Evaporation Measurements Research Articles

A comprehensive study across methods and time scales to estimate surface fluxes from Lake Kinneret, Israel

Summary Estimation of surface fluxes in general, and evaporation rates over lake surfaces in particular, is a difficult task. While previous studies of Lake Kinneret, Israel, have looked at evaporation rate using a single timescale or single measurement method, this is the first comprehensive study to reconcile and explain the differences between a variety of measurement and calculation methods (pan evaporation, water and heat balance, aerodynamic methods and Penman simplified methods) across a multitude of temporal scales (10 min, 1 h, daily, monthly). The cross validation of the methods suggests that the hourly calculation with the aerodynamic methods may result in a better estimation of the monthly Bowen Ratio for the energy balance methods, and therefore may increase its reliability. We also show that comprehensive measurements and calculations of monthly heat storage change in the lake can be used to verify the hourly results of the hydrodynamic method, and the daily calculations of the Penman’s equation method. Moreover, using the Penman’s equation it is suggested that daily Pan A evaporation measurements are nearly similar to the evaporation from the deep water lake when the heat storage change is not taken into account. It is suggested that subject to data availability, the cross validation of multiple methods increases the reliability and confidence of the measured and calculated surface fluxes.

A Comparative Study of Daily Pan Evaporation Estimation Using ANN, Regression and Climate Based Models

Evaporation estimates are needed for efficient management of water resources at a farm scale as well as at a regional or catchment scale. This paper presents application of artificial neural networks (ANN), statistical regression and climate based models viz.: Penman, Priestley–Taylor and Stephens and Stewart, for estimation of daily pan evaporation. Six different measured weather variables comprising various combinations of maximum and minimum air temperature, sun shine hours, wind speed, relative humidity I and II were used. Randomly selected 1,096 daily records were used to develop the models of ANN and regression, and 365 daily records were used as independent data set for performance evaluation, which was not used previously in any of the model development process. The results of the developed ANN and multiple linear regression (MLR) models along with Penman, Priestley-Taylor and Stephens and Stewart models were compared statistically with observed pan evaporation values. Comparison showed that there is slightly better agreement between the ANN estimations and measurements of daily pan evaporation than other models.

Cr Evaporation of Metallic Interconnects: A Novel Method for Quantification

Chromium evaporation from metallic interconnects in SOFCs is commonly poisoning the cathode of SOFCs. To reduce chromium evaporation, rather thick coatings have been deposited using various coating techniques. In this study a 22% Cr ferritic steel has been coated with thin films (<1µm) of metallic cobalt. The Cr evaporation measurements are carried out with a denuder technique which allows time resolved quantification of volatile chromium species. Downstream of the samples a denuder tube is placed which collects any CrO2(OH)2 evaporating from the samples. The experiments are carried out at 850{degree sign}C in a N2-20%O2-3%H2O atmosphere. The investigated Co coatings proved to be very efficient and reduce the Cr evaporation by approxiamatly 90%. Microscopic studies show that both coated and uncoated samples develop two-layered oxide scales with the bottom part consisting of Cr-rich corundum type oxide. The uncoated samples develop a top layer consisting of (Cr,Mn)3O4 oxide while the Co coated samples exhibit a top layer consisting of (Co,Mn)3O4.

Evaluation of Class A Pan Coefficient Models for Estimation of Reference Crop Evapotranspiration in Cold Semi-Arid and Warm Arid Climates

Evapotranspiration and evaporation measurements are important parameters for many agricultural activities such as water resource management and environmental studies. There are several models which can determine pan coefficient (KPan), using wind speed, relative humidity and fetch length conditions. This paper analyses seven exiting pan models to estimate KPan values for two different climates of Iran. Monthly mean reference crop evapotranspiration (ET0) was calculated according to the pan-ET0 model. The results showed that estimated pan coefficients by majority of the suggested models were not statistically accurate to be used in the pan-ET0 conversion method. However, for the cold semi-arid climate condition, the best KPan models for estimation of ET0 were Orang and Raghuwanshi–Wallender, respectively. Also, the Snyder and Orang models were best fitted models for warm arid climate, respectively. The mean annual value of KPan, determined by Penman–Monteith FAO 56 (PMF-56) standard model for warm arid sites, was approximately 32% higher than the corresponding value in the cold semi-arid climate. Similarly, the mean annual ET0 in the warm arid sites was 66% higher, compared to the ET0 of the cold semi-arid sites. These types of warm arid and semi-arid climates are found widely throughout the world.

