ALL4's Dan Dix presented at the Pennsylvania Chamber Environmental and Energy Conference & Trade Show about the basics of air dispersion modeling and what facilities can do to determine where they stand with the NAAQS.

1. Attainment with the New
NAAQS and What You Need to
Know About Air Dispersion
Modeling
Dan P. Dix
Pennsylvania Chamber Environmental and
Energy Conference & Trade Show
April 18, 2012

2. Agenda
 Summary of NAAQS
 PSD Modeling Procedures
 NAAQS Implementation Updates
 Dispersion Modeling Basics and Inputs
 NAAQS Modeling Demonstration
Approach
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3. About ALL4
 Environmental consulting firm
 Founded 2002 – currently 30+ employees
 Offices in Kimberton, PA and Columbus, GA
 Specialize in air quality consulting:
• Complex air permitting and strategy development
• Air dispersion modeling
• Ambient air quality monitoring
 www.all4inc.com
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4. About ALL4
 ALL4’s customized environmental regulatory
update service.
 Includes concise tabular summary of changes
and events in state, local, and/or federal
regulations based on previous months
environmental activity.
 Also Includes a regular consulting call with an
environmental expert to discuss how the
activities in report impact your business.
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5. National Ambient Air
Quality Standards
(NAAQS)

6. NAAQS Background
 “Backdrop” of the Clean Air Act
 States design their SIPs and enforce and
implement their regulations to meet the
NAAQS
 Air quality construction permit programs are
designed around NAAQS compliance
• PSD: Maintaining NAAQS attainment
• NNSR: Getting into NAAQS attainment
 NAAQS reevaluated every 5 years
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7. NAAQS Summary
Averaging Historic Revised NAAQS
Pollutant Primary/Secondary
Period NAAQS (µg/m3) (µg/m3)
1-Hour Primary 40,000 10,000
CO
8-Hour Primary 10,000 40,000
Ozone 8-Hour Primary/Secondary 75 ppb Withdrawn
Pb 3-Month Rolling Primary/Secondary 1.5 0.15
PM10 24-Hour Primary/Secondary 150 150
24-Hour Primary/Secondary 65 35
PM2.5
Annual Primary/Secondary 15 15
1-Hour Primary N/A 188
NO2
Annual Primary/Secondary 100 100
1-Hour Primary N/A 196
3-Hour Secondary 1,300 1,300
SO2 24-hour Primary 365 Revoked
Annual Primary/Secondary 80 Revoked
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8. Attainment/Nonattainment Designations
 U.S. EPA philosophy on the SO2 NAAQS
implementation process:
• Proposed NAAQS – designations based on
ambient monitoring data
• Final NAAQS – designations based primarily
on air quality modeling data
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9. PSD Air Quality
Modeling Analysis

10. PSD Modeling Analysis
PSD Significance
Pollutant
Levels (TPY)
CO 100
Pb 0.6
So what happens PM 25
PM10 15
when PSD PM2.5 10
Significance Level NO2 40
SO2 40
are exceeded as H2SO4 10
part of a PSD TRS 10
H2S 10
permit application? VOC 40
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11. SIL Analysis
 Project related emission increases are
modeled and compared to the Significant
Impact Levels (SILs).
 A SIL exists for each NAAQS and is used
as a screening approach to determine if a
full NAAQS and PSD Increment Analysis is
required.
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12. SIL Analysis
SIL
Pollutant Averaging Period
(µg/m3)
1-Hour 2,000
CO
8-Hour 500
Pb 3-Month Rolling 1.5
PM10 24-Hour 5
24-Hour 1.2
PM2.5
Annual 0.3
NO2 1-Hour 7.5
1-Hour 7.9
SO2
3-Hour 25
If project related emissions result in predicted
concentrations greater than the SIL, a NAAQS and PSD
Increment Analysis is required.
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13. NAAQS Analysis
 Facility wide PTE emissions must be modeled
for any pollutant resulting in project related
emissions greater than the SIL.
 Local sources within 50 kilometers plus the
Significant Impact Area (SIA) must be included.
 Background concentrations from representative
monitors must be included.
 The cumulative impacts from all three must then
be compared to the NAAQS.
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14. PSD Increment Analysis
 A facility wide emission inventory must be prepared
for the actual emission increases that occurred since
the major source baseline date (the time at which
the first PSD project was completed for an area) and
compared to the PSD Increment Levels.
 Since these emissions are typically difficult to
calculate, conservatively use either the NAAQS
(PTE) emission rates or actual emissions from the
most recent two years.
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15. PSD Increment Analysis
Class I PSD Class II PSD Class III PSD
Pollutant Averaging Period Increment Increment Increment
(µg/m3) (µg/m3) (µg/m3)
1-Hour None None None
CO
8-Hour None None None
PM10 24-Hour 8 30 None
24-Hour N/A N/A None
PM2.5
Annual N/A N/A None
1-Hour N/A N/A None
NO2
Annual 2.5 25 None
1-Hour N/A N/A None
SO2
3-Hour 25 512 None
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16. AQRV Analysis
 An Air Quality Related Value (AQRV) Analysis
must be completed for any Class I area within
300 kilometers of the facility.
 The CALPUFF model is used for Class I areas
located more than 50 kilometers away.
 What is a Class I area?
• National parks greater than 6,000 acres that existed before
1977.
• Wilderness areas greater than 5,000 acres that existed before
1977 and are operated by the U.S. Forest Service or the U.S.
Fish and Wildlife Service.
 Q/d Screening Approach
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17. 17

