1. Dispersion Modeling 101: ISCST3 vs. AERMOD
Iowa Chapter AWMA
February 14, 2006
Mick Durham
Stanley Consultants, Inc.

2. What we are going to talk about
Brief History of Dispersion Modeling
Industrial Source Complex Model
AMS/EPA Regulatory Model (AERMOD)
Comparisons
The IDNR Connection
Questions & Answers

3. Brief History of Modeling
Earliest Studies Simulated the Movement of Air
G.I. Taylor, 1915, Eddy Motion in the Atmosphere
O.G. Sutton, 1932, A Theory of Eddy Diffusion
Dispersion of Pollutants (Mainly Particulate) Followed WW II
E.W. Hewson, 1945, Meteorological Control of Atmospheric Pollutants by Heavy Industry
E.W. Hewson, 1955, Stack Heights Required to Minimize Ground Level Concentrations
Gale, Stewart & Crooks, 1958, The Atmospheric Diffusion of Gases Discharged from a Chimney

4. Brief History of Modeling
Birth of Dispersion Parameters
F.A. Gifford, 1960, Atmospheric Dispersion Calculations Using the Gaussian Plume Model
F. Pasquill, 1961, The Estimation of the Dispersion of Windborne Material
D. Bruce Turner, 1967, Workbook on Atmospheric Dispersion Estimates
Briggs, Gary, 1969, Plume Rise

5. Brief History of Modeling
Modeling and the Computer Age
PTMAX, PTMIN, PTMTP, 1972
Air Quality Display Model (AQDM), 1974
Single Source (CRSTER) Model, 1977
Complex Terrain (VALLEY) Model, 1977
Multiple Source (MPTER) Model, 1980
Pollutant and Environment Specific Models
APRAC, CALINE, HIWAY Carbon Monoxide Models
BLP (Bouyant Line and Point Sources); PAL (Point Area and Line Source), 1979; TEM (Texas Episode for Urban Areas)
RPM (Reactive Plume Model) for Ozone, 1980

6. Brief History of Modeling
Guideline on Air Quality Models
The Guidelines on Air Quality Models, 1978
40 CFR Part 58, Appendix W
Refined and More Complex Models
Industrial Source Complex (ISC), 1979
Industrial Short-Term ST
Industrial Long-Term LT
Complex Terrain (COMPLEX)
Dense Gas (DEGADIS)
Urban Airshed Model (UAM)

7. Brief History of Modeling
Refined and More Complex Models (cont.)
Screening Model (SCREEN)
California Line Source (CALINE) and Mobile Source Emission Factors (MOBILE)
Puff Models (INPUFF)
Visibility (VISCREEN)

8. Brief History of Modeling
Advanced Models
Industrial Source Complex Version 2 (ISC2), 1990
Industrial Source Complex Version 3 (ISC3), 1995
California Line Source (CAL3QH3)
Urban Airshed Model (UAM-V)
Complex Terrain Dispersion Model (CTDMPLUS)
Offshore and Coastal Dispersion Model (OCD)
Bouyant Line and Point Source (BLP)
Area Locations of Hazardous Atmospheres (ALOHA)
Dense Gas Dispersion Model (DEGADIS 2.1)

9. Brief History of Modeling
Today’s Models:
AERMOD
Point, Area, Line Sources
Simple or Complex Terrain
Transport distance up to 50 km
CALPUFF
Transport from 50 to hundreds of kilometers
Visibility, Regional Haze
Dispersion in Complex Terrain
Complex Dispersion Model Plus Algorithms for Unstable Conditions (CTDMPLUSS)

10. Brief History of Modeling
Today’s Models (Continued):
Caline3 or CAL3QHC, MOBILE6
Highway Line sources
Simple Terrain
Carbon Monoxide
Buoyant Line and Point Source (BLP)
Aluminum Reduction plants with buoyant line and point sources
Rural location
Community Multi-scale Air Quality Model (CMAQ)
Ozone

11. Industrial Source Complex Model
Introduced in 1979
First adopted as Preferred Model in 1983
Major Revisions 4 times in 27 year history
Can remain acceptable as a preferred model until November 9, 2006

12. Industrial Source Complex Model
Gaussian Plume Model
Building Downwash
Particulate Deposition
Point, Area, and Line Sources
Complex Terrain
Simple Meteorological Data Input

13. Industrial Source Complex Model
Has been primary model in Iowa for 27 years
Over 100 facilities have modeled compliance with ISC
Generally the short-term standards have caused greatest predicted non-compliance

14. Industrial Source Complex Model
Problems with ISCST3:
Modeling of Plume Dispersion is Crude
Only 6 possible states (Stability Classes)
No variation in most meteorological variables with height
No use of observed turbulence data
No information about surface characteristics
Erroneous depiction of dispersion in convective conditions
Substantial overprediction in complex terrain
Crude building downwash algorithm

