Residence Time Analysis Research Articles

Reconciling Trajectory Ensemble Receptor Model Results with Emissions

Several approaches have been developed that use ensembles of air parcel back trajectories to identify likely source regions for contaminants observed at a downwind receptor site. Although the results of these models have appeared to provide results that agree with known sources, there have not been efforts to make direct comparisons between the results of such analyses with emissions inventories. In this paper, the results of residence time analysis (RTA) of data from Underhill, VT and from Brigantine, NJ are compared to the average estimated emissions of coal- and oil-fired power plants. Excellent correspondence was found between the areas of highest probability identified in the trajectory ensemble analyses and the averaged emission intensities.

Is Nutrient Contamination of Groundwater Causing Eutrophication of Groundwater-Fed Meadows?

There is an ongoing debate as to whether nutrient contamination of groundwater under agricultural fields may cause nutrient-enrichment and subsequent eutrophication in discharge areas. Often, there is only circumstantial evidence to support this supposition (proximity of agricultural fields, direction of water flow, highly productive vegetation). Research on solute transport along a flow path is necessary to evaluate the risk for eutrophication. In this paper we present results of such a study. Two transects were established in a discharge meadow, a few meters downstream from fertilized cornfields. Highly productive vegetation in parts of the meadow suggested nutrient-enrichment caused by inflow of contaminated groundwater. This supposition was supported by an analysis of groundwater flow paths, residence times and chloride as tracer for pollution. However, the fate of nutrients along the flow path indicated otherwise. While we found high concentrations of DIN (dissolved inorganic nitrogen), P and K under the cornfields, DIN and P concentrations drop below detection limit when groundwater enters the meadow. Only K progressed into the meadow but did not enter the root zone. We conclude that (1) polluted groundwater from the cornfields did not cause the nutrient-enrichment, as indicated by the highly productive vegetation. Restoration projects in discharge areas should not focus upon measures in upstream areas if only circumstantial evidence is available. Solute transport should be considered as well. (2) Because K clearly showed to be the most mobile nutrient, its importance for nutrient-enrichment in discharge wetlands merits more attention in future research.

Concentration Dependence of the Methane Structure in Silicalite-1: A Molecular Dynamics Study Using the Møller−Plesset-Based Potential

Møller-Plesset perturbation-based potentials have been used in molecular dynamics simulations to examine methane diffusion in silicalite-1. The simulation box contains 2 unit cells of silicalite-1 and varying loading numbers (n(ld)) from 1, 2, 3, to 4 methane molecules per intersection, corresponding to 8, 16, 24, and 32 molecules per simulation box, respectively. Consistent with the previous study, a preferential diffusing path for methane is close to the channel axes. The structure of the methane molecules in the silicalite-1 pore was exhibited in terms of the methane-methane radial distribution function (RDF) in which the first peak appears at 6.4 A for n(ld) < or = 2, becomes a broad maximum at n(ld) = 3, and splits into two sharp peaks centered at 6.4 and 8.6 A at n(ld) = 4. This fact can be clearly described by an intensification of the methane density in the straight and zigzag channels but a decrease in the intersection when the loading increases. These features of the observed RDFs are in contrast with the previous report using a molecular dynamics force-field potential in which the RDFs for all concentrations show first maxima at approximately 4 A. The analysis of the relative residence times of methane at different sites inside the silicalite suggests that the zigzag channel is the most favored location. The computed self-diffusion coefficients as well as the heat of adsorptions are in reasonable agreement with the available values.

Seasonal characteristics and regional transport of PM in Hong Kong

Abstract The Asiatic monsoon is the dominant meteorological feature of the Pearl River Delta (PRD) region in China, and influences the accumulation of locally emitted pollutants, as well as regional transport. To investigate the effect of meteorological characteristics on PM2.5 mass and chemical composition in Hong Kong, a major city of the PRD, 24-h (0000–2400 LST) PM2.5 samples were collected every sixth day from 6 November 2000 to 26 October 2001. The sampling network included a roadside site at Mong Kok (MK), an urban site at Tsuen Wan (TW), and a rural site at Hok Tsui (HT). Air parcel back trajectory and residence time analyses indicate a predominantly northeasterly transport during winter and fall in contrast to a southwesterly-to-southeasterly transport in summer. The highest seasonal PM2.5 concentrations were found in winter, followed by fall, at each of the sampling sites due to elevated organic material (OM) and ammonium nitrate (NH4NO3), consistent with the increased air parcel residence time over potential source regions. Though local mobile sources appeared to dominate the emission of carbonaceous material, a higher organic carbon/elemental carbon (OC/EC) ratio and water-soluble potassium (K+) concentration in winter imply an additional contribution from vegetative burning. Episodic pollution events were identified and investigated with back trajectories and species enrichment factors. Local and regional sources both contributed to the formation of a pollution episode on 28 February 2001. These findings reflect the degree of PM2.5 pollution in Hong Kong and provide valuable insights for planning future monitoring and modeling studies in the PRD region.

