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Appendix
J
A Tiered Modeling Approach for Assessing the Risks Due to Sources of Hazardous Air Pollutants
Disclaimer
This report has been reviewed by the Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, and has been approved for publication. Any mention of trade names or commercial products is not intended to constitute endorsement or recommendation for use.
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Technical Support Division
Research Triangle Park, NC 27711
February 1992
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Table Of Contents
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Figures
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Tables
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1.0 Introduction
1.1 Background and Purpose
Title III of the Clean Air Act Amendment of 1990 (CAAA) sets forth a framework for regulating major sources of hazardous (or toxic) air pollutants which is based on the implementation of MACT, the maximum achievable control technology, for those sources. Under this framework, prescribed pollution control technologies are to be installed without the a priori estimation of the health or environmental risk associated with each individual source. The regulatory process is to proceed on a source category-by-source category basis, with a list of source categories to be published by the end of 1991, and a schedule for their regulation to be published a year later. After the implementation of MACT, it will be incumbent on the United States Environmental Protection Agency (EPA) to assess the residual health risks to the population near each source within a regulated source category. The results of this residual risk assessment will then be used to decide if further reduction in toxic emissions is necessary for each source category (refer to §112(f) of the CAAA). These decisions will hinge primarily on a determination of the lifetime cancer risk for the "maximum exposed individual" for each source as well as the determination of whether the exposed population near each source is protected from noncancer health effects with an "ample margin of safety". The determination of lifetime cancer risk involves the estimation of long-term ambient concentrations of toxic pollutants whereas the determination of noncancer health effects can involve the estimation of long-term and short-term ambient concentrations.
Since the measurement of long-term and short-term ambient concentrations for each toxic air pollutant (189 pollutants as listed in §112(b)) in the vicinity of each source is a prohibitively expensive task, it is envisioned that the process of residual risk determination would involve performing analytical simulations of toxic air pollutant dispersion for all sources (or a subset of sources) within each source category. Such simulations will subsequently be coupled with health effects information and compared to available data to quantify human exposure, cancer risk, noncancer health risks, and ecological risks.
In addition to mandating the residual risk assessment process, the CAAA provide for the exemption of source categories and pollutants from the MACT-based regulatory process if it can be demonstrated that the risks associated with that source category or pollutant are below specified levels of concern. EPA-approved risk assessments would need to be performed to justify such an exemption, and the CAAA provide for petition processes to approve or deny claims that a source category or a specific pollutant should not be subject to regulation.
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The purpose of this document is to provide guidance on the use of EPA-approved procedures which may be used to assess risks due to the atmospheric dispersion of emissions of hazardous air pollutants. It is likely that the techniques described herein will be useful with respect to several decision-making processes associated with the implementation of CAAA Title III (e.g., petition to add or delete a pollutant from the list of hazardous air pollutants, petition to delete a source category from the list of source categories, demonstration of source modification offsets, etc.). In addition, the procedures may serve as the basis for the residual risk determination processes described above. The guidance addresses the estimation of long-term and short-term ambient concentrations resulting from the atmospheric dispersion of known emissions of hazardous air pollutants, and subsequently addresses the techniques currently used to quantify the cancer risks and noncancer risks associated with the predicted ambient concentrations. It describes a tiered approach which progresses from simple conservative screening estimates (provided in the form of lookup tables) to more complex modeling methologies using computer models and site-specific data. In addition to providing guidance to assist in the CAAA Title III implementation process, it is being provided to the general public to assist State and local air pollution control agencies as well as sources of hazardous pollutants in their own assessment of the impacts of these sources.
While the methods described herein comprise the most up-to-date means for assessing the impacts of sources of toxic air pollution, they are subject to future revision as new scientific information becomes available, possibly as a result of the risk assessment methodology study being conducted by the National Academy of Sciences (NAS) under mandate of section 112(o) of the CAAA (report due to Congress from NAS in May, 1993)
1.2 Risk Assessment in Title III
As mentioned above, several provisions of CAAA Title III describe the need to consider ambient concentration impacts and their associated health risks in establishing the regulatory processes for sources of toxic air pollutants. Specifically, these are:
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In the context of these provisions, decisions are to be made based on whether or not the predicted impact of a source exceeds some level of concern. For comparison to specified levels of concern, source impacts are quantified in four ways:
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These impact measures are discussed in more detail in the next few paragraphs. It is worth noting at this point that insofar as knowledge is available regarding the effects of specific hazardous pollutants on the environment, it may be possible to use ecological hazard index values to quantify such impacts. Such calculations would proceed on a track which is parallel to the calculation of health hazard index values.
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For carcinogenic pollutants, the level of concern is the risk of an individual contracting cancer by being exposed to the ambient concentrations of that pollutant over the course of a lifetime, or lifetime cancer risk. For the purposes of §112(c), the criterion specified in the CAAA is 1 in 1,000,000 lifetime cancer risk for the most exposed individual, or the individual exposed to the highest predicted concentrations of a pollutant. (For other purposes, the lifetime cancer risk specifying the level of concern may be higher or lower.) Lifetime cancer risks are calculated by multiplying the predicted annual ambient concentrations (in µg/m3) of a specific pollutant by the unit risk factor or unit risk estimate (URE)1for that pollutant, where the unit risk factor is equal to the upper bound lifetime cancer risk associated with inhaling a unit concentration (1 µg/m3)of that pollutant. Since predicted annual pollutant concentrations around a source vary as a function of position, so do lifetime cancer risk estimates. Thus, decisions involving whether the impact of a source or group of sources is above some level of concern typically focus on the highest predicted concentration (and hence the highest predicted lifetime cancer risk) outside the facility fenceline. The EPA has developed unit risk factors for a number of possible, probable, or known human carcinogens, and will be developing additional cancer unit risk factors as more information becomes available. For the purposes of this document, cancer risks resulting from exposure to mixtures of multiple carcinogenic pollutants will be assessed by summing the cancer risks due to each individual pollutant, regardless of the type of cancer which may be associated with any particular carcinogen.2
For pollutants causing noncancer health effects from chronic or acute exposure, the levels of concern are chronic and acute concentration thresholds, respectively, which would be derived from health effects data, taking into account scientific uncertainties. For purposes of estimating potential long-term impacts of hazardous air pollutants, EPA has derived for some pollutants (and will derive for others) chronic inhalation reference concentration (RfC)1 values, which are defined as estimates of the lowest concentrations of a single pollutant to which the human population can be exposed over a lifetime without appreciable risk of deleterious effects. For purposes of specific chronic noncancer risk assessment, EPA may designate the RfC value, or some fraction or multiple thereof, as the appropriate long-term noncancer level of concern. For purposes of specific acute noncancer risk assessment, the EPA may designate acute reference thresholds as the appropriate short-term noncancer level of concern. For the purposes of this document, long-term noncancer levels of concern will be referred to as chronic concentration thresholds, and short-term noncancer levels of concern will be referred to as acute concentration thresholds. For ease of implementation, acute concentration thresholds will be designated for 1-hour averaging times. This does not necessarily mean that exposure data indicate deleterious health effects from exposure times of 1 hour, but rather that the 1-hour acute
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concentration threshold has been derived such that it is protective of the exposure duration of concern.
