Luminescence Dating in Fluvial Settings: Overcoming the Challenge of Partial Bleaching

Optically stimulated luminescence (OSL) dating is a versatile technique that utilises the two most ubiquitous minerals on Earth (quartz or K-feldspar) for constraining the timing of sediment deposition. It has provided accurate ages in agreement with independent age control in many fluvial settings, but is often characterised by partial bleaching of individual grains. Partial bleaching can occur where sunlight exposure is limited and so only a portion of the grains in the sample was exposed to sunlight prior to burial, especially in sediment-laden, turbulent or deep water columns. OSL analysis on multiple grains can provide accurate ages for partially bleached sediments where the OSL signal intensity is dominated by a single brighter grain, but will overestimate the age where the OSL signal intensity is equally as bright (often typical of K-feldspar) or as dim (sometimes typical of quartz). In such settings, it is important to identify partial bleaching and the minimum dose population, preferably by analysing single grains, and applying the appropriate statistical age model to the dose population obtained for each sample. To determine accurate OSL ages using these age models, it is important to quantify the amount of scatter (or overdispersion) in the well-bleached part of the partially bleached dose distribution, which can vary between sediment samples depending upon the bedrock sources and transport histories of grains. Here, we discuss how the effects of partial bleaching can be easily identified and overcome to determine accurate ages. This discussion will therefore focus entirely on the burial dose determination for OSL dating, rather than the dose-rate, as only the burial doses are impacted by the effects of partial bleaching

Coagulation in a spatially inhomogeneous plume: Formation of bimodal size distribution

Aerosol systems are more often spatially heterogeneous than homogeneous due to various factors such as turbulence in the atmosphere, flow field in a pipe, and varying generation rates at the source. As a general result, we demonstrate that an initially monodisperse and spatially heterogeneous aerosol system evolves into a bimodal size distribution purely by coagulation. The spatial inhomogeneity in the particle number concentration initiates differential coagulation rates which lead to a distribution with larger size modes in regions with higher concentration. When averaged over space, this would appear as a bimodal size distribution. We show this effect through a free-molecular coagulation model for a spatially heterogeneous system combined with the scaling theory of self-preserving distributions. It is found that sharper the occurrence of spatial heterogeneity, more pronounced is the bimodal effect. The study clearly demonstrates spatial heterogeneity as an additional factor for the origin of bimodality in aerosols. (C) 2015 Elsevier Ltd. All rights reserved

Modeling Critical Air Exchange Rates (CAERs) for aerosol number concentrations from nano-particle sources using an "effective coagulation coefficient" approach

An important issue in the context of air pollution by indoor combustion sources pertains to the joint effect of source strength, coagulation, and ventilation rate on the ultrafine particle exposure metrics. It was recently predicted by detailed numerical analysis of the Smoluchowski coagulation equation with continuous source and sink terms that the ultrafine particle number, mass, and surface area concentrations do not monotonically decrease with increasing air exchange rate, but display peak concentrations at certain Critical Air Exchange Rates (CAERs). As these results are of considerable significance for exposure assessment as well as for implementing particle control technologies, it is necessary to assess the CAER for different aerosol characteristics. Given the fact that the numerical method of solving coagulation equation with realistic Fuchs kernel is computationally intensive, simpler semi-analytical approaches are desired for providing reasonable estimates of CAER and clearer insight into the counter-intuitive, peaking behavior. In this article, we present such an approach by replacing the Fuchs kernel by a spectrum-averaged effective coagulation coefficient, within the framework of the steady-state model. The effective coagulation coefficient is size independent but depends implicitly on the aerosol concentration thus capturing the combined effect of coagulation and removal processes. The number concentrations obtained from this method have been compared and validated against the numerical solutions. The model predicts more pronounced effects on the peaking behavior as well as larger CAER values for fractal particles as compared to compact particles. The results are further discussed

Coagulation effect on the activity size distributions of long lived radon progeny aerosols and its application to atmospheric residence time estimation techniques

