Joseph C. Chang

Hazardous gas model evaluation with field observations

Abstract Fifteen hazardous gas models were evaluated using data from eight field experiments. The models include seven publicly available models (AFTOX, DEGADIS, HEGADAS, HGSYSTEM, INPUFF, OB/DG and SLAB), six proprietary models (AIRTOX, CHARM, FOCUS, GASTAR, PHAST and TRACE), and two “benchmark” analytical models (the Gaussian Plume Model and the analytical approximations to the Britter and McQuaid Workbook nomograms). The field data were divided into three groups—continuous dense gas releases (Burro LNG, Coyote LNG, Desert Tortoise NH3-gas and aerosols, Goldfish HF-gas and aerosols, and Maplin Sands LNG), continuous passive gas releases (Prairie Grass and Hanford), and instantaneous dense gas releases (Thorney Island freon). The dense gas models that produced the most consistent predictions of plume centerline concentrations across the dense gas data sets are the Britter and McQuaid, CHARM, GASTAR, HEGADAS, HGSYSTEM, PHAST, SLAB and TRACE models, with relative mean biases of about ±30% or less and magnitudes of relative scatter that are about equal to the mean. The dense gas models tended to overpredict the plume widths and underpredict the plume depths by about a factor of two. All models except GASTAR, TRACE, and the area source version of DEGADIS perform fairly well with the continuous passive gas data sets. Some sensitivity studies were also carried out. It was found that three of the more widely used publicly-available dense gas models (DEGADIS, HGSYSTEM and SLAB) predicted increases in concentration of about 70% as roughness length decreased by an order of magnitude for the Desert Tortoise and Goldfish field studies. It was also found that none of the dense gas models that were considered came close to simulating the observed factor of two increase in peak concentrations as averaging time decreased from several minutes to 1 s. Because of their assumption that a concentrated dense gas core existed that was unaffected by variations in averaging time, the dense gas models predicted, at most, a 20% increase in concentrations for this variation in averaging time.

Boundary-layer parameterizations for applied dispersion modeling over urban areas

Many applied dispersion models require the knowledge of boundary-layer parameters such as sensible heat flux,QH, friction velocity,u*, and turbulent energy components, σw and σv. Formulas are suggested for calculating these parameters over a wide variety of types of ground surfaces, based on simple observations of wind speed near the ground and fractional cloud cover, and specification of constants such as roughness length, albedo, and soil moisture availability. Observations ofu*,QH, σw, and σv during field experiments in St. Louis and Indianapolis are used to test the formulas for urban sites. Relative errors of about ±20% in the predictions are seen to occur whenu*,QH, σw, and σv are large. However, when these quantities are small (e.g.,u* < 0.2 m/s), the errors in the predictions are as large as the mean value of the quantity itself.In addition, it is concluded from studies of available field data and theories that the magnitude of σw is not well-known at elevations above about 100m during the late afternoon and night. Some simple parameterizations for σw. are suggested that are consistent with the observed steady decrease in ground-level concentration in the afternoon and the sudden increase in concentration that can occur a few hours after sunset due to wind shears associated with a low-level jet, for continuous plumes emitted from moderate to tall stacks.

Hybrid plume dispersion model (HPDM) improvements and testing at three field sites

Abstract Descriptions of several technical improvements to the Hybrid Plume Dispersion Model (HPDM) are given. The boundary-layer meteorological preprocessor now makes use of a surface moisture availability parameter and now accounts for mechanical mixing due to buildings in urban areas. A new dispersion algorithm has been added in order to simulate conditions better when a buoyant plume “lofts” against the capping inversion and spreads laterally before dispersing down to the ground. The modified model has been evaluated using 89 h of SF 6 tracer data at an urban power plant in Indianapolis, IN, and using a full year (8760 h) of SO 2 data at the Baldwin, IL, power plant and the Summit County, OH, industrial complex. The EPAs RAM and ISC models are also included in the evaluation exercises. Emphasis is on the second-highest concentrations, the correlation, the fractional mean bias and the fraction of predictions within a factor of two of observations for 1-, 3- and 24-h averaging times. The HPDM model exhibits significantly better performance (at the 95% confidence level) than the RAM or ISC model for most sites and performance measures. In most cases, the HPDM “second-highest” predictions are within 10–20% of the observations and the fractional mean bias values are less than 10%. Furthermore, the relative error of HPDM shows little trend with input variables such as wind speed and mixing depth.

