Air Dispersion Modelling Guideline for Ontario

Transcription

1 Air Dispersion Modelling Guideline for Ontario A Proposal for Consultation Air Dispersion Models and technical information relating to Ontario Regulation 346 (under the Environmental Protection Act) as amended April 2004 Page 1

2 The results from this consultation will be used to move forward with the amendment of Regulation 346. The comment period for this document is 120 days from the date it was posted on the Environmental Registry. Please submit your comments to: Public consultation document under the Environmental Bill of Rights, 1993 EBR Number: EBR # Ministry Contact: Ministry of the Environment Standards Development Branch Attention: Cathy Grant, P.Eng., Coordinator - Air Standards (Engineering Specialist) 40 St Clair Ave West, 7th floor Toronto, Ontario M4V 1M2 Phone: (416) Fax: (416) mailto:sdb-ebr@ene.gov.on.ca Additional Comments: This document that accompanies this proposal is provided solely to facilitate public consultation under section 15 of the Environmental Bill of Rights, Should the decision be made to proceed with the proposal, the comments received during consultation will be considered during the final preparation of the policy. The content, structure and form of the draft document are subject to change as a result of the consultation process. April 2004 Page 2

3 Executive Summary Clean safe air is essential in protecting the health and environment of our people and our communities. The Ontario government is setting clear goals for cleaning up the air Ontarians breathe. The Ministry of the Environment (MOE) is committed to establishing effects-based air standards to ensure cleaner air for local communities. We continue to focus on finding solutions to problems that pose the greatest risk to human health and the environment. In March 2001, the MOE consulted on the discussion paper: Updating Ontario s Air Dispersion Models (PA 01E0003). Implementation issues such as appropriate phasein period for the models, managing in the interim, etc. were included for public consultation. The MOE received and considered numerous comments regarding this posting on the Environmental Registry. These comments, along with existing MOE experience with advanced modelling, were considered in the development of this proposed Air Dispersion Modelling Guideline for Ontario. MOE continues to be active in the development and improved implementation of new air standards in Ontario. We are committed to ensuring that our tools for predicting air emission impacts and assessing compliance are based on state-of-the-art science. The proposed new air dispersion models will allow for a more accurate assessment of environmental effects from a facility. The impact of introducing new air dispersion models coupled with new or revised air quality standards may create implementation issues for some stakeholders. Issues related to time, technology and/or economic considerations are dealt with in the companion proposal entitled: Guideline for the Implementation of Air Standards in Ontario. Up-to-date, scientifically-based, enforceable air standards and new scientifically advanced air dispersion models are vital tools to protect local air quality in communities and ensuring good air quality for all Ontarians. Proposed regulatory amendments related to introducing and implementing new standards and new models as well as other regulatory amendments aimed at improving clarity and enforceability are outlined in the position paper entitled: Updating Ontario's Regulatory Framework for Local Air Quality. MOE continues to look for innovative approaches to complement legislation and regulations. These include working with communities, industries and organizations towards finding practical, cost-effective ways to strengthen environmental protection. These three initiatives support this vision and we invite all stakeholders to comment on our proposals. April 2004 Page 3

4 Table of Contents Executive Summary... 3 Table of Contents Introduction Ontario s Regulatory Framework Proposed Positions on Modelling Phasing-in the new models Managing in the interim Simple screening methods (A Tiered Approach) Background Concentrations Averaging times Odour Issues Applying statistics to modelling Good Engineering Practice Stack Heights Preferred and alternative models Ensuring Consistent use of Models April 2004 Page 4

5 1.0 Introduction The Ministry of the Environment (MOE) is proposing to phase-out the existing Ontario Regulation 346 air dispersion models and replace them with the suite of United States Environmental Protection Agency (US EPA) air dispersion models. The adoption of the US EPA models in Ontario will promote the use of modern scientific tools to assess compliance with air quality standards. Air dispersion models are used to predict maximum ground level concentrations or Point of Impingement (POI) concentrations at locations in the vicinity of facilities that emit contaminants into the air. These concentrations are then compared to the MOE s air quality standards 1 to assess compliance. Of the current preferred US EPA model suite, the most commonly used air dispersion model is the Industrial Source Complex model 3 (ISC 3) and its screening version called SCREEN3. MOE proposes to adopt US EPA AERMOD model (which includes the PRIME algorithm) as the primary model. Other air dispersion models will also be accepted by MOE if a proponent can demonstrate that an alternate model is more appropriate to assess their facility. Following the MOE posting on the Environmental Registry of the Discussion Paper entitled "Updating Ontario's Air Dispersion Models" (consultation closed July 2001) efforts were undertaken to develop technical guidance on air dispersion modelling. The Consultants' report: Proposed Guidance For Air Dispersion Modelling (November 2003) summarized the technical inputs required to use the models in Ontario. The purpose of this technical document was to provide insight into recommended modelling approaches and provide consistency in the modelling methods. This technical document is available on the MOE technical publications webpage: ( 1 Standards can be found in Schedule 1 of Regulation 346. These standards and other guidelines are listed in the MOE publication, Summary of Point of Impingement Standards, Point of Impingement Guidelines and Ambient Air Quality Criteria (AAQCs), Standards Development Branch, Ministry of the Environment, dated September 2001 (or most recent revision). April 2004 Page 5

