8.0 Other Modelling Considerations

8.1 Dispersion Modelling Results for Annual Averages

Earlier versions of the AERMOD dispersion model (versions 14134 and earlier) have some limitations regarding the generation of annual average results when more than one year of meteorological data are used. These versions of AERMOD do not output the maximum “annual” POI for each of the 5 years in the meteorological data unless each year is modelled separately. When multi-year data sets are used, such as those used to demonstrate compliance under O. Reg. 419/05, these versions of AERMOD average the results from all of the hours of meteorology in the period (i.e. five years) to produce one “period average” POI concentration at each grid point. This is the case regardless of whether the period or annual averaging periods are selected in the model.

AERMOD versions 15181 (2015) and later have addressed this issue and the modelled annual average maximum POI concentrations are calculated based on the maxima for each of the 5 meteorological years.

Note that modellers are required to use the most recent version of AERMOD posted on the EBR Registry, unless they have received a notice under section 7 of the Regulation to use a different version. The model run should be conducted either with the appropriate 5-year regional meteorological data set or a site-specific meteorological data set approved by the ministry under section 13 of the Regulation, if applicable. Because the hourly POI concentrations are automatically converted to annual averages by AERMOD, it is not necessary or appropriate to remove any meteorological data from the model run to account for meteorological anomalies.

If a modeller uses a pre-2015 version of AERMOD, the ministry would prefer that modellers evaluate the maximum POI for each year. This can be done by running each model year individually and evaluating the outputs via post processing to determine the maximum annual POI. The maximum annual POI from those five years is then compared to the annual standard.

If a modeller wishes to avoid the above-mentioned post-processing steps for pre-2015 model versions, the maximum annual POI from the single model run described above (i.e. which in actuality is the 5-year average), is then multiplied by 140%. This is intended to act as a conservative screening check against an annual standard. If the 140% value is greater than the corresponding annual standard, then individual model runs must be conducted for each year in the meteorological data set.

8.2 Use of ASHRAE

The ASHRAE method of calculation is based on copyrighted equations that are available in the chapter on building air intake and exhaust design in the ASHRAE Handbook - HVAC Applications. The equations are used to assess points of impingement located on the same building or structure as a source, such as an air intake located downwind of an exhaust on the same building. This is known as same structure or self-contamination, and applies when the intake does not belong to the emitting facility. Another approved dispersion model such as AERMOD must also be used in order to calculate the POI concentration at all other locations (i.e. at non-same-structure point of impingement locations).

The ASHRAE Handbook presents both a flush vent approach and a more comprehensive methodology. The flush vent can be used as a conservative screening method for all sources. Both methods calculate an initial plume spread followed by a further downwind dilution with the result being a dilution factor which is the number of times the concentration of contaminant in the exhaust flow is diluted by the time it reaches the intake.

The flush vent screening method assumes that the exhaust is located flush with the roof. The equation requires:

  • the equivalent circular diameter of the exhaust;
  • the exhaust exit velocity;
  • the building height,
  • the surface roughness/roughness length, and,
  • the “stretched-string” distance between the nearest edge of the source release point and the nearest edge of the intake.

The equation shall be evaluated over a series of wind speeds and specific directions to determine the minimum dilution (maximum concentration). For flush mount assessments however, the critical wind speed is typically 2 m/s. Note that the selected roughness length should be consistent with those presented in Chapters 5.4 or 6.3 for the upwind land use, as appropriate.

The more comprehensive methodology breaks up the downwind dilution into a series of release regimes ranging from free standing stacks to flush vents. Refer to the ASHRAE Handbook for more details. The chapter on Building Air Intake and Exhaust Design can be purchased/downloaded at the ASHRAE website.

As discussed in Chapter 2.3.3 Use of ASHRAE – Same Structure Combination Model, ASHRAE is required under section 9 of the Regulation for assessing same structure contamination. ASHRAE must be used once section 20 applies to a facility. If section 19 applies to a facility and it does not wish to use ASHRAE, the facility may use the Scorer-Barrett equation set out in the Appendix to Regulation 346 to assess same structure contamination (paragraph 3 of subsection 9 (1) of the Regulation).

ASHRAE can also be used to evaluate standards with annual averaging periods. The 1-hour average value from ASHRAE can be converted to an annual average by using the formula in section 17 of the Regulation (i.e., divide by a factor of 12.5 from 1-hour to an annual average). Note that these factors assume continuous emissions.

