Ultrafast Optical Control of Exciton Diffusion in WSe2/Graphene Heterostructures Revealed by Heterodyne Transient Grating Spectroscopy.

Using heterodyne transient grating spectroscopy, we observe a significant enhancement of exciton diffusion in a monolayer WSe2 stacked on graphene. The diffusion dynamics can be optically tuned within a few picoseconds by altering the photoexcited carrier density in graphene. The effective diffusion constant in initial picoseconds in the WSe2/graphene heterostructure is (40.3 ± 4.5) cm2 s-1, representing a substantial improvement over (2.1 ± 0.8) cm2 s-1, typical for an isolated WSe2 monolayer. This enhancement can be understood in terms of a transient screening of impurities, charge traps, and defect states in WSe2 by photoexcited charge carriers in graphene. Furthermore, diffusion within WSe2 is affected by interlayer interactions, such as charge transfer, varying with the incident excitation fluence. These findings underscore the dynamical nature of screening and diffusion processes in heterostructures of 2D semiconductors and graphene and provide insights for future applications of these systems in ultrafast optoelectronic devices.

The positive environmental impact of virtual isotretinoin management.

BACKGROUND/OBJECTIVES: Other medical specialties have studied how their practices influence the environment, but environmental impact studies in the field of dermatology remain limited. With respect to dermatology, vehicle emissions by patients traveling to and from appointments are an important factor influencing climate change. This study was undertaken to determine the greenhouse gas emissions avoided by managing isotretinoin virtually at West Virginia University Hospital. METHODS: A retrospective cross-sectional study was conducted during the COVID-19 outbreak from March 25 to December 1, 2020, where travel data were acquired and converted to emission data. RESULTS: 5,137 kg of GHG emissions in CO2 equivalents were prevented by managing isotretinoin virtually during the study period. 49 400 kg of GHG emissions in CO2 equivalents would be prevented annually. This is the emission load released when 24 690 kg of coal are burned. CONCLUSIONS: Environmental impact studies in the field of dermatology remain limited. GHG emissions were significantly reduced by virtually managing isotretinoin at a single institution. The practice of dermatology could reduce its carbon footprint by managing isotretinoin virtually, even in non-pandemic periods. Given that isotretinoin management represents a small percentage of the overall carbon footprint associated with dermatology, dermatologists should identify other conditions amenable to virtual medicine to produce greater environmental impact.

Relative linkages of canopy-level CO2 fluxes with the climatic and environmental variables for US deciduous forests.

We used a simple, systematic data-analytics approach to determine the relative linkages of different climate and environmental variables with the canopy-level, half-hourly CO2 fluxes of US deciduous forests. Multivariate pattern recognition techniques of principal component and factor analyses were utilized to classify and group climatic, environmental, and ecological variables based on their similarity as drivers, examining their interrelation patterns at different sites. Explanatory partial least squares regression models were developed to estimate the relative linkages of CO2 fluxes with the climatic and environmental variables. Three biophysical process components adequately described the system-data variances. The 'radiation-energy' component had the strongest linkage with CO2 fluxes, whereas the 'aerodynamic' and 'temperature-hydrology' components were low to moderately linked with the carbon fluxes. On average, the 'radiation-energy' component showed 5 and 8 times stronger carbon flux linkages than that of the 'temperature-hydrology' and 'aerodynamic' components, respectively. The similarity of observed patterns among different study sites (representing gradients in climate, canopy heights and soil-formations) indicates that the findings are potentially transferable to other deciduous forests. The similarities also highlight the scope of developing parsimonious data-driven models to predict the potential sequestration of ecosystem carbon under a changing climate and environment. The presented data-analytics provides an objective, empirical foundation to obtain crucial mechanistic insights; complementing process-based model building with a warranted complexity. Model efficiency and accuracy (R(2) = 0.55-0.81; ratio of root-mean-square error to the observed standard deviations, RSR = 0.44-0.67) reiterate the usefulness of multivariate analytics models for gap-filling of instantaneous flux data.

The effects of large roughness elements on the in-stream transport and retention of polystyrene microplastics.

The mechanisms controlling transport and retention of microplastics (MPs) in riverine systems are not understood well. We investigated the impact of large roughness elements (LREs) on in-stream transport and retention of the ubiquitous polystyrene-microplastics (PS-MPs). Scaled experiments were conducted with and without LREs under various shear Reynolds numbers (Re*) in an ecohydraulics flume. Our results, for the first time, demonstrated a clear dependence of the MPs' velocity on Re* in LREs-dominated channel. Two distinct regimes and thresholds were identified: lower Re* (≤ 15,000) regime corresponding to higher velocities of MPs ([Formula: see text]> 0.45), and higher Re* (> 15,000) to lower [Formula: see text]< 0.45). The presence and higher density of LREs increased Re*, decreased [Formula: see text], and enhanced the PS-MPs capture. The LREs-generated turbulence kinetic energy (TKE) was found to be a good predictor of PS-MPs transport and retention rates, indicating the effectiveness of LREs in retaining PS-MPs in streams and rivers.