6 This document is the proposed Air Dispersion Modelling Guideline for Ontario (ADMGO). It proposes to adopt the technical methodology outlined in the above Consultant s report as well as other policy issues to be considered in the introduction of new air dispersion models into Regulation 346. This guideline proposes the following considerations in order to effectively implement the new models in Ontario: Ontario s regulatory framework (Section 2); Section 3 outlines the following: Phasing-in the new models Managing in the interim Simple screening methods (A Tiered Approach) Background concentrations Averaging times Odour Issues Applying statistics to modelling Good Engineering Practice Stack Heights Preferred and alternative models Ensuring consistent use of models 2.0 Ontario s Regulatory Framework Air dispersion models are important tools that can be used to assess the likelihood of compliance with Ontario's air quality standards at a particular location. Models can also be used to predict impacts from accidental releases to the environment (i.e. fires, emergency bypasses, plant malfunctions, etc.) when ambient air monitors are not available to measure actual concentrations or when there is no technology available to measure a particular component in the ambient air. Finally, models can assist in determining where an ambient air monitor might be located. There are two major regulatory applications of the air dispersion models: 1) in the Certificate of Approval (Air) process; and, 2) for abatement and compliance purposes. April 2004 Page 6

7 With certain exceptions, a Certificate of Approval (C of A) is required for all new sources of air emissions or proposed alterations to existing sources 2. Modelling is used in the C of A process to assess the likelihood of compliance with Ontario s air quality standards. This modelling assessment determines whether or not a C of A for air emissions will be issued to a company. Air dispersion models can also be used for abatement or compliance assessment purposes. Models can be used by MOE or proponents to assess the likelihood of impacts from a facility on the surrounding local community. There are a number of reasons why MOE may request an emitter to undertake a compliance assessment of a particular facility. These include, but are not limited to: a known or anticipated impact from a facility's air emissions on the local community; evidence or indications of an exceedance in ambient air quality standards that appear to be linked back to a facility; selection of monitoring location, and/or the facility is a selected target of a compliance audit by MOE. An assessment for abatement or compliance purposes could be formally requested by a Provincial Officer or legally required through a legal instrument such as an Order or a C of A issued under the Environmental Protection Act. Note: The proposed regulatory amendments to incorporate the new models can be found in the position paper: Updating Ontario's Regulatory Framework for Local Air Quality. 3.0 Proposed Positions on Modelling MOE has undergone consultation with stakeholders and used its internal working experience with the new models to address and develop a proposed position on the following key issues: Phasing-in the new models Managing in the interim Simple screening methods (A Tiered Approach) Background concentrations Averaging times Odour Issues Applying statistics to modelling 2 For exemptions from the air approvals process, please refer to: Certificate of Approval Exemption (Air), O. Regulation 524/98 (as amended) and Ground Source Heat Pumps, O. Regulation 177/98. April 2004 Page 7

8 Good Engineering Practice Stack Heights Preferred and alternative models Ensuring consistent use of models Each of these topics is discussed in the following sections. 3.1 Phasing-in the new models The MOE proposes to phase-in the use of the new models within three years of Regulation 346 being amended. A facility identifying significant implementation issues with the change to the new models prior to the end of the phase in period may be eligible for the process outlined in the proposed document entitled: Guideline for the Implementation of Air Standards in Ontario (GIASO). This guideline outlines a riskbased decision making process that includes consideration of risks, timing, technology benchmarking and economic (as needed) barriers to compliance. For further information, please see the Position Paper: Updating Ontario s Regulatory Framework for Local Air Quality. 3.2 Managing in the interim In the C of A process, MOE s proposal is to accept model results for existing facilities using either the new models or the Regulation 346 models 3 during the phase-in period. Companies submitting new C of A applications for new facilities would be required to demonstrate compliance with the standards using the new models. Introduction of the new dispersion models into Ontario will not change the status of a company's C of A. Existing C of As will still be valid during the phase-in period and after the introduction of the new models even if approval was obtained using the Regulation 346 models. Under Section 9 of the Environmental Protection Act, the Director has the authority to amend or revoke a C of A at any time. Hence, for business planning purposes, amendments to C of As should not be linked to the introduction of the new air dispersion models. If assessment using the new models leads to the need to reduce emissions in order to comply with the standards, the necessary approvals must be obtained at that time. 3 For certificate of approval applications where odour is an issue, the Environmental Assessment and Approvals Branch (EAAB) requires proponents to use a model that considers all dispersion conditions. Hence, for odour impact assessments, the US EPA models will continue to be used. April 2004 Page 8