Design of micromixers using CFD modelling

The effect of various geometrical parameters of a grooved staggered herringbone micromixer on the mixing performance has been investigated using computational fluid dynamics. Mixing quality has been quantified with spatial data statistics, maximum striation thickness and residence time analyses. The results show that the number of grooves per mixing cycle does not affect the mixing quality in an important way. On the other hand, a larger groove depth and width allow the maximum striation thickness to be rapidly reduced, without increasing the pressure drop across the mixer. Wide grooves, however, create significant dead zones in the microchannel, whereas deep grooves improve the spatial mixing quality.

LUMPED Unsteady: a Visual Basic ® code of unsteady-state lumped-parameter models for mean residence time analyses of groundwater systems

A Microsoft ® Visual Basic 6.0 (Microsoft Corporation, 1987–1998) code of 9 lumped-parameter models of unsteady flow is presented for the analysis of mean residence time in aquifers. Groundwater flow systems obeying plug and well-mixed flow models and their combinations in parallel or serial connection can be simulated by the code. Models can use tritium, tritiugenic He-3, oxygen-18, deuterium, krypton-85, chlorofluorocarbons (CFC-11, CFC-12 and CFC-113) and sulfur hexafluoride (SF 6) as the environmental tracers. The executable code runs under all 32-bit Windows operating systems. Details of the code are explained and its limitations are indicated.

Analysis of cross-flow jet-type singlet oxygen generator

A cross-flow jet-type singlet oxygen generator has been developed, tested and analyzed in order to characterize the dependence of output performance on major input parameters. A thermal-balance model, which can predict O/sub 2/(/sup 1//spl Delta/) yield, gas temperature, and gas residence time, is proposed, and the resultant theoretical results are compared to the experimental data. Combined with computational fluid dynamics-based gas residence-time analysis, the model provides good agreement with the measured value of the O/sub 2/(/sup 1//spl Delta/) yield and the gas temperature. The surface chemistry model was applied to the measured Cl/sub 2/ utilization data, and was found to be inconsistent in the regime of high Cl/sub 2/ loading on the basic hydrogen peroxide jet, indicating that depletion of HO/sub 2//sup -/ is taking place.

Volatile organic compound measurements at Trinidad Head, California, during ITCT 2K2: Analysis of sources, atmospheric composition, and aerosol residence times

We report hourly in‐situ observations of C1‐C8 speciated volatile organic compounds (VOCs) obtained at Trinidad Head CA in April and May 2002 as part of the NOAA Intercontinental Transport and Chemical Transformation study. Factor analysis of the VOC data set was used to define the dominant processes driving atmospheric chemical composition at the site, and to characterize the sources for measured species. Strong decreases in background concentration were observed for several of the VOCs during the experiment due to seasonal changes in OH concentration. CO was the most important contributor to the total measured OH reactivity at the site at all times. Oxygenated VOCs were the primary component of both the total VOC burden and of the VOC OH reactivity, and their relative importance was enhanced under conditions when local source contributions were minimal. VOC variability exhibited a strong dependence on residence time (slnX = 1.55τ−0.44, r2 = 0.98; where slnX is the standard deviation of the natural logarithm of the mixing ratio), and this relationship was used, in conjunction with measurements of 222Rn, to estimate the average OH concentration during the study period (6.1 × 105 molec/cm3). We also employed the variability‐lifetime relationship defined by the VOC data set to estimate submicron aerosol residence times as a function of chemical composition. Two independent measures of aerosol chemical composition yielded consistent residence time estimates. Lifetimes calculated in this manner were between 3–7 days for aerosol nitrate, organics, sulfate, and ammonium. The lifetime estimate for methane sulfonic acid (∼12 days) was slightly outside of this range. The lifetime of the total aerosol number density was estimated at 9.8 days.

Ambient silver concentration anomaly in the Finnish Arctic lower atmosphere.