18. NAAQS
Implementation
Updates

19. SO2 NAAQS Implementation
 NAAQS Implementation Schedule:
• June 2011: Initial state nonattainment
recommendations to U.S. EPA (most counties were
“unclassifiable”)
• June 2012: EPA to finalize attainment status (most
states will still be “unclassifiable” or attainment)
• June 2013: Maintenance SIP submittals including
individual facility modeling to achieve compliance
with the NAAQS (including air quality modeling for
individual facilities)
• August 2017: Full NAAQS compliance in all areas
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20. Implementation Update
 Draft guidance for states to evaluate designations using
AERMOD was released on September 22, 2011
 Numerous comments received on draft guidance.
 On April 12, 2012 Gina McCarthy (U.S. EPA Assistant
Administrator) issued a letter to all States stating that
modeling demonstrations showing attainment of the
standard for areas initially designated “unclassifiable” will
no longer be required for the June 2013 SIP Submittals.
 Instead U.S. EPA will be conducting “Stakeholder
Outreach” in order to discuss a workable approach for
implementation of the new standard.
 http://www.epa.gov/airquality/sulfurdioxide/implement.html
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21. SO2 Maintenance SIP Submittals
 U.S. EPA: Revising PSD/NNSR programs to include
new NAAQS is not sufficient. Five components are
required:
• “Attainment Emission Inventory”
• Maintenance Demonstration
• Control Strategy
• Contingency Plan
• Verification of Continued Attainment
 Maintenance SIP will list enforceable 1-hour emission
limits (August 2017)
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22. SO2 NAAQS Implementation
 State SIPs will be based on AERMOD dispersion
modeling for the following individual facilities (by
order of priority):
• SO2 Actual Emissions > 100 tons per year
• SO2 PTE > 100 tons per year
• Smaller facilities “with a potential to cause or
contribute” to a NAAQS violation
 States are considering other options based on
population
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23. SO2 NAAQS Implementation
 Facilities may wish to install on-site
meteorological tower, co-located SODAR,
and ambient SO2 measurements
• Collection of one-year of on-site meteorological data
• Collection of 3+ years of measurement data for SO2
NAAQS
• Track concurrent hourly SO2 emissions
• Evaluate performance of AERMOD or EPA-approved
alternative model
• Propose modeling approach using evaluation results
as guidance
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24. Dispersion Modeling
Basics and Inputs

25. AERMOD Process
Hourly Wind Speed
Hourly Wind Direction
Hourly Ambient
Temperature
Land Use Patterns Predicted Ground Level
Topography Ambient
Concentrations (µg/m3)
Building Dimensions for all averaging times
Stack Dimensions
Exhaust Velocity
Exhaust Temperature
Emission Rates
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26. AERMOD Modeling System
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27. Air Quality Modeling Steps
1. Emission Inventory
2. Meteorological Data
(AERMET/AERSURFACE)
3. Terrain Data (AERMAP)
4. Building Downwash (BPIPPRM)
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28. Emission Inventories
 Short-term (1-hour) emission rates
 Potential to be used as permit limits
 Intermittent emission units (e.g., emergency
generators, intermittent emission scenarios such as
startup/shutdown operations or alternative fuels)
• Latest guidance indicates following form of
standard as guideline for what to include (i.e., 99th
percentile (4th highest) for SO2 or 98th percentile
(8th highest) for NO2 & PM2.5)
 Stack characteristics (height, temperature, velocity,
diameter, location)
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29. Meteorological Data
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30. Meteorological Data
 5 years of National Weather Service data
 Minimum of 1 year of onsite data
 Surface characteristics and topography
surrounding the facility should be similar to
(representative of) those surrounding the
meteorological station
 If no representative meteorological data are
available, SO2 implementation guidance
suggests possibility of using AERSCREEN (with
agency approval)
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31. Terrain Data
 “Ambient Air”
 Public access must be
restricted in some way
(e.g., fence, security
guard) in order for
onsite receptors to be
disregarded in the
modeling analysis
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32. Building Downwash
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33. Building Downwash
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34. NAAQS Modeling
Demonstration
Approach