The risk with respect to long- or short-term deleterious noncancer health effects associated with exposure to a pollutant or group of pollutants is quantified by the hazard index. The chronic noncancer hazard index is calculated by dividing the modeled annual concentration of a pollutant by its chronic concentration threshold value. The acute noncancer hazard index is calculated by dividing the modeled 1-hour concentration of a pollutant by its acute concentration threshold value. If multiple pollutants are being evaluated, the (chronic or acute) hazard index at any location is calculated by dividing each predicted (annual or 1-hour) concentration at that location by its (chronic or acute) concentration threshold value and summing the results.2If the hazard index is greater than 1.0, this represents an exceedance of the level of concern at that location. For pollutants which can cause deleterious health effects from acute exposures, exceedances of a level of concern may occur at any location and at any time throughout the modeling period. Thus, the frequency with which any location experiences an exceedance also becomes a measure of the risk associated with a modeled source. Frequency of acute hazard index exceedances is only addressed by the most refined analysis methods referred to in this document.
Information on UREs and RfCs is accessible through the Integrated Risk Information System (IRIS), EPA Environmental Criteria and Assessment Office (ECAO) in Cincinnati, Ohio, (513) 569-7254.
1.3 Overview of Document
This document is divided into three major sections, each section addressing a different level of sophistication in terms of modeling, referred to as "tiers". The first tier is a simplified screening procedure in which the user can estimate maximum off-site ground-level concentrations without extensive knowledge regarding the source and without the need of a computer. The second tier is a more sophisticated screening technique which requires a bit more detailed knowledge concerning the source being modeled and, in addition, requires the execution of a computer program. The third tier involves site-specific computer simulations with the aid of computer programs and detailed source parameters. Since the effects of toxic air pollutants may be of concern from both a long-term and a short-term perspective, each tier is divided into two parts. The first part addresses dispersion modeling to assess long-term ambient concentrations (important from a cancer-causing or chronic noncancer effects standpoint) and the second addresses dispersion modeling for the estimation of short-term concentrations (important from an acute toxicity perspective).
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It should be noted that this document is intended to be used in conjunction with the User's Guides for the models described: SCREEN,3TOXST,4and TOXLT.5It is not intended to replace or reproduce the contents of these documents. In addition, the reader may wish to consult the "Guideline on Air Quality Models (Revised)"6for more detailed information on the consistent application of air quality models. Modelers may also wish to use the EPA's TSCREEN7modeling system to assist in the Tier 2 computer simulation of certain toxic release scenarios. It should be noted, however, that toxic pollutant releases which TSCREEN treats as heavier-than-air are not to be modeled using techniques described herein. Atmospheric dispersion of such pollutants requires a more refined analysis, such as those described in Reference 8. Model codes, user's guides, and associated documentation referred to in this document can be obtained through the Technology Transfer Network (TTN) of the EPA's Office of Air Quality Planning and Standards (OAQPS), and access information is provided in Appendix A.
The modeling tiers are designed such that the concentration estimates from each tier should be less conservative than the previous one. This means that, for a given situation, a Tier 1 modeled impact should be greater than, or more conservative than, the Tier 1 modeled impact, and the Tier 2 modeled impact should be more conservative than the Tier 3 modeled impact. Progression from one tier of modeling to the next thus involves the use of levels of concern, as defined above. For example, if the results of a Tier 1 analysis indicates an exceedance of a level of concern with respect to either (1) the maximum predicted cancer risk, (2) the maximum predicted chronic noncancer hazard index, or (3) the maximum predicted acute hazard index, the analyst may wish to perform a Tier 2 analysis. If all three of these impact measures are below their specified levels of concern, there should be no need to perform a more refined simulation, and thus, there should be no need to progress to the next tier of modeling. Since the establishment of levels of concern for each specific hazardous air pollutant is not a part of this effort, this document will refer to generic levels of concern, and users will need to consult subsequent EPA documents to determine the specific levels of concern for their particular pollutant or pollutant mixture and for the particular purpose of their modeling efforts.
1.4 General Modeling Requirements, Definitions, and Limitations
This document describes modeling methologies for point, area, and volume sources of atmospheric pollution. A point source is an emission which emanates from a specific point, such as a smokestack or vent. An area source is an emission which emanates from a specific, well-defined surface, such as a lagoon, landfarm, or open-top tank. Sources referred to as having "fugitive" emissions (e.g., multiple leaks within a specific processing area) are typically mod-
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eled as area sources. The methods used in this document are generally considered to be applicable for assessing impacts of a source from the facility fenceline out to a 50 km radius of the source or sources to be modeled. There is no particular upper or lower limit on emission rate value for which these techniques apply.
For the purposes of this document, "source" means the same thing as "release", and "air toxic" means the same as "hazardous air pollutant". It should be noted that ''area source" as defined in the previous paragraph is not the same as the "area source" defined by the CAAA. Modeling techniques described in this document are specifically intended for use in the simulation of a finite number of well-defined sources, not for simulation of a large number of ill-defined small sources distributed over a large region, as might well be the case for some "area sources" specified in the CAAA. Simulation of the acute and chronic impacts of such area sources may utilize the RAM model9and the CDM 2.0 model,10respectively. Consult the "Guideline on Air Quality Models (Revised)"6for additional information. The reader should note that relatively small, well-defined groups of sources, however, may be modeled using the techniques described herein.