Bowen ratio evaporation measurement in a remote montane grassland: Data integrity and fluxes

Summary Evaporation measurements using two Bowen ratio energy balance (BREB) systems in a remote high altitude montane grassland catchment of the Drakensberg Mountains, Cathedral Peak, South Africa are reported on. Various methods of data verification and rejection of inaccurate measured air temperature and water vapour pressure gradients are examined. A theoretical analysis, based on the equivalent temperature, results in data rejection procedures using the measurement of the air temperature profile difference. Data rejection is necessary whenever the Bowen ratio approaches −1, resulting in extremely inaccurate and impossibly large positive or negative sensible heat and latent energy fluxes. Using the simplified energy balance, it is shown that when the Bowen ratio approaches the limit of −1, for which the available energy flux density approaches 0 W m −2 , conditions are pseudoadiabatic and isobaric and that such conditions can be depicted by the wet-bulb temperature isolines of the psychrometric chart. Disregarding evaporation estimates for which the Bowen ratio values are between arbitrarily chosen values remedies the problem to some extent. With this method, daily total evaporation may be reasonable but 20-min values unreasonable during mainly early morning and late afternoon periods. A more sensitive and dynamic approach is used to prevent BREB data from being excluded unnecessarily and to prevent rogue values escaping detection. Once the rejection procedures were applied, the 20-min BREB latent energy flux estimates compared well with measurements from a weighing lysimeter adjacent the site. Three methods were used to estimate the exchange coefficient K which allowed flux estimation for when BREB data are invalid or lacking. One method involved calculating K from wind speed only and the second method was based on the MOST-dependent temperature-variance method for which the 20-min standard deviation of 1-Hz air temperature data were used. From independent measurements of sensible heat H and latent energy LE , a time-invariant exchange coefficient K was also determined from measurements of the air temperature profile difference. These methods were used when there were invalid water vapour pressure data due to condensation in the hoses or problems with the cooled dew point mirror or when the fine-wire thermocouples were damaged or when there were unreliable estimates of the air temperature gradient. The time-invariant value for K used in one of the methods, 0.239 m 2 s −1 , was confirmed for a mixed grassland catchment using independent eddy covariance and surface-layer scintillometer measurements of H and Bowen ratio measurements of the air temperature profile difference.

Long-term results of evaporation rate in xerothermic Oak altitudinal vegetation stage in Southern Slovakia

The paper presents the results of the model computation of actual and potential evapotranspiration as well as evaporation measurements from the GGI-3000 Pan (evaporimeter) at 3 selected stations of Slovakia for the periods 1971–2000 (2 stations) and 1986–2000 (1 station). The localities are situated in the most southern parts of Slovakia in 1st Oak stage and on southern slopes of the Carpathian promontories. A model computation of the monthly totals of actual and potential evapotranspiration was performed by a common solution of energy and water balance equations of the top one-meter layer of the soil. The model outputs are compared to the measurement data. The dependence of the ratio of actual evapotranspiration/ evaporation measured by the GGI-3000 Pan (E/Ep) on relative evapotranspiration (E/Eo), when E, Eo are calculated, is linear with a high correlation coefficient during the season from April to October. The paper also gives the comparison results of the mean monthly and annual air temperatures, temperatures of active and saturated surface with the mean monthly temperature of free water surface in the GGI-3000 Pan at these 3 stations (Bratislava-Koliba, Somotor and Žiharec). It was found that the mean free water surface temperature for the period from April to October is close to the mean temperature of saturated surface during that period. The mean temperatures of free water surface in the GGI-3000 Pan from April to October tend to increase for the period 1971–2000.

