scholarly journals Mercury vapor air–surface exchange measured by collocated micrometeorological and enclosure methods – Part II: Bias and uncertainty analysis

2015 ◽  
Vol 15 (4) ◽  
pp. 4627-4676
Author(s):  
W. Zhu ◽  
J. Sommar ◽  
C.-J. Lin ◽  
X. Feng
Keyword(s):  
Flux Measurement ◽  
Bowen Ratio ◽  
Vertical Gradient ◽  
Systematic Evaluation ◽  
Data Set ◽  
Surface Exchange ◽  
Concentration Difference ◽  
Vegetation Height ◽  
Gradient Based ◽  
Flux Quantification

Abstract. Dynamic flux chambers (DFCs) and micrometeorological (MM) methods are extensively deployed for gauging air–surface Hg0 gas exchange. However, a systematic evaluation of the precision of the contemporary Hg0 flux quantification methods is not available. In this study, the uncertainty in Hg0 flux measured by relaxed eddy accumulation (REA) method, aerodynamic gradient method (AGM), modified Bowen-ratio (MBR) method, as well as DFC of traditional (TDFC) and novel (NDFC) designs is assessed using a robust data-set from two field intercomparison campaigns. The absolute precision in Hg0 concentration difference (Δ C) measurements is estimated at 0.064 ng m−3 for the gradient-based MBR and AGM system. For the REA system, the parameter is Hg0 concentration (C) dependent at 0.069+0.022C. 57 and 62% of the individual vertical gradient measurements were found to be significantly different from zero during the campaigns, while for the REA-technique the percentage of significant observations was lower. For the chambers, non-significant fluxes are confined to a few nighttime periods with varying ambient Hg0 concentration. Relative bias for DFC-derived fluxes is estimated to be ~ ±10%, and ~ 85% of the flux bias are within ±2 ng m−2 h−1 in absolute term. The DFC flux bias follows a diurnal cycle, which is largely dictated by temperature controls on the enclosed volume. Due to contrasting prevailing micrometeorological conditions, the relative uncertainty (median) in turbulent exchange parameters differs by nearly a factor of two between the campaigns, while that in Δ C measurements is fairly stable. The estimated flux uncertainties for the triad of MM-techniques are 16–27, 12–23 and 19–31% (interquartile range) for the AGM, MBR and REA method, respectively. This study indicates that flux-gradient based techniques (MBR and AGM) are preferable to REA in quantifying Hg0 flux over ecosystems with low vegetation height. A limitation of all Hg0 flux measurement systems investigated is their incapability to obtain synchronous samples for the calculation of Δ C. This reduces the precision of flux quantification, particularly the MM-systems under non-stationarity of ambient Hg0 concentration. For future applications, it is recommended to accomplish Δ C derivation from simultaneous collected samples.

scholarly journals Mercury vapor air–surface exchange measured by collocated micrometeorological and enclosure methods – Part II: Bias and uncertainty analysis

2015 ◽  
Vol 15 (10) ◽  
pp. 5359-5376 ◽  
Author(s):  
W. Zhu ◽  
J. Sommar ◽  
C.-J. Lin ◽  
X. Feng
Keyword(s):  
Flux Measurement ◽  
Vertical Gradient ◽  
Systematic Evaluation ◽  
Night Time ◽  
Data Set ◽  
Surface Exchange ◽  
Concentration Difference ◽  
Vegetation Height ◽  
Gradient Based ◽  
Flux Quantification

