This document discusses air dispersion modeling for environmental permitting. It provides an overview of the EPA-approved models for different types of areas and pollutants. It also summarizes key aspects of modeling including significance thresholds, modeling stages, averaging periods, sources, receptors, meteorological data requirements, and potential challenges. Common questions around stack height, downwash effects, and acceptable input tolerances are also addressed.

1. What Every EHS Staff
Should Know about
Monitoring and Modeling
Robynn Andracsek, P.E.
randracsek@burnsmcd.com
816-822-3596
www.burnsmcd.com/columns
May 11, 2016

2. The Air Really is Getting Cleaner…
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3. The EPA Approved Models
 Class II Area Modeling
– AERMOD – current EPA-accepted model
 NAAQS and Class Prevention of Significant Deterioration
(PSD) II increment
 Class I Area Modeling
– CALPUFF model
– Visibility, Deposition and PSD Class I increment
 Front-end software usually used
– Breeze, BEEST, Lakes

4. Modeling Stages
(Construction Permitting)
1. Stage 1 – Model the Project
– Modeling significance
– Monitoring significance
– If below both, modeling for PSD is done
2. Stage 2 – Refined (Cumulative) Modeling
– National Ambient Air Quality Standards
– Prevention of Significant Deterioration Increment
Standards
– Neighboring sources included

5. Significance Modeling
 Model the new (Project)
emissions
 If modeling significant
impact level is exceeded
– Model entire facility and
inventory sources within
radius of impact plus 50 km
 If monitoring de minimis
level is exceeded
– Install your own monitor and
gather one year of data
– Use an existing monitor
(if state will allow that)
Pollutant Average
Class II
Modeling
Significant
Impact Level
(SIL)
µg/m3
Monitoring
de Minimis
Level
µg/m3
PM10 24-hour 5 10
PM2.5
24-hour 1.2 4
Annual 0.3 --
SO2
1-hour 7.8 --
3-hour 25 --
24-hour -- 13
NO2
1-hour 7.52 --
Annual 1 14
CO
1-hour 2,000 --
8-hour 500 575

6. Radius of Impact (ROI)
 Model only the new sources
 Find farthest distance to an impact greater
than or equal to SIL (example shows 17 km
as significance impact area)
 Add 50 km to get ROI
 ROI is 67 km
 Include all neighboring
sources inside ROI

7. NAAQS vs. PSD Increment
 National Ambient Air Quality Standards
– Ceiling of air quality pollution allowed
– Most point sources at facility and neighbors within 50-100 km
radius
– Plus add in background
 Prevention of Significant Deterioration Increment
– Incremental degradation allowed above baseline in areas
with good air quality
– Only sources modified after baseline date
– Specifies the maximum extent to which the ambient
concentration may be allowed to increase above the legally
defined baseline concentration in an area with clean air.

8. Baseline Date Rule of Thumb
 Anything constructed before 1977 is NOT PSD
increment consuming

9. NAAQS
Pollutant Average µg/m3 ppm
PM10 24-hour 150 --
PM2.5
24-hour 35 --
Annual 12 --
SO2
1-hour 195 0.075
3-hour 1,300 0.5
NO2
1-hour 188 0.1
Annual 100 0.053
Ozone
1-hour -- 0.12
8-hour -- 0.070
CO
1-hour 40,000 35
8-hour 10,000 9
Lead 3-hour rolling 0.15 --
Notes:
• Models results are
in terms of
micrograms per
cubic meter (µg/m3)
• Standards for
gaseous pollutants
are in parts per
million (ppm)
• Ozone (volatile
organic compounds
[VOC]) is not
usually modeled

10. NAAQS Averaging Periods
By Average
 Annual
– PM2.5, NOX
 24-hour
– PM2.5, PM10
 3-hour
– SO2
 1-hour,
– CO, SO2, NOX
 8-hour
– CO , Ozone
By Pollutant
 PM2.5
– 24-hour, annual
 PM10
– 24-hour
 NOX
– 1-hour, annual
 SO2
– 1-hour, 3-hour
 CO
– 1-hour, 8-hour
 Ozone

11. More Than Just Emissions
Emergency Generator Coal-fired Boiler

12. Dispersion Modeling – Challenges
 NOx
– Previously only annual average
– 1-hour NO2 NAAQS Standard
– 1-hour NO2 level that triggers inclusion of nearby
sources
– May use “Tier 3” methods for reducing impacts
 New methods are being reviewed (ARM2)
 PM2.5 NAAQS standard
– Low standard
– High backgrounds
– Increment hard to meet

13. “How Tall Should the Stack Be?”
 Good Engineering Practice (GEP) stack height
– Maximum stack height allowed
in model
– Roughly 2.5 times building height
– Dominant building can be a building
on which the stack is not located
 Minimum stack height dictated by dispersion
modeling to meet NAAQS, increment
– Not every project requires modeling

14. Modeling Issues to Expect
 Site-layout/building/stack design to play a significant role
 Downwash from buildings can impact results significantly
 Issues with neighboring sources
 Background values can be very high
 Modeling operating loads and startup/shutdown emissions
 Site size
 If the project will be installed in phases, expect to
perform modeling for the entire project to avoid any
surprises
 Can look at combining stacks

15. Sources
 Point
– Stacks
 Volume
– Drop points
– Roads
 Area
– Piles (wind erosion)
– Roads (depends on state)

16. Point Sources
 Temperature
 Velocity
 Diameter
 Height
 Location
 Base elevation
 Emission rate

17. Initial Stack Dispersion
}
}
Temperature
Velocity

18. Rain Caps
} Temperature
Velocity = 0
Use “tractor flap”
exhaust if possible
Rain Caps are BAD

19. Downwash “Shadow”

20. Receptors
 Fence line
– Physical barrier
– Not the same as the
property line
 Spacing
– Along fence
– Larger spacing farther away from plant
– “Hot Spot” grids
 Import terrain elevations

21. Receptor Grid Example
•Along the fence line
boundary at a
spacing of 25 meters
•25-meter out to 100
meters
•100-meter out to one
kilometer
•500-meters out to
five kilometers
•1,000-meter out to
twelve kilometers.

22. Meteorological Data
 5 years of hourly data
– Surface plus upper air
– Usually a nearby
airport
 Some states provide
data sets that must
be used
 State may require on-
site meteorological
data (1 year delay)

23. Windrose

24. Graphical Output

25. How Accurate Do Modeling
Inputs Need to Be
My Personal Rules of Thumb for Tolerance
 Height 1 foot or < 10%
 Diameter 6 inches or < 5%
 Velocity < 5%
 X,Y location ± 5 feet
 Elevation ± 5 feet
 Temperature ±20°F
 Emissions ± 2%

26. What To Do If Model Doesn’t Pass
 Apply advanced methods
 Use hour-of-day limits
 Raise stacks
 Add controls
 Water roads or piles
 Make stacks vertical unobstructed
 Move fence line out to property line
 Move equipment
 Limit operation of “intermittent” sources

27. Modeling Review
 Modeling is an art and a science
 Start conservative and work towards reality
 Rain caps are evil
 Ambient air is outside the physical fence
 Good Engineering Practice (GEP) stack height
is maximum stack height allowed (not
recommended or minimum)
 Limits in modeling become limits in permit

28. Questions?
Robynn Andracsek, P.E.
randracsek@burnsmcd.com
816-822-3596
www.burnsmcd.com/columns