small molecule, the SAM layer suffers low thermal tolerance in comparison with other metal oxide or polymers, rendering poor efficiency in solar device with high-temperature (> 160°C) fabricating procedures. In this study, a dual modification approach involving AlOx and F-doped phenyltrimethylammonium bromide (F-PTABr) layers is introduced to enhance the buried interface. The AlOx dielectric layer improves the interface contact and prevents the upward diffusion of SAM molecules during the vapor-solid reaction at 170°C, while the F-PTABr layer regulates crystal growth and reduces the interfacial defects. As a result, the AlOx/F-PTABr-treated perovskite film exhibits a homogeneous, pinhole-free morphology with improved crystal quality compared to the control films. This leads to a champion power conversion efficiency of 21.53% for the inverted perovskite solar cells. Moreover, the encapsulated devices maintained 90% of the initial efficiency after 600h of ageing at 85°C in air, demonstrating promising potential for silicon/perovskite tandem application.

The underestimated role of manganese in modulating the nutrient structure in a eutrophic seasonally-stratified reservoir

Accumulation and subsequent release of nutrients have great potential to trigger algal blooms in lakes and reservoirs. We conducted high vertical resolution (2 m interval) monitoring at ∼monthly intervals over a year for hydrological parameters, Chl-a, ammonium (NH4+), nitrate (NO3−) and different species of phosphorus (P) and manganese (Mn) in a 40-meter-deep subtropical reservoir (Shanmei Reservoir) in Fujian, southern China. In this seasonally stratified reservoir featured with high nutrient loading, the consistent trend in the ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) between the euphotic zone and the hypolimnion, coupled with its mirrored correlation with Chl-a concentration indicates that upward flux from the hypolimnion affects phytoplankton growth in the euphotic zone. The monthly variation of the depth-integrated multiple species of N and P indicates that during the stratification period in the hypoxic hypolimnion, approximately 80% of the DIP is removed, leading to a remarkable decoupling phenomenon between NH4+ and DIP. This process effectively increases the ratio of DIN to DIP in the hypolimnion, thereby significantly reducing the potential of algal blooms caused by the upward flux. A robust positive linear correlation between iron-manganese bound phosphorus (CBD-P) and particulate Mn was observed during stratification period implying that DIP was scavenged by sediment-released Mn throughout the water column. Vertical profiles during stratification showed that upward diffusion of Mn2+ facilitated the formation of Mn oxide zones near the oxycline. The most significant decrease in DIP inventory occurs when Mn oxide zones migrate either upwards from the bottom or downwards from the oxycline, indicating that the migration of Mn oxides on the vertical profile is a key factor in the decoupling of NH4+and DIP.Our findings underscore the importance of Mn cycling as an underappreciated DIP self-immobilization process in the water column of reservoirs characterized by high nutrient loading. Furthermore, we propose that denitrification and Mn cycling establish a consecutive feedback mechanism, preventing excessive nutrient accumulation in low oxygen bottom water. In the context of global changes, we anticipate a heightened prominence of this feedback mechanism.

Estimation of vertical transport timescales of sediment in the Changjiang Estuary and its adjacent regions

The concept of resuspension age was employed to assess the timescales of vertical sediment transport in the Changjiang Estuary (CJE) and its adjoining areas. Resuspension age refers to the duration elapsed since particles last made contact with the seabed. Energetic tidal current in the study area results in a significant amount of sediment being eroded into the water during spring tides, causing the resuspension age to consistently remain lower during spring tides compared to neap tides. Spatially, consistently low resuspension ages occur in the North Branch and turbidity maximum because suspended sediment in these areas primarily originates from local erosion. While advective sediment dominates in the HZB, leading to highest resuspension age there. During the wet season, the South Branch experiences an average resuspension age surpassing 10 days, primarily due to particles predominantly transported from the river. Sediments deposited on the seabed during the wet season become the primary source of suspended sediment after being eroded into the water column during the dry season, leading to a reduction in resuspension age. Likewise, the decrease in resuspension age in the Subei Coast during the dry season is closely associated with the appearance of sediment mass advected by the northern branch plume. When stratification occurs, the upward diffusion of sediment is impeded, constraining newly eroded sediment close to the bed. As a result, these sediments exhibit lower ages than those in the upper water column, where sediment transport is primarily dominated by horizontal advection.