This document does not address the simulation of facilities located in complex terrain. Those interested in modeling facilities with possible complex terrain effects are directed to consult the "Guideline on Air Quality Models (Revised)"6or their EPA Regional Office modeling contact for assistance in this area (see listing Appendix B).
In order to conduct an impact assessment, it is necessary to have estimates of emission rates of each pollutant from each source or release point being included in the assessment. Emission rates may be best estimated from experimental measurements or sampling, where such test methods are available. Alternatively, mass balance calculations or use of emission factors developed for specific types of processes may be used to quantify emission rates. The procedures discussed in this document do not address the emission estimation process. Guidance for source-specific emission rate estimation and emission test methods is available in other EPA documentation (e.g., see References 11 through 15). Additional information concerning specific emission measurement techniques is available through the OAQPS TTN (see Appendix A).
Since many sources of hazardous air pollutants are intermittent in nature (e.g., batch process emissions), the techniques in this document have been developed to allow the treatment of intermittent sources as well as continuous types of sources. It is important to understand the different treatment of emission rates for both types of sources when carrying out either the analysis of a long-term
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impact or a short-term impact. In a long-term impact analysis, the emission rate used for modeling is based on the amount of pollutant emitted over a 1-year period, regardless of whether the emission process is a continuous or intermittent one. In addition, to assess the worst-case impact of a source or group of sources, long-term emission rates used in model simulations should reflect the emission rates for a plant or process which is operating at full design capacity. In a short-term impact analysis, the emission rate used for modeling is based on the maximum amount of pollutant emitted over a 1-hour period, during which the source is emitting. The Tier 1 and Tier 2 procedures evaluate the combined worst-case impacts of intermittent sources as if they are all emitting at the same time, whereas the Tier 3 procedures incorporate a more realistic treatment of intermittent sources by turning them on and off throughout the simulation period according to user-specified frequency of occurrence of each release. This frequency of occurrence should reflect the normal operating schedule of the source when operating at maximum design capacity.
In addition to emission rate estimates, it is necessary to have quantitative information about the sources to conduct a detailed impact assessment. Tier 1 analyses require information about the height of the release above ground level and the shortest distance from the release point to the facility fenceline. Higher tiers of analysis require additional information including, but not limited to:
Stack height
Inside stack diameter
Exhaust gas exit velocity
Exhaust gas exit temperature
Dimensions of structures near each source
Dimensions of ground-level area sources
Exact release and fenceline location
Exact location of receptors for determining worst-case impacts
Land use near the modeled facility
Terrain features near the facility
Duration of short-term release
Frequency of short-term release
Where appropriate, this document will address the best means of obtaining these input data. In some more complex cases, the modeling contact at the nearest EPA Regional Office may need to be consulted for specific modeling guidance (see listing in Appendix B).
Depending on the specific purpose of the impact assessment, it may be difficult for the modeler to decide which sources (or release points) and which pollutants should be included in a particular analysis or simulation. Since these
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questions pertain to the particular purposes for which the impact assessment is being performed, they are not addressed by this document. Instead, this document refers to and provides guidance for modeling various scenarios including single-source, multiple-source, single-pollutant, and multiple-pollutant scenarios. Subsequent EPA documents will address the questions of which sources and which pollutants should be included in an impact analysis for a specific regulatory purpose.
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2.0 Tier I Analyses
2.1 Introduction
Tier 1 analysis of a stationary source (or group of sources) of toxic pollutant(s) is performed to address the question of whether or not the source has the potential to cause a significant impact. This "screening" analysis is performed by using tables of lookup values to obtain the "worst-case" impact of the source being modeled. The analysis is performed to assess both the potential long- and short-term impacts of the source. If the predicted screening impacts are less than the appropriate levels of concern, no further modeling is indicated. If the predicted screening impacts are above any levels of concern, further analysis of those impacts at a higher Tier may be desireable to obtain more accurate results.
The Tier 1 "lookup tables" have been created as tools which may be easily used to estimate conservative impacts of sources of toxic pollutants with a minimal amount of information concerning those sources. The normalized annual and 1-hour concentration tables were created based on conservative simulations of toxic pollutant sources with the EPA's SCREEN model.3In this context, "conservative" simulations use conservative assumptions regarding meteorology, building downwash, plume rise, etc. Conservative annual concentrations were derived from SCREEN 1-hour estimates using the conservative multiplication factor of 0.10.
2.2 Long-term Modeling
Long-term modeling of toxic or hazardous air pollutants is aimed at the estimation of annual average pollutant concentrations to which the public might be exposed as the result of emissions from a specific source or group of sources. From the EPA regulatory viewpoint, this "public" does not include employees of the facility responsible for the emissions (this is the jurisdiction of the Occupational Safety and Health Agency, OSHA). Thus, the impact assessment focuses on estimating concentrations "off-site", or outside the facility boundary. For carcinogens, the calculation of cancer risk proceeds by multiplying annual concentrations by pollutant-specific cancer potency factors derived from health effects data. The impacts of pollutants with chronic noncancer effects are generally assessed by comparing predicted annual concentrations with chronic threshold concentrations which are again derived from experimental health data. For the purposes of protecting the general public against "worst-case" pollutant concentrations, the analysis is focused on predicting the worst-case, or maximum annual average concentrations.
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2.2.1 Maximum Annual Concentration Estimation
A long-term tier 1 analysis requires the following information:
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Once these five items are determined for each release (or source), screening estimates of normalized maximum annual concentrations resulting from each release are obtained from Table 1 using the following procedure.
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For example, consider the situation in which a toxic pollutant A is released at a rate of 11.6 T/yr from a vent-pipe that is 40m tall, and which is attached to a building that is 4m tall, 10m long, and 5m wide. The nearest boundary of the facility is located 65m from the pipe. A value of 35m should be selected for the emission height, because all larger entries in the table exceed the actual height of release of 40m. Concentrations should be estimated for a distance of 50m, because once again, all greater entries in the table exceed the actual distance of 45m. The appropriate normalized maximum annual concentration is 1.13 (µg/m3)(T/yr). Multiplying by the emission rate of 14.6 T/yr results in a maximum annual concentration estimate for screening purposes equal to 16.5 µg/m3.
