scholarly journals Ventilation Induced in an Isolated Subsurface Structure by Natural Forces: Method Development and Application

2019 ◽  
Vol 4 (4) ◽  
pp. 68
Author(s):  
Thomas Neil McManus ◽  
Assed Haddad
Keyword(s):  
Vertical Profile ◽  
Method Development ◽  
Narrow Band ◽  
Operating Time ◽  
Gas Transfer ◽  
Tracer Gas ◽  
Engine Exhaust ◽  
Near Surface ◽  
Subsurface Structures ◽  
Development And Utilization

It is believed that isolated subsurface structures of an infrastructure do not ventilate through opening(s) in manhole covers. The literature has almost no information on this topic. This study reports on considerations involved in the development and utilization of a method to study this question. Carbon monoxide (CO) is readily obtainable in engine exhaust, easily detectable at very low concentration, and is relatively safe to handle, which makes it ideal for use as a tracer gas. Transfer into the airspace of the structure was carried out using metal tubing. This study examined the engine operating time and the number of openings in a manhole cover. CO was measured using four instruments in the vertical profile. It was found to generally decrease in a narrow band, initially linearly through a curvilinear region and a linear tail. Clearance of most of the contaminant occurred rapidly during the first part of the process. A decrease to 25 ppm required from 439 min (7 openings) to 1118 min (1 opening). Ambient temperature and near-surface air flow likely influenced these values. The measurement profiles strongly suggest that the atmosphere in the airspace was rapidly and thoroughly well-mixed. The methodology developed and reported here is suitable for a more expanded investigation, the intent being to identify modifications of the design to optimize the process.

scholarly journals Drivers of diffusive CH<sub>4</sub> emissions from shallow subarctic lakes on daily to multi-year timescales

2020 ◽  
Vol 17 (7) ◽  
pp. 1911-1932 ◽  
Author(s):  
Joachim Jansen ◽  
Brett F. Thornton ◽  
Alicia Cortés ◽  
Jo Snöälv ◽  
Martin Wik ◽  
...  
Keyword(s):  
Wind Speed ◽  
Water Column ◽  
Atmospheric Stability ◽  
Water Concentration ◽  
Gas Transfer ◽  
Air Concentration ◽  
Subarctic Lakes ◽  
Surface Renewal ◽  
Near Surface ◽  
Functional Relations

Abstract. Lakes and reservoirs contribute to regional carbon budgets via significant emissions of climate forcing trace gases. Here, for improved modelling, we use 8 years of floating chamber measurements from three small, shallow subarctic lakes (2010–2017, n=1306) to separate the contribution of physical and biogeochemical processes to the turbulence-driven, diffusion-limited flux of methane (CH4) on daily to multi-year timescales. Correlative data include surface water concentration measurements (2009–2017, n=606), total water column storage (2010–2017, n=237), and in situ meteorological observations. We used the last to compute near-surface turbulence based on similarity scaling and then applied the surface renewal model to compute gas transfer velocities. Chamber fluxes averaged 6.9±0.3 mg CH4 m−2 d−1 and gas transfer velocities (k600) averaged 4.0±0.1 cm h−1. Chamber-derived gas transfer velocities tracked the power-law wind speed relation of the model. Coefficients for the model and dissipation rates depended on shear production of turbulence, atmospheric stability, and exposure to wind. Fluxes increased with wind speed until daily average values exceeded 6.5 m s−1, at which point emissions were suppressed due to rapid water column degassing reducing the water–air concentration gradient. Arrhenius-type temperature functions of the CH4 flux (Ea′=0.90±0.14 eV) were robust (R2≥0.93, p<0.01) and also applied to the surface CH4 concentration (Ea′=0.88±0.09 eV). These results imply that emissions were strongly coupled to production and supply to the water column. Spectral analysis indicated that on timescales shorter than a month, emissions were driven by wind shear whereas on longer timescales variations in water temperature governed the flux. Long-term monitoring efforts are essential to identify distinct functional relations that govern flux variability on timescales of weather and climate change.

scholarly journals Surface Air Movement: An Important Contributor to Ventilation of Isolated Subsurface Structures?

