Air dispersion models

Air Pollution Dispersion Application of air dispersion modeling principles and EPA tools to assessing environmental impacts from stack and area releases of pollutants Dispersion theory Gaussian plume model Ground-level concentrations Worst case scenarios Air quality impact assessments Stationary source emissions... 

In this section we will learn how to use one of EPA s simpler air dispersion models for evaluating the air quality impact of stationary sources. The model... 

AERMOD AERMOD is the next generation air dispersion modeling system and consists of 3 components - AERMOD (air dispersion model), AERMET (meteorological data preprocessor) and AERMAP (terrain preprocessor). A brief overview of the model can be found in the mod-desc.txt file which can be downloaded from the site. 

Boilers and most industrial furnaces must follow a tiered system for the regulation of both hydrogen chloride and chlorine gas. The owner and operator determine the allowable feed or emission rate of total chlorine by selecting one of three approaches, called tiers. Each tier differs in the amount of monitoring, and in some cases, air dispersion modeling (i.e., modeling the air pathways through which pollutants may travel) that the owner and operator are required to conduct. 

Because actual exposure measurements are often unavailable, exposure models may be used. For example, chemical emission and air dispersion models are used in air quality studies to predict the air concentrations for down-wind residents. Residential wells located down-gradient from a site may not currently show signs of contamination, but they may become contaminated in the future as chemicals in the groundwater migrate to the well site. In these situations, groundwater transport models may estimate the period of time that chemicals of potential concern will take to reach the wells. 

Within the refinery fence Une, maximum observed and/or calculated airborne concentrations of chemicals reported in the airnual TRI were below OSHA action and permissible exposure levels for 8-hour time-weighted average exposures. Impacts on air quality by the refinery were calculated using air dispersion modeling techniques and the emissions inventory developed during this study. 

Flux is a somewhat abstract entity and does not relate this information directly. It quantifies the chemical movement rate across an interface plane into a receiving media such as the air boundary layer (BL) in the above example. Only when it is coupled with an air dispersion model does it produce concentrations in air. In the case of a large soil surface area source, a simple relationship exists between flux and concentration. For neutral air stability conditions in the atmospheric BL with steady-state wind speed v (m/sec), the concentration in air, c (mg/m ), can be approximated by... 

Some users may want to see a display from CAMEO s air dispersion model, which requires additional input. Guidance as to reasonable input for the dispersion model would be very beneficial to emergency planners as well as to interested citizens, and would help ensure that results are meaningful. Assistance in interpreting the plume display would also be helpful to citizens. 

See, e.g. Pierce and Gutfrevno, Evidentiary Aspect of Air Dispersion Modeling and Air Quality Measurements in Environmental Litigation and Administrative Proceedings, 25 Fed n. of Ins. Counsel Q. 341-347 (1975). 

Evans (1999) discusses the use of the ALOHA model to model this incident. The author points out that oleum is a mixture of two chemicals (sulfur trioxide and sulfuric acid), but ALOHA is (as of 1999) designed to model release and dispersion of pure chemicals only. Therefore, oleum was not in the ALOHA database. The user may still model the release as sulfur trioxide, but ALOHA displayed a warning message saying that sulfur trioxide reacts with any water to produce sulfuric acid and water, and therefore ALOHA cannot model chemically reactive substances and cannot accurately predict the air hazard. Fortunately, in this incident the plume cloud escaping from the oleum tankcar was clearly visible. Visual observation allowed responders to track its movements more accurately than any air dispersion model could. 

Spicer, T. O., and J. A. Havens. 1986. Development of a Heavier-than-Air Dispersion Model for the US Coast Guard Hazard Assessment Computer System, in Heavy Gas and Risk Assessment HI, ed. S. Hartwig, Battelle Institute, Frankfurt am Main, Germany, pp. 73-121. 

Bossert and Wilson s model applies to a source resting on a non-adsorbing plane surface with no adjacent boundaries. Mankin et al. (1980a) present a still air dispersion model expanded to include the effects of boundaries with various degrees of odor adsorption at various distances from the source. 

Fleischer, M. T. (1980.). SPILLS An Eveporation/Air Dispersion Model far Chemical Spills on Land. Houston, TX Shell Development Center. 

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