Feasibility of compacted attapulgite/diatomite amended clayey soils as gas barrier materials

Compacted clay liners are extensively used as barriers to control the upward diffusion of vapors of volatile or semi-volatile organic contaminants released from unsaturated contaminated soils at industry-contaminated sites. This study aimed to investigate the gas diffusion barrier performance of compacted clayey soils amended with three agents including attapulgite and diatomite individually, and attapulgite/diatomite mixture. The properties including water retention, volumetric shrinkage, gas diffusion, and unconfined compressive strength were evaluated through a series of laboratory tests of amended compacted clayey soils. The results demonstrate that the decrease in volume proportions of inter-aggregate pores leads to an increase in unconfined compressive strength (qu). Both hydrophilic groups and microstructures of attapulgite and diatomite result in an increase in water retention percent (Wt) of compacted clayey soil specimens after amendment regardless of the type of agent or initial water content (w0). Furthermore, the ratio of the gas diffusion coefficient (De) to the gas diffusion coefficient in the air (Da) was significantly reduced owing to a decrease in volume proportions of inter-aggregate pores, hydrophilic group, and microstructures of attapulgite and diatomite. Scanning electron microscope analyses revealed that rod-shaped attapulgite filled the inter-aggregate pores formed by clay particles, whereas the disc-shaped diatomite particles, characterized by micropores, failed to obstruct the inter-aggregate pores due to their larger particle size. Mercury intrusion porosimetry (MIP) analyses showed a reduction in pore volume in the inter-aggregate pores, leading to a reduction in the total pore volume for both the attapulgite and attapulgite/diatomite mixture amended clays, which is in accordance with the scanning electron microscope (SEM) results. The findings are pertinent to the practical application of compacted clay liners as gas barriers against the upward migration of volatile or semi-volatile organic contaminants at contaminated sites.

Influence of coal rock on tight sandstone reservoirs in coal seam roofs: A case study of the lower jurassic in the Taibei sag, Turpan-Hami Basin, China

After shale gas, coalbed methane and gas hydrate, coal measure gas has become an important new unconventional resource, for which exploration to depths of >3000 m are taking place in China. However, owing to unusual geological conditions and as yet inadequate exploration technology the current level of exploration is low. The acidic water medium in coal-bearing strata affects the diagenesis of adjacent tight sandstone reservoirs, thereby influencing reservoir quality. However, the current understanding of the impact mechanism of coal seams on neighboring tight reservoirs is insufficient, constraining the further exploration of coal measure gas. In order to reveal the influence of coal rock on a tight sandstone reservoir in its roof, we study the coal-bearing strata of the Xishanyao Formation in the Taibei Sag of the Turpan-Hami Basin using cast thin sections, physical property tests, XRD analysis, carbon and oxygen isotopes, high-pressure mercury intrusion and low-temperature N2 adsorption. The carbonate cement in the sandstone reservoir of the coal seam roof was formed by the decarboxylation of organic acid. The organic acid, containing CO2 generated by coal rock hydrocarbon generation, has modified the tight sandstone reservoir of the coal seam roof via dissolution. This has improved the physical properties and pore throat structure of the tight sandstone reservoir close to the coal-sandstone interface. The amount of improvement extends to about 2 m into the sandstone. During the upward transport of fluid, the pre-existing carbonate cement dissolves and then precipitates, which causes the CO2 generated by the organic acid decarboxylation to exchange carbon with the carbon isotopes in the pre-existing carbonate cement. Due to the slow upward diffusion rate and limited diffusion distance of the CO2-bearing fluids, the carbonate cement exhibits gradually heavier δ13CPDB values with increasing distance from the coal-sandstone interface.