Solar heating of the soil and evaporation from a soil surface

In 1994 a system for classifying landfills for municipal solid waste by local climate was introduced in South Africa. If it can be shown that potential evaporation from the landfill surface will exceed average annual rainfall, it is accepted that no significant leachate will be generated in the landfill, and a leachate collection system, impervious underliner and leachate treatment system is not legally required. This represents very large financial savings on the establishment of a new landfill or the extension of an existing one. Hence the measurement and prediction of evaporation from a soil surface has become a subject of considerable importance to the waste disposal industries in South Africa and neighbouring countries (e.g. Botswana and Namibia) that have adopted similar climatic approaches. Over the past 20 years it has become apparent that the long-established solar energy balance approach to assessing evaporation from a soil may be in error. This is because it regards solar energy expended on heating the soil as reducing the energy available for supplying energy of evaporation. This paper describes experiments aimed at exploring the role of heating of the soil in the evaporation process, and concludes that, far from subtracting energy from the process, it actually provides most of the energy for evaporation.

Introduction: Can Water Use Efficiency Be Modeled Well Enough to Impact Crop Management?

Crop water use efficiency (WUE, yield per unit of water use) is key for agricultural production with limited water resources. Policymakers and water resource managers working at all scales need to address the multitudinous scenarios in which cropping systems and amounts, timing and methods of irrigation, and fertilizer applications may be changed to improve WUE while meeting yield and harvest quality goals. Experimentation cannot address all scenarios, but accurate simulation models may fill in the gaps. The nine papers in this special section explore how four simulation models were used to simulate yield, water use, and WUE of cotton (Gossypium hirsutum L.), maize (Zea mays L.), quinoa (Chenopodium quinoa Willd.), and sunflower (Helianthus annuus L.) in North and South America, Europe, and the Middle East. All the models simulated WUE adequately under well‐watered conditions, but tended to misestimate WUE under conditions of water stress, which limits their use for exploration of deficit irrigation scenarios or rain‐fed or dryland situations with expected soil water deficits. None of the experimental conditions reported involved separate measurements of evaporation (E) and transpiration (T); so there was no opportunity to test the separation of E and T simulated in the newest of the models, AquaCrop. The lack of separate E measurements also limited the authors in exploring reasons why WUE was not simulated well under water stress conditions. Future studies exploring WUE simulation should include E or T measurements so that effects of management methods that reduce E can be studied.

Eddy covariance measurements of surface energy budget and evaporation in a cool season over southern open water in Mississippi

Eddy covariance measurements of sensible (H) and latent (LE) heat fluxes were made over a large southern open water surface of Ross Barnett Reservoir (the Reservoir hereafter) in Mississippi during the cool season with frequent incursions of cold fronts from 1 September 2007 to 31 January 2008. The eddy covariance tower was located in the middle of the main body of the Reservoir with the tower fetches exceeding 2.0 km in all directions. The Reservoir was ice‐free in winter and the water temperatures always decreased with depth. Over the entire cool season, the averaged water surface temperatures were 1.8°C higher than the overlying air (i.e., positive temperature gradients that led to thermally convective conditions) and the averaged vapor pressure near the water surface was 0.8 kPa greater than the overlying air (i.e., positive vapor pressure gradients), though occasionally negative gradients for temperature and vapor pressure were also observed for short periods. On average, the wind speeds were considerably large (3.9 m s−1) to maintain adequate turbulent mixing mechanically. As a consequence of the combined effect of thermally and mechanically generated turbulent mixing, consistently positive H (with a mean H of 20.0 W m−2) and LE (with a mean LE of 80.0 W m−2) occurred during the entire season. These continuous energy losses via H and LE resulted in release of a large amount of energy stored in the water to the atmosphere. The mean Bowen ratio was low for this open water surface (i.e., 0.3), suggesting that most of the energy released from the water fueled evaporation rather than sensible heating of the atmosphere. Nighttime evaporative water losses were substantial, contributing to 45% of the total evaporative water loss in this cool season. Frequent incursions of cold fronts with windy, cold, and dry air masses significantly promoted turbulent exchanges of sensible and latent heat through enhanced turbulent mixing thermally and mechanically, leading to large H and LE events. Daily H and LE (i.e., evaporation) during the passages of cold fronts were on average 2.7 and 7.3 times those in nonevent days, respectively. Given the fact that large H and LE events occurred 26% of the time for our site, these cold front events caused an increase in the seasonal H by 42% and LE by 157%. Therefore changes in frequency, intensity, and duration of synoptic weather events, particularly the incursions of cold fronts, have significant impacts on the surface energy budget and evaporation over water at this site.

