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IntroductionModeling of Stationary Source Class II (minor) and non-PSD Class I (Title V) Facilities:This document is intended to provide general guidance to permit applicants in preparing a modeling
protocol in support of their submittal of an environmental evaluation to the Nevada Division of
Environmental Protection, Bureau of Air Quality (NDEP-BAPC). An air quality modeling analysis (e.g.,
dispersion modeling demonstration) is used to assess whether a source will show attainment with applicable
Nevada and Federal ambient air quality standards and is an integral part of the environmental evaluation
requirement. Prior to performing and submitting any air quality modeling analyses, NDEP-BAPC
recommends that the applicant first prepare and submit a modeling protocol prior to submitting the actual
permit application and environmental evaluation. However, the permit applicant is still responsible for
meeting all regulatory time-frames for timely submission of a permit application should he/she request that
NDEP-BAPC review his/her modeling protocol. Therefore, it is recommended that the permit applicant
prepare and submit their modeling protocol well ahead of the statutory deadlines in which a Class I/II
permit application must be submitted to NDEP-BAPC. Although submission of a modeling protocol ahead
of the permit application is not required by the NDEP-BAPC regulations, it does allow NDEP-BAPC the
opportunity to provide comments, if they are warranted, prior to the applicant’s time and expense in
performing an environmental evaluation.
Review of the modeling protocol by NDEP-BAPC allows our office the opportunity to respond either
verbally (informally) or in writing (formally) to any perceived deficiencies and/or technical issues raised by
the proposed air quality modeling prior to receipt of a formal permit application. The permit applicant is
to note that submission of an incomplete or inconclusive modeling analysis as part of a permit application
may result in NDEP-BAPC either deeming the application as incomplete or providing sufficient technical
reasons for denial of the application. Denial of the proposed permit application is mandated when an
independent modeling analysis performed by NDEP-BAPC, or that submitted by the applicant, conclusively
shows that the proposed permit action would potentially violate an applicable air quality standard, as
documented by an approved regulatory air model.
As part of the permit application administrative and technical review process, NDEP-BAPC internally
reviews and performs an independent critique of all modeling analyses it receives prior to making a final
decision (e.g., permit issuance or permit denial) on an air quality operating permit. Therefore, even if a
facility’s proposed emissions fall below the modeling thresholds, the permit application must still contain
sufficient required information and data to allow NDEP-BAPC to make an independent environmental
evaluation. The required information generally pertains to but is not limited to a proposed emission
inventory of all regulated air pollutants including those from insignificant activities, stack parameters (e.g.,
height, diameter, flowrate, temperature, location, etc.), siting of emission units, facility plot plan, building
locations and dimensions, fenceline and/or property boundary limits, terrain features (e.g., United States
Geological Survey (USGS) topographic quadrangle maps and any detailed map inserts), etc.
Please note that NDEP-BAPC does not provide dispersion model training or model instructional guidance
to permit applicants (and their consultants) who are required to provide modeling demonstrations.
Modelers seeking dispersion model training classes and other instructional media may find training
opportunities available from either the U.S. EPA Air Pollution Training Institute (APTI) or the various
dispersion model software vendors. Since there may be several feasible ways in which to model a
particular emissions scenario, proper modeling techniques are more a factor of prior experience than an
exact and unvarying blueprint that is valid for all situations. A response from our office in regards to your
modeling protocol or environmental evaluation is generally warranted if there are perceived deficiencies or
fundamental issues raised from a review of your approach to modeling your facility. It is the applicant’s
obligation to understand and follow correct modeling logic and procedures as was intended by the
developers of the various dispersion models. Therefore, the time and expense of providing an environmental
evaluation is the obligation of the permit applicant.
Modeling of PSD Class I (PSD-major stationary source) Facilities: For applicants preparing a modeling protocol in support of a Prevention of Significant Deterioration (PSD)
air quality modeling analysis, it is recommended that the applicant first contact the NDEP-BAPC Class
I/PSD Permitting Branch Supervisor at (775) 687-9337. PSD air quality analyses are handled on a case-by-
case basis and are prepared using guidance from the U.S. EPA document entitled New Source Review
Workshop Manual: Prevention of Significant Deterioration and Nonattainment Area Permitting,
DRAFT, October 1990.
