Guideline A-11: Air Dispersion Modelling Guideline for Ontario

Version 3.0

PIBs # 5165e03

Foreword

The “Air Dispersion Modelling Guideline for Ontario”, (the ADMGO Document), July 2016 version 3, provides guidance on complying with the dispersion model requirements of Ontario Regulation 419/05: Air Pollution – Local Air Quality (“the Regulation”).

This guideline is intended to provide guidance to ensure the 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.

The ADMGO Document is intended to provide guidance to ensure the fair and consistent implementation of the Regulation and to be used in combination with the ministry document, “Procedure for Preparing an Emission Summary and Dispersion Modelling Report, version 4, 2016”.

The ministry may periodically publish a list of questions and answers to assist in the interpretation of the ADMGO Document. The contents of this document may also be updated from time to time based upon public consultation consistent with the Ontario Environmental Bill of Rights legislation. All web site addresses referred to in this document were current at the time of release.

While every effort has been made to ensure the accuracy of the information contained in this ADMGO Document, it should not be construed as legal advice. In the event of conflict with requirements identified in the Regulation, then the regulatory requirements shall determine the appropriate approach.

For further details or to obtain copies of the approved dispersion models, regional meteorological files, or terrain data please contact:

Ministry of the Environment and Climate Change’s Public Information Centre
135 St. Clair Avenue West, 1^st floor, Toronto, ON M4V 1P5

• Tel: 416-325-4000
• Toll-free: 1-800-565-4923

Website: Ministry of the Environment and Climate Change - Rules on air quality and pollution.

For any addenda or revisions to this guide please visit the Ministry of the Environment and Climate Change website. Information may also be obtained from:

Ontario Ministry of the Environment and Climate Change
Air Monitoring and Transboundary Sciences Section
Environmental Monitoring and Reporting Branch (EMRB)
125 Resources Rd, Toronto, ON M9P 3V6
General Inquiries: 416-235-6171

1.0 Introduction

Ontario’s local air quality regulation (O. Reg.: Air Pollution – Local Air Quality or the “Regulation”) works within the province’s air management framework by regulating air contaminants released into communities by various sources, including local industrial and commercial facilities^{footnote 1[1]} The Ontario Ministry of the Environment and Climate Change (the ministry) regulates contaminants in air to protect communities who live close to these sources. The Regulation aims to limit substances released into the air that can affect human health and the environment, and requires industry to operate responsibly under a set of rules that is publicly transparent.

The Regulation includes three compliance approaches for regulated facilities to demonstrate environmental performance, and make improvements when required. These compliance approaches include meeting air standards prescribed in the Regulation, requesting, receiving and meeting a site-specific standard or registering under and meeting the requirements of a technical standard (if available). All three compliance approaches are allowable under the Regulation.

Provincial air standards are used to assess a facility’s individual contribution of contaminants to air. They are set based solely on science without taking into account technological or economic factors. As such some regulated facilities and industry sectors may not be able to meet some standards due to unique technical or economic limitations. In these cases, industries or sectors look to technology and best practices to improve their environmental performance and comply with the Regulation by either requesting a site-specific standard or registering under a technical standard (if available).

The Regulation places limits on the concentration of contaminants in the natural environment that are caused by emissions from a facility. The concentrations in the natural environment are calculated at a location referred to as a “point of impingement” which is defined in section 2 of the Regulation, as follows:

The Regulation requires that where a facility discharges a contaminant into the air from one or more sources, the concentration at any point of impingement (POI) resulting from that combined discharge must be less than the standard prescribed in the Regulation. The ministry also uses a broader list of point of impingement limits (ministry POI Limits)^{footnote 2[2]} and other screening tools,^{footnote 4[4]} to assist in preventing adverse effects that may be caused by local sources of air pollution.

Demonstration of compliance with the air standards compliance approach under the Regulation begins with development of an Emission Summary and Dispersion Modelling (ESDM^{footnote 5[5]}) report that includes a summary of total property air emissions. These emissions are then converted to POI concentrations using approved mathematical air dispersion models. In addition, a facility may use an approved air dispersion model in combination with monitoring or measurement to determine compliance.

