11. Dispersion modelling

Dispersion models are used to predict how a contaminant concentration is diluted as it moves through the atmosphere. The concentration of a contaminant at a specific POI is a function of a variety of parameters including meteorological conditions in the vicinity of the source, POI, contaminant emission rate(s) and physical characteristics of the source and terrain in the vicinity of both the source and receptor. Atmospheric dispersion models use a combination of data inputs for these parameters in conjunction with mathematical algorithms that describe both the temporal and spatial variation of contaminants as they move away from the source.

Sections 6 through 17 of the Regulation contain the dispersion modelling requirements, including what models are to be used, required inputs into the models and the averaging periods that must be used for the models. References to an approved dispersion model means that sections 9 through 17 of the Regulation have been complied with. Section 17.1 of the Regulation states:

17.1 A person who is required under this Regulation to prepare or update a report in accordance with section 26 and who uses an approved dispersion model for that purpose shall comply with sections 9 to 17.

The ministry document “Air Dispersion Modelling Guideline for Ontario, (as amended) (ADMGO) provides details on the appropriate use of the approved dispersion models listed in the Regulation. This Chapter 11 Dispersion Modelling provides guidance on the inclusion, in an ESDM report, of dispersion modelling inputs and outputs. As per subparagraph 14 ii of subsection 26 (1) of the regulation, the Emission Summary Table should include the models used in the assessment including the version code for the model used.

11.1 Inclusion of dispersion modelling input data and output results in an ESDM report

Paragraphs 10 through 13 of subsection 26(1) of the Regulation describe the requirements for the inclusion of dispersion modelling results in an ESDM report as follows:

Under Subsection 26 (1) of the Regulation – Contents of ESDM report:

  1. A description of the local land use conditions, if meteorological data described in paragraph 2 of subsection 13 (1) was used when using an approved dispersion model for the purpose of this section.
  2. For each contaminant listed under paragraph 4, a statement identifying the approved dispersion model that was used for the purpose of this section and a description of the way in which the approved dispersion model was used that is sufficient to show compliance with sections 8 to 17.
  3. For each contaminant listed under paragraph 4, an electronic copy of the input files that were used with, and the output files that were produced by, the approved dispersion model that was used for the purpose of this section.
  4. A description of the terrain data that was employed when using an approved dispersion model for the purpose of this section, if section 16 required terrain data to be employed.

In addition, paragraph 14 of subsection 26 (1) of the Regulation requires submission of an Emission Summary Table and this requirement is discussed in Chapter 12 Emissions Summary Table And Interpretation Of Results of this Procedure Document.

11.1.1 Localized land use conditions

Paragraph 10 of subsection 26 (1) of the Regulation requires inclusion of a description of the local land use conditions that were used if, according to paragraph 2 of subsection 13

(1) of the Regulation, the meteorological data used in the AERMOD dispersion model has been refined to reflect local land use. Nevertheless, a description of the local land use conditions should be provided to support the choice of the regional meteorological data used in the AERMOD modelling.

Applying for an ECA

Applicants for an ECA are required to submit a Land Use Designation Plan as part of the supporting documentation for the noise assessment.

For more information see the ministry document, “Guide to Applying for an Environmental Compliance Approval”, as amended.

11.1.2 Guidance for describing the dispersion modelling

See Appendix D for a suggested tabulated format for providing the description, required under paragraph 11 of subsection 26 (1) of the Regulation, of the way in which the approved dispersion model was used that is sufficient to show compliance with sections 9 to 17 of the Regulation. Documenting compliance with sections 10, 11, and 12 was discussed in Chapter 8 Operating Conditions and 9 Emission Rate Estimating And Data Quality of this Procedure Document. The necessary descriptions for some of these sections may already have been included elsewhere in the ESDM report and can be simply referenced in a summary table. However, it is anticipated that it will be necessary to include, within the tabulation or a separate section of the ESDM report, descriptions of the way in which the approved dispersion model was used that is sufficient to show compliance with the following sections:

  • section 9 of the Regulation – Same Structure Contamination
  • section 13 of the Regulation – Meteorological Conditions
  • section 14 of the Regulation – Area of Modelling Coverage
  • section 15 of the Regulation – Stack Height for Certain New Sources of Contaminant
  • section 16 of the Regulation – Terrain Data
  • section 17 of the Regulation – Averaging Periods

The ADMGO provides further guidance on the above-noted sections of the Regulation. Also, see Table 11-1 Dispersion Modelling Input Summary Table: Guidance On Providing A Description Of The Way In Which The Approved Dispersion Model Was Used for additional guidance on satisfying the requirements of paragraph 11 of subsection 26 (1) of the Regulation.