2019 ◽  
Vol 4 (2) ◽  
pp. 23 ◽  
Author(s):  
Thomas Neil McManus ◽  
Assed Haddad
Keyword(s):  
Air Flow ◽  
Helmholtz Resonator ◽  
Worker Safety ◽  
Time Base ◽  
Near Surface ◽  
Helmholtz Resonators ◽  
Subsurface Structures ◽  
Air Movement ◽  
Air Speed ◽  
Almost All

This study reports on near-surface airspeed measured using a fast-responding thermoanemometer during an investigation of ventilation of an isolated subsurface structure induced by natural forces. Air speed changes continuously, rapidly, and unpredictably when assessed on the time base of one or two seconds. Zero, the most common air speed, occurred in almost all tests throughout the year but especially during cool and cold months. The most probable non-zero air speed, 10.7 m/min (35 ft/min), occurred in all tests. This air speed is below the level of detection by the senses. The number of zero values and the height of the peak at 10.7 m/min follow a repetitive annual cycle. Isolated subsurface structures containing manhole covers share the characteristics of Helmholtz resonators. Grazing air flow across the opening to the exterior induces rotational air flow in the airspace of a Helmholtz resonator. Rotational flow in the airspace potentially influences the exchange of the confined atmosphere with the external one. Ventilation of the airspace occurs continuously and without cost and is potentially enhanced by the unique characteristics of the Helmholtz resonator excited by surface air movement. These results have immense importance and immediate applicability to worker safety.

ADCP Observation and Numerical Model Prediction of Tidal Currents Near Kozu Island

2018 ◽  
Author(s):  
Tsubasa Kodaira ◽  
Takuji Waseda
Keyword(s):  
Numerical Model ◽  
Model Prediction ◽  
Vertical Profile ◽  
General Circulation ◽  
Circulation Model ◽  
Tidal Currents ◽  
Ocean Current ◽  
Near Surface ◽  
The Kuroshio ◽  
Adcp Observation

Izu island chain south of Tokyo, Japan has been considered as a prospective area for ocean renewable energy development because of relatively strong tidal currents, and the Kuroshio current. An Acoustic Doppler Current Profiler (ADCP) measurement near one of the islands, Kozu island, was conducted to know ocean current variability. The ADCP, Signature500, was installed 700m away from the coast and the water depth is 32m there. The observation period is for a month from January 8th, 2017. Vertical profile of the currents was recorded every 1 m at 1 minute interval. Based on the observation, the peak tidal currents were often more than 1m/s, and the maximum total speed exceeded 2m/s. The maximum flow is probably related to the Kuroshio intrusion because temperature increased abruptly. Typical vertical profile of the current follows the logarithmic law except for the near surface area. To study the spatial distribution of the current around Kozu island, a numerical model is developed based on MIT General Circulation Model (MITgcm). The model prediction of tidal sea surface variation is in a good agreement with the observation. The model underestimated the tidal currents, but the phase is in a reasonable agreement with the ADCP observation result.

Tropospheric NO2 and HCHO derived from dual-scan MAX-DOAS measurements in Uccle (Belgium) and application to S5P/TROPOMI validation

2020 ◽  
Author(s):  
Ermioni Dimitropoulou ◽  
Francois Hendrick ◽  
Martine M. Friedrich ◽  
Gaia Pinardi ◽  
Frederik Tack ◽  
...  
Keyword(s):  
Vertical Profile ◽  
A Priori ◽  
Elevation Angle ◽  
Optical Absorption Spectroscopy ◽  
Azimuthal Direction ◽  
Inversion Algorithm ◽  
Vertical Column ◽  
Near Surface ◽  
Mixing Ratios ◽  
Horizontal Variation