Metabolic scaling of stream dissolved oxygen across the U.S. Atlantic Coast.

We investigated the hypothesis of emergent 'biogeochemical' similitude (parametric reduction) and scaling of dissolved oxygen (DO) in coastal streams across the U.S. Atlantic Coast by employing dimensional analysis methodology from fluid mechanics and hydraulic engineering. Two mechanistically meaningful dimensionless numbers were discovered as the stream 'metabolic' number and the fraction of 'DO saturation' number. The 'metabolic' number represented the synergistic control on stream DO from various climatic, hydrologic, biochemical, and ecological drivers (e.g., water temperature, atmospheric pressure, stream width and depth, total phosphorus, pH, and salinity). A graphical exploration of the 'metabolic' versus the 'DO saturation' numbers led to collapse of data during 1998-2015 from diverse coastal streams into an emergent process diagram, indicating three metabolism regimes (high, transitional, and low). The high and low metabolism regimes were, respectively, characterized by the most and least favorable environmental conditions for stream DO depletion-through reduced dissolution and reaeration, as well as increased organic decomposition, respiration, and nitrification. The emergent process diagram led to a generalized power law scaling relationship of the 'DO saturation' number as a function of the 'metabolic' number (exponent ~ 1/3; Nash-Sutcliffe Efficiency, NSE = 0.83-0.85). The metabolic scaling law was leveraged to develop a generalized empirical model to successfully predict DO in diverse streams across the U.S. Atlantic Coast (NSE = 0.83). The emergent process diagram, metabolic scaling law, and prediction model of DO would help understand and manage water quality and ecosystem health of coastal streams in the U.S. and elsewhere.

Machine learning techniques to increase the performance of indirect methane quantification from a single, stationary sensor.

Researchers are searching for ways to better quantify methane emissions from natural gas infrastructure. Current indirect quantification techniques (IQTs) allow for more frequent or continuous measurements with fewer personnel resources than direct methods but lack accuracy and repeatability. Two IQTs are Other Test Method (OTM) 33A and Eddy Covariance (EC). We examined a novel approach to improve the accuracy of single sensor IQT whereby the results from both OTM and EC were combined with two machine learning (ML) models, a random forest (RF) and a neural network (NN). Then, models were enhanced with feature reduction and hyper-parameter tuning and compared to traditional quantification methods. The NN and RF improved upon the default OTM by an average of 44% and 78%, respectively. When compared to traditional OTM estimates with low Data Quality Indicators (DQIs), RF and NN models reduced 1σ errors from ±66% to ±13% and ±34%, respectively. Models also reduced the standard deviation of estimates with 93% and 85% of estimates falling within ±50% of the known release rate. This approach can be deployed with single sensor systems at well sites to improve confidence in reported emissions, reducing the number of anomalous overestimates that would trigger unnecessary site evaluations. Additional improvements could be realized by expanding training datasets with more methane release rates. Further, deployment of such models in a variety of situations could enhance their ability help close the gap between bottom-up inventory and top-down studies by enabling continuous monitoring of temporal emissions that could identify with improved confidence, atypically higher emissions. Accurate remote single sensor systems are key in developing an improved understanding of methane emissions to enable industry to identify and reduce methane emissions.

Soil carbon consequences of historic hydrologic impairment and recent restoration in coastal wetlands.

Coastal wetlands provide key ecosystem services, including substantial long-term storage of atmospheric CO2 in soil organic carbon pools. This accumulation of soil organic matter is a vital component of elevation gain in coastal wetlands responding to sea-level rise. Anthropogenic activities that alter coastal wetland function through disruption of tidal exchange and wetland water levels are ubiquitous. This study assesses soil vertical accretion and organic carbon accretion across five coastal wetlands that experienced over a century of impounded hydrology, followed by restoration of tidal exchange 5 to 14 years prior to sampling. Nearby marshes that never experienced tidal impoundment served as controls with natural hydrology to assess the impact of impoundment and restoration. Dated soil cores indicate that elevation gain and carbon storage were suppressed 30-70 % during impoundment, accounting for the majority of elevation deficit between impacted and natural sites. Only one site had substantial subsidence, likely due to oxidation of soil organic matter. Vertical and carbon accretion gains were achieved at all restored sites, with carbon burial increasing from 96 ± 33 to 197 ± 64 g C m-2 y-1. The site with subsidence was able to accrete at double the rate (13 ± 5.6 mm y-1) of the natural complement, due predominantly to organic matter accumulation rather than mineral deposition, indicating these ecosystems are capable of large dynamic responses to restoration when conditions are optimized for vegetation growth. Hydrologic restoration enhanced elevation resilience and climate benefits of these coastal wetlands.

