Tracer Dispersion through Karst Conduit: Assessment of Small-Scale Heterogeneity by Multi-Point Tracer Test and CFD Modeling

Tracer tests are widely used for characterizing hydrodynamics, from stream-scale to basin-wide scale. In karstic environments, the positioning of field fluorometers (or sampling) is mostly determined by the on-site configuration and setup difficulties. Most users are probably aware of the importance of this positioning for the relevance of data, and single-point tests are considered reliable. However, this importance is subjective to the user and the impact of positioning is not well quantified. This study aimed to quantify the spatial heterogeneity of tracer concentration through time in a karstic environment, and its impact on tracer test results and derived information on local hydrodynamics. Two approaches were considered: on-site tracing experiments in a karstic river, and Computational Fluid Dynamics (CFD) modeling of tracer dispersion through a discretized karst river channel. A comparison between on-site tracer breakthrough curves and CFD results was allowed by a thorough assessment of the river geometry. The results of on-site tracer tests showed significant heterogeneities of the breakthrough curve shape from fluorometers placed along a cross-section. CFD modeling of the tracer test through the associated discretized site geometry showed similar heterogeneity and was consistent with the positioning of on-site fluorometers, thus showing that geometry is a major contributor of the spatial heterogeneity of tracer concentration through time in karstic rivers.

Velocity Dependence and Tracer Dispersion in Newtonian Fluids Undergoing Creeping Flow

The dynamics of tracer particles in a viscous Newtonian fluid is studied analytically and numerically through channels of varying thickness for fluids undergoing creeping flow. Exact analytical solutions of mass conservation equations of tracer particles including consideration for pressure forces are obtained. Results of the analysis indicates that Stokes velocity is an indispensable parameter and is dependent on parameters such as channel thickness (height), viscosity of the fluid, pressure gradient driven the fluid and Reynolds number corresponding to the channel thickness. The accuracy of the solution obtained is verified by comparing its velocity profiles with those obtained from finite-element-based numerical simulation studies.

Classroom aerosol dispersion: desk spacing and divider impacts

A study of aerosol dispersion was conducted in a university classroom using a CO2 tracer gas emitted from three source locations in a steady release, one source location per test. The tracer gas emitted from the single source location represented the potentially infectious aerosol droplets emitted from a single student and was thus a way to examine the influence of one sick student on the rest of the class. Two parameters were adjusted during the testing—the spacing of the desks, which included a spread and compressed configuration, and the inclusion of three-sided clear dividers attached to the student desk surfaces. Tracer dispersion was measured through the use of monitors in 13 locations within the classroom, with eight monitors representing seated student locations, four monitors representing a standing instructor along the classroom front, and one monitor at the return vent in the ceiling. As expected, spacing strongly influenced concentration levels at desks adjacent to the source location. The use of dividers reduced overall student and instructor location tracer concentrations when compared to desks without dividers in most cases. Finally, the influence of air change differences on the results was noted with consistent trends. The experimental construct provides a systematic means for classroom testing that may be broadly applicable to various configurations of classrooms beyond the one tested. Graphic abstract

The interaction between a low-level jet and the boundary layer in complex topography

<p>A study investigating the effect of a low-level jet (LLJ) event on the boundary-layer (BL) turbulent structure is presented.  During a radiosounding campaign aimed at investigating the atmospheric circulation in the area of Mount Carmel and Haifa Bay in Israel (35E - 33N), characterized by a complex terrain and a winding and jagged coastline, a couple of consecutive profiles showed a significant LLJ in the morning of January 7, 2010. Since there are no previous measurements or information about frequency or characteristics of the LLJ in this region, the scarcity of observed data does not allow addressing the nature and features of the LLJ. Therefore, its characteristics and development, and also its impact on tracer dispersion, have been explored through model simulations, using RAMS atmospheric model. RAMS was configured with four nested grids with resolution from 32 km to 500 m. A high vertical resolution in the inner grid was achieved with 15 levels below 400 m, using a vertical nesting with a rather novel approach not frequently adopted. RAMS simulated variables were verified against the available observations, providing a reliable reproduction of the LLJ pattern. An elevated inversion characterized the temperature profiles and the LLJ was located at the bottom of such inversion. An analysis of the turbulence kinetic energy (TKE) versus a jet-Richardson number showed that a strongly turbulent weakly-Stable-BL was characterizing the LLJ episode. At the hours of the peak of the LLJ event, 0700 and 0800 UTC (0900 and 1000 LT), two separate maxima, generated below and above the layer affected by the LLJ, appeared in the TKE vertical profiles due to the strong wind shear. Being a morning LLJ, when buoyancy-driven vertical motions started to develop they acted sustaining the turbulence below the LLJ, then decaying at higher elevation opposed also by the strong wind speed at the LLJ level. These and other results are presented and discussed, as a contribution to the understanding of LLJ dynamics and its impact on the boundary layer in complex topography.</p>

