Vertical Concentration Profiles Research Articles

Comparing Airborne and Along-Sidewall Ground-Based Measurements to Retrieve Thermal and PM10 Concentration Profiles in Two Alpine Valleys

Abstract The paper aims at investigating the effectiveness of estimating vertical profiles of air temperature and PM10 concentrations in Alpine valleys through ground stations positioned at different altitudes on one valley sidewall (i.e., pseudovertical profiles). Two case studies in the Italian Alps are investigated: Chiese Valley in Trentino Province and Camonica Valley in the Lombardy region. Vertical profiles of temperature and PM10 concentrations were derived from airborne measurements at the center of the two valleys by means of low-cost sensors installed on a drone during summer 2019 and a tethered balloon during winter 2020. At the same time, five stations, equipped with the same kind of low-cost sensors, simultaneously monitored the same variables on one mountain slope. Comparisons between pseudoprofiles and airborne soundings revealed that ground stations well approximated temperature and PM10 soundings during the night and early morning, while temperatures along the slopes were higher than in the center of the valley during daytime, due to solar radiative heating, with larger differences in summer than in winter. On the contrary, some episodes with PM10 concentrations slightly higher in the valley center than on the slope were recorded, due to transport events and upslope winds. Nonetheless, the pseudoprofiles based on slope ground measurements faithfully reproduced the vertical gradients of both air temperature and PM10 if compared to those assessed from the soundings performed at the center of the two valleys. Results show that pseudovertical profiles can be a reliable and inexpensive method for continuous monitoring of vertical air temperature and PM10 distribution in mountain valleys. Significance Statement The purpose of this study is to understand whether ground-based measurements of air temperature and particulate matter on mountain slopes can be used as suitable surrogates of vertical atmospheric soundings with tethered balloons or drones in mountain environment. This is important because balloon and drone soundings are expensive and regulative constrained. The knowledge of the vertical distribution of air temperature is crucial to predict the distribution of air pollutants in valleys, namely, the particulate matter emitted by wood burning. Our results showed that ground-based measurements on mountain slopes made with low-cost sensors can satisfactorily reproduce vertical gradients of air temperature and distribution of particulate matter in a reliable and inexpensive way.

Vertical profile measurements for ammonia in a Japanese deciduous forest using denuder sampling technique: ammonia emissions near the forest floor

Ammonia (NH3) has received considerable attention as a major reduced nitrogen. However, accurate estimates of the deposition amount are difficult due to its complex behavior characterized by bidirectional exchange between the atmosphere and the surface. We observed the vertical profile of NH3 concentration in a deciduous forest in Japan for 1 year to further advance the studies on NH3 bidirectional exchange in Asia, especially focusing on the process near the forest floor. The observation period lasted from September 29, 2020, to September 28, 2021, including leafy and leafless periods. Using the denuder sampling technique, we measured NH3 concentration in the forest at three heights (above the forest canopy, 30 m, and near the forest floor, 2 m and 0.2 m). NH3 concentrations tended to be highest at the top of the canopy (30 m). Focusing on the concentration near the forest floor, the concentrations at 0.2 m were frequently higher than those at 2 m regardless of the leafy and leafless period, thus suggesting NH3 emissions from the forest floor. NH3 concentration near the forest floor showed strong positive correlations with air temperature during the leafy period. The NH3 emissions from the forest floor during the leafy period were possibly due to the decomposition of leaf litter with increased air temperature. The decrease in leaf area index might induced the increase in NH3 concentration and emission. NH3 emission during the leafless period was also possibly dependent on the state of the deposition surface, apart from air temperature, relative humidity, and leaf area index.Graphical

Mass concentrations, compositions and burial fluxes of nano- and micro-plastics in a multi-species saltmarsh

