Using PC-based air dispersion models to predict pollutant concentrations

WASTE MANAGEMENT, Vol. 13, pp. 97-101, 1993 Printed in the U.S.A. All rights reserved.

0956-053X/93 $6.00 + .00 Copyright © 1993 Pergamon Press Ltd.

COMPUTERS

U S I N G PC-BASED AIR DISPERSION MODELS TO PREDICT POLLUTANT CONCENTRATIONS Laura Redmon and Dan Lipsher Trinity Consultants, Inc., 12801 N. Central Expressway, Suite 1200, Dallas, TX 75243, U.S.A.

EDITORIAL NOTE. Why a computer column? Because the new Editor-in-Chief wants one. Actually, the reason is not so simple. Bill Cawley and I feel that computers and PCs (and all the many capabilities that accompany them) are quickly becoming an integral part of today's society. Industrial-waste management professionals have to reflect that society to address the problems that arise from it, and they should utilize all the tools and technologies that society makes available to them in addressing these problems. Hopefully, this column will be a resource to you in addressing some of the problems you are facing. I would like to address two audiences with this column. The first audience is that group of senior engineers, scientists, and managers who learned their trade and developed their work habits before the current proliferation of computers. I have noticed that this group has a reluctance to change established work methods, has trouble visualizing how effective a tool the computer can be in the work place, and has no place to become knowledgeable on the capabilities and uses of computers. Yet they are often the same people who are responsible for authorizing the purchase and use of computers for work group members. I would like this column to help them make an informed decision. The second audience is that group of professionals who routinely use computers in their work. For them, I want this column to provide information on other methods of applying computers to the hazardous waste management area, including descriptions of environmentally oriented programs that are available from various sources. I also want this column to address the many diverse problems (not just those unique to waste management) that computers can be employed to handle within a professional work group. To meet these goals I'm going to need your help. I invite anyone who is interested to submit articles for inclusion in the column. The ground rules are to meet the goals as stated above with the addition that the article should not be a sales pitch. So, let me hear from you. Now, on to what I will usually be doing, i.e., introducing this issue's article. One of the strengths of computers is mathematical modeling, and as the capability of PCs increases so will the amount of modeling that is being conducted. This issue's article is an introduction to one use of PC-based models and the programs one company has developed that use them in the industrial-waste management field. I hope you find it interesting.--Tom Pinson

As a result o f federal and state regulations arising f r o m the 1990 Clean Air Act A m e n d m e n t s , managers o f industrial facilities m u s t m e e t increasingly stringent reporting requirements in order to operate plants that emit air pollutants. A facility usually must obtain an air quality operating permit from the appropriate state or local regulatory agency, and if the pollutant in question is federally controlled, compliance with national air quality standards m u s t also be demonstrated. Moreover, in geographic regions where specific air quality deficiencies exist (known as n o n a t t a i n m e n t areas), analyses m u s t be performed to indicate a facility's contribution to the a m b i e n t pollution levels in the atmosphere. Finally, state and local air toxics regulations require the estimation of chronic risk associated with emissions o f k n o w n carcinogens or designated toxics. Over the past 20 years, computerized versions of

air dispersion models have been developed under the direction o f the United States Environmental Protection Agency (U.S. EPA) and private industry. PCcompatible versions o f these models, such as the B R E E Z E series from Trinity Consultants, Inc., allow users with basic c o m p u t e r skills and a sufficient scientific background to produce estimates o f groundlevel concentrations that result from pollutants released to the atmosphere. These models take into account specific emission variables such as effluent flow rate, stack exit velocity, and release temperature, and use meteorological conditions generally represented by an historical set of data to estimate concentrations at user-specified receptor locations. The predicted concentrations are averaged over a specific period o f interest, ranging f r o m one h o u r to several years. These concentrations are c o m p a r e d to regulatory standards for a m b i e n t air quality or to health risk 97

98

L. R E D M O N A N D D. LIPSHER

levels expressed in terms of the probability of an individual developing cancer (in the case of carcinogens) as a result of exposure to a toxicant (1). Waste management applications of air quality dispersion models include predicting pollutant concentrations from sludge ponds, landfills, and waste incinerators. The emissions from sewage and chemical plant treatment ponds are generally modeled as area sources. Models such as the Industrial Source Complex-Short Term (ISCST2) and the Point Area Line (PAL) programs can be used to analyze these sources. Municipal landfills are primarily concerned with fugitive dust emissions. The Fugitive Dust Model (FDM) incorporates refinements of the algorithms in ISCST to predict both concentrations and total deposition of fugitives from point, area, and line sources. Incinerators produce emissions from a stack, which is represented as a point source in models such as ISCST2. In a typical modeling application, an environmental engineer at a large waste incinerator is charged with performing an air dispersion modeling analysis to calculate maximum pollutant concentrations of the chronic toxicant chromium. Modeled results can then be used to establish the waste feed limits or pollution control requirements for the facility, in compliance with local air toxics guidelines. The air dispersion model most appropriate for this analysis is the Industrial Source Complex-Long Term (ISCLT2) model, developed by the U.S. EPA (2). The model uses three files to create a single input file containing parameters for the algorithms used to estimate ground-level concentrations. The source file (Fig. 1) contains data about the location, height, emission rate, and size (diameter of a stack, square area,