Mean silver concentrations in weekly particle samples collected at Kevo, northern Finland, were determined for the period of October 1964-March 1978 by neutron activation analysis. Two distinct periods were observed in the silver concentration levels over this time frame. During 1964-1970, mean weekly silver concentration levels were found in the range of 0.01-190 ng/m3 with an arithmetic mean of 2.19 ng/m3. A few very high silver concentration levels (>10 ng/m3) were observed in this period, some of which simultaneously occurred with some of the highest bromine and iodine concentration levels. During 1971-1978, silver concentration levels were in the range of 0.02-0.89 ng/m3 with a mean value of 0.09 ng/m3. The observed concentration levels in the later period matched well the data from the early 1990s reported at Sevettijärvi, northern Finland, about 60 km east of Kevo. Data analysis, historical records for this region, and residence time analysis (RTA) using wind back-trajectories show that occasional smelting of silver-rich Norilsk ores at the Nikel smelter, Kola Peninsula, was probably a significant contributor to elevated mean silver concentration levels during 1964-1970. RTA alone was not able to unambiguously identify the most probable source region for highest silver impacts at Kevo due to the weekly integrated nature of the samples collected. Critical examination of wind back-trajectories (24 per day) for specific high silver, bromine, and iodine concentration weeks was carried out to supplement the ensemble RTA analysis (2 back-trajectories per day). The supplemental back-trajectory analysis revealed that deposition of the smelter component silver as well as the sea components (bromine and iodine) could occur together at Kevo during these weekly sampling periods. The study implies that data from weekly integrated samples are insufficiently time-resolved for RTA methods alone to unambiguously resolve the sources contributing to ambient atmospheric concentrations at Kevo, Finland.

Regional background ozone and carbon monoxide variations in remote Siberia/East Asia

Continuous measurements of O3 and CO were made during 1997–1999 at Mondy, a remote mountain site in East Siberia, in order to quantify their mixing ratios and their climatology in the “background” troposphere of continental Eurasia. The seasonal cycles of O3 and CO show the spring maximum‐summer minimum similar to that previously reported in the remote Northern Hemisphere. The influences of Siberian forest fires on the variations of CO mixing ratios at Mondy were observed both on a local and a regional scale during spring 1997 and fall 1998, respectively. We further evaluate the possible impact of European pollution export to the remote atmosphere of Siberia using trajectory analysis. It was found that the O3 and CO mixing ratios in the air masses transported from Europe are higher than those from Siberia and high‐latitude regions for most of the year. The medians of O3 and CO mixing ratios associated with the European air masses are 44.2 and 134 ppb, respectively, in comparison with 42.7 and 128 ppb in the Siberian air masses, and 41.0 and 110 ppb in the high‐latitude air masses. The residence time analysis of air masses transported from the European continent indicates that CO mixing ratios significantly decrease with longer transport time of air masses from Europe, while rapid air motion retains higher CO mixing ratios in every season due to the admixture of polluted European air into the continental background air during air mass transport over Eurasia and photochemical loss by OH. Because of a shorter lifetime in summer, CO mixing ratios decrease at a rate of 6–7 ppb per day, while they decrease at a rate of 2–4 ppb per day in winter and spring. The similar trend is found for O3 but only in summer, at a rate of 2–3 ppb per day. From this analysis, we are able to identify that European pollution exerts an influence, though not very strong, on the background O3 and CO at Mondy in remote Siberia/East Asia.

LUMPED: a Visual Basic code of lumped-parameter models for mean residence time analyses of groundwater systems

A Microsoft ® Visual Basic 6.0 (Microsoft Corporation, 1987–1998) code of 15 lumped-parameter models is presented for the analysis of mean residence time in aquifers. Groundwater flow systems obeying plug and exponential flow models and their combinations of parallel or serial connection can be simulated by these steady-state models which may include complications such as bypass flow and dead volume. Each model accepts tritium, krypton-85, chlorofluorocarbons (CFC-11, CFC-12 and CFC-113) and sulfur hexafluoride (SF 6) as environmental tracer. Retardation of gas tracers in the unsaturated zone and their degradation in the flow system may also be accounted for. The executable code has been tested to run under Windows 95 or higher operating systems. The results of comparisons between other comparable codes are discussed and the limitations are indicated.