Plastic pollution poses a serious threat to marine ecosystems; yet quantifying the mass concentrations of nano- and microplastics (NMPs) in saltmarsh sediments at the ocean-land interface remains a critical research gap. Here, the study employed reliable and efficient analytical techniques, namely pressurized liquid extraction and the double-shot model of thermal desorption/pyrolysis-gas chromatography-mass spectrometry, to quantify six different types of NMPs in the sediment of a multi-species saltmarsh, providing the first comprehensive assessment of NMP mass concentration and burial in this saltmarsh environment. The results demonstrate that polyethylene, polyvinyl chloride, and polypropylene dominated the NMP composition in sediments, constituting 72.6%, 17.3%, and 4.5% of the total NMPs, respectively. The measured NMPs represent an anthropogenic intrusion, constituting 0.10%–0.23% of the carbon storage in the saltmarsh. By examining the vertical concentration profiles, this study unveiled the influence of saltmarsh vegetation on NMP deposition in sediments, establishing a connection with local sedimentation patterns and the historical zonation of plant species such as Scirpus mariqueter, Phragmites australis and Spartina alterniflora. These findings underscore the crucial role of saltmarsh vegetation in facilitating NMP settling and retention, highlighting the necessity of considering vegetation dynamics in examining the emerging NMP pollution in coastal wetlands.

Soil gas gradient method for estimating natural source zone depletion rates of LNAPL and specific chemicals of concern

This paper presents a simplified approach for the soil gas gradient method for estimating natural source zone depletion (NSZD) rates of specific contaminants of concern (COCs) at sites contaminated by light non-aqueous phase liquids (LNAPL). Traditional approaches to quantify COC-specific NSZD rates often rely on numerical or analytical reaction-transport models that require detailed site-specific data. In contrast, the proposed method employs simple analytical solutions, making it more accessible to practitioners. Specifically, it requires only the maximum soil gas concentration, the effective diffusion coefficient, and the diffusive reaction length calculated from vertical soil gas concentration profiles. The simplified approach was validated against a reactive transport numerical model reported in the literature, showing consistent results within the same order of magnitude for BTEX NSZD rates at a gasoline spill site in South Carolina. Further validation using a larger dataset involved comparing NSZD rate estimates for benzene and total petroleum hydrocarbons (TPH) against those obtained using BioVapor, utilizing empirical soil gas data from the USEPA Petroleum Vapor Intrusion Database. Results demonstrated a strong correlation between NSZD rates and maximum soil gas concentrations, allowing the development of a rapid screening approach based only on the measured soil gas concentrations and literature values for diffusion coefficients and diffusive reaction lengths. This approach aligned well with previous modeling studies and was consistent with literature values for TPH NSZD rates. Overall, both the simplified and screening approaches offer practical, easy-to-use tools for evaluating temporal variability in natural attenuation rates, supporting baseline assessments and ongoing performance evaluations of remediation at LNAPL sites.

Sensitivity of AERMOD (V21112) RLINEXT dispersion model outputs by source type to variability in single noise barrier height and separation distance

The U.S. Environmental Protection Agency (USEPA) introduced the RLINEXT modeling feature in the latest versions of AERMOD (since version v21112) to predict traffic-induced pollutant concentration when noise barriers are present. The research presented in this paper explores the impacts of noise barrier characteristics on AERMOD-predicted concentrations. The research finds that because AERMOD can currently only attach one barrier to each road side, and because that barrier only impacts the source to which it is attached, it is also important to split links so that they properly pair with barriers. Given the sensitivity of AERMOD-predicted concentrations to barrier characteristics (i.e., barrier heights and distances to roadway), the research also concludes that barriers must be appropriately matched with input links. This study investigated the sensitivity of CO concentration predicted by the latest AERMOD under various noise barrier conditions (barrier heights and distances between road and barrier) and meteorological conditions (wind directions and wind speeds). The results indicate that ground-level concentration of downwind receptors decreases with increased barrier heights, and that distant barriers have less of an impact on predicted concentrations. This study also explored the impact of noise barrier on both horizontal and vertical concentration profiles, indicating that concentrations rise behind the barrier as plumes are predicted to loft over the barrier. The sensitivity analysis associated with splitting roadway links to match with barriers indicated an impact on predicted concentration for certain receptors of up to 10%, but the overall the impact on maximum concentrations was marginal.