NAC Requirements for Modeling:Modeling Threshold Requirements for New or Renewed Operating Permits: For a new stationary source or a renewal of an operating permit for an existing stationary source, Nevada Administrative Code (NAC) 445B.310.2.(a) requires that an environmental evaluation be provided by the applicant for all sources which emit, or have the potential to emit, greater than 25 tons of any one regulated pollutant per year. Regulated air pollutants include PM (total particulate matter), PM10 (particulate matter less than 10 microns in diameter), SO2 (sulfur dioxide), NOx (nitrogen oxides), CO (carbon monoxide), VOC/O3 (volatile organic compounds and the resultant secondary pollutant of ozone or photochemical smog), Pb (lead), and HAPs (hazardous air pollutants of which EPA currently designates 189 air pollutants as such). The reader is to note that although an ambient standard no longer exists for PM, the Potential-To- Emit (PTE) of PM emissions is considered by NDEP-BAPC in determining whether an environmental evaluation will be required of the applicant (for modeling demonstration purposes of any facility which will emit particulate matter, the PM10 component of PM is modeled). Please note that for renewal of operating permits, NDEP-BAPC generally does not accept as complete, an environmental evaluation performed (5) five years or more ago when the last permit was issued. The reason for this is that background conditions, model source codes, and BAPC modeling policy undergo periodic adjustments. Therefore, the permit applicant is responsible for providing a new (e.g., recent) demonstration of the modeling results on the latest model source code as published by the U.S. EPA.When the applicant is required by NAC 445B.310 to provide an environmental evaluation for a new
stationary source or permit renewal, please note that the modeling analysis must include a demonstration
of all criteria air pollutants that will be emitted, including those less than the 25 tpy threshold. The
information required as part of an environmental evaluation is referenced in NAC 445B.311. Modeling
of sources is to include pollutants from both permitted emission units and emissions from insignificant
activities (e.g., emergency generators, small storage tanks, space heaters, etc.).
Modeling Threshold Requirements for Modification of an Existing Operating Permit: Note to Permit Applicants Exempt from Modeling Requirements: In order to expedite the internal decision process for a permit application, permit applicants who fall below
the modeling thresholds stated above may elect to provide the environmental evaluation to assist NDEP-BAPC in
processing the application. However, this is not required by NAC 445B.310 and is left as an
option for the applicant.
Selection of an Air Quality Model:As part of the modeling analysis, NDEP-BAPC will consider air quality models recommended and approved
by the U.S. EPA for regulatory determinations. The U.S. EPA maintains the Support Center for
Regulatory Air Models (SCRAM) which contains for public distribution the various regulatory air models,
meteorological inputs, and user’s guides at the following website address:
http://www.epa.gov/ttn/scram/
The reader is to note that the SCRAM website generally supports DOS-based model versions. For an
additional fee, Windows-based regulatory air models which incorporate the EPA’s source code are
available for purchase from various software vendors. It is to be noted that NDEP-BAPC cannot endorse
or recommend the purchase of any one specific regulatory model software package. However, it should
be noted that many modelers find that the Windows-based model versions are generally easier to work with
and are more intuitive than the DOS-based versions. In addition, the Windows-based vendor models
provide a Graphical-User-Interface (GUI) from which the modeler can view both input and output data.
ISCST3 Model: Generally, for most regulatory applications, NDEP-BAPC prefers and recommends that the applicant
generate their modeling analysis using the most recent update (e.g., either from EPA or the software
vendor) of the ISCST3 (Industrial Source Complex Short Term 3) dispersion model, which is available
in DOS-based format from SCRAM. The prospective modeling candidate will find several Windows-based
versions of the ISCST3 available for a fee from software vendors, as well. When any modeling
analysis is performed and submitted, NDEP-BAPC requests that all relevant file inputs and outputs be
provided in both electronic (e.g., diskette) and hardcopy formats.
Screening Models: SCRAM supports less-refined dispersion models than ISCST3, which are used for simple and fast
screening analyses. An example is the SCREEN3 model. However, it is to be noted that screening models such as SCREEN3 are intended for modeling a single
emissions source only (e.g., modeling of a facility with only one baghouse stack, for instance). Therefore,
for multi-emissions unit modeling scenarios (e.g., modeling of more than one emissions unit), NDEP-BAPC
recommends that the applicant generate a modeling analysis using a more refined application such as
ISCST3. In addition, since screening models such as SCREEN3 use a generic (default) meteorology data
set, these models generally tend to predict more conservative (e.g., screening techniques predict higher
concentrations) facility impacts than the more refined dispersion models such as ISCST3. Therefore, in
general, it will be more difficult for the permit applicant to show compliance with a screening technique than
a more refined approach.
Screening techniques do, however, have their utility in modeling applications. For example, when one must
assess the maximum distance from a facility’s fenceline in which to place modeling receptors, a model such
as SCREEN3 could be quickly run to determine the maximum ground-level impacts from a stack at various
distances away from the emissions source.