The long lived naturally occurring radon progeny species in the atmosphere, namely (210)pb, Bi-210 and Po-210, have been used as important tracers for understanding the atmospheric mixing processes and estimating aerosol residence times. Several observations in the past have shown that the activity size distribution of these species peaks at larger particle sizes as compared to the short lived radon progeny species an effect that has been attributed to the process of coagulation of the background aerosols to which they are attached. To address this issue, a mathematical equation is derived for the activity-size distribution of tracer species by formulating a generalized distribution function for the number of tracer atoms present in coagulating background particles in the presence of radioactive decay and removal. A set of these equations is numerically solved for the progeny chain using Fuchs coagulation kernel combined with a realistic steady-state aerosol size spectrum that includes nucleation, accumulation and coarse mode components. The important findings are: (i) larger shifts in the modal sizes of Pb-210 and Po-210 at higher aerosol concentrations such as that found in certain Asian urban regions (ii) enrichment of tracer specific activity on particles as compared to that predicted by pure attachment laws (iii) sharp decline of daughter-to-parent activity ratios for decreasing particle sizes. The implication of the results to size-fractionated residence time estimation techniques is highlighted. A coagulation corrected graphical approach is presented for estimating the residence times from the size-segregated activity ratios of Bi-210 and Po-210 with respect to Pb-210. The discrepancy between the residence times predicted by conventional formula and the coagulation corrected approach for specified activity ratios increases at higher atmospheric aerosol number concentrations (>1010 #/m(3)) for smaller sizes (<1 pm). The results are further discussed. (C) 2014 Elsevier Ltd. All rights reserved

Survival of aerosol particles in a puff with spatially inhomogeneous size spectrum

Aerosol particles released from emission sources undergo various atmospheric and aerosol processes before they become a part of the background aerosols. Coagulation and dispersion, the two important processes that governs evolution of particle characteristics in a puff with an inhomogeneous size distribution of particles in space is considered in this study. This specific case consists of an initial Gaussian aerosol packet in which larger particles are preferentially segregated to farther distances from the centre of the packet. The coagulation-dispersion equation is solved using Jaffe approximation technique for obtaining essential results such as survival fraction, that is, fraction of particles surviving due to the simultaneous action of coagulation and dispersion. Analytical results are developed for the temporal variations of the number concentration, mean size and the standard deviation. The asymptotic results yield the final characteristics of the spectra of the particles which form a part of the background aerosols. These quantities are useful for defining the "effective aerosol source terms" in the general dynamic equations for background aerosols, long-range transport of aerosols, and geo-engineering applications. The results are further discussed

Ventilation dependence of concentration metrics of Ultra-fine Particles in a coagulating household smoke

Role of Ultra-fine Particles (UFPs) in causing adverse health effects among large population across the world, attributable to household smoke, is being increasingly recognized. However, there is very little theoretical perspective available on the complex behavior of the UFP metrics with respect to controlling factors, such as ventilation rate and particle emission rate from the combustion sources. This numerical study examines through coagulation dynamics, the dependence of UFP metrics, viz., number (PN), mass (PM0.1) and surface area (PA(0.1)) concentrations below 0.1m diameter, on ventilation and the number emission rate from household smoke. For strong sources, the steady-state concentrations of these metrics are found to increase initially with increasing Air Exchange Rate (AER), reach a peak value and then decrease. Counter correlations are seen between UFP metric and PM2.5 concentrations. The concepts of Critical Air Exchange Rate (CAER) and Half-Value Air Exchange Rate (HaVAER) have been introduced which indicate a feasibility of mitigation of PM0.1 and PA(0.1), unlike PN, by ventilation techniques. The study clearly brings forth complex differential behavior of the three UFP metrics. The results are further discussed

Axisymmetric viscous interfacial oscillations - theory and simulations

We study axisymmetric, free oscillations driven by gravity and surface tension at the interface of two viscous, immiscible, radially unbounded fluids, analytically and numerically. The interface is perturbed as a zeroth-order Bessel function (in space) and its evolution is obtained as a function of time. In the linearised approximation, we solve the initial value problem (IVP) to obtain an analytic expression for the time evolution of wave amplitude. It is shown that a linearised Bessel mode temporally evolves in exactly the same manner as a Fourier mode in planar geometry. We obtain novel analytical expressions for the time varying vorticity and pressure fields in both fluids. For small initial amplitudes, our analytical results show excellent agreement with those obtained from solving the axisymmetric Navier-Stokes equations numerically. We also compare our results with the normal mode approximation and find the latter to be an accurate representation at very early and late times. The deviation between the normal mode approximation and the IVP solution is found to increase as a function of viscosity ratio. The vorticity field has a jump discontinuity at the interface and we find that this jump depends on the viscosity and the density ratio of the two fluids. Upon increasing the initial perturbation amplitude in the simulations, nonlinearity produces qualitatively new features not present in the analytical IVP solution. Notably, a jet is found to emerge at the axis of symmetry rising to a height greater than the initial perturbation amplitude. Increasing the perturbation amplitude further causes the jet to undergo end pinch off, giving birth to a daughter droplet. This can happen either for an advancing or a receding jet, depending on the viscosity ratio. A relation is found between the maximum jet height and the perturbation amplitude. Hankel transform of the interface demonstrates that at large perturbation amplitudes higher wavenumbers emerge, sharing some of the energy of the lowest mode. When these additional higher modes are present, the interface has pointed crests and rounded troughs