Evaluations of CALPUFF, HPAC, and VLSTRACK with Two Mesoscale Field Datasets

Abstract Results of evaluations of transport and dispersion models with field data are summarized. The California Puff (CALPUFF), Hazard Prediction and Assessment Capability (HPAC), and Chemical/Biological Agent Vapor, Liquid, and Solid Tracking (VLSTRACK) models were compared using two recent mesoscale field datasets—the Dipole Pride 26 (DP26) and the Overland Along-wind Dispersion (OLAD). Both field experiments involved instantaneous releases of sulfur hexafluoride tracer gas in a mesoscale region with desert basins and mountains. DP26 involved point sources, and OLAD involved line sources. Networks of surface wind observations and special radiosonde and pilot balloon soundings were available, and tracer concentrations were observed along lines of whole-air samplers and some fast-response instruments at distances up to 20 km. The models were evaluated using the maximum 3-h dosage (concentration integrated over time) along a sampling line. It was found that the solutions were highly dependent upon the di...

Evaluation of fourteen hazardous gas models with ammonia and hydrogen fluoride field data

Abstract Fourteen hazardous gas models are evaluated using data from the Desert Tortoise ammonia (NH 3 ) and Goldfish hydrogen fluoride (HF) field experiments

Use of Salt Lake City URBAN 2000 Field Data to Evaluate the Urban Hazard Prediction Assessment Capability (HPAC) Dispersion Model

After the terrorist incidents on 11 September 2001, there is a greatly heightened concern about the potential impacts of acts of terrorism involving the atmospheric release of chemical, biological, radiological, and nuclear (CBRN) materials in urban areas. In response to the need for an urban CBRN model, the Urban Hazard Prediction Assessment Capability (Urban HPAC) transport and dispersion model has been developed. Because HPAC is widely used by the Department of Defense community for planning, training, and operational and tactical purposes, it is of great importance that the new model be adequately evaluated with urban datasets to demonstrate its accuracy. This paper describes evaluations of Urban HPAC using the “URBAN 2000” urban tracer and meteorological field experiment data from Salt Lake City, Utah. Four Urban HPAC model configuration options and five plausible meteorological input data options—ranging from data-sparse to data-rich scenarios—were considered in the study, thus leading to a total of 20 possible model combinations. For the maximum concentrations along each sampling arc for each intensive operating period (IOP), the 20 Urban HPAC model combinations gave consistent mean overpredictions of about 50%, with a range over the 20 model combinations from no overprediction to a factor-of-4 overprediction in the mean. The median of the random scatter for the 20 model combinations was about a factor of 3 of the mean, with a range over the 20 model combinations between a factor of about 2 and 9. These performance measures satisfy previously established acceptance criteria for dispersion models.

Representativeness of wind measurements on a mesoscale grid with station separations of 312 m to 10 km

A field experiment was carried out in which wind speed and direction were measured over flat terrain at a height of 10 m using 13 identical instruments spaced logarithmically along two perpendicular 10 km lines. Station separations ranged from 312 m to 10 km. One-minute data from 11 sampling periods of duration 6 to 10 h were studied. p ]The statistics showed little dependence on whether the line of instruments was oriented along the wind or across the wind. The correlation coefficients between wind fluctuations at two stations separated by distance δx were found to vary exponentially with δx, with an integral distance scale on the order of 1 km. The integral time scale derived from the variation of the single station variances with averaging time was found to equal several minutes. At a station separation of 10 km, the correlation coefficients between the wind components at the two sites were calculated to be 0.24, 0.37, and 0.47 for averaging times of 1, 10, and 60 min, respectively. These values for the correlation coefficients correspond to root-mean-square differences in wind speed at the two stations of about 1.3, 1.0, and 0.7 m/s, respectively.Exponential formulas based on dimensional analysis are suggested for fitting these observations. It is found that the observations of spatial correlations are best fit if two independent integral distance scales are used — a boundary-layer distance scale of about 300 m that best applies to small station separations and a mesoscale distance scale of about 10 km that applies to larger station separations.