9 3.3 Simple screening methods (A Tiered Approach) The modelling assessment can use a series of conservative screens or a tiered approach to assess contaminant concentrations against MOE air quality standards. Initial screens using 1 hour averaging periods would make conservative assumptions about on-site meteorological conditions and contaminant concentrations. A proponent can then assess their ground level concentrations under these conditions, and if they meet the screen (i.e. the proposed 1 hour POI); it is also likely to meet the MOE air quality standards. More refined modelling techniques would need to be applied each time a successive screen showed an exceedance of MOE air quality standards. The last screen or tier would require the most sophisticated or refined modelling techniques, and would be the most representative of actual on-site conditions and contaminant concentrations. Figure A outlines the proposed MOE screening methods. Each of the 3 tiers allows for a 1 hour average screening value to be assessed. Tier 1 involves the US EPA SCREEN3 model which requires no meteorological data. This model is normally used as an initial screening tool to assess single sources of emissions. SCREEN3 can be applied to multi-source facilities by conservatively summing the maximum concentrations for the individual emission sources. As newer models (e.g. AERSCREEN) are introduced by the US EPA and adopted by the MOE, screening assessments for multiple emission sources would be possible. Screening results are compared against the standards at the 1 hour POI averaging time. Tier 2 involves the use of the US-EPA AERMOD model using regional meteorological data sets made available by the MOE. MOE has created the regional meteorological data sets (used in Tier 2) and are available from the MOE and posted on An even more representative modelling scenario would be using the most refined modelling technique with local meteorological data (Tier 3) to assess contaminant concentrations. Local meteorological data (used in Tier 3) would be provided by the proponent and agreed to by the MOE. MOE also have local data sets available for some geographical locations. These tiers (2 and 3) could be based on a variable emission rate scenario. Tier 3 would also consider the use of alternative models (see Section 3.9) if approved by MOE. Any proponent may choose to go directly to the more refined modelling techniques (Tier 2 or 3) and skip the initial Tier 1 of the proposed screening methods if they so choose. In some situations, such as with tall stacks, the MOE may require a proponent to proceed to Tier 2 or 3 directly. April 2004 Page 9

10 Figure A: A Tiered Approach to Assessing Air Quality Standards 4 4 MOE will adopt AERSCREEN to replace SCREEN3 once it is adopted by the US EPA. April 2004 Page 10

11 3.4 Background Concentrations Emissions of contaminants into the atmosphere occur from both natural and anthropogenic sources, and atmospheric turbulence and meteorological conditions normally serve to disperse and dilute ambient air concentrations of these contaminants to well below ambient air quality criteria. At high levels of emissions, meteorological dispersion alone is not sufficient in achieving dilution and a residual accumulation of these contaminants occurs resulting in a measurable ambient concentration. Often background includes two components: 1) local background consisting of significant nearby sources (other than the source being evaluated); and 2) regional background, consisting of natural sources, distant anthropogenic sources and other minor sources. Regional background issues normally involve criteria pollutants and are dealt with using regional airshed strategies to address issues such as Smog. For most contaminants in Ontario, background concentrations are much less than the air quality standards. MOE may on occasion request ambient air concentrations to be considered in a modelling assessment. 3.5 Averaging times Air dispersion models mathematically predict the behaviour of a stack plume and how it may be impacted by atmospheric and meteorological conditions. The output from the air dispersion model is a contaminant concentration at a certain location which is then assessed against the air quality standard. One of the other key features of the new proposed air dispersion models is their ability to determine contaminant concentrations over a range of averaging times from 1 hour up to 1 year. This feature of the new models allows from a more realistic assessment of health and environmental impacts from a facility by using the most appropriate averaging time for exposure. The effects of a contaminant can then be assessed using modelled exposure concentrations over the most appropriate averaging time for that contaminant. For example, the effects-based averaging time for health-based air standards is often considered on a 24 hour (or daily) basis. Standards with variable averaging times for specific pollutants are typically referred to as the Ambient Air Quality Criteria (AAQC). A Tier 1 modelling assessment using the new models would consider the maximum one hour emission rate to determine the maximum ground level concentration. This would then be compared to the equivalent screening level concentration or one hour POI for that contaminant. One hour averaging times can also be used in Tier 2 and 3 assessments as a screen. April 2004 Page 11