Geochemistry of Erosion Processes on Badland Slopes

Understanding erosion processes is important to prevent natural disasters such as slope failure and bedrock erosion in immature sedimentary rocks. Pliocene–Pleistocene illite-rich, non-smectite mudstone of the Gutingkeng Formation is distributed over 250 km2 in southern Taiwan, forming badlands (locally called moon-world) with mud volcanoes nearby. These volcanoes erupt saline water and natural gas, and producing a Na+, Ca2+, Cl-, and SO42- rich unsaturated Popcorn crust, which covers the mudstone slope surfaces in the moon-world area. In the crust porewater, ion strength reaches about 10 mol/L; zeta potential on particle surfaces shows a highly positive voltage. Repulsion occurs between particles under this high voltage in the crust, which is rapidly slaked to form mud by heavy precipitation. The zone that is rapidly slaked by precipitation reaches 10-20 cm beneath the crust surface. Ion strength of porewater of fresh mudstone is 0.5 mol/L approaches 0 mV (range 0.2-0.5 mol/L, pH 4-6). The surface charge of particles decreases with the infiltration of precipitation into the crust and fresh rock, with a minus surface charge occurring with increased rain infiltration. This leads to many cracks forming on the surface of mudstone, which is different from the mechanism of rapid slaking. Evaporation from the 10-20 cm-thick zone between the crust and the underlying fresh mudstone would stop if water was not supplied from depth, which is supported by in-situ measurements of water evaporation in the field. These mudstones erode readily under high precipitation because of the repulsion caused by the high ion strength of porewater. High-salinity porewater including mudstone is distributed near the active mud volcanoes where saline water rises and there is a rapid uplift rate. Rapid slaking occurred with some elements in the concentrated crust and near the surface drying zone.

Short-term climate variability and the commercial barramundi (Lates calcarifer) fishery of north-east Queensland, Australia

The sustainable productivity of estuarine fisheries worldwide is threatened by over-fishing, habitat destruction and water impoundment. In some cases, the natural variability of freshwater inputs has been shown to affect catch when low flows reduce nutrient input and inundated nursery habitats. Historically, the annual commercial catch of barramundi (Lates calcarifer) in Queensland has been highly variable for reasons not fully understood. In conjunction with a life-cycle model, statistical analyses of climate variables and barramundi catch data from the Princess Charlotte Bay area identified several significant relationships. Warm sea surface temperatures, high rainfall, increased freshwater flow and low evaporation (all measures of an extensive and productive nursery habitat) were significantly correlated with barramundi catch 2 years later and suggest that young barramundi survival is enhanced under these conditions. Catchability was significantly increased with high freshwater flow and rainfall events in the year of catch. A forward stepwise ridge regression model that included a measure of rainfall and evaporation 2 years before catch explained 62% of the variance in catch adjusted for effort. It is recommended that the impact of climate variability be considered in the management of wild barramundi stocks and possibly other species not yet examined.

Modelling field-data of preferential flow in paddy soil induced by earthworm burrows

Dye tracer studies revealed that earthworm burrows in the compacted plough pan of a Chinese paddy rice field can serve as preferential flow paths. It is, however, unclear whether the observed bypass of the compacted soil horizon might be explained by differences in hydraulic properties between the plough pan, the worm burrows with a surrounding denser drilosphere and the un-compacted subsoil, or by lower-permeable macropore walls. The objective is to separately analyse effects of the individual flow domains and to identify possible limiting factors (bottlenecks) in the flow system for better soil drainage management. Hydraulic properties are inversely estimated from in situ measurements of pressure heads and evaporation by using HYDRUS_1D code. Field data of 2D pressure head progression after dye tracer infiltration in the vicinity of worm burrows are simulated using HYDRUS_2D. The axisymmetric 2D flow model considers a highly permeable cylindrical macropore region in the centre of the flow domain, assuming Darcy's law is valid. The match between simulated and measured pressure head fields improved after including a lower-permeable drilosphere pore domain. Scenario simulations show that the inflow into the 'bypass-flow' domain are reduced by the homogenized topsoil (i.e., after puddling) and limited if the macropore domain is relatively shallow. The results suggest that basic structural features may in this concept be considered as one possibility to describe observed preferential flow patterns. The separate consideration of soil structural effects may help developing and improving management strategies for manipulation of preferential flow in soils of paddy fields.

