This guide provides a comprehensive overview of air dispersion modelling in Ontario, focusing on compliance with Ontario Regulation 419/05: Air Pollution – Local Air Quality (“the Regulation”). It serves as A 11 Guide for navigating the complexities of air dispersion modelling, ensuring fair and consistent implementation of the Regulation. This document updates the previous Ontario Ministry of the Environment and Climate Change (the ministry) document PIBs # 516502 dated March 2009.
1.0 Introduction to Air Quality Regulations in Ontario
Ontario’s air quality regulation (O. Reg. 419/05) operates within the province’s air management framework. It aims to protect communities by regulating air contaminants released by various sources, including industrial and commercial facilities. The ministry regulates these contaminants to safeguard the health of communities near these sources. The goal is to limit substances released into the air that can negatively impact human health and the environment, ensuring that industries operate responsibly and transparently.
The Regulation outlines three compliance approaches for facilities to demonstrate environmental performance and implement improvements:
- Meeting air standards prescribed in the Regulation.
- Requesting and meeting a site-specific standard.
- Registering under and meeting the requirements of a technical standard (if available).
Provincial air standards are the baseline for assessing a facility’s contribution to air contamination. These standards are science-based, disregarding technological or economic factors. This can pose challenges for certain facilities facing unique limitations. In such instances, industries can improve their performance by requesting a site-specific standard or registering under a technical standard.
The Regulation sets limits on contaminant concentrations in the natural environment resulting from a facility’s emissions. These concentrations are calculated at “points of impingement” (POI), defined in Section 2 of the Regulation:
Points of Impingement
“2. (1) A reference in this Regulation to a point of impingement with respect to the discharge of a contaminant does not include any point that is located on the same property as the source of contaminant.
(2) Despite subsection (1), a reference in this Regulation to a point of impingement with respect to the discharge of a contaminant includes a point that is located on the same property as the source of contaminant, if that point is located on,
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a child care facility; or
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a structure, if the primary purpose of the property on which the structure is located, and of the structure, is to serve as,
i. a health care facility,
ii. a senior citizens’ residence or long-term care facility, or
iii. an educational facility.”
Facilities discharging contaminants into the air must ensure that the combined discharge concentration at any POI is below the standard prescribed in the Regulation. The ministry also uses a broader list of POI limits to prevent adverse effects from local air pollution sources.
Demonstrating compliance with air standards involves creating an Emission Summary and Dispersion Modelling (ESDM) report summarizing total property air emissions. These emissions are converted to POI concentrations using approved air dispersion models. Facilities can also use these models in conjunction with monitoring or measurement to determine compliance.
The “Procedure for Preparing an Emission Summary and Dispersion Modelling Report” (Procedure Document) guides the content of an ESDM report, as outlined in Section 26 of the Regulation. This Procedure Document should be used together with this “Air Dispersion Modelling Guideline for Ontario” (ADMGO), ministry documents, and technical bulletins. This guidance is intended for facilities using the prescribed air standards compliance approach, applying for an Environmental Compliance Approval (ECA), or requesting a site-specific standard.
1.1 Dispersion Models and Their Role
The Regulation mandates the use of approved dispersion models for assessing compliance with ministry POI Limits. Earlier versions of the regulation (Regulation 346) included suggested models in the Appendix. However, these are being phased out and replaced by models developed by the United States Environmental Protection Agency (US EPA).
The Regulation outlines a phased approach (2010-2020) for phasing out the models in the Appendix to Regulation 346, based on industrial sector.
Subsection 6 (1) of the Regulation lists the “approved dispersion models,” which include:
- The US EPA dispersion models: SCREEN3 and AERMOD.
- The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) method for Building Air Intake and Exhaust Design.
- The models in the Appendix to Regulation 346.
Note that ISC-Prime was an approved dispersion model until February 1, 2012.
These “approved dispersion models” are required when assessing compliance with the air standards in Schedules 2 and 3. Approved dispersion models are also used to assess conformance with ministry guideline values. However, the models in the Appendix to Regulation 346 are considered “approved” only if Section 19 applies to the facility’s discharges.
The models and standards work together as follows:
- Any approved dispersion model can be used to demonstrate compliance with the half-hour average air standards in Schedule 2 (after time averaging period conversion as per Section 17).