The “Procedure for Preparing an Emission Summary and Dispersion Modelling Report” (Procedure Document) provides guidance on complying with the requirements of the Regulation that govern the content of an ESDM report, that are set out in section 26 of the Regulation. The Procedure Document should be used in conjunction with this “Air Dispersion Modelling Guideline for Ontario” (ADMGO) (as amended from time to time), other ministry documents, and associated technical bulletins. The guidance in these documents is primarily intended for facilities that are operating under the prescribed air standards compliance approach or are required to do an ESDM report because they are applying for an Environmental Compliance Approval (ECA) or requesting a site-specific standard.

1.1 Application of the Dispersion Models in the Regulation

The Regulation requires the use of specified approved dispersion models to assess compliance with ministry POI Limits when using the air standards compliance approach. Earlier versions of the regulation (previously known as Regulation 346 – General Air Pollution Regulation) included a set of suggested air dispersion models in the Appendix to Regulation 346. The models in the Appendix to Regulation 346 are being phased out and replaced with new air dispersion models developed by the United States Environmental Protection Agency (US EPA).

The Regulation provides a staggered phase-out (between 2010 and 2020) of the models in the Appendix to Regulation 346^{footnote 6[6]}, according to a schedule that varies by industrial sector (defined by the North American Industry Classification System (NAICS)).

Subsection 6 (1) of the Regulation lists the “approved dispersion models” which include:

1. the US EPA dispersion models: SCREEN3 and AERMOD;
2. the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) method of calculations for Building Air Intake and Exhaust Design (see section 9 of the Regulation, Same Structure Contamination); and
3. the models in the Appendix to Regulation 346.

Please note that ISC-Prime was an approved dispersion model until February 1, 2012.

The “approved dispersion models” in the Regulation are required to be used when assessing compliance with the air standards in Schedules 2 and 3^{footnote 7[7]} (in accordance with sections 19 and 20 of the Regulation). Approved dispersion models are also used to assess conformance with ministry guideline values. Note, however, that the models in the Appendix to Regulation 346 are only considered to be an “approved dispersion model” if section 19 applies to discharges from the facility.

The models work in conjunction with the standards as follows:

• To demonstrate compliance with the half-hour average air standards listed in Schedule 2 of the Regulation, any of the approved dispersion models may be used (after appropriate conversion of time averaging periods is made as per section 17 of the Regulation, if necessary).
• To demonstrate compliance with the air standards listed in Schedule 3 of the Regulation, any of the approved dispersion models, except for the models in the Appendix to Regulation 346, may be used (after appropriate conversion of time averaging periods is made as per section 17 of the Regulation, if necessary).

Section 6 of the Regulation defines approved dispersion models for the purposes of parts of the Regulation as follows:

The US EPA models referred to in section 6 of the Regulation are available on the US EPA website or at the ministry Public Information Center. As noted in the Regulation, these models are updated from time to time. This means that there is a specific version of each of these models that is considered to be an “approved dispersion model”. Under subsection 177 (6) of the Environmental Protection Act, the adoption of an amendment to a document that has been incorporated by reference (e.g. AERMOD/AERMET/ASHRAE), comes into effect once the ministry publishes a notice of the amendment 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 and a licence to use this method must be purchased from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE Handbook - Building Air Intake and Exhaust Design).

1.2 Key Phase-In Dates

The ministry continues to develop and update air standards to regulate contaminants. All facilities operating under the air standards compliance approach are required to comply with any new or revised air standards by the posted phase-in dates or follow one of the other compliance approaches (e.g. site-specific standards or technical standards). Compliance with air standards is primarily assessed using air dispersion models. Newer air dispersion models are being phased in by sector as the models in the Appendix to Regulation 346 are being phased out.

The staged phase-out of the models in the Appendix to Regulation 346 is set out in sections 19 and 20 of the Regulation. In general, the phase-out is as described below:

• Until February 1, 2020, facilities may use any of the models listed in section 6 of the Regulation, including the models in the Appendix to Regulation 346, to assess compliance with the air standards in Schedule 2. However, facilities within the sectors listed in Schedule 4^{footnote 8[8]} or Schedule 5 may only choose from SCREEN3 or AERMOD to assess compliance with the air standards in Schedule 3 and must assess same structure contamination using the ASHRAE method.
• On or After February 1, 2020, all facilities may only choose from SCREEN3 or AERMOD to assess compliance with the air standards in Schedule 3. All facilities must also use the ASHRAE method to assess same structure contamination.