Temporal variability of methane emissions from a closed landfill at Denmark

<p>Methane (CH<sub>4</sub>) emissions from landfills contribute to global warming, impacting significantly the environment and human health. Landfill CH<sub>4</sub> emissions strongly depend on changes in barometric pressure, inducing short-term CH<sub>4</sub> emission variation of several orders of magnitude. Estimating the temporal variability of CH<sub>4</sub> emitted into the atmosphere could help us reducing the uncertainties of annual emission estimates from landfills. In this study, we focus on the temporal variability of CH<sub>4</sub> emissions under the impact of barometric pressure changes.</p><p>CH<sub>4</sub> emissions of a closed landfill (Skellingsted, Western Zealand, Denmark) were measured with two different methods from December 2019 to June 2020; continuously with the eddy covariance method (EC) and discretely with the dynamic tracer dispersion method (TDM). The EC method allows continuous measurements from a confined surface area, with most likely limited representativeness of the whole landfill site due to the considerable horizontal heterogeneity. The TDM method is able to quantify the emission from the whole site insensitive of the topography with the limited representativeness for the temporal variability.</p><p>CH<sub>4</sub> emissions to the atmosphere measured by the TDM and fluxes measured by the EC ranged from to 0 to almost 100 kg h<sup>-1</sup> and from 0 to 10 μmol m<sup>-2</sup> s<sup>-1</sup>, respectively. The CH<sub>4</sub> fluxes measured continuously using the EC method were highly correlated with the emissions from the periodic measurements using the TDM and fluctuated according to the pressure tendency. Under decreasing barometric pressure the highest CH<sub>4</sub> emissions where observed, while increasing barometric pressure suppressed them almost to 0.</p><p>Our results demonstrate the value of implementing two different complementary measurement techniques in parallel that will help to quantify total annual CH<sub>4</sub> emission from a landfill. EC method provides continuous measurements describing accurately the temporal variation of emissions, while TDM method is able to quantify emissions from the whole site.</p>

Can statistics of turbulent tracer dispersion be inferred from camera observations of SO₂ in the ultraviolet? A modelling study

Atmospheric turbulence and in particular its effect on tracer dispersion may be measured by cameras sensitive to the absorption of ultraviolet (UV) sunlight by sulfur dioxide (SO2), a gas that can be considered a passive tracer over short transport distances. We present a method to simulate UV camera measurements of SO2 with a 3D Monte Carlo radiative transfer model which takes input from a large eddy simulation (LES) of a SO2 plume released from a point source. From the simulated images the apparent absorbance and various plume density statistics (centre-line position, meandering, absolute and relative dispersion, and skewness) were calculated. These were compared with corresponding quantities obtained directly from the LES. Mean differences of centre-line position, absolute and relative dispersions, and skewness between the simulated images and the LES were generally found to be smaller than or about the voxel resolution of the LES. Furthermore, sensitivity studies were made to quantify how changes in solar azimuth and zenith angles, aerosol loading (background and in plume), and surface albedo impact the UV camera image plume statistics. Changing the values of these parameters within realistic limits has negligible effects on the centre-line position, meandering, absolute and relative dispersions, and skewness of the SO2 plume. Thus, we demonstrate that UV camera images of SO2 plumes may be used to derive plume statistics of relevance for the study of atmospheric turbulent dispersion.