A DEM study on the criteria for particle movement over a granular bed

The collision of a spherical particle onto a granular bed, generating a splash of ejected grains, is a crucial process in the study of wind-blown sand. Previous research has faced challenges in clearly distinguishing between particles in saltation, reptation, and creep motion. This study employs the Discrete Element Method (DEM) to simulate particle trajectories during sand-bed collisions, providing a systematic analysis of the vertical mass concentration profiles and the probability density functions (PDFs) for splash velocity, splash angle, and the number of splashed particles associated with saltation, reptation, and creep particles. Our results reveal that reptation motion predominantly occurs within a vertical height range of 0.064 mm to 8 mm above the surface, creep motion occurs below 0.032 mm, and saltation occurs at heights greater than 8 mm.

Impact of atmospheric vertical profile on hyperspectral simulations over bright desert pseudo-invariant calibration site

ABSTRACT This paper investigates how atmospheric vertical profiles of pressure, temperature, and concentration affect molecular absorption calculation. This sensitivity analysis is performed in preparation to the upcoming TRUTHS mission, anticipated to provide hyperspectral TOA BRF records with a radiometric accuracy better than 1%. Two methods for characterizing the atmospheric vertical profile are compared: rescaling the H2O and O3 concentrations of an AFGL U.S. Standard vertical profile, and using customized profiles based on CAMS data. The study investigates the effects of those methods on multi-spectral observations in key spectral regions affected, respectively, by water vapour, ozone and methane, as well as on hyperspectral observations covering the visible to SWIR region. Results show that when molecular transmittance exceeds 97%, the choice of method has minimal impact, with less than 1% uncertainty. When the molecular transmittance decreases from 97% to 75%, the corresponding uncertainty on the TOA BRF simulation increases from 1% up to 5%. For transmittance below 75%, using CAMS data for vertical profile characterization is recommended. The study also highlights how pressure and temperature profiles influence Rayleigh optical thickness estimation, particularly affecting TOA BRF in the blue spectral region.

Vertical concentrations gradients and transport of airborne microplastics in wind tunnel experiments

Abstract. Microplastics are an ubiquitous anthropogenic material in the environment, including the atmosphere. Little work has focused on the atmospheric transport mechanisms of microplastic nor its dispersion, despite it being a potential pollutant. We study the vertical transport of airborne microplastics in a wind tunnel, a controllable environment with neutral stability, to identify the necessary conditions for the long-range atmospheric transport of microplastics. An ultrasonic disperser generated airborne water droplets from a suspension of polystyrene microsphere microplastics (MPs) with a diameter of 0.51 µm. The water droplets were injected into the airflow, evaporating and releasing single airborne MPs. The disperser allowed for time-invariant and user-controlled concentrations of MPs in the wind tunnel. MPs were injected at 27, 57, and 255 mm above the ground. A single GRIMM R11 optical particle counter (OPC) and three Alphasense OPCs measured time-averaged MP concentration profiles (27, 57, and 157 mm above the ground). These were combined with turbulent airflow characteristics measured by a hotwire probe to estimate vertical particle fluxes using the flux-gradient similarity theory. The GRIMM R11 OPC measured vertical concentration profiles by moving its sampling tube vertically. The three Alphasense OPCs measured particle concentrations simultaneously at three distinct heights. Results show that maximum concentrations are not measured at the injection height but are rather shifted to the surface by gravitational settling. The MPs experience higher gravitational settling while they are part of the larger water droplets. For the lowest injection at 27 mm, the settling leads to smaller MP concentrations in the wind tunnel, as MPs are lost to deposition. Increasing the wind speed decreases the loss of MPs by settling, but settling is present until our maximum friction velocity of 0.14 m s−1. For the highest injection at 255 mm and laminar flow, the settling resulted in a net MP emission, challenging the expectation of a net MP deposition for high injection. Turbulent flows reverse the MP concentration profile giving a net MP deposition with deposition velocities of 3.7 ± 1.9 cm s−1. Recognizing that microplastics share deposition velocities with mineral particles bridges the gap in understanding their environmental behavior. The result supports the use of existing models to evaluate the transport of microplastics in the accumulation mode. The similar deposition velocities suggest that microplastics transported in the atmosphere can be found in the same places as mineral particles.