A notice can be issued by a ministry Director prior to the phase-in period to assess compliance with Schedule 3 standards, using a newer US EPA model set out in subsection 6 (1) (please refer to subsection 24 (2) of the Regulation). However, such facilities would not be required to comply with the Schedule 3 standards until their phase-in date.

If a facility is not in a sector listed in Schedule 4 or 5 they can request a “speed up” to Schedule 3 standards in advance of the phase-in dates (as per subsection 20 (4)). This requires them to comply with Schedule 3 standards only for the individual contaminants that are set out in the notice. Alternately, the Director can issue an Order under subsection 20 (5) of the Regulation to require the Schedule 3 standards to apply. That facility would then be governed under similar rules of Schedule 4 or 5 for the contaminants set out in the Order.

Note that the ASHRAE method may be used by any facility in advance of the mandatory phase-in dates of the Schedule 3 standards to assess same structure contamination using Schedule 2 half-hour air standards. Similarly, the SCREEN3 or AERMOD models may be used in advance of the mandatory phase-in dates to assess maximum POI concentrations using Schedule 2 half-hour air standards (see section 17 of the Regulation). If a facility wishes to use the Schedule 3 standards as their compliance point, they may request that the Director issue a notice under subsection 20 (4) of the Regulation, as described earlier.

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 as well as through 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 so that industry can take actions needed to comply with the regulation. Generally, new or updated air standards are phased in over a three to five-year period.

1.3 Specified Dispersion Models

Section 7 of the Regulation generally states that where a model is at least as accurate for that facility as an approved dispersion model, the Director may add it to the list of approved dispersion models for that facility. Conversely, where a model is less accurate than an approved dispersion model (or another model that the facility has been notified to use under section 7), the Director may remove it from the list of approved dispersion models for that facility.

In summary, subsection 7 (1) allows that where the Director feels that a model may predict more accurately than another, the Director may require a person to use:

• a specific approved dispersion model;
• another dispersion model (e.g. alternative model); or
• a combination of models or a combination of models and monitoring.

Specifically, subsections 7 (1) to 7 (3) of the Regulation state:

For example, in the appropriate circumstances, the Director may remove SCREEN3 and AERMOD and add CALPUFF to the list of approved dispersion models for a facility. Accordingly, the facility would then be required to assess compliance using only CALPUFF. A ministry Director would implement the above, by written notice. Note, however, that such a notice would not be effective for compliance purposes until section 20 began applying to the facility (see subsections 7 (8) and (9) of the Regulation). As outlined previously, the application of section 20 can be “sped up” if the facility requests a notice under subsection 20 (4) or if the Director orders it under subsection 20 (5).

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.

Subsection 7 (5) of the Regulation 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, before the required use of the specified model, is only applicable if the facility is not in compliance with ministry POI Limits (i.e. it is not applicable if the specified model would enable the facility to demonstrate compliance with the standard).

2.0 Overview of the Approved Dispersion Models

2.1 Modelling Overview

Air dispersion modelling is the mathematical assessment of contaminant impacts from emissions sources on air contaminant concentrations within a study area. Factors that impact the fate and transport of contaminants in the atmosphere include meteorological conditions, site configuration, emission release characteristics and surrounding terrain, amongst others.

2.2 Overview of the Models in the Appendix to Regulation 346

The models in the Appendix to Regulation 346 consist of the following three models:

• The Scorer and Barrett model/equation for use in calculating concentrations at points of impingement on the same building or structure and up to 5 metres horizontally from the building or structure on which the point of emission is located.
• The Virtual Source dispersion model for use in calculating point of impingement concentrations from stacks and vents that are generally less than twice the height, above grade, of the building that the stack or vent is on.
• The point source dispersion model for use with stacks that are generally greater than twice the height, above grade, of the building that the stack or vent is on.

