The new EPA guideline on air quality models makes several changes, including adopting AERMOD version 16216r as the new default model. It establishes a two-tiered approach for modeling ozone and secondary PM2.5 formation, using existing empirical relationships (Tier 1) or chemical transport models (Tier 2). CALPUFF is no longer preferred for long-range transport modeling beyond 50 km. The guideline also allows the use of prognostic meteorological data in some cases. While the changes aim to promote consistency, the increased flexibility may lead to legal challenges and delays.

1. www.cppwind.comwww.cppwind.com
New Guideline on Air
Quality Models and the
Electric Utility Industry
EUEC 2017, San Diego, CA
Sergio A. Guerra, PhD
February 9, 2017

2. Outline
1. Background
2. Highlights of Revisions to Modeling Guideline
3. Main Changes
2 New Guideline on Air Quality Models and the Electric Utility Industry

3. Regulatory Modeling History
Appendix W from 40 CFR Part 51: Guideline on Air Quality Modeling
– Purpose is to promote consistency
– Periodically revised to ensure that new model developments or
expanded regulatory requirements are incorporated
– Defines the accepted regulatory models
• 1978 Original Guideline
• 1986 Revised Guideline
• 1987 Supplement A- Preferred Models
• 1993 Supplement B- Alternative Models
• 1995 Supplement C – Updates and revisions
• 2003 Revised Guideline
• Nov 2005 Revised Guideline
• Jan 2017 Revised Guideline*
3 New Guideline on Air Quality Models and the Electric Utility Industry

4. Appendix W Revisions
• Proposed on July 2015
• 11th Conference on Air Quality Modeling: August 12
& 13 https://www3.epa.gov/scram001/11thmodconf.htm
• 90 day public comment period
• Finalized on December 20, 2016
– Included AERMOD version 16216
• January 17, 2017: Published in the FR
– Including a revised AERMOD version 16216r
– Effective day February 16, 2017
• Jan 26, 2017 effective day delay until March 21, 2017
4 New Guideline on Air Quality Models and the Electric Utility Industry

5. Related EPA Actions
• 8-hour Ozone NAAQS strengthened from
75 to 70 ppb
• Draft SIL values
– 1-ppb for 8-hour Ozone and
– 1.2 mg/m3 for 24-hr PM2.5
– 0.2 mg/m3 for annual PM2.5
New Guideline on Air Quality Models and the Electric Utility Industry5

6. AERMOD Changes
• AERMOD v16216r
– New Default Options
• Adjusted u-star
• Tier 2 and Tier 3 Options for NOx to NO2 conversion
• Horizontal and capped stacks
– New Adjusted u-star
• “Bug” change between v15181 vs v16216r
– Tall stacks near small urban areas
6 New Guideline on Air Quality Models and the Electric Utility Industry

7. AERMOD Changes
NOx to NO2 Conversion:
• Tier 2: ARM2 replaces ARM
– NO2/NOx Minimum Ambient Ratio (MAR) of 0.5
– Alternative lower MARs need justification/approval
New Guideline on Air Quality Models and the Electric Utility Industry7
Porez, M. (2015), An update to the ambient ratio method for 1-h NO2 air
quality standards dispersion modeling, Atmos. Env.
http://dx.doi.org/10.1016/j.atmosenv.2014.12.021

8. AERMOD Changes
NOx to NO2 Conversion:
• Tier 3: OLM and PVMRM now regulatory default
options
• PVMRM2 replaces PVMRM
New Guideline on Air Quality Models and the Electric Utility Industry8

9. Adjusted u-star “Bug” Fix
• Adjusted u-star is an option in AERMET to
adjust the surface friction velocity (u*) to
address issues with AERMOD model
overprediction under stable, low wind speed
conditions
• AERMET v 16216r reduced ADJ_U* for wind
speeds approaching zero by a factor of 2
• This action diminishes the effectiveness of the
low wind correction and can lead to increases
in concentrations
9 New Guideline on Air Quality Models and the Electric Utility Industry

10. Backburner
• Probabilistic methods
• Lowwind 3 option
• 3rd party review of updates to AERMOD
New Guideline on Air Quality Models and the Electric Utility Industry10

11. Guideline Changes
Revisions include:
– O3 and Secondary PM2.5 modeling
– Delisting of CALPUFF for LRT
– Prognostic Meteorological Data
11 New Guideline on Air Quality Models and the Electric Utility Industry

12. O3 and Secondary Formation of PM2.5
• July 29, 2010 Sierra Club submitted petition to
address secondary PM2.5 and ozone
• January 4, 2012 EPA granted Sierra Club’s
petition
12 New Guideline on Air Quality Models and the Electric Utility Industry

13. O3 and Secondary Formation of PM2.5
App W establishes a two tiered approach through:
"Guidance on the Development of Modeled Emission Rates for Precursors (MERPs) as a
Tier I Demonstration Tool for Ozone and PM2.s under the PSD Permitting Program”
https://www3.epa.gov/ttn/scram/guidance/guide/EPA-454_R-16-006.pdf
Comment period extended to March 31, 2017
Tier 1
– Use existing empirical relationships between
precursors and secondary formation
• Tier 2
– Use a chemical transport model
13 New Guideline on Air Quality Models and the Electric Utility Industry

14. O3 and Secondary Formation of PM2.5
Tier 1
• Use existing empirical relationships between
precursors and secondary formation
• Based on existing AQ Modeling e.g., modeling
performed for SIP, permit actions, similar
policy assessment, AQ modeling of
hypothetical industrial sources
• Also screening approaches based on full
science chemical transport modeling systems
(e.g., reduced form models)
14 New Guideline on Air Quality Models and the Electric Utility Industry