8.3 Use of Specified Models

There are some unique situations and/or settings (i.e. a tall stack located at a shoreline) where AERMOD may not be the most appropriate dispersion model to accurately predict the maximum POI concentrations from a facility. In these cases, the Director may specify the use of a model that is not listed in subsection 6 (1) of the Regulation (an “alternative model”) in a notice issued under section 7 of the Regulation. Acceptable Alternative Models and their use are further described in Appendix A.

A facility may request the use of an alternative model, or the ministry may require the use of an alternative model. Facilities requesting the use of an alternative model must provide the ministry with the rationale for choosing a particular alternative model, as well as demonstrate an understanding of the model, its data requirements, and the quality of data required for the model. A form to request that a notice under section 7 of the Regulation be issued is available on the ministry website.

Use of alternative models is considered to be a Tier 3 modelling assessment and requires consultation with the ministry prior to making the request for a section 7 notice. Note that when the Director issues a notice under section 7 of the Regulation, the model specified in the notice becomes an “approved” model for that facility.

Specified Dispersion Models

7. (1) The Director may give written notice to a person who discharges or causes orpermits 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,

  1. one or more dispersion models specified in the notice would predict concentrations of the contaminant at least as accurately as an approved dispersion model;
  2. 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;
  3. 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
  4. one or more approved dispersion models specified in the notice would predict concentrations of the contaminant less accurately than,
    1. a dispersion model or combination specified under clause (a), (b) or (c), or
    2. 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,

  1. to include a dispersion model or combination specified under clause (1) (a), (b) or (c); and
  2. not to include a dispersion model that is specified under clause (1) (d).

(4) Revoked

(5) Subsection (3) does not apply to a discharge of a contaminant until,

  1. three months after the notice is given under subsection (1), unless clause (b) applies; or
  2. 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 22 (1.1) or (1.2), 23 (3), 24 (2), 30 (5) or 33 (6) 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 22 (1.2), 23 (3), 24 (2),30 (5) or 33 (6) applies.

(10) If a notice given under subsection (1) is amended by a notice given under section 52, the notice under section 52 takes effect, and subsection (3) begins to apply to the amended notice,

  1. three months after the notice is given under section 52, unless clause (b) or (c) applies;
  2. one year after the notice is given under section 52, if the amendment adds a statement described in clause (1) (c) to the notice given under subsection (1), unless clause (c) applies; or
  3. on the day the notice is given under section 52, if the application of subsection (3) to the amended notice would have the effect of permitting a discharge that would otherwise be prohibited.

(11) If a notice given under subsection (1) is revoked by a notice given under section 52, the notice under section 52 takes effect, and subsection (3) ceases to apply to discharges of the contaminant,

  1. three months after the notice is given under section 52, unless clause (b) applies; or
  2. on the day the notice is given under section 52,if the revocation of the notice given under subsection (1) would have the effect of permitting a discharge that would otherwise be prohibited.

8.4 Use of Modelled Results in Combination with Monitoring Data

Monitoring and modelling should be considered complementary tools in determining concentrations of contaminants at point of impingements. If monitors measure accurately and are well located they can provide information on the magnitude and variability of a facility’s emissions in addition to their potential impact. However, monitoring data is usually limited to a few locations and a limited number of measurements which can bias the interpretation of the results. Conversely, modelling allows estimates of concentrations over a large number of receptors and a wide range of meteorological conditions. However, modelling results can also be biased by various factors including uncertainties or omissions in the quality of the emission data, or available information on local meteorological conditions. Using modelling results in combination with monitoring data provides a more complete and realistic analysis of the potential maximum POI concentrations.

The US EPA Guideline on Air Quality Models states that modelling is the approved method for determining concentrations and that monitoring alone would normally not be accepted. To assess compliance with ministry POI Limits, the ministry would consider model results along with monitoring data. Compliance cannot be demonstrated by monitoring information alone. If model results do not agree with measured data, a number of factors which could contribute to the disagreements shall be considered. For example, the facility’s source characteristics emissions, and meteorological data are factors which shall be reviewed. The adequacy of the locations and amount of monitoring data along with the data accuracy shall also be considered. In some cases, the Director may specify the values of dispersion modelling parameters under a subsection 13.1 (1) Notice. For more information on combined modelling/monitoring analysis and the requirements of the Regulation, see the ESDM Procedure Document and sections 10, 11, and 12 of the Regulation.

In some situations, the use of a combination of modelling and monitoring data to determine facility emission rates is required by subsection 12 (1) of the Regulation unless certain conditions are met. For further information on this methodology, please refer to the Technical Bulletin on “Combined Assessment of Modelled and Monitored Results as an Emission Rate Refinement Tool” available from the ministry Standards Development Branch.

