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Kennebunkport, ME, United States

Hanna S.R.,Hanna Consultants | Chang J.C.,U.S. Homeland Security Studies and Analysis Institute
Proceedings of the 15th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, HARMO 2013 | Year: 2013

Large amounts of chlorine released in a few minutes from pressurized liquefied storage such as a railcar may form a dense two-phase (gas plus small liquid aerosol drops) cloud at ground level. The time duration of release from the storage tank could be as small as a few seconds for a large hole with diameter exceeding 20 or 30 cm. But even if the release from the tank occurs in a few seconds, a dense two-phase cloud may remain in the area of the tank for many minutes if winds are light enough and/or there is a terrain depression. The 2010 Jack Rabbit (JR) field experiments at Dugway Proving Ground, Utah, demonstrated that a 30 to 60 s release of one or two tons of pressurized liquefied chlorine or anhydrous ammonia gas would result in an initial hold-up of about 30 min of the dense two-phase cloud at winds less than about 2 m/s. In this case, the chlorine gas would be slowly detrained from the surface of the two-phase cloud around the source location, following the 1990 theory of Briggs. The observed cloud hold-up time is proportional to the cube of the wind speed, in agreement with the theory. This detrainment process can be treated as an area source as far as downwind concentrations are concerned. We have carried out an analysis, considering instrument thresholds, mean biases, and uncertainties, of three types of JR concentration samplers (MiniRae, Jaz, and Canary) operated on arcs at distances from 25 to 500 m from the source. The resulting best estimates of arc-maximum observed 10 min averaged concentrations (important for health effects) at each distance are compared, assuming the two-phase cloud hold-up time as estimated using Briggs' theory.

Hanna S.,Hanna Consultants | Chang J.,U.S. Homeland Security Studies and Analysis Institute | Huq P.,University of Delaware
Atmospheric Environment | Year: 2016

As part of planning for a series of field experiments where large quantities (up to 20 tons) of pressurized liquefied chlorine will be released, observations from previous chlorine field experiments are analyzed to estimate the ranges of chlorine concentrations expected at various downwind distances. In five field experiment days during the summer 2010 Jack Rabbit I (JR I) field trials, up to two tons of chlorine were released and concentrations were observed at distances, x, from 25 to 500 m. In the 1927 Lyme Bay (LB) experiments, there were four days of trials, where 3-10 tons of chlorine were released in about 15 min from the back of a ship. Concentrations were sampled at LB from four ships sailing across the cloud path at downwind distances in the range from about 350 to 3000 m. Thus, the distances from which JR I concentrations were available slightly overlapped the LB distances. One-minute arc-maximum chlorine concentrations, C (g/m3), were analyzed from four JR I trials and two LB trials. Normalized concentrations (Cu/Q) were plotted versus x (m), where u (m/s) is measured wind speed at heights of 2-10 m and Q (g/s) is continuous mass release rate. It is found that the JR I and LB Cu/Q observations smoothly merge with each other and fall along a line with approximate slope of -2 at distances beyond about 200 m (i.e., Cu/Q is proportional to x-2). At x < 200 m, where dense gas effects are more important, the slope is less (about -1.5). Most of the data points are within a factor of two of the "best-fit" line. © 2015 Elsevier Ltd.

Hanna S.R.,Hanna Consultants | Chowdhury B.,Sage Management
Air and Waste Management Association - Guideline on Air Quality Models 2013: The Path Forward | Year: 2013