Modeling Protocol Content:As general guidance, NDEP-BAPC recommends that at a minimum, the modeling protocol submitted by
the applicant contains a brief discussion of the following items: Dispersion Model selection and version to be used (e.g., specify model such as ISCST3). The modeler is to note that since U.S. EPA periodically updates the model source codes on SCRAM, the software vendors also update their versions as well. Thus, it is the responsibility of the modeler to contact their software vendor, if a Windows-based version is used, to determine whether they are using the most recent update available.Air Pollutants to be modeled. Please note that NAC 445B.391 contains the Nevada and Federal ambient air quality standards for criteria pollutants that are to be modeled. Footnote A in this table states that the ambient concentrations must not be exceeded in areas where the general public has access, according to Nevada’s ambient air quality standards. Therefore, all modeling analyses submitted to NDEP-BAPC must report the highest, first-high concentration for each pollutant and averaging time as specified by NAC 445B.391. Please note that the Nevada ambient standards are based on a high, first-high basis which does not allow any exceedances of an ambient standard in areas in which the public has access. The Federal ambient standards are based on a high, second-high basis which therefore permits a maximum of one exceedance per year, except for ozone and those pollutants based on an annual averaging time (see footnote B in NAC 445B.391).Ozone (O3) Modeling - Ozone is a secondary pollutant formed in the atmosphere from a series of 3 complex photochemical reactions involving VOC, NOx, and other ozone precursor pollutants such as CO. Conservatively, ozone may be modeled by assuming 100% conversion of VOC pollutants into ozone emissions. This approach tends to overestimate ozone emissions because there will be a lag from the time of release of VOC from an emissions unit until the formation of ozone emissions in the atmosphere during daylight hours (e.g., photochemical reactions require sunlight as the driving mechanism). Alternatively, ozone emissions can be modeled using screening techniques such as the U.S. EPA
VOC/NOx Point Source Screening Tables, Richard D. Scheffe, September 1990. This document
may be obtained from either the U.S. EPA or the National Technical Information Service (NTIS),
which are both offices of the federal government. It is left to the permit applicant to determine how
they will address modeling issues if VOC pollutants, and subsequently ozone, will be emitted from
the facility. Emission Units and Source Types to be modeled. The list of emission units and insignificant activities to be modeled can be generated once the applicant has prepared the permit application forms and a spreadsheet (e.g., numerical tabulation) of all of the facility’s proposed emission units and insignificant activities that will be operated at the facility. The modeler is to note that all quantifiable emissions of regulated pollutants from insignificant activities must be modeled along with the permitted emission units.The emission source types to be modeled generally include the following:
Meteorological Data Set to be used. The applicant is to note that SCRAM has available for downloading surface meteorological data for the following locations in Nevada: Las Vegas, Reno, Ely, Elko, Winnemucca, Lovelock, Tonopah, and Desert Rock (Nevada Test Site). Upper air data is available in SCRAM for Winnemucca (e.g., Northern Nevada facilities) and Desert Rock (e.g., Southern Nevada facilities). NDEP-BAPC has pre-processed a number of these data sets for direct use in the ISCST3 model and can provide them upon request. In addition, NDEP-BAPC also has available for public distribution other non-SCRAM meteorological data sets which have been used previously in other permit determinations and represent source locations such as mining facilities, mineral processing plants, and electrical generating stations in Nevada that are located outside of Clark and Washoe Counties. If the SCRAM meteorological data sets available are not representative of your facility’s location, then please contact our office for further details on these additional meteorological data sets.Modeling Receptors to be used. NDEP-BAPC recommends that the protocol include a discussion of the placement of modeling receptors at the facility boundary (e.g., starting at the fence) and their extension out and into publicly accessible areas. NDEP-BAPC prefers that the resolution of initial receptors at the facility fenceline or property boundary be placed at intervals of not greater than 50 to 100 feet, if feasible, to adequately assess the protection of ambient standards where the highest impacts are expected to occur. The furthest extent of the receptors from the facility’s boundary is left to the applicant to discuss on a case-by-case basis. However, for those facilities which can have air quality impacts beyond the fence (e.g., tall stacks, combustion sources, etc.), an adequate number of receptors should be considered. The modeler is cautioned that modeling analyses that use too coarse of a receptor grid (e.g., a grid spacing of 250 meters or greater) may miss localized “hot spots” of high predicted pollutant concentrations. Therefore, it is best that a denser receptor grid be considered given that most modern personal computers (PCs) can readily process a modeling scenario with a hundred to a few thousand or more modeling receptors in under an hour. Also, please note that prior to issuing a final decision on a permit application, NDEP-BAPC will independently verify an applicant’s modeling receptor grid and predicted concentrations for accuracy and completeness. This may mean that modeling demonstrations received by our office with a “coarse” receptor grid network may have to be modeled internally to assess whether localized “hot spots” have been missed.For an ambient standards modeling analysis, the modeler should not present to NDEP-BAPC model
concentration results from any receptors that are “within the fenceline”, e.g., located in areas