Abstract. Dynamic flux chambers (DFCs) and micrometeorological (MM) methods are extensively deployed for gauging air–surface Hg0 gas exchange. However, a systematic evaluation of the precision of the contemporary Hg0 flux quantification methods is not available. In this study, the uncertainty in Hg0 flux measured by the relaxed eddy accumulation (REA) method, the aerodynamic gradient method (AGM), the modified Bowen ratio (MBR) method, as well as DFC of traditional (TDFC) and novel (NDFC) designs, are assessed using a robust data set from two field intercomparison campaigns. The absolute precision in Hg0 concentration difference (ΔC) measurements is estimated at 0.064 ng m−3 for the gradient-based MBR and AGM systems. For the REA system, the parameter is Hg0 concentration (C) dependent at 0.069 + 0.022C. During the campaigns, 57 and 62 % of the individual vertical gradient measurements are found to be significantly different from 0, while for the REA technique, the percentage of significant observations is lower. For the chambers, non-significant fluxes are confined to a few night-time periods with varying ambient Hg$^{0}$ concentrations. Relative bias for DFC-derived fluxes is estimated to be ~ ±10, and ~ 85% of the flux bias is within ±2 ng m−2 h−1 in absolute terms. The DFC flux bias follows a diurnal cycle, which is largely affected by the forced temperature and irradiation bias in the chambers. Due to contrasting prevailing micrometeorological conditions, the relative uncertainty (median) in turbulent exchange parameters differs by nearly a factor of 2 between the campaigns, while that in ΔC measurement is fairly consistent. The estimated flux uncertainties for the triad of MM techniques are 16–27, 12–23 and 19–31% (interquartile range) for the AGM, MBR and REA methods, respectively. This study indicates that flux-gradient-based techniques (MBR and AGM) are preferable to REA in quantifying Hg0 flux over ecosystems with low vegetation height. A limitation of all Hg0 flux measurement systems investigated is their inability to obtain synchronous samples for the calculation of ΔC. This reduces the precision of flux quantification, particularly in the MM systems under non-stationarity of ambient Hg0 concentration. For future applications, it is recommended to accomplish ΔC derivation from simultaneous collected samples.

scholarly journals Mercury vapor air–surface exchange measured by collocated micrometeorological and enclosure methods – Part I: Data comparability and method characteristics

2015 ◽  
Vol 15 (2) ◽  
pp. 685-702 ◽  
Author(s):  
W. Zhu ◽  
J. Sommar ◽  
C.-J. Lin ◽  
X. Feng
Keyword(s):  
Temporal Variability ◽  
Temporal Trends ◽  
Elemental Mercury ◽  
Bowen Ratio ◽  
Mercury Vapor ◽  
Bare Soil ◽  
Agricultural Field ◽  
Flux Chamber ◽  
Surface Exchange ◽  
Flux Quantification

Abstract. Reliable quantification of air–biosphere exchange flux of elemental mercury vapor (Hg0) is crucial for understanding the global biogeochemical cycle of mercury. However, there has not been a standard analytical protocol for flux quantification, and little attention has been devoted to characterize the temporal variability and comparability of fluxes measured by different methods. In this study, we deployed a collocated set of micrometeorological (MM) and dynamic flux chamber (DFC) measurement systems to quantify Hg0 flux over bare soil and low standing crop in an agricultural field. The techniques include relaxed eddy accumulation (REA), modified Bowen ratio (MBR), aerodynamic gradient (AGM) as well as dynamic flux chambers of traditional (TDFC) and novel (NDFC) designs. The five systems and their measured fluxes were cross-examined with respect to magnitude, temporal trend and correlation with environmental variables. Fluxes measured by the MM and DFC methods showed distinct temporal trends. The former exhibited a highly dynamic temporal variability while the latter had much more gradual temporal features. The diurnal characteristics reflected the difference in the fundamental processes driving the measurements. The correlations between NDFC and TDFC fluxes and between MBR and AGM fluxes were significant (R>0.8, p<0.05), but the correlation between DFC and MM fluxes were from weak to moderate (R=0.1–0.5). Statistical analysis indicated that the median of turbulent fluxes estimated by the three independent MM techniques were not significantly different. Cumulative flux measured by TDFC is considerably lower (42% of AGM and 31% of MBR fluxes) while those measured by NDFC, AGM and MBR were similar (<10% difference). This suggests that incorporating an atmospheric turbulence property such as friction velocity for correcting the DFC-measured flux effectively bridged the gap between the Hg0 fluxes measured by enclosure and MM techniques. Cumulated flux measured by REA was ~60% higher than the gradient-based fluxes. Environmental factors have different degrees of impacts on the fluxes observed by different techniques, possibly caused by the underlying assumptions specific to each individual method. Recommendations regarding the application of flux quantification methods were made based on the data obtained in this study.