15. Step 1: Compare with Most
Conservative MERPs
• Table 7.1 Most Conservative (Lowest)
Illustrative MERP Values (tons per year) by
Precursor, Pollutant and Region.
15 New Guideline on Air Quality Models and the Electric Utility Industry
https://www3.epa.gov/ttn/scram/appendix_w/2016/MERPs_WebinarPresentation_01192017.pdf

16. Step 2: Develop Case Specific MERP
MERP = Cthr x ModE-rate / ModImpact
MERP= in tpy
Cthr = Critical AQ Threshold in (e.g., SIL) in µg/m3 or ppb
ModE-rate = Modeled Emission Rate from hypothetical source in
g/s
ModImpact = Modeled air quality impact in µg/m3 or ppb
New Guideline on Air Quality Models and the Electric Utility Industry16

17. MERP Development
Scenario B Example
Ozone Analysis for Source in Southeast region:
• 0 tpy PM2.5 (primary)
• 0 tpy VOC
• 310 tpy of NOx
• 75 tpy of SO2
New Guideline on Air Quality Models and the Electric Utility Industry17

18. MERP Development
Scenario B Example
Ozone Analysis for Source in Southeast region:
• 0 tpy PM2.5 (primary)
• 0 tpy VOC
• 310 tpy of NOx > 126 tpy from table 7-1
• 75 tpy of SO2 < not an ozone precursor
MERP = Cthr x ModE-rate / ModImpact
= 1.0 ppb x 500 tpy / 1.53 ppb (Source 19 , H in Table A-1)
MERP = 327 tpy > 310 tpy of NOx = no Tier 2 needed
New Guideline on Air Quality Models and the Electric Utility Industry18

19. MERP Development
Scenario B’ Example
Ozone Analysis for Source in Southeast region:
• 0 tpy PM2.5 (primary)
• 0 tpy VOC
• 500 tpy of NOx > 126 tpy from table 7-1
• 75 tpy of SO2 < not an ozone precursor
MERP = Cthr x ModE-rate / ModImpact
= 1.0 ppb x 500 tpy / 1.53 ppb (Source 19 , H in Table A-1)
MERP = 327 tpy < 500 tpy of NOx = MERP exceeded!
New Guideline on Air Quality Models and the Electric Utility Industry19

20. O3 and Secondary Formation of PM2.5
Tier 2
• If above MERPs, chemical transport
models (CTMs) to be used for ozone
and secondary PM2.5 impacts
• Photochemical grid models seem to be
preferred (i.e., CMAQ and CAMx)
• Lagrangian puff models (e.g.,
SCICHEM) may be appropriate also
20 New Guideline on Air Quality Models and the Electric Utility Industry

21. O3 and Secondary Formation of PM2.5
MERPs: Tier 1
• Information from a different region may be used
based on comparisons of:
– Avg. and peak temperatures
– Humidity
– Terrain
– Rural or urban location
– Nearby regional sources (e.g., biogenics or anthropogenic
sources)
– Ambient concentrations of relevant pollutants
21 New Guideline on Air Quality Models and the Electric Utility Industry

22. Long Range Transport Modeling
• CALPUFF no longer the preferred model for
LRT (> 50 km)
• Still allowed for screening purposes
• In near field (< 50 km) still alternative model
for complex terrain and complex winds
• CALPUFF still the Federal Land Managers
(FLM) preferred model for Class I Air Quality
Related Value (AQRV)
New Guideline on Air Quality Models and the Electric Utility Industry22

23. Meteorological Data
Prognostic meteorological data
• Allowed when no representative NWS station
and infeasible to collect site-specific data
• No fewer than 3-years of data
• Mesoscale Model Interface Program (MMIF)
developed to read prognostic data (i.e., WRF)
to create data for input into AERMET and
AERMOD
New Guideline on Air Quality Models and the Electric Utility Industry23

24. Model Clearinghouse
• Applicant sends request to regulatory agency
• Regulatory agency reviews and submits
recommendation to EPA Regional Office
• EPA RO reviews and sends to Model
Clearinghouse
• MCH engages and consults on solutions
• MCH issues formal concurrence
• MCH decisions are archived in Searchable
database (MCHISRS): http://cfpub.epa.gov/oarweb/MCHISRS/
New Guideline on Air Quality Models and the Electric Utility Industry24

25. Summary
The good:
• Default options will help achieve compliance
– NOx-NO2 conversion
– Adjusted u-star
• Prognostic meteorological data potential of savings
in cost and time for new projects
New Guideline on Air Quality Models and the Electric Utility Industry25

26. Summary
The challenges:
• Changes to the adjusted u-star option
• Tier 2 requirements for Ozone and Secondary
PM2.5
• Long Range Transport
• Too much flexibility may lead to legal
challenges and potential delays
New Guideline on Air Quality Models and the Electric Utility Industry26

27. Take Away
• Case-by-case basis will be the norm
• Modeling protocols will be very important
• Communication and coordination with
regulatory agency, EPA Regional Office and
Model Clearinghouse is paramount
New Guideline on Air Quality Models and the Electric Utility Industry27

28. Sergio A. Guerra, PhD
sguerra@cppwind.com
Mobile: + 612 584 9595
CPP, Inc.
2400 Midpoint Drive, Suite 190
Fort Collins, CO 80525
+ 970 221 3371
www.cppwind.com @CPPWindExperts
Questions?
28 New Guideline on Air Quality Models and the Electric Utility Industry