scholarly journals Evaluation of the flux gradient technique for measurement of ozone surface fluxes over snowpack at Summit, Greenland

2011 ◽  
Vol 4 (1) ◽  
pp. 1021-1059
Author(s):  
F. Bocquet ◽  
D. Helmig ◽  
B. A. Van Dam ◽  
C. W. Fairall
Keyword(s):  
Gradient Method ◽  
Deposition Velocity ◽  
Surface Fluxes ◽  
Sensitivity Threshold ◽  
Lower Sensitivity ◽  
Ozone Deposition ◽  
Ozone Flux ◽  
Gradient Technique ◽  
Measurement Height ◽  
Polar Snow

Abstract. A multi-step procedure for investigating ozone surface fluxes over polar snow by the tower gradient method was developed and evaluated. These measurements were then used to obtain four months of turbulent ozone flux data at the Summit research camp located in the center of the Greenland ice shield. Turbulent fluxes were determined by the aerodynamic gradient method incorporating tower measurements of (a) ozone gradients measured by commercial ultraviolet absorption analyzers, (b) ambient temperature gradients using aspirated thermocouple sensors, and (c) wind speed gradients determined by cup anemometers. All gradient instruments were regularly inter-compared by bringing sensors or inlets to the same measurement height. The developed protocol resulted in an uncertainty on the order of 0.1 ppbv for 30-min averaged ozone gradients that were used for the ozone flux calculations. This protocol facilitated a lower sensitivity threshold for the ozone flux determination of −8 × 10−3 μg m−2 s−1, respectively ~0.01 cm s−1 for the ozone deposition velocity for typical environmental conditions encountered at Summit. Uncertainty in the 30-min ozone exchange measurements (evaluated by the Monte Carlo statistical approach) was on the order of 10−2 cm s−1. This uncertainty typically accounted to ~20–100% of the ozone exchange velocities that were determined. These measurements are among the most sensitive ozone deposition determinations reported to date. This flux experiment, deployed at Summit for a period of four months, allowed for measurements of the relatively low ozone uptake rates encountered for polar snow, and thereby the study of their environmental and seasonal dependencies.

scholarly journals Evaluation of the flux gradient technique for measurement of ozone surface fluxes over snowpack at Summit, Greenland

2011 ◽  
Vol 4 (10) ◽  
pp. 2305-2321 ◽  
Author(s):  
F. Bocquet ◽  
D. Helmig ◽  
B. A. Van Dam ◽  
C. W. Fairall
Keyword(s):  
Gradient Method ◽  
Deposition Velocity ◽  
Surface Fluxes ◽  
Sensitivity Threshold ◽  
Lower Sensitivity ◽  
Ozone Deposition ◽  
Ozone Flux ◽  
Gradient Technique ◽  
Measurement Height ◽  
Polar Snow

Abstract. A multi-step procedure for investigating ozone surface fluxes over polar snow by the tower gradient method was developed and evaluated. These measurements were then used to obtain five months (April–August 2004) of turbulent ozone flux data at the Summit research camp located in the center of the Greenland ice shield. Turbulent fluxes were determined by the gradient method incorporating tower measurements of (a) ozone gradients measured by commercial ultraviolet absorption analyzers, (b) ambient temperature gradients using aspirated thermocouple sensors, and (c) wind speed gradients determined by cup anemometers. All gradient instruments were regularly inter-compared by bringing sensors or inlets to the same measurement height. The developed protocol resulted in an uncertainty on the order of 0.1 ppbv for 30-min averaged ozone gradients that were used for the ozone flux calculations. This protocol facilitated a lower sensitivity threshold for the ozone flux determination of ∼8 × 10−3μg m−2 s−1, respectively ∼0.01 cm s−1 for the ozone deposition velocity for typical environmental conditions encountered at Summit. Uncertainty in the 30-min ozone exchange measurements (evaluated by the Monte Carlo statistical approach) was on the order of 10−2 cm s−1. This uncertainty typically accounted to ~20–100% of the ozone exchange velocities that were determined. These measurements are among the most sensitive ozone deposition determinations reported to date. This flux experiment allowed for measurements of the relatively low ozone uptake rates encountered for polar snow, and thereby the study of their environmental and spring-versus-summer dependencies.

