AERMOD CHANGES AND UPDATES

AERMOD CHANGES AND UPDATESSergio Guerra27th Annual Conference on the EnvironmentNovember 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

Outline1. Gently Sloping Terrain2. AERMINUTE3. 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 TerrainComparison of AERMOD Modeled 1-hour SO2Concentrations to Observations at Multiple Monitoring Stations in North DakotaPaper 2012-A-353-AWMAMary 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

Kaplan, M.; Paine, R. (2012). “Model Evaluation of AERMOD at a Monitoring Network in North Dakota.” Presented at the 10th EPA Modeling Conference, March 15, 2012.http://www.epa.gov/ttn/scram/10thmodconf/presentations/3-11-Evaluation_of_AERMOD_against_Monitoring_Data_in_North_Dakota_Paine.pdf



Gently Sloping Terrain EvaluationTest 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. GuerraEnvironmental EngineerPhone: (651) [email protected]

Education

AERMOD CHANGES AND UPDATES