15. AERMOD
AERMOD stands for American Meteorological Society/ Environmental Protection Agency Regulatory Model
Formally Proposed as replacement for ISC in 2000
Adopted as Preferred Model November 9, 2005

16. AERMOD 3 COMPONENTS 2 SUPPORT TOOLS
AERMET – THE METEOROLOGICAL PREPROCESOR
AERMAP – THE TERRAIN DATA PREPROCESSOR
AERMOD – THE DISPERSION MODEL
2 SUPPORT TOOLS
AERSURFACE – PROCESSES SURFACE CHARACTERISTICS DATA
AERSCREEN – PROVIDES A SCREENING TOOL

17. AERMOD AERMOD IS SIMILAR TO ISC IN SETUP
THE CONTROL FILE STRUCTURE IS THE SAME
VIRTUALLY ALL THE CONTROL KEYWORDS AND OPTIONS ARE THE SAME

18. AERMOD AERMOD IS DIFFERENT FROM ISC
REQUIRES SURFACE CHARACTERISTICS (ALBEDO, BOWEN RATIO, SURFACE ROUGHNESS) IN AERMET
HAS PRIME FOR BUILDING DOWNWASH AND THE BUILDING PARAMETERS ARE MORE EXTENSIVE
REQUIRES LONGER COMPUTER RUN TIMES (up to 5 times longer!)

19. Comparison of Dispersion Model Features: Meteorological Data Input
– ISCST3:
• One level of data accepted
– AERMOD:
• An arbitrarily large number of data levels can be accommodated

20. Comparison of Dispersion Model Features: Plume Dispersion and Plume Growth Rates
ISCST3:
• Based upon six discrete stability classes only
• Dispersion curves are Pasquill-Gifford
• Choice of rural or urban surfaces only
AERMOD:
• Uses profiles of vertical and horizontal turbulence variable with height
• Uses continuous growth function
• Uses many variations of surface characteristics

21. Comparison of Dispersion Model Features: Complex Terrain Modeling
ISCST3:
• Elevation of each receptor point input
• Predictions are very conservative in complex terrain
AERMOD:
• Controlling hill elevation and point elevation at each receptor are input
• Predictions are nearly unbiased in complex terrain

22. Comparisons ISC Vs AERMOD
CONSEQUENCE ANALYSIS - ratios of AERMOD predicted high concentrations to ISCST3 predicted high concentrations:
flat and simple terrain
point, volume and area sources.
1hour 3hour 24hour annual
average
high
low
Total
AN OVERVIEW FOR THE 8TH MODELING CONFERENCE SEPTEMBER 22, 2005

23. Comparisons ISC Vs AERMOD
CONSEQUENCE ANALYSIS - ratios of AERMOD predicted high concentrations to ISCST3 (and PRIME) predicted high concentrations:
flat terrain
point sources with significant bldg downwash
ANNUAL H2H H2H
AER/ISC3 AER/ISCP AER/ISC3 AER/ISCP AER/ISC3 AER/ISCP
ave
max
min
No cases
AN OVERVIEW FOR THE 8TH MODELING CONFERENCE SEPTEMBER 22, 2005

24. Comparisons ISC Vs AERMOD
Duane Arnold Energy Center Data (Palo, IA)
Ratio of Modeled Conc to Observed:
AERMOD: (1-hr avg 46m release)
ISC-Prime: (1-hr avg 46m release)
AERMOD: (1-hr avg 24m release)
ISC-Prime: (1-hr avg 24m release)
AERMOD: (1-hr avg 1m release)
ISC-Prime: (1-hr avg 1m release)

25. Comparisons ISC Vs AERMOD
Presentation at EUEC conference by Bob Paine, TRC:
AERMOD consistently showed better or comparable performance with ISCST3
In flat terrain, AERMOD and ISCST3 predictions are comparable, but AERMOD has higher annual averages
In complex terrain, AERMOD predictions are markedly lower
Building downwash predictions will often be lower, especially for stacks located some distance from controlling buildings
Overall, more confidence in accuracy of AERMOD results

26. Comparisons ISC Vs AERMOD
Our Recent Experience:
Annual concentrations higher with AERMOD by 10-15%
Short term concentrations similar without downwash
Short-term concentrations generally lower with building downwash by 20%

27. The IDNR Connection
IDNR will allow use of either ISCST3 or AERMOD until November 9, 2006
Meteorological Data will be provided by IDNR for eight stations
Compliance with ISCST3 and non-compliance by AERMOD must be addressed

28. Questions & Answers

30. AERMOD

31. AERMOD

32. AERMOD