12 Screening levels are used with the theory that if a facility meets the screening level or maximum 1 hour concentration, then it is likely to meet the AAQC. If a Tier 1 assessment indicates the 1hour value is not achieved, then proponents should assess the air quality standards using the most appropriate averaging time (as specified in the Regulation) for that contaminant and a variable emission rate scenario. This would be a Tier 2 or 3 emission scenario. For example, for facilities where emissions only occur for part of the day or vary significantly during the day, emission rates for the actual operating times could be used in assessing compliance with a 24 hour air quality standard. Note: The MOE proposes to introduce a new schedule in Regulation 346 to reflect the varying averaging times. For more information on this, please refer to the Position Paper: Updating Ontario's Regulatory Framework for Local Air Quality Conversion Factors for Averaging Times Various averaging time conversions have been used which account for different stability classes. Since AERMOD does not make use of Pasquill-Gifford stability class, this information will not be available. The Duffee, O Brien and Ostojic (1991) stability dependent formula to convert modelled values to alternative averaging times is: Equation 1 C l = C 0 x (t 0 /t l ) n where t 1 = the longer averaging time t 0 = the shorter averaging time n = the stability dependant exponent (see below) Stability Class n A&B 0.5 C 0.33 D 0.20 E&F For example, a 1 hour concentration can be converted to a 24 hour average concentration as follows: April 2004 Page 12

13 Equation 2 C 24 hour = C 1 hour x 60 min/[24 hr *(60 min/hr)]) n where: C 1 hour and C 24 hour are the maximum average concentrations respectively. Equation 1 above would use the value of n for the stability that gave the highest modelled 1 hour concentration. To estimate a maximum 10 minute average for odour standards, equation 1 with 10 minutes as the shorter averaging time would be used. Again the value of n would be for the stability class that gave the highest modelled 1 hour concentration. A wide range in power law exponents is found in the literature, the corresponding 60-minute to 10-minute factors would vary from less than 1.4 to 2.5. The MOE has historically relied on a power exponent of 0.28 (C&D stabilities), which would result in a 1.65 factor. A summary of MOE conversion factors are provided in Table A. Considering the modelling limitations and with a goal to reduce the complexity for proponents, the MOE proposes to retain the historical power exponent of 0.28 for conversion. A power exponent of 0.28 is close to the average of the stability dependent exponents given above. A proponent or the MOE may, in special situations, demonstrate that the use of an alternative conversion approach may be more appropriate or required. Table A: Averaging Time Conversion Factors Convert to Convert from 10 min ½ hour 1 hour 8 hour 24 hour Annual 10 min 1 1/1.36* ½ hour 1.36* 1 1/ /3 1/15 1 hour / hour 1/0.5 1/ hour Annual Note: Comments were made during the 2001 public consultation that the MOE is using conversion factors based on stability classes when AERMOD is not based on stability classes. Using an average power exponent of 0.28 would avoid the complexity of post processing model results to account for stability dependent conversion factors. April 2004 Page 13

14 3.6 Odour Issues While AAQC serve to protect against adverse health and environmental effects, one of the most common observable effects for air pollution is odour. In situations where there are existing odour impacts, or in situations where a proposed facility can be anticipated to be a significant source of odour, the facility will be required to demonstrate that the 10-minute average concentration of odour at the most impacted Sensitive Receptor resulting from the operation of the facility does not exceed one (1) odour unit. For odour assessments, a Sensitive Receptor means any location where routine or normal activities occurring at reasonably expected times would experience adverse effect(s) from odour discharges from the Facility, including one or a combination of: residences or facilities where people sleep (eg: single and multi-unit dwellings, nursing homes, hospitals, trailer parks, camping grounds, etc.), or institutional facilities (eg: schools, churches, community centres, day care centres, recreational centres, etc.), or outdoor public recreational areas (eg: trailer parks, play grounds, picnic areas, etc.), or other outdoor public areas where there are continuous human activities (eg: commercial plazas and office buildings). The 10-minute average concentration of odour at the most impacted Sensitive Receptor can be calculated as follows: first calculate one-hour average concentration of odour at the Point of Impingement and at the most impacted Sensitive Receptor, employing the AERMOD Prime atmospheric dispersion model or with another atmospheric dispersion model acceptable to the MOE that employs at least five (5) years of hourly local meteorological data. This can provide results reported as individual one-hour odour concentrations; and then convert each of the one-hour average concentrations predicted over the five (5) years of hourly local meteorological data to a 10-minute average concentration. For conversion factors, please refer to Section April 2004 Page 14