Climate and land cover change impacts on stormwater runoff in large-scale coastal-urban environments.

This study aims to evaluate the individual and synergistic controls of climatic and land cover changes on stormwater runoff regimes, and perform a comparative synthesis of the historical and future runoffs for complex coastal-urban environments. A large-scale (7117 km2) mechanistic hydrologic model was developed for Florida Southeast Coasts Basin as the study area using U.S. Environmental Protection Agency (EPA)'s Storm Water Management Model 5.1. The model was calibrated and validated with daily streamflow observations (Nash-Sutcliffe Efficiency = 0.74 to 0.92) during 2004-2013 (termed 2010s), computing the corresponding runoff volume as a historical reference. Runoffs for 2050s (2044-2053) and 2080s (2076-2085) were quantified by incorporating climatic projections from 20 General Circulation Models and land cover projections from EPA under the Representative Concentration Pathways (RCP) 4.5 and 8.5 scenarios. We found a predominant climatic control on the potential runoff changes and a high vulnerability in the coastal-urban environments. The concurrent changes in climate and land cover led to synergistic (stronger than the sum of individual effects) nonlinear responses of runoff. The projected changes in climate and land cover together would increase the annual basin runoff volume by 118%, 106%, 86%, and 80% under the 2080s-RCP 4.5, 2050s-RCP 4.5, 2050s-RCP 8.5, and 2080s-RCP 8.5 scenarios, respectively. Greater increases in runoff were noted at and around the urban centers than that at the non-urban areas across the basin. The relative increases in runoff were higher during the dry season and transitional months (October-May) than the wet season (June-September). Our findings would guide stormwater management and ecosystem protection for southeast Florida and coastal built environments across the world.

Ecological parameter reductions, environmental regimes, and characteristic process diagram of carbon dioxide fluxes in coastal salt marshes.

We investigated the ecological parameter reductions (termed "similitudes") and characteristic patterns of the net uptake fluxes of carbon dioxide (CO2) in coastal salt marshes using dimensional analysis method from fluid mechanics and hydraulic engineering. Data collected during May-October, 2013 from four salt marshes in Waquoit Bay and adjacent estuary, Massachusetts, USA were utilized to evaluate the theoretically-derived dimensionless flux and various ecological driver numbers. Two meaningful dimensionless groups were discovered as the light use efficiency number (LUE = CO2 normalized by photosynthetically active radiation) and the biogeochemical number (combination of soil temperature, porewater salinity, and atmospheric pressure). A semi-logarithmic plot of the dimensionless numbers indicated the emergence of a characteristic diagram represented by three distinct LUE regimes (high, transitional, and low). The high regime corresponded to the most favorable (high temperature and low salinity) condition for CO2 uptake, whereas the low regime represented an unfavorable condition (low temperature and high salinity). The analysis identified two environmental thresholds (soil temperature ~ 17 °C and salinity ~ 30 ppt), which dictated the regime transitions of CO2 uptake. The process diagram and critical thresholds provide important insights into the CO2 uptake potential of coastal wetlands in response to changes in key environmental drivers.

Relative linkages of stream water quality and environmental health with the land use and hydrologic drivers in the coastal-urban watersheds of southeast Florida.

A systematic data analytics was employed to determine the relative linkages of stream water quality and environmental health with the land use and hydrologic drivers in the coastal-urban watersheds of southeast Florida. Power law-based partial least squares regression models were developed to reliably estimate the linkages by appropriately resolving multicollinearity (Nash-Sutcliffe efficiency = 0.72-0.95). The analytics indicated Everglades as the external and the largest source of total nitrogen (TN) in the coastal-urban streams for both wet (June-October) and dry (November-May) seasons. The "external driver" exhibited 1.5-2 times stronger control on stream TN than that of the watershed "land use," "hydrology," and the "upstream reach" contributions. In contrast, Everglades appeared to be a minor source of in-stream total phosphorus (TP), which was predominantly controlled by the internal watershed processes. TP was most strongly linked with the upstream reach concentrations and watershed land uses in the wet and dry seasons, respectively. Despite the predominantly built-up fraction (74%) of the study area, agricultural land was the most substantial watershed source of in-stream nutrients. The linkages of algal biomass (Chl a) with the drivers indicated TP as the limiting nutrient. Stream dissolved oxygen was most strongly influenced by the adjacent groundwater depth and watershed land uses, respectively, in the wet and dry seasons. The estimated relative linkages and insights would be useful to identify the management targets and priorities to achieve healthy coastal-urban stream ecosystems in southeast Florida and around the world.