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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