15 3.7 Applying statistics to modelling One of the features of these air dispersion models is that, if prompted, the model can output when an exceedance is likely to occur, and how often it will occur for various averaging times. For assessments of 1 hour average concentrations, MOE will consider the 8 th highest concentration. MOE proposes to allow the consideration of the 2 nd highest concentration for specific compounds where the air quality standard and corresponding air dispersion modelling is based on a 24-hr averaging time period. This approach is for modelling results only and does not apply to actual measured or monitored concentrations. This statistical consideration would be restricted to contaminants where there are no known acute impacts. Note: Comments were made during the 2001 public consultation that the MOE should be considering a percentile (i.e. 90 th, 98 th, etc) for compliance/modelling assessments. A thorough analysis of this issue by the MOE s consultant indicated that no other jurisdiction allows for such a percentile consideration for all contaminants. In isolated cases, for specific criteria pollutants, some jurisdictions allow for the 98 th percentile to be considered when monitoring for background concentrations of Particulate Matter for example. Other jurisdictions that used percentiles made the decision based on toxicological considerations for specific contaminants. The MOE is proposing to adopt Alberta s approach for allowing 8-1 hour values to be discarded in modelling results and the US EPA approach of considering the 2 nd highest value for 24 hour modelled concentrations. 3.8 Good Engineering Practice Stack Heights Good Engineering Practice (GEP) Stack Heights are the stack heights that US EPA requires to avoid having to consider building downwash effects in dispersion modelling assessments. The definition of GEP is stack heights equal to the building height plus 1.5 times the smaller of the building height or width. The US EPA modelling guideline document prohibits the use of stack height credits in excess of the Good Engineering Practice stack height when using dispersion models to assess emission limits for the source. MOE will adopt this approach and limit the stack heights to the Good Engineering Practice stack heights for dispersion modelling assessments of new or modified installations. 3.9 Preferred and alternative models Preferred models indicate standard models that are expected to be used for air quality studies. The MOE s preferred models include SCREEN3 for screening analyses and AERMOD (with PRIME) for refined modelling analyses. Screening assessments for multiple emission sources could be done with AERSCREEN when it is available. April 2004 Page 15

16 Alternative models may be used if conditions warrant their use and are approved by the MOE. The following list contains alternative models that are currently accepted by MOE for consideration. CALPUFF CAL3QHCR SDM - Shoreline Dispersion Model Self Contamination - ASHRAE Physical Modelling MOE will consider other models were a proponent can demonstrate that the alternate air model is more appropriate for their facility 3.10 Ensuring Consistent use of Models The technical portion of this guideline is available on the MOE publications webpage ( This document is not designed to provide theoretical background on the models it discusses. Technical documents covering these topics can be easily obtained from several U.S. EPA sources. The technical portion of this guideline provides details on performing a successful modelling study including: Model Background and Applicability Model Selection and Study Approach Tiered Approach to Assessing Compliance Model Input Data Requirements Geographical Information April 2004 Page 16

17 Meteorological Data Requirements and Acquisition Information for Inclusion in an Assessment This proposed Air Dispersion Modelling Guideline is designed to provide guidance on methods for air dispersion modelling in the Province of Ontario. This proposed guideline is meant to provide insight into recommended modelling approaches and provide consistency in the modelling methods implemented by applicants. In order to ease the modelling exercise and standardize the input data, the MOE has developed and made available a set of regional meteorological files to stakeholders (for use with AERMET/AERMOD). This data has been posted on MOE has also developed an Ontario terrain digital elevation model data set available to stakeholders for air modelling purposes. Note: As outlined in the position paper: Updating Ontario s Regulatory Framework for Local Air Quality, MOE proposes to update the available guidance material for Emission Estimation Methodology. In adopting the use of effects-based limits, and utilizing the capabilities of the more advanced models that allow for atmospheric dispersion modelling for different averaging time periods, the methodology of estimating emissions should be updated to correspond to the modelled averaging-time period. Emission rates are key model input parameters, and their variations are proportional to variations in the model output. April 2004 Page 17