PLUME DISPERSION MODEL FOR A SINGLE POINT SOURCE by rt3463df

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									           ENVE5103 – Lecture 3b

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

Worst case stability - wind speed combinations
Regardless of their likelihood to occur at the given location
ONTARIO’S
   TIERED
   APPROACH
   FOR
   ASSESSING
   COMPLIANCE
   WITH AIR
   STANDARDS &
   GUIDELINES
     ONTARIO’S TIERED APPROACH FOR ASSESSING
                COMPLIANCE WITH AIR
             STANDARDS & 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
• 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.
     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
        Ontario's Plan for Clean Air
                      Guideline 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
     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.
    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)
     THE U.S. EPA SCREEN(3) MODEL
          Meteorology Options

1) Full: complete set of stability - wind speed
   combinations examined for worst case
   scenario at each downwind location
2) Stability class: worst case scenarios for
   predetermined wind speeds
3) Stability class - wind speed combination:
   calculations reported for only the
   combination specified by user
Table 2 SCREEN User Guide



       • Wind speed and
         stability class
         combinations used by
         SCREEN
     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)
 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).
  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.
       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.
   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.

								
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