&lt;p&gt;Ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of aerosols, tropospheric nitrogen dioxide (NO&lt;sub&gt;2&lt;/sub&gt;) and formaldehyde (HCHO) have been carried out in Uccle, Brussels, during two years (March 2018 &amp;#8211; March 2020). The MAX-DOAS instrument has been operating in both UV and visible (Vis) wavelength ranges in a dual-scan configuration consisting of two sub-modes: (1) an elevation scan in a fixed viewing azimuthal direction (the so-called main azimuthal direction) pointing and (2) an azimuthal scan in a fixed low elevation angle (2&lt;sup&gt;o&lt;/sup&gt;). By applying a vertical profile inversion algorithm in the main azimuthal direction and an adapted version of the parameterization technique proposed by Sinreich et al. (2013) in the other azimuthal directions, near-surface &amp;#160;concentrations (VMRs) and vertical column densities (VCDs) are retrieved in ten different azimuthal directions.&lt;/p&gt;&lt;p&gt;The present work focuses on the seasonal horizontal variation of NO&lt;sub&gt;2 &lt;/sub&gt;and HCHO around the measurement site. The observations show a clear seasonal cycle of these trace gases. An important application of the dual-scan MAX-DOAS measurements is the validation of satellite missions with high spatial resolution, such as TROPOMI/S5P. Measuring the tropospheric &amp;#160;VCDs in different azimuthal directions is shown to improve the spatial colocation with satellite measurements leading to a better agreement between both datasets. By using &amp;#160;vertical profile information derived from the MAX-DOAS measurements, we show that a persistent systematic underestimation of the TROPOMI &amp;#160;data can be explained by uncertainties in the a-priori NO&lt;sub&gt;2&lt;/sub&gt; profile shape in the satellite retrieval. A similar validation study for TROPOMI HCHO is currently under progress and preliminary results will be presented.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Sinreich, R., Merten, A., Molina, L., and Volkamer, R.: Parameterizing radiative transfer to convert MAX-DOAS dSCDs into near-surface box-averaged mixing ratios, Atmos. Meas. Tech., 6, 1521&amp;#8211;1532, https://doi.org/10.5194/amt-6-1521-2013, 2013.&lt;/p&gt;

scholarly journals Air-sea exchange of acetone, acetaldehyde, DMS and isoprene at a UK coastal site.

2021 ◽  
Author(s):  
Daniel Phillips ◽  
Frances Hopkins ◽  
Thomas Bell ◽  
Charel Wohl ◽  
Claire Reeves ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic ◽  
Atmospheric Chemistry ◽  
North Atlantic Ocean ◽  
Open Water ◽  
Proton Transfer Reaction ◽  
Gas Transfer ◽  
Gaseous Emissions ◽  
Near Surface ◽  
Direct Measurements

&lt;p&gt;Volatile organic compounds (VOCs) are ubiquitous in the atmosphere and are important for atmospheric chemistry. Large uncertainties remain in the role of the ocean in the atmospheric VOC budget because of poorly constrained marine sources and sinks. There are very few direct measurements of air-sea VOC fluxes near the coast, where natural marine emissions could influence coastal air quality (i.e. ozone (O&lt;sub&gt;3&lt;/sub&gt;), aerosols) and terrestrial gaseous emissions could be taken up by the coastal seas.&lt;/p&gt;&lt;p&gt;To address this, we present air&amp;#8211;sea fluxes of acetone, acetaldehyde and dimethylsulfide (DMS) at the coastal Penlee Point Atmospheric Observatory (PPAO) in the South-West UK during the spring (Apr-May 2018). Fluxes are quantified simultaneously by eddy covariance (EC) using a proton transfer reaction quadrupole mass spectrometer. Comparisons are made between two wind sectors representative of different air-water exchange regimes: the open water sector facing the North Atlantic Ocean and the fetch-limited Plymouth Sound fed by two estuaries.&lt;/p&gt;&lt;p&gt;Mean EC (&amp;#177; 1 standard error) fluxes of acetone, acetaldehyde and DMS from the open-water wind sector were &amp;#8209;8.01&amp;#177;0.77, &amp;#8209;1.55&amp;#177;1.44 and 4.67&amp;#177;0.56 &amp;#956;mol m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt; respectively (- sign indicates air-to-sea deposition). These measurements are generally comparable (same order of magnitude) to previous measurements in the Eastern North Atlantic Ocean at the same latitude. In comparison, the terrestrially influenced Plymouth Sound wind sector showed respective fluxes of -12.93&amp;#177;1.37, -4.45&amp;#177;1.73 and 1.75&amp;#177;0.80 &amp;#956;mol m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;. The greater deposition fluxes of acetaldehyde and acetone within the Plymouth Sound were largely driven by higher atmospheric concentrations from the terrestrial wind sector. The reduced DMS emission from the Plymouth Sound was caused by a combination of lower wind speed and likely lower dissolved concentrations as a result of the freshwater estuarine influence (i.e. dilution).&lt;/p&gt;&lt;p&gt;In addition, we measured the near surface seawater concentrations of acetone, acetaldehyde, DMS and isoprene from a marine station 6 km offshore. Comparisons are made between EC fluxes from the open water and diffusive VOC fluxes calculated with a two-layer (TL) model of gas transfer using air/water concentrations. The calculated TL fluxes are largely consistent with our direct measurements in the directions and magnitudes of fluxes. Generally, the TL model predicted acetone and acetaldehyde fluxes that were ~12&amp;#8211;33 % higher (greater deposition) than the EC measurements. This could be due to sea surface processes that produce these carbonyl compounds that were not accounted for by the TL technique.&lt;/p&gt;