Modelling of Granular Sediment Transport in Steady Flow over a Mobile Sloped Bed

This paper introduces a three-layer system, proposing a comprehensive model of granular mixture transport over a mobile sloped bed in a steady flow. This system, consisting of the bottom, contact, and upper zones, provides complete, continuous sediment velocity and concentration vertical profiles. The aim of this study is to develop and experimentally verify this model for sediment transport over a bottom locally sloping in line with or opposite the direction of sediment flow. The model considers gravity’s effect on sediment transport in the bottom (dense) layer when the component of gravity parallel to the bottom acts together with shear stresses associated with water flow. This is a crucial factor often overlooked in previous studies. This effect causes an increase in velocity in the mobile sublayer of the dense layer and significantly affects the vertical distributions of velocity and concentration above this layer. The proposed shear variation due to the interaction between fractions and an intensive sediment mixing and sorting process over a mobile sloped bed adds to the novelty of our approach. The data sets used for the model’s validation cover various conditions, including slopes, grain diameters, densities, and grain mobility conditions, from incipient motion to a fully mobilized bed. This extensive validation process instils confidence in the theoretical description and its applicability to real-world scenarios in the design of hydraulic infrastructure, such as dams, barrages, bridges, and irrigation, and flood control systems.

Limnological Characteristics and Relationships with Primary Productivity in Two High Andean Hydroelectric Reservoirs in Ecuador

Studies on limnology are essential to reservoir management; nevertheless, few are known about the limnological features of the Andean reservoirs in Ecuador. To overcome this limitation in the information, from December 2018 to December 2019, the limnological characteristics of El Labrado and Chanlud reservoirs in the Machángara river basin (Ecuador south) were examined. Using the light/dark bottles technique, the primary productivity (PP) of phytoplankton was studied in conjunction with (1) vertical profiles of oxygen concentrations, water temperature, nitrogen, phosphorus, alkalinity, and heterotrophic bacteria; (2) Secchi disk transparency; and (3) meteorological factors such as wind force, precipitation, and water level. Data indicate that both reservoirs are polymictic, with alkaline waters, low nutrients, and low PP rates. Despite this, a principal component analysis revealed that Chanlud exhibits higher nitrogen, alkalinity, heterotrophic bacteria, and PP values. In two approaches through multiple linear regression analysis, each per reservoir, the PP was explained mainly by water temperature, depth, light, heterotrophic bacteria, and meteorological parameters. The low concentrations of nutrients and the low residency time explain the low PP values. Likewise, the altitudinal factor (i.e., both reservoirs are 3400 m above sea level) and the low human perturbations in surrounding reservoir zones play a crucial role in explaining their poor PP. Notwithstanding the low metabolic rates, clear seasonal trends were observed in both reservoirs; the lowest PP rates occurred during the cold season. To our knowledge, this is the first limnological study of high Andean reservoirs in Ecuador. These findings should be part of Andean reservoir management protocols, contributing significantly to local conservation efforts. Additionally, they could be extrapolated as a frame of reference to similar eco-hydrological systems.