2.2.2 Cancer risk assessment
Once the maximum annual concentration has been estimated for each release being modeled, upper bound lifetime individual cancer risk may be estimated by mutiplying the maximum annual concentration estimates of each carcinogenic pollutant by the unit cancer risk factor for that pollutant and then summing results. This approach assumes that all cancer risks are additive, regardless of the organ system which may be affected. It should be noted that this approach assumes that all worst-case impacts occur at the same location. While this assumption may not be very realistic, it does help to insure that Tier 1 results are conservative, and, therefore protective of the public.
As an example of this approach, suppose one is simulating a plant which emits 2 pollutants A and B, through 4 different stacks such that pollutant A is released from stacks 1 and 2, and pollutant B is released from stacks 2, 3, and 4. In this example, stack 1 is the same as that described in the example above. After going through the above procedure to estimate the maxium annual concentrations of each pollutant from each stack, the results are:
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Suppose that the unit cancer risk factors for pollutants A and B are know to be 1.0 × 10-7 and 2.0 × 10-7 (µg/m3)-1, respectively. The Tier 1 maximum cancer
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risk is calculated for the individual releases and pollutants and summed as follows:
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If we are assessing the impact of this group of sources in relation to the CAAA specificed level of concern of 1 × 10-4 lifetime cancer risk, and since the maximum Tier 1 risk is greater than the CAAA specified concern level of 1 × 10-4, this source warrants further modeling on the basis of cancer risk (note that this does not rule out the need to investigate acute or chronic nonccancer risk).
2.2.3 Chronic Noncancer Risk Assessment
For all pollutants which pose a chronic noncancer threat to health, an assessment of the magnitude of this threat is made using the hazard index approach. The chronic noncancer hazard index is calculated by summing the maximum annual concentrations for each pollutant divided by the chronic threshold concentration value for that pollutant. if the calculated hazard index is greater than 1.0, the release or releases being simulated may pose a threat to the public, and further modeling may be indicated. It should again be noted that, for the sake of erring conservatively, this approach assumes that the worst-case impacts of all releases occur at the same location.
As an example of the above procedure, suppose that pollutants A and B in the example above pose a chronic noncancer health risk, and their respective chronic concentration threshold values are 20.0 and 5.0 µg/m3, respectively. The chronic noncancer hazard index would be formulated as follows:
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In this case, one of the individual hazard index values exceeds 1.0, the total hazard index for this modeled facility exceeds 1.0, and further modeling at a higher Tier may be desired.
2.3 Short-term Modeling
Since short-term modeling of toxic or hazardous air pollutants is aimed at the estimation of 1-hour average pollutant concentrations to which the public might be exposed as the result of emissions from a specific source or group of sources. Again, from the EPA regulatory viewpoint, this "public" does not include employees of the facility responsible for the emissions (this is the jurisdiction of OSHA). Thus, the impact assessment focuses on estimating concentrations "off-site", or outside the facility boundary. From the short-term perspective, the health effects of most concern vary, but they are those which create detrimental health effects as the result of short-term exposure to toxic pollutants. The risks associated with such exposures are generally assessed by comparing 1-hour predicted concentrations with acute threshold concentrations which are derived from experimental health data. For the purposes of protecting the general public against "worst-case" pollutant concentrations, the analysis is focused on predicting the worst-case, or maximum 1-hour average concentrations.
2.3.1 Maximum Hourly Concentration Estimation
A short-term Tier 1 analysis requires the following information:
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Once these five items are determined for each release, screening estimates of maximum 1-hour average concentrations resulting from each release are obtained from Table 2 using the following procedure.
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For example, again consider the situation in which toxic material A is released from a vent-pipe that is 40m tall, and which is attached to a building that is 4m tall, 10m long, and 5m wide. The nearest boundary of the facility is located 65m from the pipe. For the short-term assessment, it has been determined that the maximum emissions of A that can occur during any hour of the year is 1800g, therefore the emission rate for short-term assessment is 1800g/3600s = 0.50g/s. A value of 35m is again selected for the emission height, because all larger entries in the table exceed the actual height of release. Concentrations are estimated for a distance of 50m, because once again, all greater entries in the table exceed the actual distance of 65m. The appropriate normalized maximum 1-hour average concentration is 3.94E = 2 (µg/m3)/(g/s). Multiplying by the emission rate of 0.50g/s results in a maximum hourly concentration estimate for screening purposes equal to 197 µg/m3.
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2.3.2 Acute Hazard Index Assessment
For all pollutants which pose a threat to health based on acute exposure, an assessment of the magnitude of this threat is made using the acute hazard index approach, similar to that used in chronic noncancer risk assessment. In this case, however, the acute hazard index is calculated by summing the maximum 1-hour concentrations for each pollutant divided by the acute concentration threshold value for that pollutant. It should again be noted that, for the sake of erring conservatively, this approach assumes that the worst case impacts of all releases can occur simultaneously at the same location. Similar to the chronic risk assessment, if the calculated hazard index is greater than 1.0, the release or releases being simulated may pose a threat to the public, and further modeling at a higher Tier may be indicated.
As an example of the acute hazard index approach, consider the same plant being simulated in Section 2.2.2, but this time the maximum 1-hour concentrations are determined using the procedure in Section 2.3.2 to be the following:
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Further suppose that pollutants A and B pose health problems from acute exposures with acute threshold concentration values of 200 and 100 µg/m3, respectively. The acute hazard index is calculated as follows:
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In this case, 4 of the individual hazard index values exceeds 1.0, the total hazard index for the modeled plant exceeds 1.0, and further modeling at a higher Tier may be desired.
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3.0 TIER 2 ANALYSES
3.1 Introduction
Tier 2 analysis of a stationary source (or group of sources) of toxic pollutant(s) may be desired if the results of a Tier 1 analysis indicate an exceedance of a level of concern with respect to one or more of the following: (1) the maximum predicted cancer risk; (2) the maximum predicted chronic noncancer hazard index, or; (3) the maximum predicted acute hazard index. Note that in situations where only one or two of the Tier 1 criteria are exceeded, only those analyses which exceed the Tier 1 criteria may need to be performed at the higher Tier. For example, if the Tier 1 analysis showed cancer risk and chronic noncancer risks to be of concern while the acute risk analysis showed no cause for concern, only long-term modeling for cancer risk and noncancer risk may need to be performed at Tier 2. Tier 2 analyses are slightly more sophisticated than Tier 1 analyses, and therefore require additional input information as well as a computer for their execution. Tier 2 analyses are structured around the EPA's SCREEN model and its corresponding documentation entitled ''Screening Procedures for Estimating the Air Quality Impact of Stationary Sources."3The SCREEN model source code and documentation is available through the OAQPA TTN (see Appendix A).