Mediterranean-like “fall dump” events in the Baltic Sea

In the Mediterranean Sea, organic carbon-rich sapropels have been deposited periodically over the last fifteen million years. Some sapropels are characterized by high contents of the mat-forming, planktonic diatom Pseudosolenia calcar-avis and the planktonic diatom Thalassionema nitzschioides as a result of their mass sinking in autumn (the so-called “fall dump”). The present study shows that fall dump events also occurred in the brackish Baltic Sea around 6300–5800 calibrated years before present (cal yr BP; present = AD 1950). In sediments from the northern Baltic Sea these events are evidenced by high contents of a lipid specific for P. calcar-avis (a C25:2 highly branched isoprenoid alkene) and corroborated by fossil remains of both P. calcar-avis and T. nitzschioides. A biomarker index based on long-chain alkyl diol lipids indicates that mat-forming Proboscia diatoms were also present. High contents of calanoid resting eggs further suggest that the copepod population was stressed being unable to feed on such large diatoms. The fall dump events occurred during a complete stratification of the water column and euxinic conditions, as reflected by the redox-sensitive trace metal uranium, allowing upward diffusion of nutrients and the growth of rhizosolenid mat-forming diatoms in a deep chlorophyll maximum. Synchronous remains of P. calcar-avis in the central Baltic Sea further suggest that these events occurred on a basin-wide scale.

Downdraft influences on the differences of PM2.5 concentration: insights from a mega haze evolution in the winter of northern China

A significant haze event occurred in northern China from 16 to 21 November 2022. This study analyzed the haze spatial evolution, and meteorological influences by integrating ground and satellite measurements. Most data were obtained using aerosol lidar and wind lidar observations in suburban (Nanjiao Observation Station, NJOS) and urban Beijing (Haidian Observation Station, HDOS). The observations at NJOS and HDOS indicate the presence of a distinct layer of haze restricted to a height of up to 1500 m above the surface. However, the aerosol intensity at HDOS was comparatively lower (aerosol extinction coefficient: 1.39 ± 0.27 km−1) than at NJOS (1.77 ± 0.38 km−1), with approximately one day of time lag in response to the southerly winds. Though NJOS and HDOS presented a similar wind stratification structure, the downdraft under 1000 m influenced the surface air quality were significantly different. The intense downdraft at the lower height at HDOS prevented the vertical upward diffusion of accumulated ground pollutants, whose effect was similar to that of the inversion layer. That led to a more stable increasing trend of PM2.5 at HDOS, with the shallowest planet boundary layer height of 242 m on 20 November. By contrast, NJOS in the transportation path was more regularly influenced by the southerly flow and presented cyclical PM2.5 concentration. This study shows downdraft in urban environments acting as an accelerator for urban episodic PM2.5 pollution, suggesting the complicated contribution from meteorological factors.

Discerning the sulfur geochemical features of turbidites and methane-rich sediments from the South China sea