Low-cost UAV coordinated carbon observation network: Carbon dioxide measurement with multiple UAVs

Improving the mitigation of global greenhouse effects requires drastic reductions in carbon emissions. Effective carbon management at different spatial scales is therefore crucial to a more sustainable path of economic development. Regional and local carbon management currently rely on the traditional bottom-up method of emission reports, which are known for their opaque and complex procedures. Novel top-down carbon monitoring methods including space-based, aerial and ground-based observations are tantalising alternatives to more reliable data sources of carbon emissions as well as sinks. Unmanned Aerial Vehicles (UAVs) based carbon monitoring is an upcoming field because of its lower costs, higher spatial-temporal resolution and operability at a wider range of weather conditions compared to satellite observations. UAVs equipped with Non-dispersive Infrared (NDIR) carbon dioxide (CO2) sensors can measure in the air with faster response and can intelligently navigate themselves within areas of greater interest. With the help of lighter NDIR payloads and multiple UAVs flying in synchrony, larger cross sections or areas for maximising information acquisition of emission plumes can be covered. In this article, we first introduce the design of a miniaturised NDIR payload, which is compatible with a lightweight UAV to achieve a single system weighing less than 4 kg. Then, we test our design via comparisons against another NDIR payload to determine its accuracy (−0.82 ppm and −0.44 ppm) and precision (0.12 ppm and −0.03 ppm) in measurements of two vertical concentration profiles. Finally, we deployed four UAVs on two major anthropogenic emission source monitoring campaigns with coordinated flight patterns. To fully understand the benefit of multi-UAV carbon monitoring, we attempted two flight patterns and computed power plant emission rates via the cross-sectional flux method. While all flights successfully captured the concentration enhancements downwind of the power plants, the single-sectional flight pattern measured an emission rate of 10ktCO2/day which is 44% less than that reported monthly and the multi-sectional flight pattern yielded relatively varied results (8.5ktCO2/day on average, which is 54% less) for the respective sections due to wind underestimations, plume section undersampling and wind fluctuations.

Estimation of the suspended sediment concentration in ice-covered channels based on the gravitational theory

Rivers in cold high latitude northern regions usually freeze during winter, thereby forming an ice cover on the water surface. The additional ice cover significantly changes the flow characteristics of the rivers and hence affects the process of water and sediment transport. Understanding the vertical profile of suspended sediment concentration (SSC) in ice-covered channel flows is helpful for investigating the sediment deposition and the bed morphology of rivers located in cold regions. To predict the SSC distribution in ice-covered channels, this study is based on the gravitational theory and puts forward a new SSC model by dividing the flow underneath the ice cover into the upper ice layer and the lower bed layer with the maximum streamwise velocity and the zero Reynolds shear stress as the interface. The calculated results of the new SSC model agree well with the collected experimental data, and the accuracy of the prediction results is evaluated by error analysis. Besides, the differences in the governing equations and the prediction accuracy between the gravitational theory and the diffusion theory are discussed, and thus the pros and cons of the proposed SSC model are illustrated. Although the gravitational theory has the controversial point, the validation results and discussions demonstrate that adopting the gravitational theory to estimate the SSC distribution in ice-covered channels is effective. The theoretical approach used herein can provide some guidance for studying the sediment-laden flow of freezing rivers in high-latitude northern regions.

A Numerical Study on the Influence of Riparian Vegetation Patch on the Transportation of Suspended Sediment in a U-Bend Channel Flow