Again, similar to the Tier 1 analysis, if any of the predicted model impacts from Tier 2 are above the appropriate levels of concern, further modeling is indicated at a higher Tier.
3.2 Long-term Modeling
Long-term Tier 2 modeling utilizes the SCREEN3model to estimate 1-hour maximum concentrations, and then utilizes a conservative conversion factor to derive maximum annual concentration values from the SCREEN predictions.16,17These maximum annual concentration estimates are used to assess cancer risk and chronic noncancer risk exactly as in Section 2.2.2 and 2.2.3 of this document.
3.2.1 Maximum Annual Concentration Estimation
In addition to the information required to perform a Tier 1 analysis, a Tier 2 analysis requires the following information:
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Refer to the "Guideline on Air Quality Models (Revised)"6for additional guidance on this determination.
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Once these items are determined for each release being modeled, estimates of maximum concentrations from each release are obtained through individual SCREEN runs for each release. Recommendations for each SCREEN run are as follows:
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*Note: The maximum horizontal dimension is defined as the largest possible alongwind distance the structure could occupy.
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As an example of the Tier 2 long-term analysis, consider Stack 1 from the Tier 1 example. To consider downwash possibilities, the maximum horizontal dimension is first estimated as {(10m)2 + (5m)2}1/2 = 11.2m. The dimension L is then 4m, and the maximum stack height for which downwash is possible would be 4m + 1.5 × 4m = 10m. Since the actual stack height is 40m, downwash need not be considered in the SCREEN simulation. The emission rate specified in the example of 14.6 T/yr is converted to g/s to be used in the SCREEN simulation, resulting in an emission rate of 14.6/34.73 = 0.42 g/s. In addition to the actual stack height (40m) and minimum fenceline distance (65m), input parameters for the SCREEN simulation are:
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The results from the SCREEN simulation indicates that the maximum 1-hour concentration at or beyond 65m is 32.5 µg/m3, occurring 165m downwind. Using the recommended conversion factor of 0.09, the maximum annual concentration is estimated at 2.6 µg/m3 (this value can be contrasted with the Tier 1 estimation of 16.5 µg/m3).
3.2.2 Cancer Risk Assessment
Maximum annual concentrations for all releases of carcinogens should be multiplied by the appropriate unit cancer risk factor and summed to estimate the maximum cancer risk. It should be noted that this approach, as in Tier 1, presumes that all worst-case impacts occur at the same location. While this assumption may not be very realistic, it does help insure that the results of a Tier 2 analysis are conservative and therefore protective of the public. More receptor-specific risk calculations are addressed in the Tier 3 analyses.
Borrowing again from the Tier 1 example, maximum annual impacts for each source and pollutant combination are estimated using the SCREEN model. Risk estimates are then made by summing the risk due to each release, regardless of downwind distance to maximum impact. The results are:
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For this example, the maximum lifetime cancer risk estimated using the Tier 2 methods is a factor of 6 lower than that estimated in the Tier 1 analysis. However, the cancer risk level still exceeds 1 × 10-6, indicating that modeling at a higher Tier may be desireable.
3.2.3 Chronic Noncancer Risk Assessment
As in Tier 1, maximum annual concentrations are divided by their chronic concentration threshold values and summed to calculate the hazard index values. Again, this approach conservatively assumes that all worst-case impacts occur at the same location.
Continuing with the example, the chronic noncancer hazard index is recalculated using the Tier 2 estimated long-term impacts. Threshold concentration values for chronic noncancer effects again are taken as 20.0 and 5.0 µg/m3 for pollutants A and B, respectively. The following results:
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The chronic noncancer hazard index estimated in Tier 2 is a good deal less than that estimated for the same sources in Tier 1. Even though none of the individual source/pollutant combinations exceeds a chronic threshold concentration value, the total hazard index exceeds 1.0, and further analysis at Tier 3 is indicated for chronic noncancer effects.
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3.3 Short-term Modeling
Short-term Tier 2 modeling utilizes the SCREEN3model to estimate 1-hour maximum concentrations directly. These maximum 1-hour concentration estimates are used to assess acute hazard index values exactly as in Section 2.3.2 of this document.
3.3.1 Maximum Hourly Concentration Estimation
In addition to the information required to perform a Tier 1 short-term analysis, a Tier 2 analysis requires the following information for stack sources:
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Once these items are determined for each release being modeled, estimates of maximum concentrations from each release are obtained through individual SCREEN runs for each release. Recommendations for each SCREEN run are as follows:
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Using this approach with the Stack 1 example, the SCREEN model is exercised with the stack parameters specified in Section 3.2.1. The maximum short-term emission rate of 0.50 g/s (see Section 2.3.1), however, is used to estimate the maximum 1-hour source impact. The results of the SCREEN model indicate that the maximum 1-hour concentration is 38.8 µg/m3, again occurring 165m downwind.
3.3.2 Acute Hazard Index Assessment
As in Tier 1, maximum 1-hour concentrations are divided by their acute threshold concentration values and summed to calculate the acute hazard index values. Again, this approach conservatively assumes that all worst-case impacts can occur simultaneously at the same location.
To illustrate this procedure, short-term impacts from the example plant are assessed using the hazard index approach. Again the acute threshold concentration values are taken as 200 and 100 µg/m3, respectively. The results are:
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For this example, the acute hazard index estimated in Tier 2 is roughly 20% of that estimated for the same sources in Tier 1. However, since the total hazard index exceeds 1.0, further analysis at Tier 3 is indicated for health effects resulting from acute exposures.