Relative source contribution analysis using an air trajectory statistical approach

An air trajectory statistical approach was developed to estimate the relative contributions of various sources within a planning domain for a receptor site during high air pollution. The proposed approach is based on the coupling of residence time analysis and a known emission inventory. The theoretical basis of the approach is detailed. The approach was applied to investigate the relative contributions of various anthropogenic volatile organic compound (VOC) sources to the ozone formation potential of a receptor site in southern Taiwan. One hundred and ninety‐seven ozone events (defined as those with an hourly ozone concentration that exceeded 120 ppb standard) over 1994–1998 were selected to undergo the residence time analysis. The VOC emission inventory was adjusted to reflect the different ozone formation potentials of the various sources by considering the source VOC profiles and the maximum incremental reactivity scales of each VOC compound. The results show that the sources that influenced the evaluated receptor site were located in northwestern coastal regions; the relative contributions of the point, line, and area sources were in the ration of approximately 5:2:3. Two districts, Kaohsiung City and Kaohsiung Hsien, were the dominant contributors of the five investigated districts. The proposed method may act as a preliminary tool to efficiently select the potential source region/category, such that mitigation and control strategies can be targeted, and/or used to guide a comprehensive modeling study.

Reversal of flagellar rotation is important in initial attachment of Escherichia coli to glass in a dynamic system with high- and low-ionic-strength buffers.

The attachment rates of wild-type, smooth-swimming, tumbly, and paralyzed Escherichia coli to glass was measured at fluid velocities of 0.0044 and 0.044 cms(-1) (corresponding to shear rates of 0.34 and 3.4 s(-1), respectively), in 0.02 and 0.2 M buffer solutions. At the highest ionic strength, we did not observe a significant difference in the attachment rate of wild-type and paralyzed cells at either fluid velocity. However, when the ionic strength was reduced, paralyzed bacteria attached at rates 4 and 10 times lower than that of the wild type under fluid velocities of 0.0044 and 0.044 cms(-1), respectively. This suggested that the rotation of the flagella assisted in attachment. We then compared the attachment rates of smooth-swimming (counterclockwise rotation only) and tumbly (clockwise rotation only) cells to the wild type to determine whether the direction of rotation was important to cell attachment. At 0.0044 cms(-1), the smooth-swimming cells attached at rates similar to that of the wild type in both buffer solutions but significantly less at the higher fluid velocity. Tumbly cells attached at much lower rates under all conditions. Thus, the combination of clockwise and counterclockwise flagellar rotation and their coupling appeared to be important in cell attachment. We considered a number of hypotheses to interpret these observations, including a residence time analysis and a comparison of traditional Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to soft-particle theory.

A three-dimensional residence-time analysis of potential summertime atmospheric transport to Summit, Greenland

AbstractThe interannual variations in atmospheric transport patterns to Summit, Greenland, are studied using twice-daily, three-dimensional, 10 day backward trajectory data corresponding to the summers (1 June–31 August) of 1989–98. While previous trajectory climatology studies have been prepared for Summit, the present work considers both the horizontal and vertical components of transport. A three-dimensional residence-time methodology is employed to account for both horizontal and vertical components of transport. the vertical transport component is quantified by passing all trajectories through a three-dimensional grid and tracking the time spent (i.e. the residence time) in each gridcell. This method also allows inspection of trajectory altitude distributions corresponding to transport from upwind regions of interest. the three-dimensional residence-time methodology is shown to be a valuable tool for diagnosing the details of long-range atmospheric transport to remote locations. for Summit, we find that the frequent transport from North America tends to occur at low altitudes, whereas transport from Europe is highly variable. Mean summertime flow patterns are described, as are anomalous patterns during 1990,1996 and 1998.

Comparative application of multiple receptor methods to identify aerosol sources in northern Vermont.

This study applies and compares results of four receptor modeling techniques to a common set of speciated fine particle measurement data collected ata remote site in northwestern Vermont between 1988 and 1995. Two multivariate mathematical models, positive matrix factorization and UNMIX, were applied to the measurement data and identified seven "common" sources that had similar compositions and similar fine mass contributions in both models. Two ensemble backward trajectory techniques, potential source contribution function and residence-time analysis, were also applied to evaluate and interpret the mathematical model results. The trajectory techniques indicate a strong regional character to the upwind locations associated with aerosol contributions from most of the sources identified independently by the mathematical models and help in the interpretation of those results. The process of model comparison provides insights on the strengths and limitations of the individual and combined source attribution techniques. Convergent results among the multiple methods provide a degree of confidence that each of the receptor methods may represent useful tools for future air quality management. Divergent or inconsistent results among the models can help identify limitations of the individual models and of the underlying aerosol and meteorological data sets.

