Permit Guidance: Modelling Guidelines (Environmental Evaluation)

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, NO_x, 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/NO_x 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:

Point Sources (e.g., emissions from stacks, baghouses, chimneys, exhaust fan vents, tank vents, etc.). Volume Sources (e.g., process fugitive emission sources from conveyor transfer points, ore stockpiles, ore dump hoppers, rock screens, open building roof vents and ventilation windows, etc.). The user is to follow the appropriate modeling guidance to provide the initial dimensions of the volume of the release (e.g., and values). Area Sources (e.g., surface-based emissions from landfills, waste lagoons, roadway surfaces, etc.). Area sources can also be elevated such as the emissions from an open tank surface (e.g., cold-cleaning or dipping operation). Open Pit Sources (e.g., the applicant’s modeling protocol must also include a discussion on handling the particulate depositional characteristics when using a model such as ISCST3 since an open pit source incorporates some estimate of the deposition of emitted particulate back onto the pit floor).

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 , NO_x , 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.

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