Coupling of heat, water vapor, and liquid water fluxes to compute evaporation in bare soils

The quantification of soil evaporation and of soil water content dynamics near the soil surface are critical in the physics of land-surface processes on regional and global scales, in particular in relation to mass and energy fluxes between the ground and the atmosphere. Although it is widely recognized that both liquid and gaseous water movement are fundamental factors in the quantification of soil heat flux and surface evaporation, their computation is still rarely considered in most models or practical applications. Moreover, questions remain about the correct computation of key factors such as the soil surface resistance or the soil surface temperature. This study was conducted to: (a) implement a fully coupled numerical model to solve the governing equations for liquid water, water vapor, and heat transport in bare soils, (b) test the numerical model with detailed measurements of soil temperature, heat flux, water content, and evaporation from the surface, and (c) test different formulations for the soil surface resistance parameter and test their effect on soil evaporation. The code implements a non-isothermal solution of the vapor flux equation that accounts for the thermally driven water vapor transport and phase changes. Simulated soil temperature, heat flux, and water content were in good agreement with measured values. The model showed that vapor transport plays a key role in soil mass and energy transfer and that vapor flow may induce sinusoidal variations in soil water content near the surface. Different results were obtained for evaporation calculations, depending on the choice of the soil surface resistance equation, which was shown to be a fundamental term in the soil-atmosphere interactions. The results also demonstrated that soil water dynamics are strongly linked to temperature variations and that it is important to consider coupled transport of heat, vapor and liquid water when assessing energy dynamics in soils.

Measurement of evaporation from the forest floor in a deciduous forest throughout the year using microlysimeter and closed‐chamber systems

AbstractEvaporation from the forest floor (EFF) in a deciduous broadleaf forest was measured using microlysimeter and closed‐chamber systems. The microlysimeter was used at six points in the experimental basin, and measurements gave different EFF values at different points. This could be attributed to the local photoenvironment of each sampling point, rather than to litter conditions, if the spatial variation in air temperature (Ta) or vapour pressure deficit (VPD) at the forest floor was small within this basin. A detachable microlysimeter measured condensation in the litter layer during the night, indicating that the litter layer, as well as the mulch layer, played a role in preventing evaporation from the soil layer. The closed‐chamber system made it possible to continuously measure long‐term EFF. EFF was closely related to VPD; even during the night, when solar radiation was zero, EFF amounted to 14·0% of the daily EFF. The daily EFF was 0·20 ± 0·13 mm day−1 during the study period, with two seasonal peaks: in late spring (0·31 mm day−1 in April) and early fall (0·22 mm day−1 in September). The former peak has been reported from two deciduous forests in Japan and is strongly related to the solar radiation reaching the forest floor when the trees are dormant. Copyright © 2008 John Wiley &amp; Sons, Ltd.

The effect of shrub canopy upon surface temperatures and evaporation in the Negev Desert

AbstractAlthough known as ‘islands of fertility’ or ‘resource islands’, information regarding the effect of shrubs upon microclimate in deserts is scarce. Here we report on measurements of evaporation and temperatures that were carried out in and around a pair of shrubs at the Nizzana research site in the western Negev Desert during 1993–94 and during the growing season (November–March) of 1994–95 and 1996–1997. Whereas evaporation was measured monthly using mini‐atmometers (10 cm diameter and 10 cm tall) at an exposed site and under and around the shrub (at the eastern, southern, western and northern aspects), temperature was measured under a shrub canopy, at its northern aspect, and at an exposed habitat. Evaporation was aspect dependent with increasing rates in the following order: exposed &gt; south‐facing &gt; west‐facing ≈ east‐facing &gt; north‐facing &gt; under canopy. Except from the northern aspect, the under‐canopy habitat showed substantially lower rates of evaporation in comparison with all other habitats. The differences between the under‐canopy and the exposed habitat were larger during wintertime (with the under‐canopy habitat having 0·53 times the evaporation rate than that of the exposed habitat) although higher differences in temperatures characterized both habitats in summertime (up to 14·4 °C in summer as compared with 6·9 °C only in winter). The results were explained by extended surface wetness that characterized the under‐canopy habitat following rainstorms. While already being dried out at the exposed habitat, surface wetness at the under canopy habitat persisted for several days afterwards, resulting, following one rainstorm, in vapour pressure of 2·15–2·39 kPa in comparison with only 0·82–0·83 kPa of the exposed habitat. The substantially lower evaporation rates that characterize the under‐canopy habitat may thus play a pivotal role in providing preferential conditions for lush under‐canopy annual growth. Copyright © 2008 John Wiley and Sons, Ltd.