Engineering of layered, lipid-encapsulated drug nanoparticles through spray-drying

Drug-containing nanoparticles have been synthesized through the spray-drying of submicron droplet aerosols by using matrix materials such as lipids and biopolymers. Understanding layer formation in composite nanoparticles is essential for the appropriate engineering of particle substructures. The present study developed a droplet-shrinkage model for predicting the solid-phase formation of two non-volatile solutes-stearic acid lipid and a set of drugs, by considering molecular volume and solubility. Nanoparticle formation was simulated to define the parameter space of material properties and process conditions for the formation of a layered structure with the preferential accumulation of the lipid in the outer layer. Moreover, lipid-drug demarcation diagrams representing a set of critical values of ratios of solute properties at which the two solutes precipitate simultaneously were developed. The model was validated through the preparation of stearic acid-isoniazid nanoparticles under controlled processing conditions. The developed model can guide the selection of solvents, lipids, and processing conditions such that drug loading and lipid encapsulation in composite nanoparticles are optimized. (C) 2017 Elsevier B.V. All rights reserved

Theoretical analysis of formation of many-drop arrays in a quadrupole electrodynamic balance

Electrodynamic (ED) levitation of one or a few charged droplets using Paul traps offers opportunities to understand the Rayleigh instability as well as the role of various inter-particle forces on pattern formations. In this work, we develop a theory for two-dimensional patterns formed during levitation of two to a few droplets in an ED balance, based on an extension of the classical Dehmelt approximation to interacting particle systems. The theory presents closed-form solutions to the inter-particle separations, secular frequencies, orientation stabilities of two-drop systems, and collapse of droplets onto the XY plane. Based on symmetry considerations, the theory is extended to predict the sizes and shapes of closed structures formed for few-drop systems. The analysis specifically helps one to understand the charge independence of inter-particle separations for equal charge to mass ratio drops. This and several other results obtained in this analysis provide functional relationships for the various quantities examined in numerical simulations and experiments. The study thus significantly advances our understanding of many-body levitation in electrodynamic balance

Estimation of critical supersaturation solubility ratio for predicting diameters of dry particles prepared by air-jet atomization of solutions

Air-jet atomization of solution into droplets followed by controlled drying is increasingly being used for producing nanoparticles for drug delivery applications. Nanoparticle size is an important parameter that influences the stability, bioavailability and efficacy of the drug. In air-jet atomization technique, dry particle diameters are generally predicted by using solute diffusion models involving the key concept of critical supersaturation solubility ratio (Sc) that dictates the point of crust formation within the droplet. As no reliable method exists to determine this quantity, the present study proposes an aerosol based method to determine Sc for a given solute-solvent system and process conditions. The feasibility has been demonstrated by conducting experiments for stearic acid in ethanol and chloroform as well as for anti tubercular drug isoniazid in ethanol. Sc values were estimated by combining the experimentally observed particle and droplet diameters with simulations from a solute diffusion model. Important findings of the study were: (i) the measured droplet diameters systematically decreased with increasing precursor concentration (ii) estimated Sc values were 9.3 +/- 0.7, 13.3 +/- 2.4 and 18 +/- 0.8 for stearic acid in chloroform, stearic acid and isoniazid in ethanol respectively (iii) experimental results pointed at the correct interfacial tension pre-factor to be used in theoretical estimates of Sc and (iv) results showed a consistent evidence for the existence of induction time delay between the attainment of theoretical Sc and crust formation. The proposed approach has been validated by testing its predictive power for a challenge concentration against experimental data. The study not only advances spray-drying technique by establishing an aerosol based approach to determine Sc, but also throws considerable light on the interfacial processes responsible for solid-phase formation in a rapidly supersaturating system. Until satisfactory theoretical formulae for predicting CSS are developed, the present approach appears to offer the best option for engineering nanoparticle size through solute diffusion models. (C) 2017 Elsevier Inc. All rights reserved