Upward diffusion of dissolved methane and associated anaerobic oxidation of methane (AOM) coupled with microbial sulfate reduction (MSR) are widespread in normal hemipelagic sediments of continental margins. The occurrence of authigenic sulfur enrichment in such sediments is usually regarded as an indicator of AOM; however, extensive MSR and authigenic iron sulfide generation also occur in turbidites. Therefore, research attention should be directed toward determining the geochemical features of authigenic sulfur mineralization in methane-rich background sediments and turbidites. Here we report the sedimentology and geochemical composition of sediments from four piston cores (CL30, CL44, CL47, and CL3A) containing turbidite layers from methane-rich areas of the South China Sea. The turbidite layers were deposited at 17–15 kyr BP or earlier during the last glacial period and were located above the present sulfate–methane transition zone (SMTZ). The median grain size, proportion of sand-sized grains, and Si, Ti, K, and Zr contents show marked elevation in the turbidite layers compared with the background sediments, indicating the input of large amounts of terrigenous detrital matter with coarser grains. High total sulfur (TS) contents and low δ34SAIS values (AIS: acid-insoluble sulfur) (−35.2‰ to −26.1‰) are observed within turbidite layers relative to the background hemipelagic sediments in core CL44. These results suggest that intensive sulfidization occurred as a result of enhanced organoclastic sulfate reduction rates in turbidite horizons, which were probably caused by the resultant decrease in downward oxygen flux and waning bioturbation after rapid deposition of turbidites. In comparison, TS contents are higher and δ34SAIS values are positive (0.2‰–46.8‰) within the SMTZ on account of the rapid consumption of sulfate and build-up of isotopically heavier hydrogen sulfide pools via intensive AOM. Moreover, authigenic barium (Ba) enrichments commonly occur in the depth interval immediately above the SMTZ rather than within turbidite layers; therefore, they serve as a useful indicator of the position of the SMTZ. This study presents distinct differences in the geochemical compositions of turbidity deposits and methane-rich background sediments and highlights that a multi-proxy approach including sedimentological and geochemical analyses should be used to constrain the types of MSR in deep-sea sediments, given the common occurrence of turbidites and subsurface methane release in marginal deep-sea areas. It is also suggested that if applied with caution, S–C–Ba geochemical patterns can be used to identify palaeo-SMTZs and palaeo methane-rich zones in turbidite-containing strata.

Three in One: Superoleophilic, Chemically and Mechanically Resistant ZIF-7 and ZIF-11 Percolation Networks for Selective Permeation of Oils and Chlorinated Solvents.

Imbuing superwetting functions to organic-inorganic hybrid networks displaying chemical resistance, self-cleaning ability, and selective permeation of liquids has received increasing attention in recent years. Here we report superhydrophobic ZIF-7 and ZIF-11 on multilayer fluorinated graphene (FG) nanosheets with long-lasting water-repellent features. By exploring the solution processing of these chemically resistant dispersions, superoleophilic FG-ZIF-7 stainless steel mesh (FG-ZIF-7-SSM) and FG-ZIF-11 over cotton cloth (FG-ZIF-11-CC) possessing superior adhesion were fabricated. These permselective oil-liking prototypes were explored toward mesitylene and crude oil pickup from chemically harsh marine conditions such as seawater, acidic water, and alkaline water, with a separation efficiency of 96-94% up to 10 cycles. Furthermore, using an FG-ZIF-11-CC-wrapped glass pipet, upward diffusion of chloroform from sea, acidic, and alkaline water in 45 s was demonstrated with a separation efficacy of 94% up to 20 cycles. In addition to the chemical resistance and reusability, the mechanical stability of FG-ZIF-7-SSM and FG-ZIF-11-CC was investigated through folding, tape peeling, and dragging through sandpaper up to 250 cycles, showing no signs of changes in the hydrophobic responses. This research sheds light on the application of physiochemically resistant percolation coatings based on fluorinated graphene multilayers supporting ZIF-7 and ZIF-11 toward oil/water separation.

Effect of the anti-seepage masking layer on the microstructure evolution of Ni-based superalloy during co-deposition of the Al-Si coating

This study optimizes turbine blade microstructures during production, specifically focusing on surface treatments for blade airfoils and dovetails. Blade airfoils need to prepare the Al-Si coating to improve oxidation resistance, while blade dovetails must prepare the anti-seepage masking layer to hinder the deposition and diffusion of active atoms into the substrate. The effect of the anti-seepage masking layer on the microstructure evolution of IN718 superalloys during co-deposition of the Al-Si coating was systematically revealed. Results indicate that by adding Si elements to modify the aluminide coating, a uniform and dense Al-Si coating was prepared on IN718 superalloys at 950 ℃ for blade airfoils, enhancing oxidation resistance. Meanwhile, the anti-seepage masking layer with 42 µm could effectively prevent the deposition and diffusion of Al and Si atoms into the substrate for blade dovetails. Defects such as interface layers, micro-voids, and cracks between the masking layer and substrate facilitate the removal of the anti-seepage masking layer from the substrate after heat treatment. On the contrary, the thicker anti-seepage masking layer will induce the upward diffusion of Cr and Mo atoms and form a stronger metallurgical bonding with the substrate, which is difficult to remove and will harm the properties of the substrate.