- Any approved dispersion model, except for those in the Appendix to Regulation 346, can be used to demonstrate compliance with the air standards in Schedule 3 (after time averaging period conversion as per Section 17).
Section 6 of the Regulation defines approved dispersion models for the purposes of the Regulation:
Approved Dispersion Models
6. (1) For the purposes of this Part, the following are approved dispersion models for discharges of a contaminant, except as otherwise provided:
- The AERMOD dispersion model made available on the Internet by the United States Environmental Protection Agency, as amended from time to time, or a copy of that model that is available from the ministry.
- The ASHRAE method of calculation.
- Revoked: O. Reg. 507/09, s. 4 (1).
- The SCREEN3 dispersion model made available on the Internet by the United States Environmental Protection Agency, as amended from time to time, or a copy of that model that is available from the ministry.
- The method of calculation required by the Appendix to Regulation 346, if section 19 applies to the discharges.
(2) The ministry shall make copies of the approved dispersion models referred to in paragraphs 1, 4 and 5 of subsection (1) available through a website maintained by the ministry on the Internet or through the ministry’s Public Information Centre.
The US EPA models are available on the US EPA website or at the ministry’s Public Information Center. These models are updated periodically, meaning that specific versions are considered “approved.” Under subsection 177 (6) of the Environmental Protection Act, amendments to documents incorporated by reference (e.g., AERMOD/AERMET/ASHRAE) take effect upon the ministry publishing a notice in either The Ontario Gazette or the environmental registry under the Environmental Bill of Rights (EBR), 1993. Modellers should refer to the ministry’s Rules on air quality and pollution website for information on air dispersion model versions to be used to demonstrate compliance with O. Reg. 419/05.
The ASHRAE method of calculation is copyrighted, requiring a license from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE Handbook – Building Air Intake and Exhaust Design).
1.2 Important Phase-In Dates for Compliance
The ministry continues to update air standards for regulating contaminants. Facilities operating under the air standards compliance approach must comply with new or revised standards by their posted phase-in dates or use alternative compliance approaches. Air dispersion models are the primary tool for assessing compliance with these standards. Newer models are being phased in by sector as the models in the Appendix to Regulation 346 are phased out.
Sections 19 and 20 of the Regulation detail the phase-out of the models in the Appendix to Regulation 346. The general schedule is as follows:
- Until February 1, 2020, facilities could use any model listed in Section 6, including those in the Appendix to Regulation 346, to assess compliance with the air standards in Schedule 2. However, facilities in Schedule 4 or Schedule 5 could only use SCREEN3 or AERMOD to assess compliance with Schedule 3 and had to use the ASHRAE method for same structure contamination.
- On or After February 1, 2020, all facilities can only use SCREEN3 or AERMOD to assess compliance with the air standards in Schedule 3 and must use the ASHRAE method for same structure contamination.
A ministry Director can issue a notice prior to the phase-in period to assess compliance with Schedule 3 standards using a newer US EPA model as per subsection 6 (1). However, facilities would not be required to comply with the Schedule 3 standards until their phase-in date.
Facilities not listed in Schedule 4 or 5 can request a “speed up” to Schedule 3 standards before the phase-in dates (as per subsection 20 (4)). This requires compliance with Schedule 3 standards only for the contaminants specified in the notice. Alternatively, the Director can issue an Order under subsection 20 (5) to require the Schedule 3 standards to apply. The facility would then be governed under similar rules of Schedule 4 or 5 for the contaminants specified in the Order.
The ASHRAE method can be used by any facility before the mandatory phase-in dates of the Schedule 3 standards to assess same structure contamination using Schedule 2 half-hour air standards. Similarly, SCREEN3 or AERMOD can be used to assess maximum POI concentrations using Schedule 2 half-hour air standards (see Section 17). If a facility wishes to use the Schedule 3 standards as their compliance point, they may request a notice from the Director under subsection 20 (4).
The models in the Appendix to Regulation 346 are available from the ministry’s rules on air quality and pollution website. Copies of SCREEN3 and AERMOD can be obtained from the US EPA’s dispersion modelling website and the ministry’s rules on air quality and pollution website or Public Information Centre. The ASHRAE method of calculation is copyrighted and a licence to use this method must be purchased from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE Handbook).