The virtual source and the point source models have been translated into a software program known as the Regulation 346 Dispersion Modelling Package available from the ministry’s rules on air quality and pollution website. This software package has been set up to search through the slightly unstable and neutral meteorological conditions^{footnote 9[9]} specified in the Appendix to Regulation 346 to identify the meteorological condition which will give the highest half-hour average concentration at a point of impingement. The program is designed to search through all ground-level receptors^{footnote 10[10]} off the facility’s property to find the maximum half-hour average concentration. In addition, the Regulation 346 Dispersion Modelling Package can calculate the concentration at specific points of impingement, such as air intakes on the roofs of nearby buildings or impingement on the sides or roof of an apartment building. See Appendix C for instructions on the use of the models in the Appendix to Regulation 346.

2.3 Overview of SCREEN3, AERMOD and ASHRAE

Ontario’s approved dispersion models include SCREEN3 for screening analyses and AERMOD for more sophisticated modelling analyses. In addition, the ASHRAE model must be used as necessary to assess potential for contamination of building air intakes that are located on the same structure as the source of the contaminant (see section 9 of the Regulation). SCREEN3 or AERMOD are used for assessment of POI concentrations at receptors that are not located on the same structure as the source of contaminant. A brief overview of each of these models can be found in the following subsections. For appropriate model selection, please review Chapter 2.3.1 – 2.3.3, as appropriate that outline the details of the following air dispersion models:

• AERMOD
• SCREEN3
• ASHRAE (same structure contamination)

2.3.1 AERMOD

The AERMIC (American Meteorological Society/EPA Regulatory Model Improvement Committee) Regulatory Model, AERMOD^(1,2,3,4) was specially designed to support the US EPA's regulatory modelling programs. AERMOD was developed to replace the Industrial Source Complex Model-Short Term (ISCST3) as US EPA's approved model for most small-scale regulatory applications^(5,6). It incorporates concepts such as planetary boundary layer theory and advanced methods for handling complex terrain. The Plume Rise Model Enhancements (PRIME)⁽⁷⁾ building downwash algorithms were incorporated into AERMOD as well. This provides a more realistic treatment of downwash effects than previous approaches.

The PRIME algorithm was designed to incorporate two fundamental features associated with building downwash:

• Enhanced plume dispersion coefficients due to the turbulent wake.
• Reduced plume rise caused by a combination of the descending streamlines in the lee of the building and the increased entrainment in the wake.

BPIP (Building Profile Input Program) must be used to generate the necessary PRIME downwash parameters which then form part of the input file for AERMOD. For more guidance on how buildings are defined, refer to section 4.6.2.

AERMOD is a steady state, Gaussian plume model that is currently the US EPA's regulatory air dispersion model. Some of the significant improvements in AERMOD compared to previous US EPA regulatory models, such as the Industrial Source Complex (ISC) model, are:

• incorporates non-Gaussian plume shapes where appropriate;
• dispersion is a function of horizontal and vertical turbulence that varies with height, and uses continuous growth functions rather than discrete stability parameters;
• calculates effective wind speed through the plume;
• allows the use of measured turbulence in meteorological data preparation;
• mixing heights are calculated using hourly meteorological data rather than interpolating from morning and evening estimates;
• more realistic treatment of terrain impacts;
• plumes may partially penetrate the inversion at the mixing height; and
• impact of urban heat islands on turbulence is taken into account.

Some of the AERMOD modelling capabilities are that the model:

• may be used to model contaminant emissions from many sources;
• can handle multiple source types, including point, volume, area and open pit sources. Line sources may also be modelled as a string of volume sources or as elongated area sources;
• enables emission rates to be treated as constant or varied by month, season, hour-of-day, or other optional periods of variation. These variable emission rate factors may be specified for a single source or a group of sources;
• can account for the effects of aerodynamic downwash due to nearby structures on point source emissions;
• contains algorithms for modelling the effects of settling and removal (through dry deposition) of large particles and for modelling the effects of precipitation scavenging (through wet deposition) for gases or particulates;
• has receptor locations that can be specified as gridded or discrete receptors in a Cartesian or polar coordinate system;
• AERMOD requires two types of meteorological data files, a file containing hourly surface parameters and a file containing vertical profiles. These two files are produced by the US EPA AERMET meteorological pre-processor program⁽⁸⁾;
• for applications involving elevated terrain, a hill height scale along with receptor elevation is required at each receptor. The US EPA AERMAP terrain pre-processor program^(9,10) can be used to generate hill height scales, as well as terrain elevations for all receptor locations;
• an urban option which results in altering dispersion parameters due to the urban heat island effect is included. Factors that affect the selection of the urban option include the population/urban intensity and the location of a facility relative to the urban core. The urban population is an input to this option; and
• yields results that can be output for concentrations, total deposition flux, dry deposition flux, and /or wet deposition flux.

