# 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

• 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

• 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

• 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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