Key Phase-In Dates for Air Standards
New/updated air standards apply to all sectors, regardless of the models used to assess POI concentrations. Air standards are phased in over a period of time to allow industry to comply. Generally, new or updated standards are phased in over three to five years.
1.3 Specifying Dispersion Models: Director’s Authority
Section 7 of the Regulation allows the Director to add a model to the approved list for a specific facility if it is deemed at least as accurate as an approved model. Conversely, the Director can remove a model if it is less accurate.
Subsection 7 (1) allows the Director to require a person to use:
- A specific approved dispersion model.
- Another dispersion model (e.g., alternative model).
- A combination of models or a combination of models and monitoring.
Subsections 7 (1) to 7 (3) of the Regulation state:
Specified Dispersion Models
“7. (1) The Director may give written notice to a person who discharges or causes or permits the discharge of contaminants from a property stating that the Director is of the opinion that, with respect to discharges of a contaminant from that property,
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one or more dispersion models specified in the notice would predict concentrations of the contaminant at least as accurately as an approved dispersion model;
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a combination specified in the notice of two or more dispersion models would predict concentrations of the contaminant at least as accurately as an approved dispersion model;
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a combination specified in the notice of one or more dispersion models and one or more sampling and measuring techniques would predict concentrations of the contaminant at least as accurately as an approved dispersion model; or
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one or more approved dispersion models specified in the notice would predict concentrations of the contaminant less accurately than,
i. a dispersion model or combination specified under clause (a), (b) or (c), or
ii. another approved dispersion model.
(2) Before the Director gives a person a notice under subsection (1), the Director shall give the person a draft of the notice and an opportunity to make written submissions to the Director during the period that ends 30 days after the draft is given.
(3) If a notice is given under subsection (1) with respect to discharges of a contaminant from a property, a reference in this Part to an approved dispersion model shall be deemed, with respect to those discharges,
- to include a dispersion model or combination specified under clause (1) (a), (b) or (c); and
- not to include a dispersion model that is specified under clause (1) (d).”
For example, the Director could remove SCREEN3 and AERMOD and add CALPUFF to the list of approved dispersion models for a facility via written notice. Compliance with this notice would not be effective until Section 20 applied to the facility (see subsections 7 (8) and (9) of the Regulation).
Subsections 7 (5) to 7 (11) of the Regulation include requirements on how the specified dispersion models will be used in terms of timing; reference to “approved dispersion models”, ESDM reports and standards.
Specified Dispersion Models
“7. (5) Subsection (3) does not apply to a discharge of a contaminant until,
- three months after the notice is given under subsection (1), unless clause (b) applies; or
- one year after the notice is given under subsection (1), if the notice includes a notice under clause (1) (c).
(6) Subsection (5) does not apply for the purpose of preparing a report to which subsection 23 (3), 24 (2), 30 (5) or 32 (16) applies.
(7) Subsection (5) does not apply to a discharge of a contaminant if subsection (3) would have the effect of permitting the discharge.
(8) If a notice is given to a person under subsection (1) and section 20 does not apply to the person in respect of a contaminant, subsection (3) applies to the person in respect of the contaminant only after section 20 begins to apply to the person in respect of the contaminant.
(9) Despite subsection (8), if a notice is given to a person under subsection (1) and section 20 does not apply to the person in respect of a contaminant, subsection (3) applies to the person in respect of the contaminant for the purpose of preparing a report to which subsection 23 (3), 24 (2), 30 (5) or 32 (16) applies.
(10) The Director may, by written notice, revoke a notice given under subsection (1).
(11) Subsection (3) ceases to apply to discharges of the contaminant three months after the notice is given under subsection (10).”
Subsection 7 (5) states that the specified use of a dispersion model as an approved dispersion model is not applicable until three months (or one year if monitoring is involved) after the issuance of a written notice by the Director. Subsection 7 (7) clarifies that the three-month delay is only applicable if the facility is not in compliance with ministry POI Limits.
2.0 Approved Dispersion Models: An Overview
2.1 Understanding Dispersion Modelling
Air dispersion modelling is the mathematical evaluation of contaminant impacts from emission sources on air contaminant concentrations within a study area. Factors influencing the fate and transport of contaminants in the atmosphere include meteorological conditions, site configuration, emission release characteristics, and surrounding terrain.