Modeling accidental releases to the atmosphere of a dense reactive chemical (Uranium hexafluoride)

Abstract In order to model the atmospheric transport and dispersion of dense reactive chemicals such as uranium hexafluoride (UF6), it is necessary to include algorithms that account for heat exchanges due to chemical reactions and phase changes. UF6 may be released accidentally at uranium-enrichment plants as a warm gas from a pipeline rupture, or as a flashing liquid from a pressurized tank or line break. The resulting plume is initially very dense due to the large molecular weight of UF6, but may become lighter-than-air as the UF6 reacts with water vapor to form HF, which has a molecular weight less than that of air, and which may cause an increase in plume temperature due to the exothermic reaction. The major chemical and thermodynamic processes related to UF6 have been incorporated in a modified version of an existing dense gas model, HGSYSTEM. The same general approach could be used to include other reactive chemicals in the modeling system. New modules that are applicable to any type of chemical release have also been added to HGSYSTEM to account for building downwash, lift-off of warm plumes from the ground, and deposition. The revised HGSYSTEM/UF6 model has been evaluated with field data from UF6 tests. The sensitivities of the model predictions to variations in input parameters have been assessed.

Lift-off of ground-based buoyant plumes

Abstract If a ground-based plume has enough buoyancy to overcome the effects of ambient turbulence and other physical processes, it will rise or lift-off the ground, thus reducing the health and environmental impacts of chemicals released accidentally to the atmosphere. The approach described below was developed using wind tunnel observations of plumes for which buoyancy was conserved, but we also propose it for use for plumes whose buoyancy flux varies with distance; this can occur due to the presence of aerosols, depolymerization, reactions with water vapor or other chemicals to form new products, and evaporation and condensation processes. It is assumed that the lift-off phenomenon can be parameterized by defining a dimensionless buoyancy flux, F ∗∗= F / u 3 W , where F is the local plume buoyancy flux, u is the local effective wind speed advecting the ground-based plume, and W is the local lateral plume width. All variables can vary with plume travel time or downwind distance. It is suggested that the effects of plume lift-off can be accounted for by multiplying the calculated ground-level concentration in the absence of lift-off by the term exp(−6 F ∗∗ 0.4 ). Special emphasis is given to the development of simple empirical lift-off equations for buoyant plumes which are trapped in building wakes. In this case, the empirical formula that is proposed combines the exp(−6 F ∗∗ 0.4 ) term with four additional terms related to the spread of plumes in building wakes, and has been demonstrated to agree with wind tunnel observations.

Analysis of historical ozone episodes in the SCCCAMP region and comparison with SCCCAMP 1985 field study data

Abstract A comprehensive air quality and meteorological monitoring project entitled the South-Central Coast Cooperative Aerometric Monitoring Program (SCCCAMP 1985) was conducted in the Santa Barbara Channel and adjacent areas from Point Sal to Point Dume during a five-week period in September–October 1985. As part of a larger study to analyze the SCCCAMP 1985 observations and related databases, an analysis has been conducted of a six-year historical ozone and meteorological database. The objectives of the historical data analysis study were to 1) characterize meteorological and ozone concentration patterns during a six-year historical period (1979–1984), 2) identify relationships between meteorological variables and high ozone concentrations in the SCCCAMP region, and 3) compare the meteorological conditions and ozone concentrations observed during the SCCCAMP 1985 study with those during the same periods in the historical database in order to assess the representativeness of the SCCCAMP 1985 study perio...