On the influence of sediment resuspension on deep-ocean Pa-231 and Th-230 cycling: Roles of turbulent mixing and differential scavenging

<p>The naturally-occurring particle-reactive radionuclides protactinium-231 (<sup>231</sup>Pa) and thorium-230 (<sup>230</sup>Th) are used as tracers of a variety of oceanic processes, both at present and in the past. Most notably, the sediment <sup>231</sup>Pa/<sup>230</sup>Th ratio has been used to infer changes in the Atlantic Meridional Overturning Circulation over the last (de)glaciation. However, recent measurements along the U.S. GEOTRACES North Atlantic transect (GA03) revealed two features which are at odds with current understanding about <sup>231</sup>Pa and <sup>230</sup>Th behaviour in the ocean: (i) a sharp decrease in dissolved <sup>231</sup>Pa and <sup>230</sup>Th activities with depth below 2000-4000 m and (ii) very large particulate <sup>231</sup>Pa and <sup>230</sup>Th activities near the bottom, at a number of stations between the New England continental shelf and Bermuda. Concomitant measurements of particulate matter concentration and potential temperature showed that both features are associated with the benthic nepheloid layer (BNL) and the bottom mixed layer (BML) that are present at these stations.</p><p>Here we develop and apply a simplified model of the exchange of particles, <sup>231</sup>Pa, and <sup>230</sup>Th between the BNL and the upper sediment, to explore the extent to which the radionuclide anomalies observed near the bottom at a number of GA03 stations can be explained by local sediment resuspension. We find that the model can broadly reproduce the observed anomalies at two stations where samples for radionuclide analyses were collected near the seafloor. Sensitivity tests with the model show that the <sup>231</sup>Pa/<sup>230</sup>Th ratio of particles in the BML and the sediment varies by a factor of 3 as the sediment resuspension rate fluctuates within a range consistent with observational estimates. The modelled variability is comparable to the spatial variability of <sup>231</sup>Pa/<sup>230</sup>Th of suspended particles in the modern North Atlantic and to the variability of Atlantic sediment <sup>231</sup>Pa/<sup>230</sup>Th records across the last (de)glacial period. Two factors are found to contribute to the modelled sensitivity of the sediment <sup>231</sup>Pa/<sup>230</sup>Th to sediment resuspension rate: the vertical turbulent mixing in the BML and the differential scavenging intensity of Pa and Th due to variation in particle concentration. Overall, our study indicates that the exchange of material between the BNL and the upper sediment can affect the particulate <sup>231</sup>Pa/<sup>230</sup>Th ratio in the bottom water and the sediment, which may complicate the use of sediment <sup>231</sup>Pa/<sup>230</sup>Th as a palaeoceanographic tracer.</p>

Sources and Temporal Variations of Coarse Particulate Matter (PM) in Central Tehran, Iran

In this study, we used the positive matrix factorization (PMF) model to evaluate the sources of ambient coarse particulate matter (PM) and their temporal variations in two sampling sites, i.e., a school dormitory and a retirement home, located in central Tehran. 24-h ambient PM samples were collected using low-volume air samplers from May 2012 to June 2013. The collected filters were analyzed for their chemical components, including water-soluble ions, metals, and trace elements, which were used as the input to the PMF model. Our results indicated annual averages of 45.7 ± 3.8 µg/m3 and 36.2. ± 4.0 µg/m3 for coarse PM at the School dormitory and Tohid retirement home, respectively. Moreover, higher ambient coarse PM mass concentrations were observed in the warm season (53.3 ± 5.8 µg/m3 for school dormitory and 43.1 ± 6.1 µg/m3 for Tohid retirement home) as opposed to the cold season (41.4 ± 4.7 µg/m3 for school dormitory and 28.7 ± 4.6 µg/m3 for Tohid retirement home). Our PMF analysis also identified road dust, soil, and industry, and atmospherically processed coarse PM as the three sources of ambient coarse PM in central Tehran. Road dust, soil, and industry were the major sources of ambient coarse PM, contributing respectively to 74 ± 9% and 19 ± 2% of the total coarse PM mass concentration, while atmospherically aged aerosols had a rather minimal contribution of 7 ± 1% to total coarse PM mass concentration. The temporal trends of the resolved factors also revealed higher contributions of road dust to total ambient coarse PM during warm season as opposed to cold season, due to the increased resuspension rate from road surfaces as a result of higher wind speeds, and temperatures, combined with lower relative humidity. Similarly, higher resuspension rate of mechanically originated particulates resulted in higher warm-season time contributions of the soil factor. Results of this study clearly revealed the key role of road dust and non-tail pipe emissions on ambient coarse PM mass concentrations in crowded areas of central Tehran, and have important implications on the potential health impacts that can be caused by these difficult to mitigate sources of coarse PM.