Decisions on the adequacy of the monitoring data will be made on a case-by-case basis in consultation with the ministry. Pre-consultation with the ministry is required under paragraph 3 of subsection 11 (1) if a comparison of model results with monitoring data is undertaken for compliance assessment purposes. A Request for Approval of a Plan for Combined Analysis of Modelled and Monitoring Results form is available on the ministry website.

For cases where reliable information is available on the emission rates and source characteristics for a facility, modelled results can identify maximum impact areas and concentration patterns that could assist in locating monitoring sites. Model runs using a number of years of meteorological data would show the variations in the locations and the magnitude of maximum concentrations. Modelling results can also be used to provide information on the potential frequency of exceedences of high concentrations, but only when used with more site-specific meteorological data. For more information, see the “Guideline for the Implementation of Air Standards in Ontario” (GIASO).

8.5 Determination of Frequency of Exceedence

As mentioned earlier, model results can provide information on the frequency of exceedance of standards or guideline values. There are several circumstances when determination of the frequency of a certain concentration being exceeded may be required. Two situations are when an URT may be exceeded and when a facility is making a request for a site-specific standard. For example, paragraph 3 of subsection 30 (7) and subparagraph 2 iii of subsection 33 (1) (site-specific standard request) generally require the following:

A written statement specifying the number of averaging periods for which the approved dispersion model predicts that discharges of the contaminant may result in a contravention of section 20 … because of the concentration of the contaminant at points of impingement … expressed as a percentage of the number of averaging periods …

The use of a local representative meteorological data set and a refined emission inventory that uses variable emissions (if applicable) are important if the frequency results are to be representative. The ministry regional data sets are not to be used to determine frequency of exceedence without prior approval from the ministry.

There are a couple of different ways to obtain frequency information. Using the MAXIFILE keyword in the model OUTPUT Pathway creates a file that lists, in date order, the concentrations that are above a specified threshold (i.e. a POI Limit) for each grid point. This allows one to calculate the frequency of exceeding a particular threshold concentration. The POSTFILE keyword in the OUTPUT Pathway allows one to obtain all modelled concentrations for each receptor. Note that a POSTFILE can get very large. The frequency analysis should be determined at every receptor that is over a ministry POI Limit. It should be noted that the grid point at which the maximum POI occurs is not necessarily the location of the highest frequency of exceedences).

8.6 Modelling Adjacent Properties

Section 4 of the Regulation sets out the conditions where, for the purpose of compliance with ministry POI Limits, two or more adjacent properties would be considered to be a single property. In these cases, section 3 requires modelling of the aggregate emissions from all sources of contaminant located within the adjacent properties and the property would be defined by the external property line around the adjacent properties.

Adjacent Properties

4. (1) Two properties are adjacent for the purposes of this Regulation if the boundary of one property touches or, were it not for an intervening highway, road allowance, railway line, railway allowance or utility corridor, would touch the boundary of the other property,

(2) For the purposes of this Regulation, except section 34, two or more properties on which different sources of contaminant are located shall be deemed to be a single property if each of the properties is adjacent to one or more of the other properties and,

  1. the persons responsible for the sources of contaminants have jointly notified the Director in writing that they wish the properties to be deemed to be a single property;
  2. the Director has reasonable grounds to believe that a contravention of section 19, or 20 may occur as a result of discharges of a contaminant from the different sources of contaminant if the properties are deemed to be a single property, and the Director has given written notice of that belief to the persons responsible for the sources of contaminant;

    (b.1) the persons responsible for the sources of contaminant are required to prepare a report to which subsection 22 (1.1) applies, the Director has reasonable grounds to believe that a contravention of section 19 may occur as a result of the discharges of a contaminant from the different sources of contaminant if section 19 applies and the properties are deemed to be a single property, and the Director has given written notice of that belief to the persons responsible for the sources of contaminant; or

  3. the persons responsible for the sources of contaminant are required to prepare a report to which subsection 22 (1.2), 23 (3), 24 (2), 30 (5) or 33(6) applies, the Director has reasonable grounds to believe that a contravention of section 20 may occur as a result of the discharges of a contaminant from the different sources of contaminant if section 20 applies and the properties are deemed to be a single property, and the Director has given written notice of that belief to the persons responsible for the sources of contaminant

(2.1) Subject to subsection (2.2), clause (2)(a) does not begin to apply until 60 days after the Director receives the notice referred to in that clause.

(2.2) Clause (2)(a) does not apply if the Director has reasonable grounds to believe that an adverse effect may occur if one or more of the properties are excluded form the single property and gives written notice of that belief to the persons responsible for the sources of contaminant.