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4.0 Tier 3 Analyses
4.1 Introduction
Tier 3 analysis of a stationary source (or group of sources) of toxic pollutant(s) may be desired if the results of a Tier 2 analysis indicate an exceedance of a level of concern with respect to one or more of the following: (1) the maximum predicted cancer risk; (2) the maximum predicted chronic noncancer hazard index, or; (3) the maximum predicted acute hazard index. Tier 3 analysis of a stationary source (or group of sources) of toxic pollutant(s) is performed to provide the most scientifically-refined indication of the impact of that source. This Tier involves the utilization of site-specific source and plant layouts as well as meteorological information. In contrast to the previous Tiers, Tier 3 allows for a more realistic simulation of intermittent sources and combined source impacts. In addition, results from short-term analyses indicate not only if a risk level of concern can be exceeded, but how often that level of concern might be exceeded during an average year. Dispersion modeling for the Tier 3 analysis procedure is based on use of EPA's Industrial Source Complex (ISC) model18and as such utilizes many of the same techniques recommended in the "Guideline on Air Quality Models (Revised)"6approach to the dispersion modeling of criteria pollutants.
To facilitate the dispersion modeling of toxic air pollutants, the EPA has developed TOXLT (TOXic modeling system Long-Term)5for refined long-term analyses, and TOXST (TOXic modeling system Short-Term)4for refined short-term analyses. The TOXLT system incorporates the ISCLT (long-term) directly to calculate annual concentrations and the TOXST system incorporates the ISCST (short-term) model directly to calculate hourly concentrations. Codes and user's guides for both TOXLT and TOXST are available via electronic bulletin board (see Appendix A).
4.2 Long-Term Modeling
Long-term Tier 3 modeling using the TOXLT5modeling system to estimate maximum annual concentrations and maximum cancer risks. The TOXLT modeling system uses the ISCLT model to calculate these annual concentrations at receptor sites which are specified by the user. A post-processor called RISK subsequently calculates lifetime cancer risks and chronic noncancer hazard index values at each receptor.
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4.2.1 Maximum Annual Concentration Estimation
In addition to the information required to perform a Tier 2 long-term analysis, the Tier 3 long-term analysis requires the following information:
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Once these data have been obtained, an input file should be prepared for execution of the ISCLT model using the guidance available in the ISC User's Guide.18The ISCLT model should then be executed using the TOXLT system. Procedures utilized should also be consistent with the TOXLT User's Guide5(available via electronic bulletin board, see Appendix A). Specific recommendations concerning the development of these inputs include:
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The printed ISCLT output will indicate the top 10 impacts for each source group, while the master file inventory will contain all of the annual concentration predictions from each source group at each receptor.
Continuing with the examples from Tiers 1 and 2, TOXLT was utilized to perform site-specific ISCLT dispersion modeling for the 4 stacks in the example. Each of the stacks was modeled as an individual source group. A STAR summary of five years of meteorological data from the nearest NWS site was utilized along with specific source and plant boundary locations according to Figure 1 below. Stacks are represented in the Figure as open circles, with stacks 3 and 4 located at the same place. A rectangular receptor grid (indicated by the filled circles) with 50m spacing outside the plant boundary was used to obtain concentration predictions. Neither pollutant was presumed to decompose in the atmosphere.
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Figure 1.
Schematic of Example Facility with Long-Term Impact Locations
The results of the dispersion modeling indicated the following maximum annual off-site concentrations for each of the source/pollutants combinations:
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It should be noted that the maximum concentrations from each source/receptor combination were not co-located. The positions of the maximum concentration from each source are indicated on Figure 1 corresponding to the letters X, Y, and Z in the table above. In general, the Tier 3 maximum concentration values are 25 to 30% as high as the Tier 2 values.
4.2.2 Cancer Risk Assessment
Concentrations from the ISCLT master file inventory are used by the RISK post-processor to calculate cancer risks at each receptor site in the ISCLT receptor array. RISK can then provide summaries of the calculated risks according to
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user specifications. Use of the RISK post-processor requires the following considerations:
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If the maximum predicted lifetime cancer risk in the receptor grid is less than the designated level of concern (e.g., 1 × 10-6), placement of additional receptors in the ISCLT receptor array should be considered as a means of ensuring that the simulation is not underestimating maximum risk. If the maximum cancer risk in the receptor array is greater than the designated level of concern, additional runs of the RISK post-processor may be performed using reduced emission rate multipliers to assess the impacts of possible emission control scenarios. If the analysis shows no cancer risk greater than the designated level of concern and the receptor array is deemed adequate, the modeled source is considered to be in compliance with the specified criterion. In the case of noncompliance, it may be desirable on the part of the modeler to conduct a more refined analysis. See Section 5.0 if this document discusses some of the possibilities for further modeling refinements.
The output of the Risk post-processor for the example plant indicates that the maximum lifetime cancer risk outside the plant boundary is 4.2 × 10-7, located at point W on Figure 1. Such a result would indicate that the facility would not cause a significant cancer risk to the public, according to the cancer risk level specified by the CAAA of 1990.
4.2.3 Chronic Noncancer Risk Assessment
In this assessment, concentrations from the ISCLT master file inventory are used by the RISK post-processor to calculate chronic noncancer hazard index values for a specific noncancer effect at each receptor site in the ISCLT receptor array. RISK can then provide summaries of the calculated index values according to user specifications. A separate risk simulation should be performed for each chronic noncancer effect being considered. Use of the RISK post-processor requires the following considerations:
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If the maximum hazard index value in the receptor grid exceeds 1.0, emission reduction scenarios can be performed (again, using reduced emission rate multipliers) to determine how this hazard index value can be reduced below 1.0. If the maximum hazard index value in the receptor grid does not exceed 1.0, the source(s) being modeled is considered to be in compliance with the specified criteria. In the case of non-compliance, it may be desirable on the part of the modeler to conduct a more refined analysis. See Section 5.0 if this document discusses some of the possibilities for further modeling refinements.
Using the chronic noncancer threshold concentration values for pollutants A and B of 20.0 and 5.0 µg/m3, respectively, the RISK post-processor was exercised for the example facility to obtain a maximum hazard index value of 0.27 located at point Z on Figure 1. This result, which is approximately 30% of the Tier 2 result, would indicate that the facility does not present significant chronic noncancer risk in its current configuration.