Bend sections are ubiquitous in natural sandy river systems. This study employs Computational Fluid Dynamics–Discrete Phase Model (CFD-DPM) methodology to analyze particle transport dynamics in U-bend channel flows, focusing on the distinctions between partially vegetated (Case No.1) and non-vegetated (Case No.2) scenarios. The research aims to unravel the intricate relationships among bending channel-induced secondary flow, vegetation blockage, and particle aggregation, employing both quantitative and qualitative approaches. (I) The key findings reveal that vegetation near the inner walls of curved channels markedly diminishes the intensity of secondary circulation. This reduction in circulation intensity is observed not only within vegetated areas but also extends to adjacent non-vegetated zones. Additionally, the study identifies a close correlation between vertical vortices and particle distribution near the channel bed. While particle distribution generally aligns with the vortices’ margin, dynamic patch-scale eddies near vegetation patches induce deviations, creating wave-like patterns in particle distribution. (II) The application of the Probability Density Function (PDF) provides insights into the radius-wise particle distribution. In non-vegetated channels, particle distribution is primarily influenced by secondary flow and boundary layers. In contrast, the presence of vegetation leads to a complex mixing layer, altering the particle distribution pattern and maximizing PDF values in non-vegetated free flow subzones. (III) Furthermore, the research quantifies spatial–temporal sediment heterogeneity through PDF variance. The findings demonstrate that variance in non-vegetated channels increases towards the outer wall in bending regions. Vegetation-induced turbulence causes higher variance, particularly in the mixing layer subzone, underscoring the significance of eddy size in sediment redistribution. (IV) The study of vertical concentration profiles in vegetated U-bend channels offers additional insights, while secondary flow in non-vegetated channels facilitates upward sediment transport and vegetation presence, although increasing the Turbulent Kinetic Energy (TKE), restricts channel space, and impedes secondary flow, thereby reducing vertical particle suspension. Sediment concentrations are found to be higher in the lower layers of vegetated bends, contrary to the pattern in non-vegetated bends. These findings highlight the complex interplay between vegetation, secondary flow, and sediment transport, illustrating the reduced effectiveness of secondary flow in promoting vertical particle transportation in bending channels due to the vegetation obstruction.

Exploring organic and inorganic contaminant histories in sediment cores across the anthropocene: Accounting for site/area dependent factors

Sedimentary records help chronologically identify anthropogenic contamination in environmental systems. This study analysed dated sediment cores from L’Albufera Lake (Valencia, Spain), to assess the occurrence of heavy metals (HMs), polycyclic aromatic hydrocarbons (PAHs), perfluoroalkyl substances (PFASs), organophosphorus flame retardants (OPFRs), pesticides and pharmaceuticals and personal care products (PPCPs). The results evidence the continuing vertical presence of all types of contaminants in this location. The sediment age was difficult to establish. However, the presence of shells together with an historical estimation and the knowledge of sedimentary rates could help. HMs contents are higher in the upper layer reflecting the most recent increase of the industrial and agricultural practices in the area since the middle 20th century. Higher availability index of these HMs in the upper sediment layers is associated with point and diffuse contamination sources in the area. PAHs and OPFRs were homogeneous distributed through the sediments with few exceptions such as phenanthrene in the North and fluoranthene in the South. Perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS) were detected throughout the sediment core while short-chain PFASs (except perfluoropentanoic acid (PFPeA)) were detected only in the top layer. Pesticides and PPCPs showed appreciable down-core mobility. The vertical concentration profiles of organic contaminants did not exhibit a clear trend with depth, then, it is difficult to develop a direct relationship between sediment age and contaminant concentrations, and to elucidate the historical trend of contamination based on dated sediment core. Consequently, linking contaminant occurrence in sediments directly to their historical use is somewhat speculative at least in the conditions of L’Albufera Lake.

Optical and Microphysical Properties of the Aerosols during a Rare Event of Biomass-Burning Mixed with Polluted Dust

A rare event of mixed biomass-burning and polluted dust aerosols was observed over Athens, Greece (37.9° N, 23.6° E), during 21–26 May 2014. This event was studied using a synergy of a 6-wavelength elastic-Raman-depolarization lidar measurements, a CIMEL sun photometer, and in situ instrumentation. The FLEXPART dispersion model was used to identify the aerosol sources and quantify the contribution of dust and black carbon particles to the mass concentration. The identified air masses were found to originate from Kazakhstan and Saharan deserts, under a rare atmospheric pressure system. The lidar ratio (LR) values retrieved from the Raman lidar ranged within 25–89 sr (355 nm) and 35–70 sr (532 nm). The particle linear depolarization ratio (δaer) ranged from 7 to 28% (532 nm), indicating mixing of dust with biomass-burning particles. The aerosol optical depth (AOD) values derived from the lidar ranged from 0.09–0.43 (355 nm) to 0.07–0.25 (532 nm). An inversion algorithm was used to derive the mean aerosol microphysical properties (mean effective radius (reff), single scattering albedo (SSA), and mean complex refractive index (m)) inside selected atmospheric layers. We found that reff was 0.12–0.51 (±0.04) µm, SSA was 0.94–0.98 (±0.19) (at 532 nm), while m ranged between 1.39 (±0.05) + 0.002 (±0.001)i and 1.63 (±0.05) + 0.008 (±0.004)i. The polarization lidar photometer networking (POLIPHON) algorithm was used to estimate the vertical profile of the mass concentration for the dust and non-dust components. A mean mass concentration of 15 ± 5 μg m−3 and 80 ± 29 μg m−3 for smoke and dust was estimated for selected days, respectively. Finally, the retrieved aerosol microphysical properties were compared with column-integrated sun photometer CIMEL data with good agreement.