Discontinuity of sediment transport from the Bohai Sea to the open sea dominated by the wind direction

The sediment transport of the Bohai Sea through the Bohai Strait is a key issue in coastal shelf research. Based on remote sensing data and numerical simulation results, the sediment transport from the Bohai Sea to the Yellow sea is temporally discontinuous, and the surface and bottom layers exhibit two distinct processes. Further analysis showed that discontinuities are caused by differences in stratification intensity due to wind directions. During strong northwesterly winds, suspended sediments from the Bohai Sea can be transported along the Laizhou Bay through the southern part of Bohai Strait. And northeasterly wind can strengthen the stratification by driving stronger the Yellow Sea Warm Current. Strong stratification significantly inhibited the upward diffusion of the bottom suspended sediments. Stratification with different intensities can affect the efficiency of sediment transport, which can overcome the limitations of the sediment transport mechanism in the Bohai Sea. The insights obtained in this study into the effect of stratification on sediment transport could benefit further exploration of other hydrological mechanisms in coastal shelf seas.

The role of the microbiota-gut-brain axis and intestinal microbiome dysregulation in Parkinson's disease.

Parkinson's disease (PD) is a complex progressive neurodegenerative disease associated with aging. Its main pathological feature is the degeneration and loss of dopaminergic neurons related to the misfolding and aggregation of α-synuclein. The pathogenesis of PD has not yet been fully elucidated, and its occurrence and development process are closely related to the microbiota-gut-brain axis. Dysregulation of intestinal microbiota may promote the damage of the intestinal epithelial barrier, intestinal inflammation, and the upward diffusion of phosphorylated α-synuclein from the enteric nervous system (ENS) to the brain in susceptible individuals and further lead to gastrointestinal dysfunction, neuroinflammation, and neurodegeneration of the central nervous system (CNS) through the disordered microbiota-gut-brain axis. The present review aimed to summarize recent advancements in studies focusing on the role of the microbiota-gut-brain axis in the pathogenesis of PD, especially the mechanism of intestinal microbiome dysregulation, intestinal inflammation, and gastrointestinal dysfunction in PD. Maintaining or restoring homeostasis in the gut microenvironment by targeting the gut microbiome may provide future direction for the development of new biomarkers for early diagnosis of PD and therapeutic strategies to slow disease progression.

Performance of a siphon-like water supplying solar distiller with helium gas enhancing the reverse film evaporation process

The multistage solar desalination method based on the interface local thermal method is an attractive and efficient desalination method. However, there are still problems such as the upward diffusion of water vapor in the air and the downward irradiation of solar light, which make the multistage device fail to effectively condense and interface salt crystallization cannot be effectively eliminated, hindering the development of large-scale and sustainable applications of this technology. Here, we use a cheap nanofiber super-hydrophilic material to promote the flow of seawater at the interface and promote the discharge of salt crystallization under the siphon-like effect. At the same time, the water vapor‑helium space is constructed by filling helium to promote the downward diffusion of water vapor in the space and solve the problem of insufficient multistage condensation. In this work, a two-stage and three-stage reverse mass transfer solar seawater desalination system with solar hot film evaporation was built, and the corresponding theoretical calculation and experimental verification were carried out. The experimental results show that under sunlight irradiation of 800 W m−2, the three-stage experimental system filled with helium can effectively enhance the condensation of water so that the GOR value of the system vapor is 1.54, the first-stage evaporation efficiency reaches 67.9 %, and the water productivity reaches 1.85 kgm−2h−1. In this study, injecting low molecular weight gas to achieve efficient condensation of the system to improve the performance of the interfacial evaporation desalination system provides a new method and reference.