accessible only to facility personnel or their authorized contractors. The purpose of an ambient
standards demonstration is to adequately demonstrate that ambient concentrations above the
regulatory limits will not be exceeded at publicly accessible areas. Generally, publicly accessible
areas include locations anywhere on or beyond (e.g., away from the facility) the fenceline. Publicly
accessible areas also include receptors located outside of a facility but on the property of an
adjacent and independently owned and controlled facility. For modeling purposes, if the
combination of your facility’s highest modeled impacts along with the appropriate background
levels exceeds an ambient standard level at public access points, then a modeling exceedance
exists. A facility that impacts into the airshed space of another neighboring facility cannot waive
their neighbor’s right for access to “clean” air (e.g., at or below ambient limits) even if their
neighbor’s facility is fenced. Therefore, a permitted facility should consider all areas outside of their
fenceline and/or property boundary limits as accessible to the public. If the facility does not have a fenceline to model, then the protocol should discuss what other
features are available to adequately restrict the public from areas in which the facility can have an
impact, e.g., property boundary limits owned, restricted, and/or under control-supervision of the
facility. BAPC will handle on a case-by-case basis those facilities located on State or Federal lands
(e.g., Bureau of Land Management (BLM)) that propose to use terrain features and/or mineral
rights as a hindrance to public access on lands that allow public recreational access such as fishing,
hunting, four-wheeling, and camping. Please keep in mind that since the ambient air quality
standards go down to averaging periods as short as 1-hour (e.g., ozone), even short-term exposure
to above-standards concentrations for the public may be grounds for denial of a proposed permit
action that fails to adequately protect the ambient standards at publicly accessible areas. Terrain to be modeled. Please indicate whether the terrain and receptors immediately surrounding the facility can be assumed relatively flat or are complex (e.g., terrain receptors located above stack (plume) height resulting in hill-side stack impacts, for instance). If complex terrain is to be used, please indicate where the terrain height receptor data has been obtained. Refined dispersion models such as ISCST3 support Digital Elevation Model (DEM) terrain data collected by the USGS.Operating Schedule to be considered. For sources that are proposed to operate 24 hours per day, no operating schedule adjustment in models such as the ISCST3 is necessary. However, for sources that are limited by permit constraints to a schedule of less than 24 hours day, it may be necessary to use an Hour of Day Emission Rate Factor (HROFDAY) in ISCST3 in order to show compliance with the ambient standards. In this fashion, the user can indicate through the use of a 0 to 100% scalar on how to account for both operational and non-operational hours in the dispersion model (e.g., model defaults to an emissions source being 100% operational in any hour period).For any hour that an emissions unit is to be operated, the emissions must be modeled at their
proposed emissions rate limit. The Hour of Day Emission Rate Factor is used to account for non-operational
hours. NDEP-BAPC does not consider the “down-scaling” of hourly emission limits
in the ISCST3 model as an appropriate method of accounting for non-operational hours.
Therefore, NDEP-BAPC will not accept modeling analyses that use “down-scaling” of permitted
emission limits in order to show compliance with the ambient standards. Building Downwash effects which should be considered. According to the ISC (Industrial Source Complex) User’s Guide available on SCRAM, for regulatory applications, a building is considered sufficiently close to a stack to cause building wake effects when the distance between the stack and the nearest part of the building is less than or equal to five times the lesser of the height or the projected width of the building. For modeling scenarios which meet these conditions, the user must use an appropriate algorithm to handle building wake effects on nearby stack (point) sources. In ISCST3, the Building Profile Input Program (BPIP) is the U.S. EPA algorithm used to calculate direction-specific building dimensions and GEP (Good Engineering Practice) stack height information. Modeling applications which will have building downwash effects on stack (point) sources must include and account for building dimensions (e.g., location, length, width, and roof height) and run the appropriate algorithms such as BPIP.Background Pollutant Concentrations should be considered, when the data relevant to a facility’s location and prevailing climate is available. Background concentration is the current level of ambient air pollution, external to the facility’s own impacts, this is the result of other point (industrial facilities), area (residential areas), and mobile (transportation) sources of pollution. Background concentrations will vary throughout the state ranging from relatively “pristine” up to levels that exceed local and National Ambient Air Quality Standards (NAAQS) in federally-designated Nonattainment Areas (e.g., presently as of this writing, certain locations within Clark and Washoe Counties).NDEP-BAPC and other regulatory agencies of Washoe and Clark Counties maintain and operate
air quality monitoring stations throughout Nevada. This data is available for public inspection in the
NDEP-BAPC Trends Report, which is published by the BAPC Monitoring Branch. This document
is available on the NDEP-BAPC website at: http://ndep.nv.gov/baqp/COVER00.htm The environmental evaluation must adequately demonstrate that the numerical addition of the
highest, first-high concentration of each pollutant (by its respective averaging time) predicted from
the facility impacts, along with consideration of appropriate background concentrations, will not
show an exceedance of any applicable ambient standards to which the public has access.