scholarly journals Structural features derived from a Multiscale Analysis and 2.75D Modelling of Aeromagnetic Data over the Pitoa-Figuil Area (Northern Cameroon)

2020 ◽  
Author(s):  
Voltaire Souga Kassia ◽  
Theophile Ndougsa-Mbarga ◽  
Arsène Meying ◽  
Jean Daniel Ngoh ◽  
Steve Ngoa Embeng
Keyword(s):  
Multiscale Analysis ◽  
Structural Features ◽  
Magnetic Data ◽  
Horizontal Gradient ◽  
Aeromagnetic Data ◽  
Near Surface ◽  
Subsurface Structures ◽  
Northern Cameroon ◽  
Processing Techniques ◽  
Geological Formations

Abstract. In the Pitoa-Figuil area (Northern Cameroon), an interpretation of aeromagnetic data was conducted. The aim of this investigation was first to emphasize lineaments hidden under geological formations and secondly to propose two 2.75D models of the subsurface structures. Different magnetic data processing techniques were used, notably horizontal gradient magnitude, analytic signal, and Euler deconvolution. These techniques in combination with the 2.75D modelling to the aeromagnetic anomaly reduced to the equator permit to understand the stratification of the deep and near surface structures, which are sources of the observed anomalies. We managed to put in evidence and characterize 18 faults and some intrusive bodies. According to Euler's solutions, anomaly sources go up to a depth of 5.3 km.

scholarly journals Subsurface structures of a quick-clay sliding prone area revealed using land-river reflection seismic data and hydrogeological modelling

2019 ◽  
Author(s):  
Silvia Salas-Romero ◽  
Alireza Malehmir ◽  
Ian Snowball ◽  
Benoît Dessirier
Keyword(s):  
Large Scale ◽  
Geophysical Methods ◽  
Coarse Grained ◽  
Magnetic Data ◽  
Near Surface ◽  
Side Scan Sonar ◽  
Reflection Seismic ◽  
Subsurface Structures ◽  
Geotechnical Investigations ◽  
Quick Clay

Abstract. Quick-clay landslides are common geohazards in Nordic countries and Canada. The presence of potential quick clays is confirmed using geotechnical investigations, but near-surface geophysical methods, such as seismic and resistivity surveys, can also help identifying coarse-grained materials associated to the development of quick clays. We present the results of reflection seismic investigations on land and in part of the Göta River in Sweden, along which many quick-clay landslide scars exist. This is the first time that such a large-scale reflection seismic investigation has been carried out to study the subsurface structures associated with quick-clay landslides. The results also show a reasonable correlation with the radio magnetotelluric and traveltime tomography models. The morphology of the river bottom and riverbanks, as e.g. subaquatic landslide deposits, is shown by side-scan sonar and bathymetric data. Undulating bedrock, covered by subhorizontal sedimentary glacial and postglacial deposits is clearly revealed. An extensive coarse-grained layer exists in the sedimentary sequence and is interpreted and modelled in a regional context. Individual fractures and fracture zones are identified within bedrock and sediments. Hydrological modelling of the coarse-grained layer confirms its potential for transporting fresh water infiltrated in fractures and nearby outcrops. The groundwater flow in the coarse-grained layer promotes leaching of marine salts from the overlying clays by slow infiltration and/or diffusion, which helps in the formation of potential quick clays. Magnetic data show coarse-grained materials at the landslide scar located in the study area, which may have acted as a sliding surface together with quick clays.

scholarly journals Accounting for surface waves improves gas flux estimation at high wind speed in a large lake