Dynamic response of water flow and sediment transport off the Yellow River mouth to tides and waves in winter

Freshwater and sediments are transported from the Yellow River mouth downstream along the coast into Laizhou Bay under the northeasterly wind in winter. Numerical experiments indicate that sediment transport shows the tendency of convergence in the river mouth, divergence in the downstream area, and convergence in the north of Laizhou Bay. Tides and waves are the two main forcings affecting the transport of water and sediments off river mouths. For the high-turbidity Yellow River mouth and the adjacent sea, tidal forcing enhances the subtidal downstream transport of water and sediments off the river mouth into Laizhou Bay, whereas wave forcing has little effect on the advection of water and sediments. Sediment resuspension is controlled by the bottom shear stress induced by tides and waves. The tide-induced bottom shear stress is higher in the north of Laizhou Bay and south of Bohai Bay due to the stronger bottom tidal current. The wave-induced bottom shear stress plays a more important role in sediment resuspension, which is higher in the nearshore region along the Yellow River Delta away from the coast to some extent on account of the maximum near-bottom wave orbital velocity. Tidal mixing strengthens the upward diffusion of the bottom suspended sediments. Without tidal forcing, there is an interesting phenomenon along the Yellow River Delta. In the nearshore region, the decreased bottom shear stress suspends less sediment above the bed. However, in the offshore region, the enhanced stratification hinders the upward diffusion of the bottom sediment due to the lack of tidal mixing, resulting in higher suspended sediment concentration (SSC) in the bottom layer.

One-dimensional γ-Al2O3 growth from the oxidation of NiAl

We report an oxide nanowire growth phenomenon that morphologically resembles the vapor-liquid-solid process but with a fundamentally different mechanism. This is demonstrated with the high-temperature NiAl oxidation that results in the unidirectional γ-Al2O3 growth with a NiO nanoparticle at the nanowire tip. The NiO nanoparticle surface serves as the active site for dissociative O2 adsorption that produces an inward flux of atomic oxygen toward the γ-Al2O3/NiAl interface for γ-Al2O3 growth by reacting with bottom-up diffusing Al3+ cations. The nanowire lengthening follows the parabolic kinetics with the upward lattice diffusion of Al3+ cations in the γ-Al2O3 nanowires as the rate-limiting factor.

Spinodal Decomposition of B2‐phase and Formation of Cr‐Rich Nano‐precipitates in AlCoCrFeNi2.1 Eutectic High‐Entropy Alloy

Herein, the occurrence of a B2‐phase separation and formation of Cr‐rich nano‐precipitates during the solidification process of AlCoCrFeNi2.1 eutectic high‐entropy alloy is addressed. Toward this end, various advanced characterizations, including high‐resolution transmission electron microscopy and atom probe tomography combined with thermodynamic calculations, are employed. The as‐solidified microstructure is composed of face‐centered cubic (FCC) dendrites and interdendritic regions consisting of a eutectic mixture of FCC and body‐centered cubic (BCC) phases. The presence of uniformly distributed Cr‐rich nano‐precipitates is traced through the BCC B2 phase in the interdendritic area. Regarding the occurrence of upward diffusion and Gibbs free energy variation, the formation of Cr‐rich nano‐precipitates is attributed to the spinodal decomposition where the critical temperature of 800 °C is passed behind during the solidification process. The formation of dense dislocation array in the interdendritic region due to thermal stress induced during solidification is introduced as a pathway for diffusion of alloying elements in the course of cooling stage.