Dispersion models often have difficulty matching observed tracer concentrations during lowwind stable conditions. Several presentations at the 10th EPA Air Modeling Conference in March 2012 addressed this topic. In particular, the current base AERMOD version has been shown to overpredict by a factor of as much as ten when compared with observed concentrations from the Oak Ridge and Idaho Falls field experiments during stable periods, when wind speeds often dropped below 1 m/s. Recent studies show that some of this overprediction tendency can be reduced by revising the meteorological preprocessor's empirical parameterization of the ratio u/u during low wind stable conditions, thus increasing the estimated σv and σw and hence the lateral and vertical dispersion rates by a factor of as much as two. Even with this revision, when standard Monin-Obukhov similarity theory (MOST) is applied during stable conditions, the estimated u, σV and σW, and mechanical mixing depth, z-i, will approach zero as the mean observed wind approaches zero. Observations of turbulence show that, even as the mean wind speed approaches zero during stable conditions, there is always significant σV and σw over time periods of 15 to 60 minutes. Some dispersion modelers have used analyses in the literature of turbulence observations over many boundary layer field experiments to define a ":minimum σv and minimum σw ". This paper focuses on the minimum turbulence (σv and σw) assumptions in AERMOD and SCICHEM. It is shown how differences in these assumptions (min σV = 0.2 m/s for AERMOD and 0.5 m/s for SCICHEM; and min σw = 0.02 m/s for AERMOD and 0.1 m/s for SCICHEM) can account for the fact that SCICHEM performs fairly well, with a minimal mean bias for the Idaho Falls data and a mean overprediction bias of a factor of about two for the Oak Ridge data, while AERMOD has a larger mean overprediction bias. Sensitivity runs with SCICHEM, where the AERMOD assumptions for minimum turbulence (σv and σw) are used instead of the SCICHEM values, show about a factor of 2.8 increase, on average, in simulated arc max concentrations for the o v changes, about a factor of 2.2 increase for the σW changes, and about a factor of 6.6 increase for both σv and σw changes. The EPA has provided results of their tests with their December 2012 modified versions of AERMOD, assuming increases in min σv and adjustments to the low-wind u/u parameterization. Our evaluations show that the revised AERMOD shows less bias, and the performance measures are included in our comparisons.

Pullen J.,Stevens Institute of Technology | Chang J.,U.S. Homeland Security Studies and Analysis Institute | Hanna S.,Hanna Consultants
Bulletin of the American Meteorological Society | Year: 2013

Evaluation of the operational air-sea modeling response to the 2011 radionuclide emissions crisis in Japan demonstrated the need for accurate source information and for international coordination among parallel modeling efforts. Several other non-Japanese governmental and international agencies also responded to the crisis using various modeling tools. The US NRC used a Lagrangian trajectory Gaussian puff model designed to simulate impacts from distances of 100 m or less to mesoscale distances. The NRC ran various scenarios in the affected areas from March 12, 2011, assuming source-emission terms based on 10% and 100% core damage. They compared their results with the limited on-site monitoring data that became available after March 14, 2011 and provided their source-term estimates to the US Department of Energy's (DOE) National Atmospheric Release Advisory Center (NARAC). Three-dimensional oceanic transport models were also employed to predict contaminant circulation patterns off the coast of Japan by many agencies and organizations.

Hanna S.R.,Hanna Consultants
Proceedings of the 15th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, HARMO 2013 | Year: 2013

During the 1992 HARMO1 (Riso) workshop, I presented an overview of limitations of the state-of-the-art short range atmospheric dispersion models. The topics were: 1) mixing depth, 2) vertical profiles of turbulence, 3) formation of the nocturnal jet, 4) non-steady-state periods, 5) surface constants (albedo, soil moisture and roughness), 6) surface energy balance relations, and 7) Lagrangian time scales. The current paper revisits these seven topics. The most progress has been made in topics that are also of interest to climate change, such as 5) and 6) on surface parameters and surface energy balance relations. Other areas, such as mixing depths and vertical profiles of turbulence and Lagrangian time scales, have seen less progress. Some current difficult topics are added to the list: 8) boundary layer profiling and dispersion in low-wind stable conditions, 9) how to handle steep terrain, building obstacles, and variations in land use, 10) whether newer technology is helping, and 11) how to handle dense plumes and chemical reactions.

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