scholarly journals Seasonal variation of ozone deposition to a tropical rain forest in southwest Amazonia

2007 ◽  
Vol 7 (20) ◽  
pp. 5415-5435 ◽  
Author(s):  
U. Rummel ◽  
C. Ammann ◽  
G. A. Kirkman ◽  
M. A. L. Moura ◽  
T. Foken ◽  
...  
Keyword(s):  
Eddy Covariance ◽  
Rain Forest ◽  
Deposition Velocity ◽  
Dry Season ◽  
Stomatal Aperture ◽  
Specific Humidity ◽  
Deposition Flux ◽  
Diel Variation ◽  
Wet Season ◽  
Ozone Deposition

Abstract. Within the project EUropean Studies on Trace gases and Atmospheric CHemistry as a contribution to Large-scale Biosphere-atmosphere experiment in Amazonia (LBA-EUSTACH), we performed tower-based eddy covariance measurements of O3 flux above an Amazonian primary rain forest at the end of the wet and dry season. Ozone deposition revealed distinct seasonal differences in the magnitude and diel variation. In the wet season, the rain forest was an effective O3 sink with a mean daytime (midday) maximum deposition velocity of 2.3 cm s−1, and a corresponding O3 flux of −11 nmol m−2 s−1. At the end of the dry season, the ozone mixing ratio was about four times higher (up to maximum values of 80 ppb) than in the wet season, as a consequence of strong regional biomass burning activity. However, the typical maximum daytime deposition flux was very similar to the wet season. This results from a strong limitation of daytime O3 deposition due to reduced plant stomatal aperture as a response to large values of the specific humidity deficit. As a result, the average midday deposition velocity in the dry burning season was only 0.5 cm s−1. The large diel ozone variation caused large canopy storage effects that masked the true diel variation of ozone deposition mechanisms in the measured eddy covariance flux, and for which corrections had to be made. In general, stomatal aperture was sufficient to explain the largest part of daytime ozone deposition. However, during nighttime, chemical reaction with nitrogen monoxide (NO) was found to contribute substantially to the O3 sink in the rain forest canopy. Further contributions were from non-stomatal plant uptake and other processes that could not be clearly identified. Measurements, made simultaneously on a 22 years old cattle pasture enabled the spatially and temporally direct comparison of O3 dry deposition values from this site with typical vegetation cover of deforested land in southwest Amazonia to the results from the primary rain forest. The mean ozone deposition to the pasture was found to be systematically lower than that to the forest by 30% in the wet and 18% in the dry season.

scholarly journals Ozone deposition impact assessments for forest canopies require accurate ozone flux partitioning on diurnal timescales

2021 ◽  
Vol 21 (24) ◽  
pp. 18393-18411
Author(s):  
Auke J. Visser ◽  
Laurens N. Ganzeveld ◽  
Ignacio Goded ◽  
Maarten C. Krol ◽  
Ivan Mammarella ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Dry Deposition ◽  
Land Surface ◽  
Large Scale ◽  
Crop Yields ◽  
Carbon Sink ◽  
Surface Ozone ◽  
Ozone Uptake ◽  
Ozone Deposition ◽  
Ozone Flux