2.2 Models in the Appendix to Regulation 346
These models consist of:
- The Scorer and Barrett model/equation for calculating concentrations at points of impingement on the same building or structure.
- The Virtual Source dispersion model for calculating point of impingement concentrations from stacks and vents generally less than twice the height of the building.
- The point source dispersion model for stacks generally greater than twice the height of the building.
The virtual source and point source models have been translated into the Regulation 346 Dispersion Modelling Package, available from the ministry’s rules on air quality and pollution website. This software searches through specified meteorological conditions to identify the condition resulting in the highest half-hour average concentration at a point of impingement. The program searches through all ground-level receptors off the facility’s property to find the maximum concentration. It also calculates the concentration at specific points of impingement, such as air intakes on building roofs. See Appendix C for instructions on using these models.
2.3 SCREEN3, AERMOD, and ASHRAE: Key Models in Use
Ontario’s approved dispersion models include SCREEN3 for screening analyses and AERMOD for sophisticated analyses. ASHRAE must be used to assess contamination of building air intakes on the same structure as the contaminant source. SCREEN3 or AERMOD are used for POI concentrations at receptors not on the same structure.
2.3.1 AERMOD
AERMOD (American Meteorological Society/EPA Regulatory Model Improvement Committee) is designed to support the US EPA’s regulatory modeling programs. It incorporates concepts like planetary boundary layer theory and advanced methods for handling complex terrain. The Plume Rise Model Enhancements (PRIME) building downwash algorithms are incorporated into AERMOD for more realistic downwash effects.
The PRIME algorithm incorporates:
- Enhanced plume dispersion coefficients due to the turbulent wake.
- Reduced plume rise due to descending streamlines and increased entrainment in the wake.
BPIP (Building Profile Input Program) is used to generate necessary PRIME downwash parameters for the AERMOD input file.
AERMOD is a steady-state, Gaussian plume model that is currently the US EPA’s regulatory air dispersion model. Improvements over previous models include:
- Incorporates non-Gaussian plume shapes where appropriate.
- Dispersion is a function of horizontal and vertical turbulence.
- Calculates effective wind speed through the plume.
- Allows use of measured turbulence in meteorological data preparation.
- Mixing heights are calculated using hourly meteorological data.
- More realistic treatment of terrain impacts.
- Plumes may partially penetrate the inversion at the mixing height.
- Impact of urban heat islands on turbulence is considered.
AERMOD capabilities include:
- Modeling contaminant emissions from many sources.
- Handling multiple source types (point, volume, area, open pit). Line sources can be modeled as volume or elongated area sources.
- Enabling emission rates to vary by month, season, hour-of-day, or other periods.
- Accounting for aerodynamic downwash due to nearby structures.
- Algorithms for modeling settling, removal (dry deposition), and precipitation scavenging (wet deposition).
- Receptor locations specified as gridded or discrete in Cartesian or polar coordinates.
- Requires two types of meteorological data files: hourly surface parameters and vertical profiles produced by the US EPA AERMET pre-processor program.
- For elevated terrain, a hill height scale and receptor elevation are required, generated by the US EPA AERMAP terrain pre-processor program.
- An urban option altering dispersion parameters due to the urban heat island effect.
- Outputs concentrations, total deposition flux, dry deposition flux, and wet deposition flux.
More details on AERMOD model formulations can be found in references 1, 2 and 3, as well as the US EPA Guidelines on Air Quality Models (Revised).
2.3.2 SCREEN3
The SCREEN3 model provides an easy-to-use method for estimating contaminant concentrations, based on the US EPA document “Screening Procedures for Estimating the Air Quality Impact of Stationary Sources.”
SCREEN3 can perform single-source short-term calculations, including:
- Estimating maximum hourly ground-level concentrations and the distance to the maximum.
- Incorporating building downwash effects.
- Estimating concentrations in the cavity recirculation zone.
- Estimating concentrations due to inversion break-up and shoreline fumigation.
- Determining plume rise for flare releases.
- Incorporating the effects of simple elevated terrain.
- Modeling simple area sources using numerical integration.
