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On January 4, 2012, the EPA committed to engage in rulemaking to evaluate updates to the Guideline on Air Quality Models (AppendixWof 40 CFR 51) and, as appropriate, incorporate new analytical techniques or models for secondary PM2.5. As a result, the National Association of Clean Air Agencies (NACAA) developed a screening method involving offset ratios to account for secondary PM2.5 formation. This method can be used to evaluate total (direct and indirect) PM2.5 impacts for permitting purposes. Therefore, the evaluation of this method is important to determine its viability for widespread use.
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EVALUATION OF SO2 AND NOXOFFSET RATIOS TO ACCOUNT FORSECONDARY PM2.5 FORMATIONSergio Guerra, Shannon Olsen, Jared AndersonAWMA Specialty ConferenceMarch 20, 2013
Background: PM2.5 Secondary Formation• On January 4, 2012, the EPA granted a petition submitted
on behalf of the Sierra Club on July 29, 2010.• EPA committed to engage in rulemaking to evaluate
updates to the Guideline on Air Quality Models as published as Appendix W to 40 CFR 51, and, as appropriate, incorporate new analytical techniques or models for ozone and secondary PM2.5.
PM2.5 Offset Ratios• EPA’s NSR implementation rule for PM2.5 (73 FR 28321,
May 16, 2008).• Ratios first introduced by the EPA for nonattainment areas to offset
emissions increases of direct PM2.5 emissions with reductions of PM2.5 precursors and vice versa.
• On July, 21 2011, the EPA changed their position and established that these offset ratios were no longer considered presumptively approvable but must be subject to a technical demonstration.
NACAA’s Recommended Approach for Assessing Secondary PM2.5
EPA’s PM2.5 Compliance Demonstration: Assessment Cases
Minnesota-specific Offset Ratios• Developed by MPCA modelers using CAMx.• Secondary PM2.5 emission rate is defined as the sum of
the SO2 emission rate divided by 10 and the NOxemission rate divided by 100.
• The total equivalent emission rate is to be used in AERMOD modeling demonstrations to show compliance with the PM2.5 NAAQS.
How were they developed• The EPA ratios are based on the 75th percentile
distribution for NOx and on the 90th percentile distribution for SO2.
• Minnesota’s offset ratios seem to be based on the absolute minimum value.
Box Plots of Concentration Over Distance
McCourtney, Margaret. Single Source Secondary PM2.5 Modeling with AERMOD and CAMx; 2012 RSL Modelers’ Workshop; Chicago, IL, 2012.http://www.cleanairinfo.com/regionalstatelocalmodelingworkshop/archive/2012/presentations/Wed/6-3_RSLWorkshop_PM25_Point_Src_Projects-McCourtney_May02_v2anigif.pdf
Box Plots of Concentration Over Distance
McCourtney, Margaret. Single Source Secondary PM2.5 Modeling with AERMOD and CAMx; 2012 RSL Modelers’ Workshop; Chicago, IL, 2012.http://www.cleanairinfo.com/regionalstatelocalmodelingworkshop/archive/2012/presentations/Wed/6-3_RSLWorkshop_PM25_Point_Src_Projects-McCourtney_May02_v2anigif.pdf
Input Parameters for four source typesInput
parameter Case 1 Case 2 Case 3 Case 4
Facility EGUTaconite
Mine
Food Processing
Facility
Pulp and Paper Mill
Emission source Boiler
Indurating furnace
Boiler Boiler
Capacity (MMBtu/hr) 270 540 200 250
Fuel(s) Coal Natural gasFuel oil / Propane /
Natural gas
Natural gas / Fuel oil /
wood
Controls ESPBaghouse /
CycloneLNB / FGR
ESP / Cyclone
Stack height (m) 60 100 50 75
Emission rate (g/s)
PM2.5
NOx
SO2
0.140 80
74.57
0.53.51.0
2.510
150
Exit temperature(degrees K)
340 320 427 450
Diameter (m) 2.5 5 1.2 1.8Exit velocity
(m/s) 22 15 15 13
Modeling Conditions• AERMOD version 12345• No terrain• Assessment of building effects (40 meters in x,y,z)• Assumed Minnesota’s Lowest 98th monitored 3-year
average concentration of 17 g/m3
Results of primary and total 24-hour PM2.5concentrations
Building Effects
Included?
Predicted Impact from
Primary Emissions
(µg/m3)
Predicted Impact from
Secondary Emissions
(µg/m3)
Total Equivalent
PM2.5(µg/m3)
Background (µg/m3)
Total Predicted
Impact PM2.5
(µg/m3)
Primary PM2.5
(% Total Pred)
Secondary PM2.5
(% Total Pred)
Case 1 (EGU)
Yes 0.20 16.72 16.92 17 33.9 0.6% 49.3%No 0.06 4.86 4.92 17 21.9 0.3% 22.2%
Case 2 (Taconite
Mine)
Yes 1.75 0.19 1.94 17 18.9 9.3% 1.0%No 1.75 0.19 1.94 17 18.9 9.3% 1.0%
Case 3 (Food Proc. Plant)
Yes 6.11 1.65 7.76 17 24.8 24.7% 6.7%
No 0.60 0.16 0.76 17 17.8 3.4% 0.9%
Case 4 (Pulp & Paper Mill)
Yes 2.71 16.38 19.09 17 36.1 7.5% 45.4%
No 1.30 7.87 9.18 17 26.2 5.0% 30.1%
Benefits from the Offset-ratio Method • Avoids the use of complex chemistry models (i.e., CAMx,
CMAQ).• Simple to use.
Uncertainties of the Offset Ratio Method• Variability of CAMx generated offset ratios.
• Distance• Season• Grid resolution• Stack height• Emission rate
• Assume primary and secondary emissions occur concurrently in time and space.
Current Sources of Conservatisms• Combining the 98th percentile modeled concentration with
the 98th percentile of monitored concentration yields 99.96%: equivalent to one exceedance every 6.8 years.
• Assumed that permitted (PTE) emissions are emitted constantly.
Conclusion• Offset ratio method may be viable option for facilities that
have low PM2.5 , NOx and SO2 emissions.• Older facilities, or facilities with large emissions of NOx
and SO2 may not be able to model compliance with this method.
Sergio A. GuerraEnvironmental EngineerPhone: (651) [email protected]