More details on AERMOD model formulations and options can be found in references 1, 2 and 3, as well as the US EPA Guidelines on Air Quality Models (Revised)⁽¹¹⁾.

2.3.2 SCREEN3

The SCREEN3 model was developed to provide an easy-to-use method of obtaining contaminant concentration estimates. These estimates are based on the US EPA document "Screening Procedures for Estimating the Air Quality Impact of Stationary Sources"⁽¹²⁾.

The SCREEN3⁽¹³⁾ model can perform all the single source short-term calculations in the EPA screening procedures document, including:

• estimating a full range of stability classes and wind speeds to find maximum hourly ground-level concentrations and the distance to the maximum. Concentrations can be calculated at a range of downwind distances.
• incorporating the effects of building downwash on the maximum concentrations for both the near wake and far wake regions.
• 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 (i.e., terrain not above stack top) on maximum concentrations.
• modelling simple area sources using a numerical integration approach.
• estimating 24-hour average concentrations due to plume impaction in complex terrain (i.e., terrain above stack top) using the VALLEY model 24-hour screening procedure.

2.3.3 Use of ASHRAE - Same Structure Contamination Model

Improper stack design and configuration can lead to impacts beyond ground level contamination. The influence of buildings on contaminant emissions is examined in Chapter 4.6 - Building Impacts. The interactions between sources and buildings can also lead to situations of emission re-entry into nearby buildings since many buildings have air-handling units located on their rooftops. As a result, it is important to ensure that emissions from rooftop sources do not allow plume impact on air-handling units on their rooftops, or other intakes located on the same building or structure. This is known as same structure contamination.

The ASHRAE Handbook describes 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, as amended or revised from time to time ^{(14, 15)}, provides analytical approaches for determining impacts on receptors (in this case, typically air intakes) for a series of stack, rooftop and/or sidewall configurations including:

• Strong Jets in Flow Recirculation Cavity
• Strong Jets on Multi-winged Buildings
• Exhausts with Zero Stack Height (flush vents)

Same structure contamination becomes especially important within industrial parks, institutional settings such as hospitals and college/university campuses, or multi-unit commercial complexes where emissions from one unit can impact neighbouring units (where the neighbouring unit is within the same structure as the emission source) through air intakes, open doors, or windows. The ASHRAE model is for use with respect to a point of impingement that is located on the same structure as the source of contaminant. Recall that “point of impingement” is defined in section 2 of the Regulation. In general terms, 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. ASHRAE is to be used as the approved dispersion model for “same structure contamination” situations. Section 9 of the Regulation states:

Section 9 of the Regulation sets out that, when the points of impingement are located on the same structure as the source of emission of the contaminant, a person may choose to use ASHRAE (with appropriate time averaging under section 17, if necessary), as the approved dispersion model 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 for that contaminant using the equations set out in section 17 of the Regulation.

The emission rate (contaminant mass release rate) used in the model must correspond to the averaging period of the ministry POI limit in accordance with section 11 of the Regulation.

It should be noted that modellers assessing same structure contamination using one of the approaches described above must also assess against the ministry POI Limits at other off-property POI locations using one of the appropriate approved dispersion models listed in section 6 of the Regulation. Chapter 8.1 contains additional guidance on ASHRAE.

2.4 Alternative Models

There are some situations where the Director may specify the use of a model that is not listed in subsection 6 (1) of the Regulation. Such models would be considered to be an “alternative model” under section 7 of the Regulation. Once a notice is issued, these alternative models may be used instead of, or to complement the ministry’s list of approved dispersion models. The following list contains alternative models that are currently considered by the ministry on a case-by-case basis. Please see Appendix A for terms of appropriate use and required supporting explanations.