Contributions of Evaporation and Other Mechanisms to Tear Film Thinning and Break-Up

To evaluate the contribution of three mechanisms-evaporation of the tear film, inward flow of water into the corneal epithelium or contact lens, and "tangential flow" along the surface of epithelium or contact lens-to the thinning of the tear film between blinks and to tear film break-up. In addition to a discussion of relevant studies, some previously unpublished images are presented illustrating aspects of tear film break-up. CONTRIBUTIONS OF THREE MECHANISMS TO TEAR FILM BREAK-UP: Inward flow of water into the epithelium or contact lens is probably unimportant, and a small flow in the opposite direction may actually occur. Tangential flow is probably important in certain special cases of tear film break-up-at the black line near the tear meniscus, over surface elevations, after partial blinks, and from small thick lipid spots in the tear film. In all these special cases it is argued that tangential flow is important initially, but evaporation may be needed for final thinning to break-up. It is argued that most of the observed tear film thinning between blinks is due to evaporation, rather than tangential flow, and that large "pool" break-up regions are the result of evaporation over an extended area. Evaporation in our "free-air" conditions may be four to five times faster than the average of the values reported in the literature when air currents are prevented by preocular chambers. However, recent evaporation measurements using "ventilated chambers" give higher values, which may correspond better to free-air conditions. Thus evaporation may be fast enough to explain many cases of tear film break-up, and to give rise to considerable increases in the local osmolarity of the tear film between blinks.

Surfactant dependent morphology of polymeric capsules of perfluorooctyl bromide: Influence of polymer adsorption at the dichloromethane–water interface

In a strategy to develop more stable ultrasound contrast agents (UCAs), we have designed a process to obtain nano/microcapsules with a single core of liquid perfluorocarbon within a biodegradable polymeric shell of homogeneous thickness. During the optimization of perfluorooctyl bromide (PFOB) encapsulation by solvent emulsion-evaporation, a marked influence of surfactants has been observed. While sodium cholate leads to spherical capsules of homogeneous thickness, sodium taurocholate induces to the formation of “acorn”-particles with one hemisphere of PFOB and another one of PLGA, and polyvinyl alcohol is responsible for the coexistence of both morphologies. Whereas the theoretical model proposed by Torza and Mason [J. Colloid Interface Sci. 33 (1970) 67] fails to predict the observed morphologies, microscopic observations of the evaporation and interfacial tension measurements provide an insight into the mechanism of formation of these structures. Most probably, there is a competition between PLGA and the surfactant stabilizing the emulsion at the dichloromethane–water interface. If PLGA is able to adsorb at the interface, the core–shell morphology is obtained, otherwise the acorn morphology is preferentially formed. When the surfactant rearrangement at the interface is long (>30 min), a coexistence of morphologies can be obtained.