scholarly journals Air–surface exchange of Hg<sup>0</sup> measured by collocated micrometeorological and enclosure methods – Part 1: Data comparability and method characteristics

2014 ◽  
Vol 14 (16) ◽  
pp. 22273-22319
Author(s):  
W. Zhu ◽  
J. Sommar ◽  
C.-J. Lin ◽  
X. Feng
Keyword(s):  
Temporal Variability ◽  
Temporal Trends ◽  
Elemental Mercury ◽  
Bowen Ratio ◽  
Mercury Vapor ◽  
Bare Soil ◽  
Agricultural Field ◽  
Surface Exchange ◽  
Flux Quantification ◽  
The Difference

Abstract. Reliable quantification of air-biosphere exchange flux of elemental mercury vapor (Hg0) is crucial for understanding global biogeochemical cycle of mercury. However, there has not been a standard analytical protocol for flux quantification, and little attention has been devoted to characterize the temporal variability and comparability of fluxes measured by different methods. In this study, we deployed a collocated set of micro-meteorological (MM) and enclosure measurement systems to quantify Hg0 flux over bare soil and low standing crop in an agricultural field. The techniques include relaxed eddy accumulation (REA), modified Bowen-ratio (MBR), aerodynamic gradient (AGM) as well as dynamic flux chambers of traditional (TDFC) and novel (NDFC) designs. The five systems and their measured fluxes were cross-examined with respect to magnitude, temporal trend and sensitivity to environmental variables. Fluxes measured by the MM and DFC methods showed distinct temporal trends. The former exhibited a highly dynamic temporal variability while the latter had much gradual temporal features. The diurnal characteristics reflected the difference in the fundamental processes driving the measurements. The correlations between NDFC and TDFC fluxes and between MBR and AGM fluxes were significant (R > 0.8, p < 0.05), but the correlation between DFC and MM instantaneous fluxes were from weak to moderate (R = 0.1–0.5). Statistical analysis indicated that the median of turbulent fluxes estimated by the three independent MM-techniques were not significantly different. Cumulative flux measured by TDFC is considerably lower (42% of AGM and 31% of MBR fluxes) while those measured by NDFC, AGM and MBR were similar (< 10% difference). This implicates that the NDFC technique, which accounts for internal friction velocity, effectively bridged the gap in measured Hg0 flux compared to MM techniques. Cumulated flux measured by REA was ~60% higher than the gradient-based fluxes. Environmental factors have different degrees of impacts on the fluxes observed by different techniques, possibly caused by the underlying assumptions specific to each individual method. Recommendations regarding the application of flux quantification methods were made based on the data obtained in this study.

scholarly journals Net ecosystem exchange and energy fluxes in a West Siberian bog

2017 ◽  
Author(s):  
Pavel Alekseychik ◽  
Ivan Mammarella ◽  
Dmitry Karpov ◽  
Sigrid Dengel ◽  
Irina Terentieva ◽  
...  
Keyword(s):  
Net Ecosystem Exchange ◽  
Bowen Ratio ◽  
Heavy Rain ◽  
West Siberia ◽  
Weather Conditions ◽  
Middle Taiga ◽  
Energy Budgets ◽  
Co2 Uptake ◽  
Surface Exchange ◽  
Rainy Weather