- Estimating 24-hour average concentrations in complex terrain using the VALLEY model 24-hour screening procedure.
2.3.3 ASHRAE for Same Structure Contamination
Improper stack design can lead to impacts beyond ground-level contamination. The interactions between sources and buildings can lead to emission re-entry into nearby buildings. It is important to ensure that emissions from rooftop sources do not impact air-handling units or other intakes on the same building or structure (same structure contamination).
The ASHRAE Handbook provides a methodology for proper stack design to avoid re-entrainment of contaminants. The Chapter titled “Building Air Intake and Exhaust Design” in the ASHRAE Applications Handbook provides analytical approaches for determining impacts on receptors for various configurations, including:
- Strong Jets in Flow Recirculation Cavity
- Strong Jets on Multi-winged Buildings
- Exhausts with Zero Stack Height (flush vents)
Same structure contamination is especially important in industrial parks, institutional settings, or multi-unit commercial complexes where emissions can impact neighboring units through air intakes. The ASHRAE model is for use when the point of impingement is on the same structure as the source. A point of impingement (POI) includes: (a) any point off-site; and (b) any point on-site that is (i) on a child care facility; or (ii) on a structure that serves primarily as a health care facility, a senior citizens’ residence or long-term care facility, or an educational facility; and (c) any point on the same structure as the source of a contaminant that does not belong to the facility. In situations where there are multiple structures located in close proximity to one another, contact the ministry (EMRB) for additional guidance regarding the modelling approach. In general, structures located within 5 meters of one another would be considered to be the “same structure” for the purposes of air dispersion modelling.
Section 6 of the Regulation lists the ASHRAE method of calculation as an approved dispersion model for “same structure contamination.” Section 9 of the Regulation states:
Same Structure Contamination
9. (1) The following approved dispersion models are the only approved dispersion models that may be used for the purposes of this Part with respect to the contaminant and a point of impingement that is located on the same structure as the source of contaminant:
- The ASHRAE method of calculation.
- A dispersion model or combination of dispersion models that, pursuant to subsection 7 (3), is deemed to be included in references in this Part to approved dispersion models, if the notice given under subsection 7 (1) states that the Director is of the opinion that the dispersion model or combination of dispersion models would predict concentrations of the contaminant at least as accurately as the ASHRAE method of calculation.
- The method of calculation required by the Appendix to Regulation 346, if section 19 applies to discharges of the contaminant.
(2) The ASHRAE method of calculation may be used for the purposes of this Part with respect to a contaminant only with respect to a point of impingement that is located on the same structure as the source of contaminant.
When points of impingement are located on the same structure as the emission source, a person may choose to use ASHRAE (with appropriate time averaging under Section 17) or the Scorer and Barrett calculation in the Appendix to Regulation 346. However, when Section 20 begins to apply, a person must use either ASHRAE or an alternative model specified by the Director in a notice issued under Section 7 of the Regulation.
The ASHRAE method can determine concentrations for a range of averaging periods. Where ASHRAE cannot accommodate the averaging period of the standard, then the 1-hour concentration should be determined and converted to the appropriate averaging period using the equations in Section 17 of the Regulation.
The emission rate used in the model must correspond to the averaging period of the ministry POI limit in accordance with Section 11 of the Regulation. Modellers assessing same structure contamination must also assess against the ministry POI Limits at other off-property POI locations using an appropriate approved dispersion model from Section 6 of the Regulation. Chapter 8.1 contains additional guidance on ASHRAE.
2.4 Alternative Models: Expanding the Toolkit
There are situations where the Director may specify the use of a model not listed in subsection 6 (1) of the Regulation, considered an “alternative model” under Section 7. Once a notice is issued, these models may be used instead of or to complement the ministry’s approved models. Alternative models currently considered by the ministry on a case-by-case basis include:
- CALPUFF
- CAL3QHCR
- SDM – Shoreline Dispersion Model
- Physical or Wind Tunnel Modelling
- AERSCREEN
The ministry may consider other models if the modeller demonstrates that the alternative dispersion model is at least as accurate as an approved dispersion model. The Director may authorize the use of an alternative model by issuing a notice, if of the opinion that the specified model would be at least as accurate as an approved dispersion model, in accordance with Section 7 of the Regulation.
