Modeling Guidance and Examples for Commonly

Asked Questions(Part II)

Reece Parker and Justin Cherry, P.E.

Air Permits Division

Texas Commission on Environmental Quality

Advanced Air Permitting Seminar 2014

What Is PM2.5?

NOx and SO2Stationary Sources

PM2.5

Direct Chemical Formation

The chemical composition of PM2.5 can vary with the local topography, source emissions, time of year, and weather.

PM2.5 StandardsNAAQS:

24-hr: 35 µg/m3

Primary Annual: 12 µg/m3

Secondary Annual: 15 µg/m3

Increments:24-hr: 9 µg/m3

Annual: 4 µg/m3

SIL*:24-hr: 1.2 µg/m3

Annual: 0.3 µg/m3

*with sufficient justification

Using the SILPM2.5 SIL justification for NAAQS:

Determine a representative background value

Subtract the background from the NAAQS

Compare the difference to the SIL

Background Value SILNAAQS

4 Assessment CasesCase 1: Direct PM2.5 < 10 tpy SER; NOx and/or SO2 < 40 tpy SER

Primary impacts only

Case 2: Direct PM2.5 ≥ 10 tpy SER; NOx and/or SO2 < 40 tpy SER

Primary impacts, still must address secondary formation

Case 3: Direct PM2.5 ≥ 10 tpy SER; NOx and/or SO2 ≥ 40 tpy SER

Primary impacts AND secondary impacts

Case 4: Direct PM2.5 < 10 tpy SER; NOx and/or SO2 ≥ 40 tpy SER

Primary impacts AND secondary impacts

Case 1Direct PM2.5 emissions < 10 tpy and SO2 and/or NOx

emissions < 40 tpy:Model direct PM2.5 emissions following guidance for a

NAAQS analysis

Case 2Direct PM2.5 emissions ≥ 10 tpy:

Model direct PM2.5 emissions following guidance for a NAAQS analysis

SO2 and/or NOx emissions < 40 tpy:

Discuss in AQA why proposed SO2 and NOx emissions are not significant to the secondary formation of PM2.5

Case 3Direct PM2.5 emissions ≥ 10 tpy:

Model direct PM2.5 emissions following guidance for a NAAQS analysis

SO2 and/or NOx emissions > 40 tpy:

Provide a qualitative, hybrid qualitative/quantitative, or quantitative assessment of the secondary formation of PM2.5

Case 3 Qualitative ApproachIdeas to consider:Peak impacts from direct

emissions and secondarily formed PM2.5 likely do not overlap

Assessment of background data and condition with the NAAQS

Case 3 Qualitative Approach (Continued)Ideas to consider:Evaluation of speciated PM2.5 data:

Magnitude of secondary PM2.5 precursor emissions from existing sources

Comparing project precursor emissions to those of existing sources

Limitations of chemical species necessary for photochemical reactions to form secondary PM2.5

Case 3 Hybrid ApproachQualitative: Follow the Case 3 qualitative assessments

General conclusions from existing photochemical modeling

Case 3 Quantitative ApproachQuantitative #1:

Assume 100% conversion from SO2 and NOx to PM2.5

Assess combined impacts of direct and equivalent direct PM2.5 emissions

Quantitative #2: Full quantitative photochemical grid modeling exercise*

*No requirement for photochemical modeling - this will be discussed further

Case 4Direct PM2.5 emissions < 10 tpy:

Model direct PM2.5 emissions following guidance for a NAAQS analysis

SO2 and/or NOx emissions ≥ 40 tpy:

Provide a qualitative, hybrid qualitative/quantitative, or quantitative assessment of the secondary formation of PM2.5

Case 3 ExampleDirect PM2.5 emissions: 62 tpy

NOx emissions: 96 tpy

SO2 emissions: 10 tpy

Need to address secondary formation of PM2.5.

Case 3 Qualitative ExampleSlow transformation and small portions of NOx emissions

can convert to PM2.5

Maximum concentration areas for secondary impacts of NOx are not likely to overlap with direct impacts of PM2.5

Case 3 Example (Cont.)Qualitative (Cont.):

Speciated PM2.5 data shows nitrates make up 2% of total PM2.5 concentration

Regional NOx emissions have a magnitude of 25,000 tons

Project emissions of NOx (96 tpy) are small and not likely to contribute to secondary formation of PM2.5

Case 3 Example (Cont.)Quantitative:

Assume 100% conversion of NOx to (NH4)NO3

Using NACAA formula: 1 µg/m3 of NOx could form 1.7391 µg/m3 of (NH4)NO3

24-hr and annual NOx from the source predicted to be 2.9 µg/m3 and 0.3 µg/m3, respectively

Using the formula, 24-hr and annual secondary formation from the source would be 5 µg/m3 and 0.5 µg/m3, respectively

Case 3 Example (Cont.)Quantitative (Cont.):

24-hr and annual predicted concentrations from the direct emissions of PM2.5 were 2 µg/m3 and 1 µg/m3, respectively

Add all components together for a total value

Pollutant

Averaging Time

Project GLCmax (µg/m3)

Secondary Formation

from Project (µg/m3)

Background

(µg/m3)

Total Predicted

Concentration

(µg/m3)

NAAQS

PM2.5 24-hr 2 5 26 33 35

PM2.5 Annual 1 0.5 9.6 11.1 12

PM2.5 Increment

What to consider:Major source baseline date - October 20, 2010Trigger date - October 20, 2011Minor source baseline date - county specific

SIL:Additional justification

Output metric:Yearly H1H vs. 5-year average

PM2.5 SIL Justification for IncrementEvaluate proposed direct PM2.5 emissions increases:

Report the maximum predictions and not a 5-year average

Provide justification for using the SILs to compare with the model predictions

PM2.5 SIL Justification for Increment

PM2.5 Monitoring for Increment5 years of monitoring data (µg/m3):

24-hrConcentratio

ns

2009 2010 2011 2012 2013

H1H 23.4 23.2 22.9 23.5 23.3

H2H 21.9 22.1 21.4 22.3 22.9

Increment Consumed 2013-2010

SILIncrement Standard

PM2.5 Increment

When predictions are greater than the SIL or if the SIL cannot be justified:

Evaluate increment affecting sources together with the project sources

Document approach to identify increment affecting sources

Receptors - the extent of the receptor grid needs to capture maximum concentrations from the project and show that concentrations are decreasing

PM2.5 Increment (continued)Further detail:PSD major sources were further evaluated:

Projects with completion dates 18 months prior to the major source baseline date up to the minor source baseline date were identified

Projects were reviewed to determine if PM2.5 was associated with project

The extent of the modeling domain used to limit search for PSD major sources:

24-hr and annual GLCmax locations, distance from property line, etc.

PM2.5 Increment (continued)

Contact InformationReece Parker

Air Dispersion Modeling Team (512) 239-1348 reece.parker@tceq.texas.gov

Justin Cherry, P.E.Air Dispersion Modeling Team (512) 239-0955 justin.cherry@tceq.texas.gov

Air Permits Division

Reece Parker

(512) 239-1348

reece.parker@tceq.texas.gov

Air Permits Division

(512) 239-0955justin.cherry@tceq.texas.gov

Justin Cherry