Table 11-1: Dispersion Modelling Input Summary Table: Guidance on Providing a Description of the Way in Which the Approved Dispersion Model was Used
Relevant section of the Regulation Section Title Guidance to Providing a Tabulated Summary of How the Approved Dispersion Model was Used

section 6

Approved dispersion models

Include model version numbers for all models including: AERMOD and ASHRAE.
section 8 Negligible Sources of Contaminant If a source(s) was not considered in the application of the dispersion models, then it is sufficient to reference the portion of the ESDM report that relates to Chapter 7 Assessment Of The Significance Of Contaminants And Sources of this Procedure Document; which provides an explanation of how it was determined that the source(s) discharge a negligible amount of contaminant.
section 9 Same Structure Contamination If same structure contamination is applicable, then provide a description of how the methods described in the ASHRAE Handbook – HVAC Applications were combined with the dispersion modelling. See the ADMGO for more detailed guidance on the ASHRAE Handbook.
section 10 Operating Conditions Reference the portion of the ESDM report that relates to Chapter 8 Operating Conditions of this Procedure Document and describes the operating condition that results, for each significant contaminant, in the POI concentration for that contaminant.
section 11 Source of Contaminant Emission Rates Reference the portion of the ESDM report that relates to Chapters 9.0 Emission Rate Estimating And Data Quality of this Procedure Document (i.e., provides an explanation of the methods used to estimate contaminant emission rates and an assessment of how accurately the methods estimate the emission rate).
section 12 Combined Effect of Assumptions for Operating Conditions and Emission Rates If, based upon emission rate estimating methodologies that are consistent with paragraph 1 of subsection 11 (1) and operating conditions consistent with subsection 10 (1) of the Regulation, predictions of POI concentration exceed ministry POI Limits then describe either: (i) the refinement of emission rate estimates according to section 12 of the Regulation (also see Chapter 8 Operating Conditions and 9 Emission Rate Estimating And Data Quality of this Procedure Document) or (ii) the abatement plan, which is an option under section 12, and its submission to a provincial officer.
section 13 Meteorological Conditions If using ministry regional meteorological data sets or regional meteorological data with local land use conditions, then simply indicate the description of the local land use conditions; or if using site-specific meteorological data, then indicate the means in which the ministry Director has approved the use of this data and the AERMET model version used. See the ADMGO for more detailed guidance on the application of meteorological data.
section 14 Area of Modelling Coverage (receptor locations) If the area of the modeling coverage is applicable to the use of the approved dispersion models then it is sufficient to provide a statement or brief description of how the receptor grid pattern that was used in the modelling includes a spacing that is no greater than the largest grid interval allowed by section 14 of the Regulation and how the bounds of the grid are large enough to capture the maximum POI as required by section 14 of the Regulation. If using different modelling grid, then indicate the means in which the ministry Director has approved the use of this approach. See the ADMGO for more detailed guidance on receptor locations.
section 15 Stack Height for Certain New Sources of Contaminant If this section is applicable (i.e., to sources constructed after November 30, 2005), then it is sufficient to provide a summary of the actual stack heights of the relevant sources and the calculated or stipulated maximum height that can be used with the approved dispersion model. See the ADMGO for more information on Stack Height for Certain New Sources of Contaminant.
section 16 Terrain Data If terrain data is applicable to the use of the approved dispersion models, then it is sufficient to:
  • indicate whether there were any points of impingement that had an elevation higher than the lowest point from which a relevant contaminant is discharged from; and if so
  • provide a brief description of how terrain data was considered or employed and provide a copy of the terrain data that was used (if applicable).footnote 19
See the ADMGO for more detailed guidance on the application of terrain data.
section 17 Averaging Periods Provide an indication of how the averaging periods for the dispersion model outputs were consistent with the requirements of this section and/or how any averaging period conversions were consistent with the requirements of this section. See Table 7-1 Averaging Period Conversion Factor (F) of this Procedure Document for an equivalent method, to section 17 of the Regulation, in converting POI concentrations to different averaging periods. Also, see the ADMGO for more additional guidance on the application of averaging periods.