3.0 A Tiered Approach to Compliance
Section 6 of the Regulation allows the use of the models in the Appendix to Regulation 346, SCREEN3, or AERMOD to assess compliance with Schedule 2 standards. Section 6 of the Regulation also sets out that any of the US EPA approved dispersion models (SCREEN3 or AERMOD) can be used to assess compliance with the standards in Schedule 3 (subject to the section 7 of the Regulation – Specified Dispersion Models).
The SCREEN3 model is simpler and generally more “conservative” in assessing point of impingement concentrations than AERMOD. Therefore, initial calculations are often performed using SCREEN3.
AERMOD uses more refined input data for parameters like meteorology, land use, and terrain. However, simplified data inputs can be used when they result in conservative assessments. A modelling assessment may consist of a series of steps, referred to as a tiered approach.
Tiered Modelling Approach
This approach focuses the required level of effort according to site requirements. Any of the three tiers may be performed, and linear progression is not necessary. More sophisticated techniques are needed when successive screens show an exceedance of ministry POI Limits. Successive screens combined with improved data quality as required by section 12 of the Regulation and in the ESDM Procedure Document, means that the last tier would require the most sophisticated modelling and emission estimating techniques, and would be the most representative of actual on-site conditions and contaminant concentrations.
A Tier 1 assessment represents the most conservative assumptions. To complete a Tier 1 assessment, a modeller shall use the SCREEN3 model.
Tier 2 and 3 assessments represent more precise modelling inputs and scenarios. The most significant difference between Tier 2 and 3 assessments is the use of more site-specific or local meteorological data inputs. To complete a Tier 2 assessment, the modeller shall use the pre-processed regional meteorological data made available by the ministry. A Tier 3 assessment requires the use of more site-specific meteorological data approved by the Director. In Tier 3 assessments, the ministry may also require alternative models that may be more representative of local conditions and produce more representative concentrations for specific contaminants.
Tier 2 and Tier 3 modelling assessments calculate results for each hour of the meteorological data set, which allows for specifying emission rates appropriate to the hour of the day and day of the week for the hourly average, 24-hour average and annual average derivations (i.e., a variable emission rate scenario).
Any modeller may choose to go directly to the more sophisticated modelling techniques (Tier 2 or 3) and skip the initial Tier 1 of the screening methods. In some situations, the ministry may require a modeller to proceed to Tier 2 or 3 directly.
3.1 Tier 1 Modelling: Screening with SCREEN3
Tier 1 is a screening level analysis and shall be completed a screening model such as the US EPA SCREEN3 model, which requires no input of real meteorological data as it includes all potential worst case meteorological conditions. If a Tier 1 conservative modelling assessment demonstrates compliance with ministry POI Limits, and emission estimates are in accordance with the guidance outlined in the ESDM Procedure Document, there is no need for additional modelling.
3.1.1 Using Conservative Dispersion Factors
Conservative dispersion factors can be used to conservatively screen out contaminants and/or sources with insignificant (negligible) emissions from further modeling analyses, consistent with section 8 of the Regulation.
A series of conservative dispersion factors (in micrograms per cubic meter per gram per second emission) have been developed using SCREEN3 for a short stack on a 6 meter tall building in combination with distances from the stack. These should be used to assess negligibility in accordance with the approach outlined in the ESDM Procedure Document.
The 1-hour concentrations can be converted for comparison to ministry POI Limits with different averaging periods in accordance with the method outlined section 17 of the Regulation.
| Distance (m) | Urban Dispersion Factor (µg/m3 per g/s emission) | Rural Dispersion Factor (µg/m3 per g/s emission) |
|---|---|---|
| Up to 20 | 8700 | 10000 |
| 40 | 6300 | 8100 |
| 60 | 4600 | 5900 |
| 80 | 3400 | 5100 |
| 100 | 2600 | 4500 |
| 150 | 1400 | 3500 |
| 200 | 900 | 2800 |
| 250 | 600 | 2300 |
| 300 | 450 | 1900 |
| 350 | 350 | 1700 |
| 400 | 300 | 1500 |
| 450 | 250 | 1300 |
| 500 | 200 | 1150 |
| 600 | 150 | 950 |
| 700 | 120 | 800 |
| 800 | 90 | 650 |
| 900 | 80 | 575 |
| 1000 | 70 | 500 |
3.1.2 Modelling Multiple Sources with SCREEN3
SCREEN3 performs single source calculations to determine maximum 1-hour average concentrations downwind of the source. SCREEN3 can be applied to multi-source facilities by conservatively summing the maximum concentrations for the individual emission sources.