(2.3) The Director shall not give a person a notice under subsection (2.2) unless the Director first gives the person a draft of the notice and an opportunity to make written submissions to the Director during the period than ends 30 days after the draft is given.

(3) Before the Director gives a person a notice under clause (2)(b), (b.1) or (c), 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.

(4) Subsection (2) applies only if every property on which a source of contaminant is located,

  1. uses raw materials, products or services from one or more of the other properties on which the sources of contaminant are located; or
  2. provides raw materials, products or services to one or more of the other properties on which the sources of contaminant are located.

(5) Clause (2)(c) only applies for the purpose of preparing the report referred to in that clause.

9.0 Glossary of Terms

AERMAP
The terrain pre-processor for AERMOD. AERMAP allows the use of digital terrain data in AERMOD.
AERMET
The meteorological pre-processor for AERMOD.
AERMIC
American Meteorological Society/Environmental Protection Agency Regulatory Model Improvement Committee.
AERMOD
The current US EPA short-range regulatory air dispersion model that was developed by AERMIC. AERMOD is a next-generation air dispersion model that incorporates concepts such as planetary boundary layer theory and advanced methods for handling complex terrain.
AERSCREEN
The screening version of AERMOD, that produces estimates of concentrations without the need for a full set of meteorological data.
Air Emissions
An example of a “discharge”, as the term is defined in subsection 1 (1) of the Environmental Protection Act.
Albedo
Portion of the incoming solar radiation reflected and scattered back to space.
Alternative Model
is a model specified in a notice issued by the Director under section 7 of the Regulation, that may be used if conditions warrant.
Air
has the same meaning as in subsection 1 (1) of the Environmental Protection Act; namely, open air not enclosed in a building, structure, machine, chimney, stack or flue.
AMS
American Meteorological Society.
Approved Dispersion Models
means a model listed in section 6 of the Regulation or specified in a notice issued under section 7 of the Regulation.
Calm
Cessation of horizontal wind, or wind speed below 1 m/s.
Complex Terrain
Terrain exceeding the height of the stack being modelled.
DEM
Digital Elevation Model. Digital files that contain terrain elevations typically at a consistent interval across a standard region of the Earth’s surface.
Dispersion Model
A group of related mathematical algorithms used to estimate (model) the dispersion of contaminants in the air due to transport by the mean (average) wind and small scale turbulence.
Diurnal
Daytime period.
Emission Factor
Typically used with a product production rate or a raw material consumption rate to assess the rate at which a contaminant is released to the air.
ESDM Report
Emission Summary and Dispersion Modelling Report (see also section 26 of the Regulation) (see also Guideline A-10: Procedure for Preparing an Emission Summary and Dispersion Modelling Report.
Flagpole Receptor
Any point of impingement located above ground level (see subsection 14 (5) of the Regulation).
Inversion
An increase in ambient air temperature with height. This is the opposite of the usual case.
Lee side
The lee side of a building is the side that is sheltered from the wind.
Mixing Height
Top of the neutral or unstable layer and also the depth through which atmospheric contaminants are typically mixed by dispersive processes.
Ministry
the Ontario Ministry of the Environment and Climate Change.
Monin-Obukhov Length
A constant, characteristic length scale for any particular example of flow. It is negative in unstable conditions (upward heat flux), positive for stable conditions, and approach infinity as the actual lapse rate for ambient air reaches the dry adiabatic lapse rate.
NWS
National Weather Service. A US government organization associated with the National Oceanic and Atmosphere Administration.
Pasquill Stability Categories
A classification of the dispersive capacity of the atmosphere, originally defined using surface wind speed, solar insolation (daytime) and cloudiness (night time). They have since been reinterpreted using various other meteorological variables.
Receptor
Air dispersion models compute the concentrations of contaminants emitted from user-specified sources at user-defined spatial points. Modellers commonly refer to these points as receptors. These receptors are used in the modelling exercise to determine concentrations of contaminants at points of impingement as defined in section 2 of the Regulation. See also section 14 of the Regulation.
Regulation
means Ontario Regulation 419/05: Air Pollution - Local Air Quality, as amended from time to time.
Screening Model
A relatively simple model or analysis technique to compute conservative concentrations to determine if a given source of contaminant is likely to pose a threat to air quality.
Simple Terrain
An area where terrain features are all lower in elevation than the top of the stack of the source.
Upper Air Data (or soundings)
Meteorological data obtained from balloon-borne instrumentation that provides information on pressure, temperature, humidity and wind away from the surface of the earth.
US EPA
United States Environmental Protection Agency (US EPA).
Wind Profile Component
The value of the exponent used to specify the profile of wind speed with height according to the power law.