Estimation the effect of accumulated long-range transported pollutants during a PM2.5 event in Taiwan

Fine particulate matter (PM2.5) pollution has attracted much attention in recent years due to its adverse impact on human health and the environment. This study examined a major PM2.5 event observed in Taiwan on March 02–03, 2022, using the Weather Research and Forecasting - Community Multiscale Air Quality (WRF-CMAQ) models. This period also coincided with an outage caused by the power failure of three power plants in Southern Taiwan on March 03, 2022, which created a unique situation to distinguish the impacts of long-range transported (LRT) pollutants and the operation of power plants on the air quality in Taiwan during the high-air-pollution event.Four numerical experiments, Baseline (real case), Replace (Baseline with power plants), No_LRT (Baseline without LRT), and No_0303_LRT (Baseline without LRT on March 03, 2022), were conducted with different initial emission scenarios. The simulation results revealed that the PM2.5 concentrations emitted from three power plants only counted a relatively small proportion in total, while the LRT pollutants were the key factor contributing to the high-concentration PM2.5 during the event. Besides, we figured out that the high PM2.5 concentrations on March 03, 2022 were dominated by the contribution of accumulated LRT. In addition, the results also suggested that the existence of a lee-side vortex, which is considered a potential key issue of the mesoscale system contributes to extreme air pollutant events in Taiwan, causing encouraging conditions for the upward diffusion and northward transport of the PM2.5 concentrations over the southern part of Taiwan, as a result, the distribution of pollution was modulated.

Satellite‐Observed Interannual Variations in Sea Surface Chlorophyll‐a Concentration in the Yellow Sea Over the Past Two Decades

AbstractOcean chlorophyll‐a concentration is a reliable indicator of phytoplankton biomass that plays an important role in controlling the marine ecosystem. Here, we investigated the interannual variations in sea surface chlorophyll‐a concentration (SSC) in the Yellow Sea and underlying mechanisms with 22 yr (1998–2019) of satellite ocean color observations. Results indicated that SSC showed a positive trend (0.039 mg m−3 yr−1 on average) during 1998–2010 and a negative trend (−0.058 mg m−3 yr−1 on average) during 2011–2019. Similar SSC trends occurred in the four seasons. These trends had larger magnitudes in inshore waters than in offshore waters. SSC variations were primarily driven by rainfall and anthropogenic nutrient emissions with a possibly secondary contribution from sea surface temperature (SST). During 1998–2010, increased rainfall and anthropogenic nutrient emissions largely enhanced the terrestrial nutrient inputs into the Bohai Sea and Yellow Sea. Coincidentally, decreased SST might have led to a weakened stratification in summer, favoring upward turbulent diffusion of deeper waters rich in nutrients and chlorophyll‐a. These processes resulted in significant increases of surface nutrient concentrations and SSC. During 2011–2019, the opposite processes occurred to generate decreased SSC. The current study highlights the change of SSC trends in the Yellow Sea in the first two decades of the 21st century and physical processes at work, enriching our understanding of ocean chlorophyll‐a dynamics in the global shelf seas.

On the Spectral Evolution of Hot White Dwarf Stars. IV. The Diffusion and Mixing of Residual Hydrogen in Helium-rich White Dwarfs

In the framework of our extensive modeling study of the spectral evolution of white dwarfs, we present here a new set of detailed calculations of the transport of residual hydrogen in helium-rich white dwarfs. First, we investigate the so-called float-up process at high effective temperatures, whereby the upward diffusion of trace hydrogen leads to the formation of a hydrogen atmosphere. We examine the dependence of this phenomenon on the initial hydrogen abundance and on the strength of the radiative wind that opposes gravitational settling. Combined with our empirical knowledge of spectral evolution, our simulations provide new quantitative constraints on the hydrogen content of the hot helium-dominated white dwarf population. Then, we study the outcome of the so-called convective dilution process at low effective temperatures, whereby the superficial hydrogen layer is mixed within the underlying helium-rich envelope. In stark contrast with previous works on convective dilution, we demonstrate that, under reasonable assumptions, our models successfully reproduce the observed atmospheric composition of cool DBA stars, thereby solving one of the most important problems of the theory of spectral evolution. This major improvement is due to our self-consistent modeling of the earlier float-up process, which predicts the existence of a massive hydrogen reservoir underneath the thin superficial layer. We argue that the trace hydrogen detected at the surface of DBA white dwarfs is, in most cases, of primordial origin rather than the result of external accretion.