11.1.3 Guidance for providing input and output files

Paragraph 12 of subsection 26 (1) of the Regulation requires submission of an electronic copy of the input files that were used with the approved dispersion model and the output files that were produced by the use of the models. Please see the ADMGO for additional guidance on the topic of providing input and output files related to use of the approved dispersion models.

Self-contamination assessed using ASHRAE calculations should include a site drawing (including a side and top view). This should include Plan and/or Elevation views of the building showing the building height, length and width, in addition to source and receptor (intake) locations and distances. Source details and parameters such as the stack height, flow rate and diameter should also be provided. Full calculations (including the wind speed ranges searched) must be provided in an Appendix, or electronically via an Excel spreadsheet.

11.1.4 Guidance for providing terrain data

If terrain data was used during modelling, the data must be submitted as a part of the ESDM report. (s.16 (1) and s. 26 (1) 3). Please see ADMGO document for more detailed guidance on the use of terrain data with the approved dispersion models.

11.1.5 Dispersion modelling for standards with annual averages

When modelling for standards with annual averaging periods, using models such as AERMOD, the emission rate inputs to the model must reflect the maximum annual operating scenario (i.e. worst case year). The maximum annual POI concentration would be compared to the annual standard to assess compliance.

The model run would be conducted either with the appropriate five-year regional meteorological data set or a site-specific meteorological data set approved by the ministry under s.13 of the Regulation, if applicable. Because the hourly POI concentrations are converted to annual averages by AERMOD, It is not necessary to remove any meteorological data from the model run.

Earlier versions of the AERMOD dispersion model will average all the meteorological conditions over five years for each grid point to produce one maximum POI concentration based on the annual emission rate. Later versions of AERMOD (2015) do output the maximum POI for each meteorological year. If a proponent is using an earlier version of AERMOD, the ministry would prefer that proponents evaluate the maximum POI for each year. The maximum annual POI from those 5 years is then compared to the annual standard. This can be done by running each model year individually and evaluating the outputs to determine the maximum annual POI. For post 2015 versions of AERMOD, the maximum POI from the five years should be used to assess against the annual standard.

For the earlier versions of AERMOD, in order to avoid complex post-processing steps, it may also be acceptable to do one model run, using the appropriate five-year regional meteorological data set or a site-specific meteorological data set approved for that site under s.13 of the Regulation, if applicable. The maximum annual POI could then be multiplied by 140% and this number would be compared to the annual standard. This is intended to act as a conservative screening check against the annual standard. If 140% of the five year averaged POI model results is more than that of the annual standard, then individual model runs must be conducted for each of the five years of meteorological data or the site-specific meteorological data. The maximum POI from each of those individual years is then assessed against the annual standard.

Note: if the above approach involving an assessment of 140% of the modelled value is used, this must be highlighted in the Emission Summary Table with a note.

11.2 Roadways, railways and bodies of Water

POI concentrations must be below the relevant ministry POI Limit at all off-site locations (as per the definition of point of impingement in section 2 of the Regulation). However, there may be site-specific situations where an off-site location would not need to be considered in the ESDM report as a POI, such as:

  1. Most public roadways between two separate parcels of a facility property.
  2. Most railway lines or railway right-of-ways.
  3. Bodies of water that are inside the boundaries of Ontario, except situations where an adverse effect may occur (e.g., in the vicinity of a marina).

Footnotes

  • footnote[19] Back to paragraph The terrain data for Ontario has been provided by the Ontario Ministry of Natural Resources in a format that can be used to run the US EPA models. Terrain data is available from the ministry’s web-site and the ministry’s Public Information Centre (on compact disc).