An Investigation of the Reaction Mechanism for the Promotion of Propane Oxidation over Pt/Al2O3 by SO2

Total oxidation of propane with oxygen in the absence and the respective presence of SO2 in the feed gas was studied over 1 wt% Pt on γ-Al2O3 using transient experiments with TAP (temporal analysis of products) and in situ DRIFT spectroscopy. The TAP experiments confirm the promoting role of SO2 on the conversion of propane and, moreover, show an inhibiting effect from SO2 on the conversion of the intermediate products propene and CO. In situ DRIFT spectroscopy reveals the formation of sulphate species on the catalyst surface and indicates the formation of an allylic intermediate in the absence of SO2 only. Interaction between hydrocarbon intermediates and the sulphate species is confirmed by the appearance of thiol fragments in the mass spectra. The transient experiments show that propane in the first step is irreversibly adsorbed at the surface. Analysis of the surface residence times of the products formed at the propane pulse in pump-probe experiments provides information about the reaction pathways. Without SO2 in the feed gas, propane reacts consecutively in the sequence propane → propene → ethane → CO → CO2. In the presence of SO2 the first step of the main route is the breaking of a C–C bond in propane producing ethane and a C1 fragment, which then form CO and eventually CO2. It is suggested that the promoting effect by SO2 on the conversion of propane is due to the formation of an acidic site at the Pt/Al2O3/SO42− interface.

Molecular dynamics simulation of the antimicrobial salivary peptide histatin-5 in water and in trifluoroethanol: a microscopic description of the water destructuring effect.

The results of 520 ps molecular dynamics simulation of histatin-5, a small peptide present in human saliva and possessing antimicrobial activity, dissolved in water and in 2,2,2-trifluoroethanol, are reported. The simulations indicate that histatin-5 is destabilized in water and begins to unfold after 250 ps, while in organic solvent it maintains a regular secondary structure throughout the trajectory. Analysis of the peptide-solvent hydrogen bonds indicates that 2,2,2-trifluoroethanol is a poorer proton acceptor than water. The fluorine atom of the alcohol is almost never engaged in a hydrogen bond and the organic solvent interacts mainly with the peptide through its hydroxyl group. For some residues analysis of the solvent residence time indicated longer values for 2,2,2-trifluoroethanol than for water. The most striking difference is related to the number of times the solvent enters and leaves the first coordination shell of the peptide. This value was more than one order of magnitude higher for water than for the alcohol, suggesting that this may be the main cause of alpha-helix destabilization perpetrated by water.

Surface residence time measurements of transient methyl radicals

We present the first measurement of the thermal desorption kinetics of methyl radicals. Neutral methyl radicals are photochemically produced within a methyl iodide film condensed on MgO(1 0 0). A substantial fraction become trapped through inelastic collisions with the film and thermalize to the film temperature prior to desorption. Novel residence time measurements are made by comparing neutral and ion flight times in a time-of-flight spectrometer. An Arrhenius analysis of methyl radical residence times provides a desorption barrier of 2.5±0.5 kcal mol −1 and a pre-factor of 10 10±1 s −1 .

Localisation and dynamics of sodium counterions around DNA in solution from molecular dynamics simulation

The localisation and dynamics of sodium counterions around the DNA duplex d(AGCGTACTAGTACGCT)2 corresponding to the trp operator fragment used in the crystal structure of the half site complex has been studied by a 1.4 ns molecular dynamics simulation in explicit solvent. A continuous and well-defined counterion density is shown to be present around the minor groove, while density patches are found in the major groove in regions where DNA bending is observed. A residence time analysis reveals the dynamic nature of these distributions. The resulting picture agrees with previous theoretical and experimental studies of A-tract DNA sequences, and is consistent with the polyelectrolyte condensation model.

Mean residence time analysis for twin screw extruders

AbstractThis paper presents and experimentally validates a physically motivated model for predicting the mean residence time in twin screw extruders. Accurate estimation of the mean residence time and the propagation delay through a plasticating extruder is critical for implementing feedback control schemes employing sensors mounted along the extruder. Experiments were carried out on a 30 mm Krupp Werner and Pfleiderer co‐rotating twin screw extruder equipped with reflectance optical probes over the melting section and mixing section and at the die. The residence time distributions for twelve operating conditions and two screw geometries are compared. The mean residence times predicted by our model are in good agreement with the experimentally measured mean residence times.