35. Full NAAQS Evaluation
 Includes facility, other local facilities, and
background concentrations
 Any modeled emission rates should be
acceptable as a 1-hour permit limit (for
NO2 and SO2) with the appropriate margin
for compliance
 Considerations for accounting for
emissions during startup and shutdown
 Emergency unit considerations
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36. Local Sources
 Same emission rate considerations apply
for local sources (although permit limit
concerns wouldn’t apply)
 State agency typically dictates which local
sources to include in evaluation
 Typically 50km + Significant Impact Area
(SIA)
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37. NAAQS Modeling Strategy
 Start with an evaluation of each individual
emission source
 Each source will have different factors that
drive resulting ambient concentrations
 The cumulative ambient concentration
from all sources (plus background) will be
evaluated against the NAAQS
 Evaluate each source against the NAAQS
as a first step
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38. NAAQS Modeling Strategy
 Big picture factors that will drive ambient
concentrations for individual sources:
• Elevated emission rates
• Stack velocity (orientation of release and
flowrate)
• Stack temperature (plume buoyancy)
• Stack height versus surrounding terrain
• Surrounding buildings and structures (i.e.,
building downwash)
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39. Hypothetical Modeling Example
 Modeling of a hypothetical facility with the
following SO2 emission sources:
• Process SO2 source
• Backup engine source
 NAAQS modeling evaluation is based on
SO2 potential-to-emit
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40. Hypothetical Facility Terrain
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41. “Process” SO2 Source
 SO2 Emission Rate: 240 lb/hr (CEMS)
 Stack Height: 290 feet
 Stack Diameter: 16.5 feet
 Exhaust Temp: 350 °F
 Exhaust Flow: 230,000 acfm
 Elevated emission rate, buoyant source,
tall stack (taller than the tallest buildings at
the facility)
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42. Process SO2 Source Impacts
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43. Process SO2 Source Impacts
 Highest impacts in complex terrain far from
facility
 Wind speed doesn’t match location of elevated
concentrations
 Impacts occur during periods of atmospheric
stability and low mixing heights (typically early
morning, low wind speed conditions)
 High concentrations due partially to the
limitations of the AERMOD dispersion model
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44. Combustion SO2 Source
 SO2 Emission Rate: 20 lb/hr (AP-42)
 Stack Height: 60 feet
 Stack Diameter: 2 feet
 Exhaust Temp: 225 °F
 Exhaust Flow: 16,000 acfm
 Buoyant source, short stack (shorter than
the tallest buildings at the facility)
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45. Engine SO2 Source
 SO2 Emission Rate: 3 lb/hr (Vendor)
 Stack Height: 10 feet
 Stack Diameter: 1.3 feet
 Exhaust Temp: 935 °F
 Exhaust Flow: Horizontal Discharge
 Horizontal discharge, short stack
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46. Engine SO2 Source Impacts
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47. Engine SO2 Source Impacts
 Elevated ambient concentrations at the
facility fenceline for two reasons:
• Low stack height (10 feet)
• No plume buoyancy due to horizontal
discharge
 Ambient air considerations become very
important (i.e., public access)
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48. Modeling Refinements
 “Process” SO2 Emission Source:
• Stack height increase is technically and
economically infeasible
• Raw materials are fixed due to product
and consumer demand
• Upgrades to the scrubber could achieve
control: ~30% more control (~170 lb/hr)
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49. Process SO2 Source Impacts (Before)
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50. Process SO2 Source Impacts (After)
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51. Modeling Refinements
 Engine SO2 Emission Source:
• Simplest fix is to change the stack
discharge orientation from horizontal to
vertical
• No changes to the vendor-guaranteed
emission rate of the engine
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52. Engine SO2 Source Impacts (Before)
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53. Engine SO2 Source Impacts (After)
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54. Cumulative Concentrations
 The facility must cumulatively comply with
the NAAQS
 Addressing each individual source helps
as a first cut
 This scenario still exceeds the 1-hour
NAAQS for SO2 when the sources are
taken cumulatively
 Haven’t even considered ambient
background concentrations
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55. Modeling Strategies
 Emissions Strategies
 Actual Distribution of Emissions
• Evaluate adequacy of emission limits
• Evaluate emissions control options
• Evaluate alternate fuels and fuel specifications
 Facility Fence Line Strategies
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56. Modeling Strategies
 Stack/Exhaust Strategies:
• Combined source exhausts
• Co-located exhaust points to increase
buoyancy
• Turn horizontal stacks vertical
• Increase stack heights
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57. Modeling Strategies
 Plume transport time
 Surrounding surface characteristics
 Wind speed monitor thresholds
 Mechanical mixing height
considerations
 Alternative models (e.g., CALPUFF)
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58. Final Thoughts
 PSD Modeling
• New 1-hour SO2 and NO2 standards extremely stringent
 Conduct modeling for planning purposes
 Consider doing through attorney under attorney-client privilege
 Your facility could be modeled if another nearby source is
conducting PSD modeling
 SIP Implementation Modeling
• Be involved in U.S. EPA’s Stakeholder Outreach process
• Be Aware that Environmental Group are modeling facilities with
publically available information
• Consider collecting site-specific meteorological and SO2 ambient
monitoring data
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59. Questions?
Dan Dix
ddix@all4inc.com
(610) 933-5246 x18
2393 Kimberton Road
PO Box 299
Kimberton, PA 19442
All4 Inc.
www.all4inc.com
www.enviroreview.com
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