Abstract. Very few studies of ecosystem-atmosphere exchange involving eddy-covariance data have been conducted in Siberia, and none in West Siberia. This work provides the first estimates of carbon dioxide (CO2) and energy budgets at a typical bog of the West Siberian middle taiga based on May-August measurements in 2015. The footprint of measured fluxes consisted of homogeneous mixture of tree-covered ridges and hollows with the vegetation represented by typical sedges and shrubs. Generally, the surface exchange rates resembled those of pine-covered bogs elsewhere. The surface energy balance closure was 90 %. Net CO2 uptake was comparatively high, summing up to 196 gC m−2 for the four measurement months, while the Bowen ratio was typical at 30 %. The ecosystem turned into a net CO2 source during several front passage events in June and July. Several periods of heavy rain helped keep the water table at a constant level, preventing a usual drawdown in summer. However, because of the cloudy and rainy weather, the observed fluxes might rather represent the special weather conditions of 2015 than their typical level.

scholarly journals Global Observations and Modeling of Atmosphere-Surface Exchange of Elemental Mercury – A Critical Review

2016 ◽  
Author(s):  
W. Zhu ◽  
C.-J. Lin ◽  
X. Wang ◽  
J. Sommar ◽  
X. W. Fu ◽  
...  
Keyword(s):  
Global Analysis ◽  
Large Data ◽  
Elemental Mercury ◽  
Physicochemical Process ◽  
Anthropogenic Sources ◽  
Flux Chamber ◽  
Surface Exchange ◽  
Environmental Media ◽  
Flux Quantification ◽  
Spatio Temporal

Abstract. Reliable quantification of air-surfaces flux of elemental Hg vapor (Hg0) is crucial for understanding mercury (Hg) global biogeochemical cycles. There have been extensive measurements and modeling efforts devoting to estimating the exchange fluxes between the atmosphere and various surfaces (e.g., soil, canopies, water, snow, etc.) in past three decades. However, large uncertainty remains due to the complexity of Hg0 bi-directional exchange, limitations of flux quantification techniques and challenges in model parameterization. In this study, we provide a comprehensive review on the state of science in the atmosphere-surface exchange of Hg0. Specifically, the advancement of flux quantification techniques, mechanisms in driving the air-surfaces Hg exchange, and modeling efforts are presented. Due to the semi-volatile nature of Hg0 and redox transformation of Hg in environmental media, Hg deposition and evasion are influenced by multiple environmental variables including seasonality, vegetative coverage and its life cycle, temperature, light, moisture, atmospheric turbulence, presence of reactants (e.g., O3, radicals, etc.) that drives the physicochemical process of Hg in the media where Hg0 exchange occurs. However, effects of these processes on flux have not been fundamentally and quantitatively determined, which limits the accuracy of flux modeling. In this study, we compile an up-to-date global observational flux database and discuss the implication of flux data on global Hg budget. Mean Hg0 flux obtained by micrometeorological measurement did not appear to be significantly greater than the flux measured by dynamic flux chamber methods over unpolluted surfaces (p=0.16, one-tailed, Mann-Whitney U test). The spatio-temporal coverage of existing Hg0 flux measurements is highly heterogeneous with large data gaps existing in multiple continents (Africa, South Asia, Middle East, South America and Australia). The magnitude of evasion flux is strongly enhanced by human activities, particularly at contaminated sites. Hg0 flux observations in East Asia are comparatively larger in magnitude than the rest of the world, suggesting substantial reemission of previously deposited mercury from anthropogenic sources. Hg0 exchange over pristine surfaces (e.g., background soil and water) and vegetation need better constrains for global analysis of atmospheric Hg budget. The existing knowledge gap and the associated research needs for future measurements and modeling efforts for the air-surface exchange of Hg0 are discussed.

scholarly journals Aerodynamic gradient measurements of the NH<sub>3</sub>-HNO<sub>3</sub>-NH<sub>4</sub>NO<sub>3</sub> triad using a wet chemical instrument: an analysis of precision requirements and flux errors