Development of a Device to Measure Mass and Resuspension Rate of Dust inside Confined Environments

A dust explosion is one of the key security issues for many industrial, pharmaceutical and agro-alimentary plants and for the safety of the workers. We have developed an optoelectronic sensor system to determine the mass of deposited dust and the resuspension rate. The authors also mount antennas on an optoelectronic sensor system to perform measurements remotely. The technique used is based on a non-invasive light absorption method. The paper reports a cost analysis in order to demonstrate the possibility to use, in our optoelectronic sensor system, several sensors to monitor large volume. In this paper the authors present the sensor system, the test and calibration of its components together with the results and the error analysis, demonstrating experimentally what is the maximum and the minimum readable range.

A Methodology for Estimating the Parameters in Three-Dimensional Cohesive Sediment Transport Models by Assimilating In Situ Observations with the Adjoint Method

The model parameters in the suspended cohesive sediment transport model are quite important for the accurate simulation of suspended sediment concentrations (SSCs). Based on a three-dimensional cohesive sediment transport model and its adjoint model, the in situ observed SSCs at four stations are assimilated to simulate the SSCs and to estimate the parameters in Hangzhou Bay in China. Numerical experimental results show that the adjoint method can efficiently improve the simulation results, which can benefit the prediction of SSCs. The time series of the modeled SSCs present a clear semidiurnal variation, in which the maximal SSCs occur during the flood tide and near the high water level due to the large current speeds. Sensitivity experiments prove that the estimated results of the settling velocity and resuspension rate, especially the temporal variations, are robust to the model settings. The temporal variations of the estimated settling velocity are negatively correlated with the tidal elevation. The main reason is that the mean size of the suspended sediments can be reduced during the flood tide, which consequently decreases the settling velocity according to Stokes’s law, and it is opposite in the ebb tide. The temporal variations of the estimated resuspension rate and the current speeds have a significantly positive correlation, which accords with the dynamics of the resuspension rate. The temporal variations of the settling velocity and resuspension rate are reasonable from the viewpoint of physics, indicating the adjoint method can be an effective tool for estimating the parameters in the sediment transport models.

Modelling for Resuspension of Multilayer Particles

The resuspension of graphite dust in High Temperature Gas-cooled Reactors is not only an essential way to estimate the concentration of particles in the normal operating core, but also a key phenomenon related to the release of radioactive products in depressurization accidents and the safety evaluation of environment. Precise resuspension modelling is an important work in safety analysis of HTRs. Current models were based on a single particle sitting on the substrate, however, multilayers of graphite dust deposit in the core during decades of operation. This paper uses recursion to deduce Rock’n’Roll model on each layer, and finally get the resuspension fraction changing with time and flow velocity. The results showed a “lag effect” of resuspension compared with those of Biasi’s modified model. Compared with the results of multilayer resuspension, the original monolayer model overestimated the resuspension rate and fraction.

Numerical solutions of Reeks-Hall equation for particulate concentration in recirculating turbulent fluid flow

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Source term is an important issue in safety assessment of nuclear power plants. Therefore, modeling of particulate concentration in reactor coolant systems during normal operation and hypothesized or real accidents is of continuing interest. We report here on exploration of a numerical solution of the Reeks and Hall equation with the use of fractional resuspension rate in its original integral form. The numerical results for particulate concentration are compared with those obtained from the exact expression given by Williams and experimental data provided by Wells et al. The numerical results agree very well with exact results and also agree well with the data of Wells et al. Applications of numerical method to problems with time dependent resuspension rate (for which exact solutions are not available), are explored and some typical results are reported. Research is carried out for three related resuspension models: Reeks, Reed, and Hall (1988), Vainshtein et al (1997), and Rock 'n Roll (2001). Results from Rock 'n Roll model show some advantages over the other two models. Since the advanced numerical technique we used may not be entirely suitable for use in large integrated computed codes, we have also explored use of a first order finite difference scheme for solving the Reeks-Hall equation. This first order scheme is sensitive to time-step size, but can work in some cases.

The regularity of wind-induced sediment resuspension in Meiliang Bay of Lake Taihu

Contaminants released by wind-induced sediment resuspension could influence the water quality in shallow lakes. This study aims to reveal the quantitative relationship between wind speed (v) and sediment resuspension rate (r) in Meiliang Bay of Lake Taihu. The study was conducted in three steps. First, the in situ wind speed and current velocity were measured over a period of 2 days in Meiliang Bay to establish the relationship between wind and hydrodynamic conditions; second, an indoor experiment was conducted in a cylindrical simulator with sediment from the study area to determine sediment resuspension rates under different hydrodynamic conditions; and third, linkages between sediment resuspension and wind were determined. The average sediment resuspension rate was highly correlated with the wind speed (R2 = 0.99), and was expressed by r = 20.72v2.034 at wind speeds in the range of 0–14 m/s. The critical wind speed for sediment resuspension is about 7 m/s. Under these conditions, the average resuspension rate could reach 1,000 g/(m2d), with a total phosphorus release rate of 1.1 g/(m2d) and a total nitrogen release rate of 18.1 g/(m2d).