Dry Eye Diagnosis

To determine the most effective objective tests, applied singly or in combination in the diagnosis of dry eye disease. Two groups of subjects--41 with dry eye and 32 with no ocular surface disease--had symptoms, tear film quality, evaporation, tear turnover rate (TTR), volume and osmolarity, and meibomian gland dropout score assessed. The subjects with dry eye had TTR, tear evaporation, and osmolarity significantly different from that of healthy normal subjects. Cutoff values between the groups were determined from distribution curves for each aspect of tear physiology, and the effectiveness of the cutoff was determined from receiver operator characteristic (ROC) curves. Values of 12%/min for TTR, 33 g/m(-2)/h for evaporation, and 317 mOsmol/L for osmolarity were found to give sensitivities, specificities, and overall accuracies of 80%, 72%, and 77%; 51%, 96%, and 67%; and 78, 78%, and 79%, respectively when applied singly as diagnostic criteria in dry eye. In combination, they yielded sensitivities, specificities, and overall accuracy of 100%, 66%, and 86% (in parallel) and 38%, 100%, and 63% (in series), respectively. Discriminant function analysis incorporating these three factors in an equation allowed diagnosis with a sensitivity of 93%, specificity of 88%, and overall accuracy of 89%. Tear osmolarity is the best single test for the diagnosis of dry eye, whereas a battery of tests employing a weighted comparison of TTR, evaporation, and osmolarity measurements derived from discriminant function analysis is the most effective.

Interaction of Hydration, Aging, and Carbon Content of Soil on the Evaporation and Skin Bioavailability of Munition Contaminants

Water plays a key role in enhancing the permeability of human skin to many substances. To further understand its ability to potentially increase the bioavailabililty of soil contaminants, artificial sweat was applied to excised pig skin prior to dosing with munition-contaminated soils. Skin was mounted in chambers to allow simultaneous measurement of evaporation and penetration and to control air flow, which changed the dwell time of skin surface water within a l-h period post application of test materials. Additional variables included type of compound, aging of spiked soil samples, and carbon content of soil. To this end, the evaporation and skin penetration of C-14 labeled hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), 2,6-dinitrotoluene (26DNT), and 2,4,6-trinitrotoluene (TNT) were determined from two soil types, Yolo, having 1.2% carbon, and Tinker, having 9.5% carbon. RDX soil samples aged 27 mo and 62 mo were compared to freshly spiked soils samples. Similarly, 26DNT samples aged 35–36 mo and TNT samples aged 18 mo were compared to freshly spiked samples. Approximately 10 μg/cm2 of radiolabeled compound was applied in 10 mg/cm2 of soil. Radiolabel recovered from the dermis and tissue culture media (receptor fluid) was summed to determine percent absorption from the soils. Radiolabel recovered from vapor traps determined evaporation. Mean skin absorption of all compounds was higher for low-carbon soil, regardless of soil age and skin surface water as affected by air flow conditions. For 26DNT, a simultaneous increase in evaporation and penetration with conditions that favored enhanced soil hydration of freshly prepared samples was consistent with a mechanism that involved water displacement of 26DNT from its binding sites. A mean penetration of 17.5 ± 3.6% was observed for 26DNT in low-carbon soil, which approached the value previously reported for acetone vehicle (24 ± 6%). 26DNT penetration was reduced to 0.35% under dryer conditions and to 0.08% when no sweat was applied. When soil hydration conditions were not varied from cell to cell, air flow that favored a longer water dwell time increased penetration, but not evaporation, consistent with a mechanism of enhanced skin permeability from a higher hydration state of the stratum corneum. Profiles of 26DNT penetration versus air flow conditions were exponential for freshly prepared soil samples, suggesting strong and weak binding sites; corresponding profiles of 26DNT penetration from aged samples were linear, suggesting a conversion of weak to strong binding sites. Absorption and evaporation was less than 5% for TNT and less than 1% for RDX, regardless of soil type and age. Fresh preparations of RDX in Tinker soil and aged samples of TNT in Yolo soil showed a significant decrease in skin absorption with loss of surface moisture. The penetration rate of radiolabel into the receptor fluid was highest during the 1–2 h interval after dosing with 26DNT or TNT. High-performance liquid chromatography (HPLC) analysis of 26DNT in receptor fluid at maximum flux indicated no metabolism or breakdown. For TNT, however, extensive conversion to monoamino derivatives and other metabolites was observed. Relatively little radioactivity was found in the dermis after 26DNT and TNT applications, and dermal extracts were therefore not analyzed by HPLC. RDX was not sufficiently absorbed from soils to allow HPLC analysis. This study has practical significance, as the use of water for dust control at remediation sites may have the unintended effect of increasing volatilization and subsequent absorption of soil contaminants. Soil in contact with sweaty skin may give the same result. Skin absorption of 26DNT from soil was over 50-fold higher than the value for dryer skin and over 200-fold higher than the value obtained when there was no sweat application. While the hydration effect was less dramatic for RDX and TNT, soil contaminants more closely matching the physical properties of 26DNT may be similarly affected by hydration.