2010 ◽  
Vol 3 (1) ◽  
pp. 187-208 ◽  
Author(s):  
V. Wolff ◽  
I. Trebs ◽  
C. Ammann ◽  
F. X. Meixner
Keyword(s):  
Nitric Acid ◽  
Ammonium Nitrate ◽  
Field Experiments ◽  
Phase Changes ◽  
Water Soluble ◽  
Individual Species ◽  
Measured Concentration ◽  
Surface Exchange ◽  
Concentration Difference ◽  
Wet Chemical

Abstract. The aerodynamic gradient method is widely used for flux measurements of ammonia, nitric acid, particulate ammonium nitrate (the NH3-HNO3-NH4NO3 triad) and other water-soluble reactive trace compounds. The surface exchange flux is derived from a measured concentration difference and micrometeorological quantities (turbulent exchange coefficient). The significance of the measured concentration difference is crucial for the significant determination of surface exchange fluxes. Additionally, measurements of surface exchange fluxes of ammonia, nitric acid and ammonium nitrate are often strongly affected by phase changes between gaseous and particulate compounds of the triad, which make measurements of the four individual species (NH3, HNO3, NH4+, NO3− necessary for a correct interpretation of the measured concentration differences. We present here a rigorous analysis of results obtained with a multi-component, wet-chemical instrument, able to simultaneously measure gradients of both gaseous and particulate trace substances. Basis for our analysis are two field experiments, conducted above contrasting ecosystems (grassland, forest). Precision requirements of the instrument as well as errors of concentration differences and micrometeorological exchange parameters have been estimated, which, in turn, allows the establishment of thorough error estimates of the derived fluxes of NH3, HNO3, NH4+, and NO3−. Derived median flux errors for the grassland and forest field experiments were: 39% and 50% (NH3), 31% and 38% (HNO3), 62% and 57% (NH4+), and 47% and 68% (NO3−), respectively. Additionally, we provide the basis for using field data to characterize the instrument performance, as well as subsequent quantification of surface exchange fluxes and underlying mechanistic processes under realistic ambient measurement conditions.

scholarly journals MODEL INDEPENDENT CONSTRAINTS ON NON-ELECTRONIC FLAVORS IN THE SOLAR BORON NEUTRINO FLUX

2006 ◽  
Vol 21 (22) ◽  
pp. 1761-1768 ◽  
Author(s):  
S. DEV ◽  
SANJEEV KUMAR ◽  
SURENDER VERMA
Keyword(s):  
General Model ◽  
Sterile Neutrino ◽  
Neutrino Flux ◽  
Flux Measurement ◽  
Data Set ◽  
The Earth ◽  
Independent Analysis ◽  
Antineutrino Flux ◽  
Model Independent ◽  
Salt Phase

We perform the most general model-independent analysis of the latest 391-Day Salt Phase SNO Data Set incorporating the super-Kamiokande ES flux measurement and obtain bounds on the antineutrino and sterile neutrino flux in the solar 8 B neutrino flux reaching the detectors on the earth. The muon/tauon antineutrino flux is found to be disallowed at 1.4σ C.L. The sterile flux is found to be nonzero at about 1.26 standard deviations.

First-arrival traveltime tomography based on the adjoint-state method

Geophysics ◽  
2009 ◽  
Vol 74 (6) ◽  
pp. WCB1-WCB10 ◽  
Author(s):  
Cédric Taillandier ◽  
Mark Noble ◽  
Hervé Chauris ◽  
Henri Calandra
Keyword(s):  
Synthetic Data ◽  
Point Of View ◽  
Data Set ◽  
Near Surface ◽  
Traveltime Tomography ◽  
Adjoint State ◽  
Gradient Based ◽  
First Arrival ◽  
State Method ◽  
Adjoint State Method

Classical algorithms used for traveltime tomography are not necessarily well suited for handling very large seismic data sets or for taking advantage of current supercomputers. The classical approach of first-arrival traveltime tomography was revisited with the proposal of a simple gradient-based approach that avoids ray tracing and estimation of the Fréchet derivative matrix. The key point becomes the derivation of the gradient of the misfit function obtained by the adjoint-state technique. The adjoint-state method is very attractive from a numerical point of view because the associated cost is equivalent to the solution of the forward-modeling problem, whatever the size of the input data and the number of unknown velocity parameters. An application on a 2D synthetic data set demonstrated the ability of the algorithm to image near-surface velocities with strong vertical and lateral variations and revealed the potential of the method.