4.3 Short-term Modeling
Short-term Tier 3 modeling uses the TOXST modeling system4to estimate maximum hourly concentrations and the receptor-specific expected annual number of exceedances of short-term concentration thresholds. For multiple pollutant scenarios, this amounts to the number of times the acute hazard index value exceeds 1.0. The model uses the ISCST model to calculate these hourly concentrations at receptor sites which are specified by the user. Acute hazard index values are subsequently calculated at each receptor by the TOXX post-processor, in which a Monte Carlo simulation is performed for intermittent sources to assess the average number of times per year the acute hazard index value exceeds 1.0 at each receptor.
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4.3.1 Maximum Hourly Concentration Estimation
In addition to the information required to perform a Tier 2 analysis, the Tier 3 short-term analysis requires the following information:
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Once these data have been obtained, an input file should be prepared for execution of the ISCLT model using the guidance available in the ISC User's Guide.18The ISCST model should then be executed using the TOXST system. Procedures utilized should also be consistent with the TOXST User's Guide5(available through the electronic bulletin board, see Appendix A). Specific recommendations concerning the development of these inputs include:
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where LACT is the lowest acute concentration threshold value in the group of pollutants being modeled, and Npoli is the number of pollutants emitted from ISCST source group i.
The printed ISCST output will indicate the top 50 impacts for each ISCST source group, and the TOXFILE will contain all of the concentrations above the cutoff value from each ISCST source group at each receptor.
The ISCST model was exercised for the example facility. The maximum 1-hour concentrations for each source/pollutant combination were determined to be as follows:
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The locations of the predicted maximum 1-hour concentrations are shown in Figure 2. The maximum impacts from each source were only slightly lower than those from the Tier 2 analysis.
4.3.2 Acute Hazard Index Exceedance Assessment
Concentrations from the ISCST master file inventory are used by the TOXX post-processor to calculate acute hazard index values for each hour of a multiple-year simulation period at each receptor site in the ISCST receptor array. The program then counts the number of times a hazard index value exceeds 1.0 (an exceedance) and prints out a summary report which indicates the average number of times per year an exceedance occurs at each receptor. The use of the TOXX post-processor requires the following considerations:
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Figure 2.
Schematic of Example Facility with Short-term Impact Locations
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If the maximum number of acute hazard index exceedances in the receptor grid is less than some specified value (e.g., 0.1, equivalent to an average of 1 hourly exceedance every 10 years), the modeled source is considered to be in compliance with the acute threshold concentration criteria. However, resimulation with placement of additional receptors in the ISCST receptor array should be considered as a means of assuring that the simulation is not underestimating the maximum acute hazard index. If the maximum number of hazard index exceedances in the receptor array is greater than the specified value, additional runs of the TOXX post-processor with reduced emissions rate multipliers may be performed to assess the impacts of possible emission control scenarios. In the case of non-compliance, it may be desirable on the part of the modeler to conduct a more refined analysis. Section 5.0 of this document discusses such possibilities.
The TOXX post-processor was exercised for the example facility using the results form the ISCST simulation. The frequency of operation for each source ranged from 0.14 to 0.84, reflecting the actual yearly frequency of "on" time for each source. The output showed that none of the receptors experienced an impact resulting in a hazard index value of 1.0 or greater. Comparing this result with the Tier 2 result indicates that the hazard index never exceeds 1.0 because in a Tier 3 analysis the maximum impacts are seen not to occur at the same place and time. This indicates that the facility does not cause a significant health risk from acute exposure in its current configuration.
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5.0 Additional Detailed Analyses
If any Tier 3 analyses indicate non-compliance with any of the user-specified criteria, it may be desireable to conduct an additional, more refined analysis. This may mean the use of on-site meteorological data or it may mean that a more appropriate modeling procedure is deemed applicable for the specific case. The determination of an appropriate alternative modeling procedure can only be made in a manner consistent with the approach outlined in the "Guideline on Air Quality Models (Revised)."6
In some cases, the EPA may allow exposure assessments to incorporate available information on actual locations of residences, potential residences, businesses, or population centers for the purpose of establishing the probability of human exposure to the predicted levels of toxic pollution near the source being modeled. In such cases, use of the Human Exposure Model (HEM II)19with the ISCLT dispersion model is preferred. Again, if the use of other modeling procedures is desired, the approval of a more appropriate alternative modeling procedure can only be made in a manner consistent with the approach outlined in Section 3.2 of the "Guideline on Air Quality Models (Revised)."6
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6.0 Summary Of Differences Between Modeling Tiers
To summarize the major differences between the 3 modeling tiers described in this document, Table 3 below briefly lists the input requirements, output parameters, and assumptions associated with each tier. This Table may be used to quickly determine whether a given scenario may be modeled at any particular tier. Within each tier, cancer unit risk estimates, chronic noncancer concentration thresholds, and acute concentration thresholds are required to convert concentration predictions into cancer risks, chronic noncancer risks, and acute noncancer risks, respectively.
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References
1. Environmental Protection Agency, 1988. Glossary of Terms Related to Health, Exposure, and Risk Assessment. EPA-450/3-88-016. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
2. Environmental Protection Agency, 1987. The Risk Assessment Guidelines of 1986. EPA-600/8-87-045. United States Environmental Protection Agency, Washington, DC 20460.
3. Brode, Roger W., 1988. Screening Procedures for Estimating the Air Quality Impact of Stationary Sources (Draft). EPA-450/4-88-010. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
4. Environmental Protection Agency, 1992. Toxic Modeling System Short-Term (TOXST) User's Guide. EPA-450/4-92-002. United States Environmental Protection Agency, Research Triangle Park, NC 27711 (in preparation).
5. Environmental Protection Agency, 1992. Toxic Modeling System Long-Term (TOXLT) User's Guide. EPA-450/4-92-003. United States Environmental Protection Agency, Research Triangle Park, NC 27711 (in preparation).
