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.

1. EVALUATION OF SO2 AND NOX
OFFSET RATIOS TO ACCOUNT FOR
SECONDARY PM2.5 FORMATION
Sergio Guerra, Shannon Olsen, Jared Anderson
AWMA Specialty Conference
March 20, 2013

2. 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.

3. 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.

4. NACAA’s Recommended Approach for
Assessing Secondary PM2.5

5. EPA’s PM2.5 Compliance Demonstration:
Assessment Cases

6. 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 NOx
emission 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.

7. 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.

8. 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_ProjectsMcCourtney_May02_v2anigif.pdf

9. 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_ProjectsMcCourtney_May02_v2anigif.pdf

10. Input Parameters for four source types
Input
parameter
Case 1
Case 2
Case 3
Case 4
Facility
EGU
Taconite
Mine
Food
Processing
Facility
Pulp and
Paper Mill
Boiler
Indurating
furnace
Boiler
Boiler
270
540
200
250
Natural gas /
Fuel oil /
wood
ESP /
Cyclone
Emission
source
Capacity
(MMBtu/hr)
Fuel(s)
Coal
Natural gas
Fuel oil /
Propane /
Natural gas
Controls
ESP
Baghouse /
Cyclone
LNB / FGR
60
100
50
75
0.1
40
80
7
4.5
7
0.5
3.5
1.0
2.5
10
150
340
320
427
450
2.5
5
1.2
1.8
22
15
15
13
Stack height
(m)
Emiss
PM2.5
ion
NOx
rate
SO2
(g/s)
Exit
temperature
(degrees K)
Diameter (m)
Exit velocity
(m/s)

11. 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

12. Results of primary and total 24-hour PM2.5
concentrations
Building
Effects
Included?
Case 1
(EGU)
Case 2
(Taconite
Mine)
Case 3
(Food
Proc.
Plant)
Case 4
(Pulp &
Paper
Mill)
Predicted
Impact
from
Primary
Emissions
(µg/m3)
Predicted
Impact
from
Secondary
Emissions
(µg/m3)
Total
Equivalent
PM2.5
(µg/m3)
Yes
No
Yes
0.20
0.06
1.75
16.72
4.86
0.19
No
1.75
Yes
Background
(µg/m3)
Total
Predicted
Impact
PM2.5
(µg/m3)
Primary
PM2.5
(% Total
Pred)
Secondary
PM2.5
(% Total
Pred)
16.92
4.92
1.94
17
17
17
33.9
21.9
18.9
0.6%
0.3%
9.3%
49.3%
22.2%
1.0%
0.19
1.94
17
18.9
9.3%
1.0%
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%
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%

13. Benefits from the Offset-ratio Method
• Avoids the use of complex chemistry models (i.e., CAMx,
CMAQ).
• Simple to use.

14. 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.

15. 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.

16. 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.

17. Sergio A. Guerra
Environmental Engineer
Phone: (651) 395-5225
sguerra@wenck.com