Grinding a Nanotube

Field evaporation phenomena, the removal of atoms from positively charged surfaces at strong field of tens of volts per nanometer, are of tremendous scientific and technological importance. The technique of field-ion microscopy (FIM) has been developed based on this phenomenon, and is historically the first technique to have achieved atomic resolution. The atom probe, a variation of FIM, came later and is generally used to identify the species of atoms that are being individually evaporated from the surface of a FIM tip. Recently, atom-probe tomography (APT) has been developed as a powerful analytical characterization technique that ties compositional information to structure. This is the only approach that is capable of determining the 3D location and elemental identity of atoms in a sample with near atomic precision. In principle, the field evaporation phenomenon can be utilized not only for materials characterization, but also for materials processing and morphology control with extremely high precision because of its unique atom-by-atom removal capability. However, FIM and atom probe investigations only provide access to structural information on the tip surface or the evaporated segments of the specimen, and it is not possible to directly observe the structural evolution of the sample. This limitation has greatly restricted the potential applications of field evaporation as a materials-processing tool. Here, we have investigated the positive field evaporation of nanomaterials by transmission electron microscopy (TEM). Direct TEM observations of the details of the structural evolution during field evaporation have been obtained for the first time. In previous work, the shortening of individual positively biased carbon nanotubes (CNTs) has been observed by scanning electron microscopy (SEM), but the observed phenomenon may perhaps arise from the electron ablation effect because of electrons emitted from the counter CNT electrodes. Detailed information about the end caps of CNTs is difficult to obtain because of the limited spatial resolution of SEM. On the other hand, structural changes in individual CNTs have also been observed in previous in situ field-emission measurements. However, it is likely that the observed structural changes do not arise from “pure” field evaporation phenomena. The reason for this is that when CNTs are negatively charged, the large field-emission current can induce very irregular tip structures and can even damage the emitters. In contrast, as will be demonstrated here, positive field evaporation is much more controllable. Indeed, we demonstrate that positive field evaporation in combination with in situ TEM may actually provide a very simple and effective means for controlling the morphology of nanomaterials with atomic precision. All the in situ experiments have been carried out using a 200 kV Tecnai G20 transmission electron microscope with a vacuum level of about 1×10 Pa. The manipulations and measurements have been made using a Nanofactory single-tilt sample holder. All CNTs used in this work are multiwalled CNTs (MWCNTs) prepared via chemical vapor deposition. In our experiment, a single CNT protruding from the edge of a Pt wire has been selected using a W tip. By running a high current, the CNT is broken into two segments with the bottom section attached to the W tip. As shown in Figure 1a, the W tip is then moved to a different location where the counter (top) Pt electrode is microscopically flat. The distance between the CNT tip and the counter electrode is usually several tens to hundreds of nanometers. We have used electronbeam-induced deposition to deposit amorphous carbon (a-carbon) on the contact area, which effectively prevents the removal of the CNT from the W tip even under extremely strong fields. A maximum voltage of ±140 V can be applied between the Pt electrode and the grounded counter W tip. The attached CNT can be positively charged for field evaporation experiments by applying a negative voltage to the Pt electrode, or negatively charged for field-emission experiments by applying a positive bias to the Pt electrode. During the field-emission or field evaporation measurements, the electron-beam (e-beam) has been blanked out. The CNT has been positively charged with a constant bias of 140 V, which may produce enough of a local field to cause field evaporation. As the CNT end is progressively shortened and becomes flat, field evaporation slows down and eventually stops. Figure 1b and c show the end cap of the CNT before and after the evaporation experiment, respectively; the corresponding field-emission current–voltage (I–V) curves are shown in Figure 1d. Here, field-emission measurements of the CNT have been conducted to provide quantitative estimates of the field conversion factor and threshold field for positive field evaporation. C O M M U N IC A TI O N