To simplify the modelling when there are many release points on a facility, a modeller may choose to:
- combine individual stacks/vents into a single stack where the stack parameters are selected to generate a conservative dispersion factor; or
- combine individual sources into area or volume sources where the size and locations of the sources are conservatively selected.
Information on which sources can be grouped and how this can be done conservatively is given in Chapter 4.5.2. Combing Individual Sources into Volume, Area and Single Point Sources. The approach of combining sources may also be used in Tier 2 and Tier 3 modelling as long as the source characteristics selected are reasonably conservative.
3.2 Tier 2 Modelling: Refining the Analysis with Regional Data
Tier 2 assessments use more advanced approved dispersion models, such as AERMOD, and regional meteorological data sets provided by the ministry. These data sets are available on the ministry’s Map: Regional Meteorological and Terrain Data for Air Dispersion Modelling website. Alternately, the meteorological data sets may also be obtained through the ministry’s Public Information Centre: 1st floor, 135 St. Clair Avenue West, Toronto, ON M4V 1P5, 416-325-4000, 1-800-565-4923.
This is consistent with subsection 13 (1) of the Regulation.
The available files include the pre-processed AERMOD-ready regional data sets in addition to the raw AERMET-ready surface and upper air files. Local land use conditions can be incorporated by using these AERMET-ready surface and upper air files to produce AERMOD-ready files. The dispersion modelling assessment should be completed with the most appropriate regional data set for the location and surrounding land use of the facility and compared to ministry POI Limits for the appropriate averaging period to demonstrate compliance.
Note: As set out in paragraph 2 of subsection 13 (1), it is possible to refine regional meteorological data sets by specifying local land use conditions when using AERMOD. This step would still be considered a part of a Tier 2 assessment, however, the AERMET model inputs must be clearly articulated and documented in the Emission Summary and Dispersion Modelling Report (ESDM Report). Modellers should consult EMRB to confirm that representative surface parameters have been selected and AERMET is used appropriately.
3.3 Tier 3 Modelling: Site-Specific Data and Advanced Techniques
For situations where ministry POI limits are not met in a Tier 2 assessment, or the regional meteorological data set is not representative of the conditions at the facility being modelled, more precise analysis with locally representative meteorological data or advanced modelling (Tier 3) shall be used to assess contaminant concentrations. Alternative models may be required under section 7 of the Regulation. A Tier 3 assessment would consider the use of alternative models if a notice is issued by the Director under section 7 of the Regulation. Paragraphs 3 and 4 of subsection 13 (1) also set out the requirements for more site-specific meteorological data and subsections 13 (2) and 13 (3)set out a notice provision where the Director may specify the type of meteorological data to be used.
For geographical locations with unique local meteorology, appropriate local meteorological data sets may be obtained from the ministry. Local meteorological data can be developed by the modeller, but all local or site-specific meteorological data must be approved by the ministry. Site-specific meteorological data can be generated using:
- Observations (e.g., Environment Canada, National Oceanic and Atmospheric Administration observations) for a representative location near the facility, and upper air data sets, available from the ministry. This data is processed through AERMET along with local land use characteristics to produce the meteorological input files.
- Using on-site data for advanced meteorological modeling using Weather Research and Forcasting Model (WRF), CALMET or other models, the ministry or the modeller can prepare more detailed and site-specific data files. In this case, the ministry requests that a plan be submitted for review in advance of the meteorological modelling. Upon acceptance of the plan, the resulting meteorological data set and supporting documentation would be submitted to the ministry for review and approval under subsection 13 (1) of the Regulation.
In either case, it is strongly recommended that any dispersion modelling should not commence until ministry approval of the data is granted under section 13 of the Regulation. A form is available on the ministry website to request approval of site-specific meteorological data [“Request for Approval under subsection 13 (1) of Regulation 419/05 for use of site-specific meteorological data” (PIBs