Mercury Mobility in Epibenthic Waters of a Deltaic Environment

AbstractMercury (Hg) cycling at the sediment‐water interfaces (SWI) encompasses multiple homogeneous and heterogeneous biogeochemical reactions whose result is not yet elucidated. Estuarine SWIs, where the organic matter mineralization is active, constitute experimental sites particularly suitable for scrutinizing Hg speciation and mobilization. Here, we present high‐resolution vertical concentration profiles of Hg species, including inorganic divalent Hg () and monomethyl Hg (MMHg) in solid and dissolved (<0.22 μm) phases, on both sides of the SWI of the proximal part of the Rhône prodelta (northwestern Mediterranean Sea) using sediment cores and dialyzers implemented for a 67‐day‐long period. Concentrations of the dissolved species were <0.10 nM in the sediment pore waters but reached up to 0.58 nM in the epibenthic water zone, a concentration level that is ∼200 times higher than that of the water column. Conversely, MMHg concentrations were low (<0.5 pM) above the SWI and increased to up to 4.6 pM in the sulfate‐reducing zones of the sediment. The dynamic of the Hg species interconversions was explored using one‐dimensional transport‐reaction equations. This model allowed us to constrain the depth intervals where various species are produced or consumed and to approximate the reaction rates. We conclude that the epibenthic zone of the Rhône prodelta is a location of intense mobilization of inorganic HgII associated with organic matter mineralization and MMHg distribution in pore water is controlled by microbiological in situ reactions, but sedimentary MMHg does not diffuse in the overlying water column.

Bubbles dominated the significant spatiotemporal variability and accumulation of methane concentrations in an ice-covered reservoir

Climate-sensitive ice-covered reservoirs are critical components of methane (CH4) release. However, the mechanisms that influence CH4 dynamics during ice-covered periods remain poorly studied. To investigate the effects of bubbles on CH4 dynamics, we conducted intensive field and incubation experiments in an ice-covered reservoir (ice growth, stability, and melt period) in Northeast China. We found that the mean dissolved CH4 concentrations in the ice (625.9 ± 2419.7 nmol L−1) and underlying water (1218.9 ± 2678.9 nmol L−1) were high, making them atmosphere CH4 sources. The visible bubble bands (bubble area) in the riverine zone and the vertical profile of the CH4 concentration in the ice reflect the distribution of trapped bubbles. The mean CH4 concentration in the ice of the bubble area (1674.8 ± 3926.8 nmol L−1) was 2 orders of magnitude higher than that of no-bubble area (53.7 ± 9.2 nmol L−1). Moreover, a large amount of CH4 accumulated under the ice in the bubble area. These findings suggest that bubbles determine the CH4 storage in ice and CH4 accumulation in the underlying water. Ice growth increases CH4 storage in ice and the underlying water because of the entrapment and re-dissolution of CH4 bubbles. However, ice melting releases the CH4 accumulated in the ice and underlying water. A comparison of the field and incubation experiments indicated that the deep-water environment of the reservoir had a CH4 burial effect. Stepwise regression analysis revealed that higher sediment organic matter content, median particle size, and porosity increased the production and release of CH4 bubbles, trapping more CH4 bubbles in ice. Overall, this study improves the mechanistic understanding of CH4 dynamics and predictability of CH4 emissions during ice-covered periods.

