Modeling 101: Intro to Dispersion Modeling Programs and Process

Transcription

1 1 Modeling 101: Intro to Dispersion Modeling Programs and Process Carolinas Air Pollution Control Association Presentation by: Pilar Johansson Trinity Consultants

2 2 Presentation Outline When are Models Needed Dispersion Modeling Fundamentals Dispersion Modeling Tools AERMOD Model Model Accuracy

3 When are Models Needed 3

4 4 Purpose of Dispersion Modeling Emissions Dispersion Modeling Ground-Level Concentrations Regulatory Engineering design Ecological Studies Health Studies

5 5 When are Models Needed? Regulatory Programs Development of regulations (SIP Revisions) Prevention of Significant Deterioration (PSD) Nonattainment areas (NNSR) Best Available Retrofit Technology (BART) State-only modeling (e.g. Air Toxics) SARA Title III planning

6 6 When are Models Needed? Engineering Stack Height or Emission Analysis Odor Assessments Demonstrate Control Equipment / Strategies Supplemental Control Systems Emergency Response Planning

7 7 When are Models Needed? Health/Safety Assessments Modeling the effects of a chemical release Risk Management Plans (112(r)) Analysis of Urban Air Toxics (112(k)) Human Health Risk Assessments

8 8 When are Models Needed? Ecological Assessments Screening Level Ecological Risk PSD Additional Impacts Analysis Litigation Deposition Studies

9 9 Advantages of Modeling Cost: Can model thousands of sites for the cost of measuring data at a single site Accuracy: Can quickly evaluate source contributions to determine source impacts Forecasting: Can analyze what-if scenarios

10 10 How Do We Choose a Modeling Methodology? Federal & State Guidance Appendix W (Guideline On Air Quality Models) New Source Review Workshop Manual State Guidance Regulatory Specific Guidance (SIP, PSD, toxics, etc.) FLAG, IWAQM, VISTAS, MOG Support Center for Regulatory Air Models (SCRAM)

11 11 State Modeling Guidelines North Carolina: South Carolina: nceandreporting/airdispersionmodeling/formsguideli nessoftware/ SCRAM (Appendix W, Other Guidance, Models, and Model Clearinghouse):

12 Dispersion Modeling Fundamentals 12

13 13 What is Modeling? Modeling is the combined mathematical simulation of atmospheric processes which gives a convenient and physically meaningful way of relating sources/emissions to ambient air impacts.

14 14

15 15 Definitions Ambient Air Ambient Air The portion of the atmosphere, external to buildings, to which the general public has access [40 CFR Part 50.1 (e)] In general, any location at or beyond the fence line of the facility. The fence line must restrict public access by a continuous physical barrier, such as a fence or a wall. If plant property is accessible to the public or if any residence is located within the restricted area, receptors should be located on-property. NC and SC non-psd modeling often use Property Line.

16 16 Definitions General Fence Line Emission Source Structures Receptors

17 17 Definitions - Downwash In most modeling cases, maximum concentrations occur at receptors affected by downwash for stacks

18 18 On structure More dispersion and descent Off structure Less dispersion, little descent

19 19 Definitions GEP Good Engineering Practice (GEP) Stack Height Cannot model a stack higher than GEP in most cases 1,215 ft

20 20 GEP Regs: Specifies Source Height Modeled Proposed height 65 meters: Use proposed height Proposed height 65 meters: Use 65 meters unless higher height is justified to avoid building or terrain downwash How to justify? By GEP formula or Wind Tunnel Study

21 21 GEP Stack Height h GEP = H b + 1.5L where: h GEP = GEP stack height, m Hb = Building height, m L = Lesser of building height or maximum projected width, m

22 22 What is Plume Rise? Final height of the plume above the physical stack height Effective heights defined by the height of the stack plus any plume rise H = h + Δh H = Effective height h = Stack height h = Plume rise

23 Stack Tip Downwash Karman vortices 23 When a fluid flows steadily over an isolated cylindrical solid barrier (stack), vortices are shed downstream.

24 24 Wind Speed and Direction Station No Columbia, SC (2006) Station No Charleston, SC (2006)

25 Turbulence 25

26 Dispersion Modeling Tools 26

27 27 Gaussian Models For non-reactive pollutants, good for short-term averaging times, limited steady state met fields: AERMOD AERSCREEN Industrial Source Complex Model (ISC) SCREEN3 BLP Buoyant Line Plume

28 Gaussian Plume Dispersion 28

29 29 Chemistry and Numerical Models For reactive pollutants, 3D met grids, data and computer intensive: CALPUFF CMAQ CAMx SCIPUFF Global Scale Models

30 Puff Dispersion 30

31 AERMOD Model 31

32 AERMOD 32

33 Model Visualization 33

34 34 Model Input: Control Data Declares model options Regulatory options specified in EPA Guideline Averaging periods relevant for analysis Non-regulatory options

35 35 Model Input: Source Data Defines locations and characteristics of emissions sources Declares source groups to logically group sources or conduct multiple analyses in one run May include data associated with downwash

36 36 Source Characterization Point (e.g. stacks) Exhaust stack or well-defined vent Vertical velocity Plume rise Area (e.g. parking lots) Fugitive releases No plume rise

37 37 Source Characterization Volume (e.g. transfer points) Fugitive releases No plume rise Surface, obstructed, or elevated Special Sources Line Open Pit Open Pit

38 38 Model Input: Terrain Data Modeling program can import Digital Elevation Model (DEM) or National Elevation Data (NED) data All raw data requires processing for use programs: AERMAP program for AERMOD

39 39 Model Input: Receptors Receptors are typically placed on grids and in discrete locations For regulatory purposes - capture maximum impact on 100-meter grid Exclude receptors located within enclosed property Any public access (e.g., roads, rail) within plant property may need to be classified as ambient

40 Typical Receptor Grids 40

41 41 Model Input: Meteorological Data Can be obtained from the agency in some cases All raw data requires processing for use programs: AERMET program for AERMOD AERMINUTE improvements to reduce number of clams and missing winds

42 42 Importance of Land Use Land Use affects mechanical and buoyancy turbulence In AERMOD, land use types are distinguished to assign values to the micrometeorological parameters of albedo, Bowen ratio, and surface roughness

43 43 Model Output Color Coded Impacts 3,764 3,762 3,760 3,758 UTM North (km) 3,756 3,754 3, to 1.0 ug/m3 1.0 to 2.0 ug/m3 2.0 to 3.0 ug/m3 3.0 to 5.0 ug/m3 5.0 to 30.0 ug/m3 3,750 3,748 3, UTM East (km)

44 Dispersion Modeling Accuracy 44

45 45 Comments on Model Accuracy Models are more reliable for estimating longer timeaveraged concentrations than for estimating shortterm concentrations at specific locations. Models are reasonably reliable in estimating the magnitude of highest concentrations occurring sometime, somewhere within an area. Section of Guideline on Air Quality Models

46 46 Comments on Model Accuracy Errors in highest estimated concentrations of 10 to 40 percent are found to be typical. Uncertainties do not indicate that an estimated concentration does not occur, only that the precise time and locations are in doubt. Section of Guideline on Air Quality Models

47 47 Sources of Uncertainty Model Formulation - Difficult to represent aspects of natural environment Model Inputs - Exact operating features not known at each time step Emission rate Stack gas velocity & temperature

48 48 Essentially, all models are wrong, but some are useful. George E. P. Box