or initial horizontal and vertical dimensions) of each source. The data file (Fig. 2) contains information about the receptors at which concentrations are to be calculated, regulatory options to be used in the model run, the period over which concentrations are to be averaged, meteorological data, and various output options. Finally, the model uses direction-specific dimensions of structures in the vicinity of the source that will alter the wind flow, and thus ground-level concentrations of the modeled pollutant. To simplify the calculation of these building dimensions, Trinity has developed BREEZE WAKE (Fig. 3), which automatically determines which buildings influence the wind flow around each source in each of 16 directions. In the present example, the facility's waste incinerator (the source) is 70 ft tall, with a stack diameter of 1.5 ft. The emission rate for chromium is 0.05102 g/sec with an exit velocity of 5680.6 ft/min. The release temperature is 145°F. The incinerator stack is located adjacent to a building that is 194.5 ft tall with a maximum width (i.e., longest diagonal) of 100 ft. The building will disturb the wind flow, affecting the plume, so building downwash is included in the analysis. The model is run with two receptor grids, centered around the source, to evaluate the distribution of chromium concentrations. In the coarse grid, receptors are spaced one km apart; in the fine grid, used to locate the point of maximum concentration, receptors are spaced at 100-m intervals. The model results indicate a maximum annual chromium concentration of 0.019 #g/m 3, occurring 100 m west of the source. When compared with the l-in-10,000 and 1-in-100,000 cancer risk levels for

General Source I n f o r m a t i o n Src # 1

Source T p e ' .tacI ~

Source Description Inc in e r a t o r

X-Coord. (H)

V-Coora. (H)

362211.0

308674Z

Base E l e v . (Ft)

Height (Ft)

1139.980

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F I G U R E 1. Example of a source data-entry screen in the B R E E Z E version of ISCLT.

U S I N G P C - B A S E D AIR D I S P E R S I O N M O D E L S

99

General Information

Number Of Source Groups Calculate Concentrations Or Depositions? D i s p e r s i o n Hode Use Begulatory Default Option7

NGBOUP ISW(1) ISM(2O)

ISW(ZS)

Concentrations Urban Yes

No

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Ha

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FIGURE 2. Example of a data-entry screen in the B R E E Z E version of ISCLT. On this screen, the user specifies whether the model should use various regulatory default options or if parameters other than those specified by the EPA should be used.

chromium (0.00833 # g / m 3 and 0.000833 pg/m 3, respectively), it is apparent that the modeled facility's emissions of chromium are well above the acceptable risk levels established by the EPA ( 1). Standard tabular output generated by Trinity's ISCLT model includes a representation of all of the input and meteorological data, the direction-specific building dimensions used for downwash analysis, and the annual ground-level concentrations at each receptor. An optional table also provides the 10 highest concentrations and the receptor locations for those

values. In order to increase the interpretive value of the modeling results, the user can generate multidimensional plots illustrating concentrations at receptor points, contouring of concentration values, and surface elevations. Figure 4, for example, shows a contour plot, using the fine grid, of the annual chromium concentrations surrounding the incinerator. Figure 5 superimposes risk levels on a post plot of chromium concentrations, indicating outlying areas at certain risk levels. (The area of impact of the l-in100,000 risk factor, 83.3 X 10 s #g/m 3, extends ap-

FIGURE 3. Example of a BREEZE WAKE data-entry screen. BREEZE WAKE automatically determines which buildings influence wind flow, and thus ground-level concentrations, from nearby sources.

100

L. R E D M O N A N D D. L I P S H E R

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FIGURE 4. Plot of the fine receptor grid ( 100-m spacing) used in the example. The plot indicates the concentration isopleths as well as the property boundary. On-property receptors were removed with a utility included in the graphics package. Note: All concentrations and contour levels must be divided by 100,000 to obtain micrograms per cubic meter.

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USING PC-BASED AIR DISPERSION MODELS

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FIGURE 6. Post plot showing the area where concentrations exceed acceptable risk levels for the fine receptor grid. Note." All concentrations and contour levels must be divided by 100,000 to obtain micrograms per cubic meter. 8.33 contour represents 1: 1,000,000 risk level; 83.3 contour represents 1:100,000 risk level.

proximately 2.55 km from the source.) Finally, Fig. 6 shows the areal extent of the l-in-10,000 risk level, which extends 470 m from the source. Computer models such as ISCLT can perform the iterative calculations necessary to estimate concentrations from numerous sources at hundreds of receptors in a fraction of the time in which they could be solved by hand. And modeling the effects of a pollutant release is far more economical than measuring actual concentrations: a computerized model run can be set up and executed for a small percentage of the cost of placing ambient air quality instrumentation and a network of receptors around a source. Trinity Consultants' BREEZE line of dispersion modeling software adds a user-friendly interface, graphics capability, and postprocessing utilities to models available from the EPA. The BREEZE HAZ models include products such as DEGADIS+ for dense gas releases, SPILLS for ground-level liquid releases, and INPUFF for "instantaneous" releases of hazardous pollutants. The BREEZE WAY models comprise a line of products for use in mobile source emissions studies such as CALINE3 (for emissions

near roadways), CAL3QHC (for emissions near signalized intersections), and MOBILE5.1 (for emissions produced by gasoline- and diesel-powered vehicles). The BREEZE AIR models include products such as ISC2, COMPLEX-I (which predicts concentrations in areas of complex terrain), and SCREEN (for screening analyses of pollutant releases). All of these models operate on 80286-, 80386-, and 80486based personal computers, can take advantage of the increased processing capabilities of math coprocessors, and come with comprehensive documentation. BREEZE software is used by regulatory agencies, industrial companies of all sizes, and air quality consultants around the world. REFERENCES 1. U.S. EPA. Cancer Risk from Outdoor Exposure to Air Toxics, Vol. 1. Office of Air Quality Planning and Standards, document reference number EPA-450/1-90-004a (1990). 2. U.S, EPA. Guideline on Air Quality Models (Revised). Office of Air and Radiation and Office of Air Quality Planning and Standards, document reference number EPA-450/2-78-027R (1986).

Open for discussion until 30 April 1993.