Abstract. Dry deposition is an important sink of tropospheric ozone that affects surface concentrations and impacts crop yields, the land carbon sink, and the terrestrial water cycle. Dry deposition pathways include plant uptake via stomata and non-stomatal removal by soils, leaf surfaces, and chemical reactions. Observational studies indicate that ozone deposition exhibits substantial temporal variability that is not reproduced by atmospheric chemistry models due to a simplified representation of vegetation uptake processes in these models. In this study, we explore the importance of stomatal and non-stomatal uptake processes in driving ozone dry deposition variability on diurnal to seasonal timescales. Specifically, we compare two land surface ozone uptake parameterizations – a commonly applied big leaf parameterization (W89; Wesely, 1989) and a multi-layer model (MLC-CHEM) constrained with observations – to multi-year ozone flux observations at two European measurement sites (Ispra, Italy, and Hyytiälä, Finland). We find that W89 cannot reproduce the diurnal cycle in ozone deposition due to a misrepresentation of stomatal and non-stomatal sinks at our two study sites, while MLC-CHEM accurately reproduces the different sink pathways. Evaluation of non-stomatal uptake further corroborates the previously found important roles of wet leaf uptake in the morning under humid conditions and soil uptake during warm conditions. The misrepresentation of stomatal versus non-stomatal uptake in W89 results in an overestimation of growing season cumulative ozone uptake (CUO), a metric for assessments of vegetation ozone damage, by 18 % (Ispra) and 28 % (Hyytiälä), while MLC-CHEM reproduces CUO within 7 % of the observation-inferred values. Our results indicate the need to accurately describe the partitioning of the ozone atmosphere–biosphere flux over the in-canopy stomatal and non-stomatal loss pathways to provide more confidence in atmospheric chemistry model simulations of surface ozone mixing ratios and deposition fluxes for large-scale vegetation ozone impact assessments.

scholarly journals Interactions among vegetation and ozone, water and nitrogen fluxes in a coastal Mediterranean maquis ecosystem

2009 ◽  
Vol 6 (1) ◽  
pp. 1453-1495 ◽  
Author(s):  
G. Gerosa ◽  
A. Finco ◽  
S. Mereu ◽  
R. Marzuoli ◽  
A. Ballarin-Denti
Keyword(s):  
Critical Level ◽  
Water Fluxes ◽  
Stomatal Flux ◽  
Gradient Technique ◽  
Dose Metrics ◽  
Measurement Height ◽  
Mediterranean Maquis ◽  
Ozone Fluxes ◽  
A Minor ◽  
Upward Flux

Abstract. Ozone, water and energy fluxes were measured over a Mediterranean maquis ecosystem from 5 May until 31 July by means of the eddy covariance technique. Additional measurements of NOx fluxes were performed by the aerodynamic gradient technique. Stomatal ozone fluxes were obtained from water fluxes by a Dry Deposition Inferential Method based on a big leaf concept. The maquis ecosystem acted as a net sink for ozone. The different water availability between late spring and summer was the major cause of the changes observed in stomatal fluxes, which decreased, together with evapotranspiration, when the season became drier. NOx concentrations were significantly dependent on the local meteorology. NOx fluxes resulted less intense than the ozone fluxes. However an average upward flux of both NO and NO2 was measured. The non-stomatal pathways of ozone deposition were investigated. A correlation of non-stomatal deposition with air humidity and, in a minor way, with NO2 fluxes was found. Ozone risk assessment was performed by comparing the exposure and the dose metrics: AOT40 (Accumulated dose over a threshold of 40 ppb) and AFst1.6 (Accumulated stomatal flux of ozone over a threshold of 1.6 nmol m−2 s−1). AOT40, both at the measurement height and at canopy height was greater than the Critical Level (5000 ppb·h) adopted by UN-ECE. Also the AFst1.6 value (12.6 mmol m−2 PLA, Projected Leaf Area) was higher than the provisional critical dose of 4 mmol m−2 PLA. The cumulated dose grew more regularly than the exposure but it showed two different growth rates in the spring and in the summer periods.

scholarly journals Spatiotemporal Controls on Observed Daytime Ozone Deposition Velocity Over Northeastern U.S. Forests During Summer

2019 ◽  
Vol 124 (10) ◽  
pp. 5612-5628 ◽  
Author(s):  
O. E. Clifton ◽  
A. M. Fiore ◽  
J. W. Munger ◽  
R. Wehr
Keyword(s):  
Deposition Velocity ◽  
Ozone Deposition

scholarly journals Measuring the biosphere-atmosphere exchange of total reactive nitrogen by eddy covariance