Automatic categorization of Web service elements

2018 ◽  
Vol 14 (2) ◽  
pp. 233-258 ◽  
Author(s):  
Efthimia Mavridou ◽  
Konstantinos M. Giannoutakis ◽  
Dionysios Kehagias ◽  
Dimitrios Tzovaras ◽  
George Hassapis
Keyword(s):  
Web Services ◽  
Web Service ◽  
Semantic Annotation ◽  
Third Party ◽  
Systematic Evaluation ◽  
Evaluation Procedure ◽  
Data Set ◽  
Content Type ◽  
Semantic Categorization ◽  
Description File

Purpose Semantic categorization of Web services comprises a fundamental requirement for enabling more efficient and accurate search and discovery of services in the semantic Web era. However, to efficiently deal with the growing presence of Web services, more automated mechanisms are required. This paper aims to introduce an automatic Web service categorization mechanism, by exploiting various techniques that aim to increase the overall prediction accuracy. Design/methodology/approach The paper proposes the use of Error Correcting Output Codes on top of a Logistic Model Trees-based classifier, in conjunction with a data pre-processing technique that reduces the original feature-space dimension without affecting data integrity. The proposed technique is generalized so as to adhere to all Web services with a description file. A semantic matchmaking scheme is also proposed for enabling the semantic annotation of the input and output parameters of each operation. Findings The proposed Web service categorization framework was tested with the OWLS-TC v4.0, as well as a synthetic data set with a systematic evaluation procedure that enables comparison with well-known approaches. After conducting exhaustive evaluation experiments, categorization efficiency in terms of accuracy, precision, recall and F-measure was measured. The presented Web service categorization framework outperformed the other benchmark techniques, which comprise different variations of it and also third-party implementations. Originality/value The proposed three-level categorization approach is a significant contribution to the Web service community, as it allows the automatic semantic categorization of all functional elements of Web services that are equipped with a service description file.

Trend enhancement in aeromagnetic maps using constrained coherence-enhancing diffusion filtering

Geophysics ◽  
2014 ◽  
Vol 79 (1) ◽  
pp. J1-J9 ◽  
Author(s):  
Meixia Geng ◽  
Qingjie Yang ◽  
Yuan Yuan
Keyword(s):  
Field Data ◽  
Mesh Size ◽  
Linear Structure ◽  
Vertical Gradient ◽  
Acute Angle ◽  
Data Set ◽  
Diffusion Filtering ◽  
Sample Spacing ◽  
Fine Spacing ◽  
Original Line

Geophysical data have to be modeled on a regular grid for various numerical procedures. However, airborne data tend to be collected with fine spacing along traverses but with much coarser spacing between traverses. Gridding only honors flight-line data when the mesh size is close to the sample spacing; otherwise, high-frequency information is always lost, which creates aliasing artifacts. For example, linear trends at an acute angle with respect to flight lines are imaged as “bull’s-eyes,” which resemble a boudinage at line intersections. The presence of boudinage artifacts can significantly distort anomalies of interest and thus lead to incorrect interpretation of shapes or sizes of causative bodies. We evaluated a method called constrained coherence-enhancing diffusion filtering that only diffuses the image in specific areas where strong anisotropy is detected. This method was tested on synthetic and field data set. Results indicated that the method can be efficiently used to enhance linear structure in multiple local directions. The images derived from this grid, such as the vertical gradient map, are also significantly improved. The original line data are honored by the constraints applied. We also used a field data set to compare the proposed approach with the approach used when diffusion is applied uniformly in all areas, irrespective of anisotropy. The proposed method was proven to produce better results with fewer artifacts.

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