Therefore, the permit applicant must consider appropriate pollutant background from either a
monitoring station located on the facility property (e.g., for those facilities that run air quality
monitoring stations) or from a nearby NDEP-BAPC monitoring station, if applicable. For example,
if a permit applicant’s facility is located in or nearby Elko, Nevada, then the applicant should
consider using the results from the NDEP-BAPC PM10 monitoring station on Elko St. as
appropriate PM10 background for the Elko city area. For PM10 background, it is appropriate to use the NDEP-BAPC arithmetic mean results from a
PM10 monitoring station, taking note to exclude incomplete years of operation, if sufficient data
exists for complete years. The PM10 arithmetic mean results in the Trends Report are to be used
for both the 24-hour and annual average concentrations. For sources of particulate matter emissions that are located sufficiently distant from any PM10
monitoring station and other sources of air pollutant emissions (e.g., rural mining facilities), then as
general practice, BAPC will consider the use of a PM10 background measured at the Lehman Caves
- IMPROVE Site (White Pine County) in the mid-1980's. This PM10 background can be
considered representative of rural Nevada locations with relatively “pristine” (e.g., clean)
background conditions. For these rural facilities, the 24-hr PM10 background can be assumed to
be 10.2 µg/m3 , and the annual-average PM10 background can be assumed to be 9.0 µg/m3. For sources that will emit any amounts of SO2 , NOx , CO, VOC/O3 , or Pb, the applicant should
only propose the assumption of a “clean” (e.g., zero concentration) background for these pollutants
if nearby NDEP-BAPC/other monitoring stations’ data for these pollutants are unavailable. The
permit applicant can contact the NDEP-BAPC Monitoring Branch at (775) 687-9350 for further
questions regarding the Trends Report or to see what ambient background data is available in
his/her area in Nevada. Effects of Nearby Facilities are to be considered on a case-by-case basis. For instance, if a facility is located sufficiently far from any other stationary source of air pollutant emissions, then it is generally reasonable to assume background conditions using a nearby monitoring station. However, if two facilities are located on the same property (e.g., within the same fenceline) or are located adjacent to one another (e.g., neighboring facilities in an industrial park), then it may no longer be correct to assume a “general” background value for all receptor locations surrounding the facility. For these facilities, the more correct approach may be to simultaneously model both sets of emission units and insignificant activities from two or more co-located facilities. These modeling cases are handled on a case-by-case basis. Please contact the NDEP-BAPC Class I or Class II Permitting Branches at (775) 687-9350 for further information on whether our office has permit emissions data on a nearby facility that may be impacting the immediate ambient airshed around your facility.Modeling Summary Analysis - Please note that a complete environmental evaluation report should include a summary narrative and tabulation of the highest modeled values presented to demonstrate attainment of the applicable ambient standards. Therefore, simply attaching the model output without any written explanation does not constitute a complete environmental evaluation. Model input and output files also need to be documented electronically in diskette or other storage format (e.g., CD-ROM). If your environmental evaluation is lacking in this information, the permit application package may be deemed incomplete and returned in its entirety to you.Facility Plot Plan - To assist the modeler and the permit reviewer, each permit application package should submit a detailed facility plot plan, regardless of whether an environmental evaluation is required. The facility plot plan is a graphical depiction of the location of emission units, facility buildings, fenceline/property-line features, terrain topography, etc. The facility plot plan should be drawn to scale and indicate any relevant directional markings such as North and other features such as public roadways. A sample plot plan is shown to give the reader an illustration of what features should be included, at a minimum.Appendix |
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