Assessment of Cryogenic Coring to Preserve Vertical Distributions of Trichloroethylene and Reaction Products in an Aquitard

AbstractThere is not a clear understanding of the extent by which naturally occurring reactions can attenuate trichloroethene (TCE) and its daughter products within low permeability zones (LPZs), and addressing this knowledge gap requires advancement of methods to accurately measure in situ volatile chemical concentrations. In this study, a soil coring method that freezes the soil in‐situ (a.k.a., cryogenic coring) was utilized to measure depth‐discrete distributions of TCE and its volatile reaction products through a TCE‐impacted silty clay aquitard, and results were compared with those from adjacent soil cores taken using a conventional coring approach. Vertical concentration profiles of TCE, cis‐1,2‐dichloroethylene (DCE), vinyl chloride (VC), ethane, and methane were all compared between the two coring methods, and results indicate the two coring methods recovered statistically equivalent concentrations of volatiles across most depths of the fine‐grained cohesive clayey soil at the study site. Biotic reductive dechlorination was the dominant TCE reaction pathway at the site; several reduced gasses that are possible markers for abiotic reduction were detected, but their concentrations and intervals of occurrence were not sufficiently consistent to indicate whether they were from abiotic TCE reduction or unrelated biological processes. Overall, cryogenic coring yielded improved recovery of sand lenses compared to conventional coring, but offered no apparent benefits for improved recovery of TCE and its volatile reaction products in the low permeability aquitard material at the site.

Evolution of suspended sediment concentration profiles in the partially and periodically stratified Changjiang Estuary during dry season

A better understanding of how the vertical profile of suspended sediment concentration (SSC) is influenced by multiple factors is required to better understand the hydrodynamics and sediment dynamics of estuaries. Based on in situ measurements obtained during a spring-neap cycle in the estuarine turbidity maximum (ETM) of the Changjiang Estuary in January 2015, we investigate the intratidal evolution and controlling mechanisms of SSC profiles. The results show that during the dry season the ETM was characterized by periodic salinity stratification, low SSC, suspended sediments with reduced grain size, and low sediment availability for resuspension. Three types of SSC profiles were identified, i.e., profiles with two-layer, linear and quasi-uniform structures. The shapes of the SSC profiles varied with time and generally deviated from the equilibrium profile due to the highly variable hydrodynamic and sediment conditions. During ebbs of spring and intermediate tides, the SSC profiles generally evolved from a two-layer to a quasi-uniform structure via a linear structure. Analysis shows that the linear and quasi-uniform profiles occurred under the combination of vertically well-mixed salinity, negligible resuspension, low settling velocity associated with very fine grains, and shallow water depth. Such an evolution pattern can be well reproduced using a one-dimensional numerical model, with the diffusion coefficient being parabolically distributed in the water column and the four factors mentioned above. The findings reported here imply a high variability of SSC profiles in estuarine environments.

Derivation of vertical concentration profile for nonuniform sediment in suspension using Shannon entropy

This research paper introduces a novel methodology for quantifying the sediment concentration of nonuniform sediment in suspension within an open channel, employing the concept of Shannon entropy. By treating the normalized sediment concentration as a stochastic variable, a probability distribution function is formulated via the maximum entropy principle. The validity of the proposed cumulative distribution function is established through a comparison with empirical data. Additionally, the newly devised concentration profile for nonuniform suspended sediment is compared with the established deterministic concentration equation and experimental data on grain-size distribution. A relationship between model parameters and Rouse number for nonuniform sediment is also provided. This study’s significance lies in introducing a new way to understand nonuniform sediment transport and its behaviors. The utilization of Shannon entropy not only showcases the versatility of this technique but also underlines its potential for unraveling the complexities inherent in nonuniform sediment dynamics. This study is the first step to introduce the entropy notion in nonuniform sediment and encourages more research to improve it.