2021 ◽  
Author(s):  
Pascal Perolo ◽  
Bieito Fernández Castro ◽  
Nicolas Escoffier ◽  
Thibault Lambert ◽  
Damien Bouffard ◽  
...  
Keyword(s):  
Surface Waves ◽  
Wind Shear ◽  
Diffusion Flux ◽  
Gas Transfer ◽  
Co2 Fluxes ◽  
Large Lake ◽  
Lake Geneva ◽  
Transfer Velocity ◽  
Near Surface ◽  
Gas Transfer Velocity

Abstract. The gas transfer velocity (k) is a major source of uncertainty when assessing the magnitude of lake gas exchange with the atmosphere. For the diversity of existing empirical and process-based k models, the transfer velocity increases with the level of turbulence near the air-water interface. However, predictions for k can vary by a factor of 2 among different models. Near-surface turbulence results from the action of wind shear, surface waves and buoyancy-driven convection. Wind shear has long been identified as a key driver, while recent lake studies have shifted the focus towards the role of convection, particularly in small lakes. In large lakes, wind fetch can however be long enough to generate surface waves and contribute to enhance gas transfer, as widely recognised in oceanographic studies. Here, field values for gas transfer velocity were computed in a large hardwater lake, Lake Geneva, from CO2 fluxes measured with an automated (forced diffusion) flux chamber and CO2 partial pressure measured with high frequency sensors. k estimates were compared to a set of reference limnological and oceanic k models. Our analysis reveals that accounting for surface waves generated during windy events significantly improves the accuracy of k estimates in this large lake. The improved k model is then used to compute k over a one-year time-period. Results show that episodic extreme events with surface waves (6 % occurrence, significant wave height > 0.4 m) can generate more than 20 % of annual cumulative k and more than 25 % of annual net CO2 fluxes in Lake Geneva. We conclude that for lakes whose fetch can exceed 15 km, k-models need to integrate the effect of surface waves.

scholarly journals Ocean bubbles under high wind conditions. Part 1: Bubble distribution and development

2021 ◽  
Author(s):  
Helen Czerski ◽  
Ian M. Brooks ◽  
Steve Gunn ◽  
Robin Pascal ◽  
Adrian Matei ◽  
...  
Keyword(s):  
Void Fraction ◽  
Breaking Waves ◽  
Surface Flow ◽  
Stokes Drift ◽  
Gas Transfer ◽  
Langmuir Circulation ◽  
Near Surface ◽  
Wind Speeds ◽  
Void Fractions ◽  
Bubble Plumes

Abstract. The bubbles generated by breaking waves are of considerable scientific interest due to their influence on air-sea gas transfer, aerosol production, and upper ocean optics and acoustics. However, a detailed understanding of the processes creating deeper bubble plumes (extending 2–10 metres below the ocean surface) and their significance for air-sea gas exchange is still lacking. Here, we present bubble measurements from the HiWinGS expedition in the North Atlantic in 2013, collected during several storms with wind speeds of 10–27 m s−1. A suite of instruments was used to measure bubbles from a self-orienting free-floating spar buoy: a specialised bubble camera, acoustical resonators, and an upward-pointing sonar. The focus in this paper is on bubble void fractions and plume structure. The results are consistent with the presence of a heterogeneous shallow bubble layer occupying the top 1–2 m of the ocean which is regularly replenished by breaking waves, and deeper plumes which are only formed from the shallow layer at the convergence zones of Langmuir circulation. These advection events are not directly connected to surface breaking. The void fraction distributions at 2 m depth show a sharp cut-off at a void fraction of 10−4.5 even in the highest winds, implying the existence of mechanisms limiting the void fractions close to the surface. Below wind speeds of 16 m s−1 or RHw = 2 × 106, the probability distribution of void fraction at 2 m depth is very similar in all conditions, but increases significantly above either threshold. Void fractions are significantly different during periods of rising and falling winds, but there is no distinction with wave age. There is a complex near-surface flow structure due to Langmuir circulation, Stokes drift, and wind-induced current shear which influences the spatial distribution of bubbles within the top few metres. We do not see evidence for slow bubble dissolution as bubbles are carried downwards, implying that collapse is the more likely termination process. We conclude that the shallow and deeper bubble layers need to be studied simultaneously to link them to the 3D flow patterns in the top few metres of the ocean. Many open questions remain about the extent to which deep bubble plumes contribute to air-sea gas transfer. A companion paper (Czerski, 2021) addresses the observed bubble size distributions and the processes responsible for them.

Sign in / Sign up

Export Citation Format

Share Document