ENVE5103 Lecture 3b

1
ENVE5103 Lecture 3b

• Gaussian dispersion modelling as a screening tool
  in the Regulatory Framework.

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DISPERSION MODELLING

• The dispersion calculations for a single point
  source under a particular meteorology can be
  repeated for
• multiple sources with additive effects
• different meteorologies that might be expected at
  different times of the day or year

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DISPERSION MODELLING - SCREENING

• What is the worst possible scenario for a given
  source?
• (What is the highest possible ground-level
  pollutant concentration?)
• Under what conditions does it occur (stability,
  windspeed)
• Where does it occur?
• These questions require calculations with the
  many permutations possible.
• Conservative estimates (I.e those leading to high
  concentrations) are used with relatively simple
  model equations

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THE U.S. EPA SCREEN(3) MODEL

• Maximum short term (1 hour) ground level
  concentrations (magnitude and location) downwind
  from a single source (point, line, area or
  volume)
• Building downwash calculations with wake and
  cavity concentration estimates
• Inversion break-up and shoreline fumigation
• Plume rise for flare releases
• Flat or Simple Elevated Terrain

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SCREENING

• Conservative estimates of averaging time
• Original dispersion data are for 10 min averaging
• Regulatory screening models (e,g, SCREEN) use
  these as 1 hour averaging results.
• An estimate of the lower limit for mixing height
• Multiple reflections off the ground and stable
  layer aloft
• Zm hs 1 (m) when plume height gt Zm.
• Worst case stability - wind speed combinations
• Regardless of their likelihood to occur at the
  given location

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• ONTARIOS TIERED APPROACH FOR ASSESSING
  COMPLIANCE WITH AIRSTANDARDS GUIDELINES

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ONTARIOS TIERED APPROACH FOR ASSESSING
COMPLIANCE WITH AIRSTANDARDS GUIDELINES

• Tier 1 is a screening level analysis which
  includes all potential worst case meteorological
  conditions. If an air quality study passes
  appropriate standards and/or guidelines there is
  no need for additional modelling.
• Tier 2 is a refined modelling analysis that makes
  use of regional meteorological data.
  Pre-processed regional meteorological data sets
  prepared by the Ontario Ministry of the
  Environment will be available to modellers

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• Tier 3 consists of refined modelling analyses
  that incorporate local meteorological data. This
  data typically must be pre-processed by the
  modeller or a Canadian meteorological data
  provider such as Environment Canada.
• Local meteorological data sets include
  site-specific parameters and meteorological
  characteristics that directly represent the site
  of consideration with a greater level of detail
  than most regional data sets.

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Ontario's Plan for Clean Air

• two new regulations introduced in 2005
• Regulation 194/05 Industry Emissions - Nitrogen
  Oxides and Sulphur Dioxide stricter (NOx) and
  (SO2) emission limits for industry
• Regulation 419/05 Air Pollution - Local Air
  Quality new air standards, emission reporting and
  dispersion modelling tools to show compliance

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Ontario's Plan for Clean AirGuideline documents

• 3614e02 Procedure for Preparing an Emission
  Summary and Dispersion Modelling (ESDM) Report
• 5165e Air Dispersion Modelling Guideline for
  Ontario (ADMGO)
• 5166e Guideline for Implementation of Air
  Standards in Ontario (GIASO)
• All available at
• http//www.ene.gov.on.ca/envision/air/regulations/
  localquality.htm

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Model Input Data SCREEN3

• Source type and characteristics (Point, Flare,
  Area or Volume)
• Building Downwash If this option is used then
  building dimensions (height, length and width)
  must be specified.
• Meteorology SCREEN3 can consider all conditions,
  or a specific stability class and wind speed can
  be provided.
• Terrain SCREEN3 support flat, elevated and
  complex terrain. If elevated or complex terrain
  is used, distance and terrain heights must be
  provided.
• Fumigation SCREEN3 supports shoreline
  fumigation. If used, distance to shoreline must
  be provided.

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THE U.S. EPA SCREEN(3) MODEL Point source inputs

• Emission rate (g/s)
• Stack height (m)
• Stack inside diameter (m)
• Stack gas exit velocity (m/s) or
• flow rate (ft3/min or m3/s)
• Stack gas temperature (K)
• Ambient temperature (K)
• Receptor height above ground (may be used to
  define flagpole receptors) (m)
• Urban/rural option (U urban, R rural)

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THE U.S. EPA SCREEN(3) MODEL Meteorology Options

• Full complete set of stability - wind speed
  combinations examined for worst case scenario at
  each downwind location
• Stability class worst case scenarios for
  predetermined wind speeds
• Stability class - wind speed combination
  calculations reported for only the combination
  specified by user

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Table 2 SCREEN User Guide

• Wind speed and stability class combinations used
  by SCREEN

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THE U.S. EPA SCREEN(3) MODEL Fumigation Options

• Inversion break-up (Figure 5-15 de Nevers)
• - pollutant release into the radiation inversion
  layer moves horizontally with little dispersion
  due to the strong stability of the inversion
  layer
• - radiation inversion starts breaking up
  mid-morning
• - when mixed layer reaches stack height high
  ground level concentrations can be experienced
  close to the stack
• shoreline fumigation (sources within 3000 m of a
  large body of water)

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SCREEN3 Non-regulatory options

• An alternative mixing height algorithm (Brode,
  1991).
• optional input of an anemometer height in place
  of the default height of 10 meters.
• an alternative building cavity algorithm
  (Schulman and Scire, 1993).

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Brode algorithm for mixing height

• The alternative mixing height is determined by
  using the maximum of a predetermined mixing
  height or a value adjusted slightly higher than
  the plume height, whichever is greater. Both the
  mixing height and adjustment values to the plume
  height are based on stability class. Selection of
  this algorithm results in concentrations that are
  generally more conservative than output from the
  ISCST3 model.

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Anemometer height ? 10 m

• The optional input of an anemometer height in
  place of the default height of 10 meters affects
  the stack top wind speeds for Choice of
  Meteorology selections 1 and 2.
• For Choice of Meteorology selection 3, the user
  is prompted to enter a 10 meter wind speed which
  is unaffected by any optionally entered
  anemometer height.

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Schulman and Scire Building Cavity Algorithm

• The published concentration results using this
  algorithm model the sampled wind tunnel test
  concentrations better than the regulatory
  algorithm for the range selected.