2012 ◽  
Vol 9 (11) ◽  
pp. 4247-4261 ◽  
Author(s):  
C. Ammann ◽  
V. Wolff ◽  
O. Marx ◽  
C. Brümmer ◽  
A. Neftel
Keyword(s):  
Eddy Covariance ◽  
Measurement Techniques ◽  
Time Lag ◽  
Fast Response ◽  
Average Concentration ◽  
Reactive Nitrogen ◽  
Gradient Technique ◽  
Measurement Height ◽  
Flux Measurements ◽  
Trace Compounds

Abstract. The (net) exchange of reactive nitrogen (Nr) with the atmosphere is an important driver for ecosystem productivity and greenhouse gas exchange. The exchange of airborne Nr includes various trace compounds that usually require different specific measurement techniques, and up to now fast response instruments suitable for eddy covariance measurements are only available for few of these compounds. Here we present eddy covariance flux measurements with a recently introduced converter (TRANC) for the sum of all Nr compounds (∑Nr). Measurements were performed over a managed grassland field with phases of net emission and net deposition of ∑Nr and alternating dominance of oxidized (NOX) and reduced species (NH3). Spectral analysis of the eddy covariance data exhibited the existence of covariance function peaks at a reasonable time lag related to the sampling tube residence time under stationary conditions. Using ogive analysis, the high-frequency damping was quantified to 19%–26% for a low measurement height of 1.2 m and to about 10% for 4.8 m measurement height. ∑Nr concentrations and fluxes were compared to parallel NO and NO2 measurements by dynamic chambers and NH3 measurements by the aerodynamic gradient technique. The average concentration results indicate that the main compounds NO2 and NH3 were converted by the TRANC system with an efficiency of near 100%. With an optimised sample inlet also the fluxes of these compounds were recovered reasonably well including net deposition and net emission phases. The study shows that the TRANC system is suitable for fast response measurements of oxidized and reduced nitrogen compounds and can be used for continuous eddy covariance flux measurements of total reactive nitrogen.

scholarly journals Turbulent surface fluxes and air–ice coupling in the Baltic Air–Sea–Ice Study (BASIS)

2001 ◽  
Vol 33 ◽  
pp. 237-242 ◽  
Author(s):  
Jouko Launiainen ◽  
Bin Cheng ◽  
Juha Uotila ◽  
Timo Vihma
Keyword(s):  
Wind Speed ◽  
Sea Ice ◽  
Drag Coefficient ◽  
Gradient Method ◽  
Sonic Anemometer ◽  
Mean Value ◽  
Surface Fluxes ◽  
Eddy Flux ◽  
Aerodynamic Roughness ◽  
The Baltic

AbstractTurbulent surface fluxes were studied using observations taken over sea ice in the Baltic Sea in March 1998. The fluxes of momentum and sensible heat were measured by a sonic anemometer and compared with fluxes derived from wind velocity and air-temperature profiles. The neutral 10 m drag coefficient showed no apparent dependence on wind speed (in the range 2–20 m s–1), resulting in a mean value of 1.0 × 10–3 for smooth snow-covered ice and 1.5 × 10−3 for deformed ice. The overall mean value was 1.28 × 10–3. The roughness length for temperature revealed a greater apparent dependence on wind speed and was slightly larger than the aerodynamic roughness for low wind speeds, and vice versa for moderate and high winds. We give an empirical expression that predicts how the scalar roughness depends on the aerodynamic roughness (drag coefficient) and wind speed. Agreement of the gradient-method results with the eddy-flux results supports the validity of the Monin-Obukhov similarity theory. Fluxes modelled by a coupled air-ice-sea model compared well with the eddy-flux and gradient methods. Surface temperature estimates by the three methods also agreed well. Tests and sensitivity analysis emphasize the need for especially accurate sensor calibration and strict information about the sensor heights for the gradient method.

