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AERMOD CHANGES AND
UPDATES Sergio Guerra
27th Annual Conference on the Environment
November 13, 2012
What does AERMOD have in common
with a crystal ball?
Both are used to predict the future
What is an Air Quality Model?
An attempt to predict or simulate the ambient
concentrations of contaminants in an area of interest
An Air Quality Model can be as simple as an algebraic
equation or more complex
AERMOD
• AERMOD is a steady-state plume model that incorporates
air dispersion based on planetary boundary layer
turbulence structure and scaling concepts, including
treatment of both surface and elevated sources, and both
simple and complex terrain
• AERMOD replaced the Industrial Source Complex
(ISCST3) model as EPA’s regulatory model on December
9, 2006
• Preprocessors include: AERMET, AERMINUTE,
AERSURFACE, AERMAP, BPIP
Outline
1. Gently Sloping Terrain
2. AERMINUTE
3. EPA Formula Height
Gibson Generating Station
• Review of IDEM’s AERMOD Evaluation for the Gibson
Generating Station
• Robert Paine and Carlos Szembek (AECOM)
Gibson Generating Station
• The Indiana Department of Environmental Management
(IDEM) conducted an evaluation of AERMOD
• Gibson is an isolated source with 4 stacks and 3 nearby
monitors
• On-site met data and hourly SO2 emission data for 2010
• Comparison of monitored versus predicted concentrations
Gibson Generating Station
Gibson Generating Station
• Low winds produced highest concentrations (~0.5 m/s)
• Plume travel distance within an hour is short of the
distance needed to reach maximum receptors
• Formulation problem or coding error related to sigma-z
(used to calculate effective mixing lid)
1. Gently Sloping Terrain
Comparison of AERMOD Modeled 1-hour SO2
Concentrations to Observations at Multiple Monitoring
Stations in North Dakota
Paper 2012-A-353-AWMA
Mary M. Kaplan, Robert Paine (AECOM)
Evaluation Opportunity in North Dakota
• Mercer County: Antelope Valley Station and Great Plains
Synfuels Plant
• Electrical generating unit sources dominate SO2
emissions – hourly data available
• Five SO2 monitors in area within about 10 km of two
nearby “central” sources
• Site‐specific PSD quality meteorological data years
available (10‐m tower)
• Major SO2 sources within 50 km were modeled
• Five recent years of data were used
Dakota Gasification Co.
• Allowable emissions used for all sources, assumed to be
constantly at peak rates
• Receptors placed at monitor sites only, using actual
terrain (even though slopes are < 2%), except to
characterize the spatial concentration pattern
• Four of the five monitors were at elevations near local
stack base, a fifth monitor was about 100 m higher
Gently Sloping Terrain Evaluation
Test of generic tall stack buoyant source
• Modeled both flat and very gentle terrain
• Terrain case was uniformly sloped upward 1% in all
directions
• Modeled entire year of meteorology
• Obtained peak concentration on each ring of receptors
out to 50 km
• Plots follow for flat and gently sloping terrain
Gently Sloping Terrain
• AERMOD has unusual prediction result for very low wind,
stable conditions and low slope
• Problem is, in part, caused by very low mixing height that
leads to very compact plume
• Mixing height is below building obstacles, which the
model does not know about
• Plume stays perfectly level; terrain should not be
considered in these cases
• With terrain, result is an unexpected plume impact “hump”
at point of terrain impact
2. AERMINUTE
• Concentrations not calculated in AERMOD for hours with
calm or missing meteorological data
• AERMINUTE supplements calm hours with hourly values
based on two minute ASOS winds
• Non-regulatory component of AERMOD
• Purpose of AERMINUTE is not to increase conservatism
but to “reclaim” data that was lost due to METAR reporting
in NWS data
• Light wind conditions may be controlling factor in some
cases due to limited dilution
2. AERMINUTE
http://www.epa.gov/scram001/10thmodconf/presentations/1-7-aerminute_update.pdf
2. AERMINUTE
http://www.epa.gov/scram001/10thmodconf/presentations/1-7-aerminute_update.pdf
2. AERMINUTE
• Effect of AERMINUTE is also introduction of very low
speed winds that can result in significantly higher
concentrations
• EPA pending clarification memo
• 1-minute ASOS :
• Data representative
• More appropriate given AERMOD’s 1-hr time step
• EPA recommends its use to supplement NWS data
• Proposed 0.5 m/s (1 knot) threshold
• AERMET update will include an option to specify wind threshold for
1-minute ASOS data.
3. EPA formula height • Good Engineering Practice (GEP) stack height
• The stack height at which a plume released from a stack
is not excessively affected by downwash
• Hgep = Hb +1.5L
• Hb = building height above stack base
• L = lesser of building height and projected building width
• Created a discontinuity in the dispersion profile
http://www.epa.gov/apti/course422/ce1.html
3. EPA formula height
Before April 2011
• Before AERMOD v.
11059, building
downwash effects were
ignored if stack height
was greater to the GEP
stack height
• Created a discontinuity in
the dispersion profile
After April 2011
• Since AERMOD v. 11059
model no longer turns off
downwash above the EPA
formula height
• Downwash effects are
included for stack higher
than the GEP stack height
3. EPA formula height
• EPA: GEP height does not represent height when
downwash effects become negligible but 40% increase in
concentrations due to downwash at GEP height
• AERMOD 11059 “miscellaneous” change
• AERMOD no longer turns off downwash above the EPA formula
height
• Issue for facilities designed with the previous downwash
assumptions that if/when modeled again will experience
increase in concentrations
• Pending clarification memo to clarify this change
Conclusion
• Compliance is a Journey
• Knowing about the challenges related to gently sloping
terrain, ARMINUTE, and the EPA formula height can help
devise a strategy to reach your goal
Questions?
Sergio A. Guerra
Environmental Engineer
Phone: (651) 395-5225