6. Environmental Protection Agency, 1988. Guideline on Air Quality Models (Revised). EPA-450/2-78-027R. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
7. Environmental Protection Agency, 1990. User's Guide to TSCREEN: A Model for Screening Toxic Air Pollutant Concentrations. EPA-450/4-90-013. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
8. Environmental Protection Agency, 1991. Guidance on the Application of Refined Dispersion Models for Air Toxic Releases. EPA-450/4-91-007. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
9. Catalano, J.A., D.B. Turner, and J.H. Novak, 1987. User's Guide for RAM - Second Edition. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
10. Irwin, J.S., T. Chico, and J.A. Catalano. CDM 2.0 - Climatological Dispersion Model-User's Guide. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
11. Environmental Protection Agency, 1991. Procedures for Establishing Emissions for Early Reduction Compliance Extensions. Draft. EPA-450/3-91-012a. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
12. Environmental Protection Agency, 1978. Control of Volatile Organic Emissions from Manufacturers of Synthesized Pharmaceutical Products. EPA-450/2-78-029. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
13. Environmental Protection Agency, 1980. Organic Chemical Manufacturing Volumes 1-10. EPA-450/3-80-023 through 028e. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
14. Environmental Protection Agency, 1980. VOC Fugitive Emissions in Synthetic Organic Chemicals
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Manufacturing Industry - Background Information for Proposed Standards. EPA-450/3-80-033a. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
15. Environmental Protection Agency, 1990. Protocol for the Field Validation of Emission Concentrations from Stationary Sources. EPA-450/4-980-015. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
16. Pierce, T.E., Turner, D.B, Catalano, J.A., Hale, F.V., 1982. "PTPLU: A Single Source Gaussian Dispersion Algorithm." EPA-600/8-82-014. United States Environmental Protection Agency, Washington, DC 20460.
17. California Air Pollution Control Officers Association (CAPCOA), 1987. Toxic Air Pollutant Source Assessment Manual for California Air Pollution Control District and Applications for Air Pollution Control District Permits, Volumes 1 and 2. CAPCOA, Sacramento, CA.
18. Environmental Protection Agency, 1987. Industrial Source Complex (ISC) User's Guide- Second Edition (Revised), Volumes 1 and 2. EPA-450/4-88-002a and b. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
19. Environmental Protection Agency, 1991. Human Exposure Model (HEM-II) User's Guide. EPA-450/4-91-010. United States Environmental Protection Agency, Research Triangle Park, NC 27711.
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Appendix A
Electronic Bulletin Board Access Information
The Office of Air Quality Planning and Standards (OAQPS) of the EPA has developed an electronic bulletin board network to facilitate the exchange of information and technology associated with air pollution control. This network, entitled the OAQPS Technology Transfer Network (TTN), is comprised of individual bulletin boards that provide information on OAQPS organization, emission measurement methods, regulatory air quality models, emission estimation methods, Clean Air Act Amendments, training courses, and control technology methods. Additional bulletin boards will be implemented in the future.
The TTN service is free, except for the cost of the phone call, and may be accessed from any computer through the use of a modem and communications software. Anyone in the world wanting to exchange information about air pollution control can access the system, register as a system user, and obtain full access to all information areas on the network after a 1 day approval process. The system allows all users to peruse through information documents, download computer codes and user's guides, leave questions for others to answer, communicate with other users, leave requests for technical support from the OAQPS, or upload files for other users to access. The system is available 24 hours a day, 7 days a week, except for Monday, 8-12 a.m. EST, when the system is down for maintenance and backup.
The model codes and user's guides referred to in this document, in addition to the document itself, are all available on the TTN in the bulletin Board entitled SCRAM, short for Support Center for Regulatory Air Models. Procedures for downloading these codes and documents are also detailed in the SCRAM bulletin board.
Documentation on EPA-approved emission test methods is available on the TTN in the bulletin board entitled EMTIC, short for the Emission Measurement Testing Information Center. Procedures for reading or downloading these documents are also detailed in the EMTIC bulletin board.
The TTN may be accessed at the phone number (919)-541-5742, for users with 1200 or 2400 bps modems, or at the phone number (919)-541-1447, for users with a 9600 bps modem. The communications software should be configured with the following parameter settings: 8 data bits; 1 stop bit; and no (N) parity. Users will be asked to create their own case sensitive password, which they must remember to be able to access the network on future occasions. The entire network is menu-driven and extremely user-friendly, but any users requiring assistance may call the system operator at (919)-541-5384 during normal business hours EST.
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Appendix B
Regional Meteorologists/Modeling Contacts
Ian Cohen
EPA Region I (ATS-2311)
J.F.K. Federal Building
Boston, MA 02203-2211
FTS: 853-3229
Com: (617) 565-3225
E-mail: EPA9136
FAX: FTS 835-4939
Robert Kelly
EPA Region II
26 Federal Plaza
New York, NY 10278
FTS: 264-2517
Com: (212)-264-2517
E-mail: EPA9261
FAX: FTS 264-7613
Alan J. Cimorelli
EPA Region III (3AM12)
841 Chestnut Building
Philadelphia, PA 19107
FTS: 597-6563
Com: (215) 597-6563
E-mail: EPA9358
FAX: FTS 597-7906
Lewis Nagler
EPA Region IV
345 Courtland Street, N.E.
Atlanta, GA 30365
FTS: 257-3864
Com: (404) 347-2864
E-mail: EPA9470
FAX: FTS 257-5207
James W. Yarbough
EPA Region VI (6T-AP)
1445 Ross Avenue
Dallas, TX 75202-2733
FTS: 255-7214
Com: (214) 255-7214
E-mail: EPA9663
FAX: FTS 255-2164
Richard L. Daye
EPA Region VII
726 Minnesota Avenue
Kansas City, KS 66101
FTS: 276-7619
Com: (913) 551-7619
E-mail: EPA9762
FAX: FTS 276-7065
Larry Svoboda
EPA Region VIII (8AT-AP)
999 18th Street
Denver Place-Suite 500
Denver, CO 80202-2405
FTS: 776-5097
Com: (303) 293-0949
E-mail: EPA9853
FAX: FTS 330-7559
Carol Bohnenkamp
EPA Region IX (A-2-1)
75 Hawthorne Street
San Francisco, CA 94105
FTS: 484-1238
Com: (415) 744-1238
E-mail: EPA9930
FAX: FTS 484-1076
Rebecca Calby
EPA Region V (5AR-18J)
77 W. Jackson
Chicago, IL 60604
FTS: 886-6061
Com: (312) 886-6061
E-mail: EPA9553
FAX: FTS 886-5824
Robert Wilson
EPA Region X (ES-098)
1200 Sixth Avenue
Seattle, WA 98101
FTS: 399-1530
Com: (206) 442-1530
E-mail: EPA9051
FAX: 399-0119
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TECHNICAL REPORT DATA FORM
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TECHNICAL REPORT DATA FORM (continued)