Retrospective comparison between single reading plus an artificial intelligence algorithm and two-view digital tomosynthesis with double reading in breast screening

2021 ◽  
pp. 096914132098419
Author(s):  
Axel Graewingholt ◽  
Stephen Duffy
Keyword(s):  
Artificial Intelligence ◽  
Detection Rate ◽  
Sensitivity Threshold ◽  
Lower Sensitivity ◽  
Digital Tomosynthesis ◽  
Tumour Type ◽  
Double Reading ◽  
Experienced Radiologist ◽  
Single Reading ◽  
Higher Sensitivity

Objective To examine the breast cancer detection rate by single reading of an experienced radiologist supported by an artificial intelligence (AI) system, and compare with two-dimensional full-field digital mammography (2D-FFDM) double reading. Materials and methods Images (3D-tomosynthesis) of 161 biopsy-proven cancers were re-read by the AI algorithm and compared to the results of first human reader, second human reader and consensus following double reading in screening. Detection was assessed in subgroups by tumour type, breast density and grade, and at two operating points, referred to as a lower and a higher sensitivity threshold. Results The AI algorithm method gave similar results to double-reading 2D-FFDM, and the detection rate was significantly higher compared to single-reading 2D-FFDM. At the lower sensitivity threshold, the algorithm was significantly more sensitive than reader A (97.5% vs. 89.4%, p = 0.02), non-significantly more sensitive than reader B (97.5% vs. 94.4%, p = 0.2) and non-significantly less sensitive than the consensus from double reading (97.5% vs. 99.4%, p = 0.2). At the higher sensitivity threshold, the algorithm was significantly more sensitive than reader A (99.4% vs. 89.4%, p < 0.001) and reader B (99.4% vs. 94.4%, p = 0.02) and identical to the consensus sensitivity (99.7% in both cases, p = 1.0). There were no significant differences in the detection capability of the AI system by tumour type, grading and density. Conclusion In this proof of principle study, we show that sensitivity using single reading with a suitable AI algorithm is non-inferior to that of standard of care using 2D mammography with double reading, when tomosynthesis is the primary screening examination.

scholarly journals Measurement and modelling ozone fluxes over a cut and fertilized grassland

2009 ◽  
Vol 6 (10) ◽  
pp. 1987-1999 ◽  
Author(s):  
R. Mészáros ◽  
L. Horváth ◽  
T. Weidinger ◽  
A. Neftel ◽  
E. Nemitz ◽  
...  
Keyword(s):  
Chemical Interaction ◽  
Deposition Velocity ◽  
Weather Conditions ◽  
Eddy Covariance Method ◽  
Tall Grass ◽  
Detailed Model ◽  
Model Calculations ◽  
Ozone Flux ◽  
Stomatal Flux ◽  
Intensively Managed

Abstract. During the GRAMINAE Integrated Experiment between 20 May and 15 June 2000, the ozone flux was measured by the eddy covariance method above intensively managed grassland in Braunschweig, northern Germany. Three different phases of vegetation were covered during the measuring campaign: tall grass canopy before cut (29 May 2000), short grass after cut, and re-growing vegetation after fertilization (5 June 2000). Results show that beside weather conditions, the agricultural activities significantly influenced the O3 fluxes. After the cut the daytime average of the deposition velocity (vd) decreased from 0.44 cm s−1 to 0.26 cm s−1 and increased again to 0.32 cm s−1 during the third period. Detailed model calculations were carried out to estimate deposition velocity and ozone flux. The model captures the general diurnal patter of deposition, with vd daytime values of 0.52, 0.24, and 0.35 cm s−1 in the first, second and third period, respectively. Thus the model predicts a stronger response to the cut than the measurements, which is nevertheless smaller than expected on the basis of change in leaf area. The results show that both cut and fertilization have complex impacts on fluxes. Reduction of vegetation by cutting decreased the stomatal flux initially greatly, but the stomatal flux recovered to 80% of its original value within a week. At the same time, the non-stomatal flux appears to have increased directly after the cut, which the model partially explains by an increase in the deposition to the soil. A missing sink after the cut may be the chemical interaction with biogenic volatile organic compounds released after the cut and exposed senescent plant parts, or the increase in soil NO emissions after fertilization. Increased canopy temperatures may also have promoted ozone destruction on leaf surfaces. These results demonstrate the importance of canopy structure and non-stomatal pathways on O3 fluxes.

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