• CALPUFF
• CAL3QHCR
• SDM – Shoreline Dispersion Model
• Physical or Wind Tunnel Modelling
• AERSCREEN

The ministry may consider other models where a modeller can demonstrate that the alternative dispersion model is at least as accurate as an approved dispersion model. In accordance with section 7 of the Regulation (see Chapter 1.3 of this ADMGO document for an excerpt of section 7 of the Regulation) 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.

3.0 A Tiered Approach for Assessing Compliance With Ministry POI Limits Using SCREEN3 or AERMOD

As set out in section 6 of the Regulation, a person may use the models in the Appendix to Regulation 346, SCREEN3 or AERMOD to assess compliance with the standards in Schedule 2 of the Regulation. In some situations, a conversion to the relevant averaging period of a standard will have to be performed (please refer to section 17 of the Regulation for authorized conversion calculations). A facility that is subject to section 19 of the Regulation (i.e. Schedule 2 standards apply) may request that the Director issue a notice under subsection 20 (4) to allow the use of the Schedule 3 standards as its compliance point in advance of the phase-in dates.

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 dispersion model is simpler and in many situations is generally more “conservative”^{footnote 11[11]} in assessing point of impingement concentrations than the AERMOD approved dispersion model listed in section 6 of the Regulation. As a result, it is often appropriate to initially calculate a point of impingement concentration using SCREEN3.

The AERMOD dispersion model uses more refined input data for parameters such as meteorology, local land use and terrain. However, it is reasonable to use simplified data inputs when the selected data results in conservative assessments of point of impingement concentrations. As a result, a modelling assessment may consist of a series of modelling steps. The ministry refers to this step-wise method to modelling as a tiered approach.

A tiered approach to air dispersion modelling is commonly used and is presented in Figure 3-1. This approach focuses the required level of effort according to site requirements. It should be noted that any of the three tiers may be performed and linear progression through each tier is not necessary. More sophisticated modelling techniques would need to be applied each time a successive screen showed an exceedence 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.

Figure 3.1: Tiered Modelling Approach Flow Diagram

A Tier 1 assessment represents the most conservative assumptions about meteorological conditions and modelling inputs. To complete a Tier 1 assessment, a modeller shall use the SCREEN3 model.

Tier 2 and 3 modelling assessments represent more sophisticated, precise and accurate 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 (see Chapter 3.2 Tier 2 Modelling below for data sets). A Tier 3 assessment requires the use of more site-specific meteorological data approved by the Director (e.g. the data required in paragraphs 3 and 4 of subsection 13 (1) of the Regulation). 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.

The 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). For more information on estimating emission rates, refer to the ESDM Procedure Document.

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, such as with complex facilities or requests for site-specific standards, the ministry may require a modeller to proceed to Tier 2 or 3 directly.

3.1 Tier 1 Modelling

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 Use of Conservative Dispersion Factors

Conservative dispersion factors can be used to conservatively screen out contaminants and/or sources with insignificant (negligible^{footnote 12[12]}) emissions from further modelling analyses, consistent with section 8 of the Regulation.

A series of conservative dispersion factors (in micrograms per cubic metre per gram per second emission) have been developed using SCREEN3 for a short stack on a 6 metre tall building in combination with distances from the stack as shown in Table 3-1. 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.

3.1.2 Modelling Multiple Sources Using 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:

1. combine individual stacks/vents into a single stack where the stack parameters are selected to generate a conservative dispersion factor; or
2. 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

Tier 2 assessments involve the use of the more advanced^{footnote 13[13]} approved dispersion models, such as AERMOD, and the use of regional meteorological data sets made available by the ministry. The ministry has created the regional meteorological data sets to be used in these assessments, which 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: 1^st 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 (for more information on meteorological data, see Chapter 6 of this guideline).

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) (see Chapter 11.1.1 of the ESDM Procedure Document and paragraph 10 of subsection 26 (1) of the Regulation). Modellers should consult EMRB to confirm that representative surface parameters have been selected and AERMET is used appropriately.

3.3 Tier 3 Modelling

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 more information, see Chapter 6 of this guideline.

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 in the vicinity of the facility (appropriately converted and reformatted into a format acceptable to AERMET), 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. Chapter 6.4 describes the information needed for verifying the meteorological data files produced by a modeller.
• Using on-site data for advanced meteorological modelling 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 # 5350e)].