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Complying with EPA's Guidance for SO2 Designations


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EPA is under a Court order to complete the remaining SO2 designations for the rest of the country in three additional rounds. On March 20, 2015 the EPA released an updated guidance for 1-hr SO2 area designations. The two options included are compliance through dispersion modeling or ambient monitoring. Of these two options, dispersion modeling is the fastest and most cost effective one to characterize SO2 air quality. However, this compliance demonstration can be challenging given that AERMOD tends to produce overly conservative concentration estimates. Source characterization techniques and probabilistic techniques may be used to achieve compliance with the 1-hour NAAQS. Three advanced methods discussed: 1) Equivalent Building Dimensions (EBD); 2) Emission Variability Processor (EMVAP); 3) 50th Percentile Background Concentrations.

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Complying with EPA's Guidance for SO2 Designations

  1. 1. Complying with EPA’s Guidance for SO2 Designations PNWIS November 6, 2015 Sergio A. Guerra, PhD – CPP Inc.
  2. 2. Outline • Background and Overview and Options (To model or to monitor) • Summary of SO2 Designation Schedule • Advanced Modeling Techniques – Equivalent Building Dimensions (EBD) – Emission Variability Processor (EMVAP) – 50th Percentile Background
  3. 3. Background • August 5, 2013- EPA issued first round of SO2 Designations. • Three lawsuits were filed against EPA for not designating all portions of the country by the June 2013 deadline. • March 2, 2015- Court ordered EPA to complete remaining SO2 designations.
  4. 4. Background • March 20, 2015- The Updated Guidance for SO2 Area Designations was released by EPA. • August 10, 2015- EPA releases the Final Data Requirements Rule for 1-hr SO2 NAAQS.
  5. 5. Round 1 Areas Associated with 2009-2011 Monitored Violations. • 7/25/2013: EPA promulgates final SO2 area designations for 29 nonattainment areas. • 10/04/2013: Effective Date.
  6. 6. Round 2 Areas Associated with 68 Power Plants & New Monitored Violations. • 9/18/2015: States may submit updated recommendations and supporting information for area designations to EPA. • 1/22/2016: EPA notifies states concerning any intended modifications to their recommendations (120-day letters). • By 7/2/2016: EPA promulgates final SO2 area designations.
  7. 7. Round 3 Modeled Areas and Areas w/o Monitors • 1/13/2017: States submit air quality modeling results for selected areas (per SO2 DRR). • By 9/1/2017: EPA notifies states of any intended modification to their recommendations. • By 12/31/2017: EPA promulgates final SO2 area designations.
  8. 8. Round 4 New Monitored Areas/All Remaining Areas • 1/1/2017: States begin to operate new monitoring network. • 5/1/2020: States certify 2019 monitoring data to calculate the 2017-2019 design value. • By 9/2/2020: EPA notifies states about any intended modification to their recommendation (120-day letters). • 12/31/2020: EPA promulgates final SO2 area designations.
  9. 9. What Does All This Mean? Large SO2 sources have two options: 1) Dispersion Modeling 2) Ambient Monitoring Preferred option is modeling however this can be challenging because of conservative nature of model.
  10. 10. Modeling Softballs December 2013 Modeling TAD: • Use of actual instead of allowable emissions (i.e., PTE) to assess violations of the standard. • Use of 3 years of meteorological data instead of 5. • Receptor placement only in locations where monitor could be placed. • Use of actual stack height instead of GEP stack height.
  11. 11. Advanced Modeling Techniques Areas Advanced Modeling Techniques Traditional Modeling Technique Building dimensions used for downwash Equivalent Building Dimensions (EBD) Use of Building Profile Input Program for PRIME (BPIP- PRM) Variable emissions Use EMVAP to account for variability Assume continuous maximum emissions Background Concentrations Combine AERMOD’s concentration with the 50th % observed Tier 1: Combine AERMOD’s concentration with max. or design value (e.g., 99th % observed for SO2) Tier 2: Combine predicted and observed values based on temporal matching (e.g., by season or hour of day).
  12. 12. Equivalent Building Dimensions
  13. 13. Building Dimension Inputs & BPIP • BPIP uses building footprints and tier heights • Combines building/structures • All structures become one single rectangular solid for each wind direction and each source • BPIP dimensions may not characterize the source accurately and may result in unreasonably high predictions
  14. 14. PRIME AERMOD’s Building Downwash Algorithm • Used EPA wind tunnel data base and past literature • Developed analytical equations for cavity height, reattachment, streamline angle, wind speed and turbulence • Developed for specific building dimensions • When buildings outside of these dimensions, theory falls apart
  15. 15. Refinery Structures Upwind - Horizontal flow Solid BPIP Structure Upwind No Structures Streamlines for Lattice Structures Should be Horizontal
  16. 16. BPIP Diagnostic Tool CPP determines Equivalent Building Dimensions (EBD) and provides them to consultant for use in the dispersion modeling analysis.
  17. 17. BPIP Diagnostic Tool
  18. 18. Long Buildings with Wind at an Angle Figure created in BREEZE® Downwash Analyst BREEZE is a registered trademark of Trinity Consultants, Inc.
  19. 19. • Equivalent Building Dimensions” (EBDs) are the dimensions (height, width, length and location) that are input into AERMOD in place of BPIP dimensions to more accurately predict building wake effects • Guidance originally developed when ISC was the preferred model – – EPA, 1994. Wind Tunnel Modeling Demonstration to Determine Equivalent Building Dimensions for the Cape Industries Facility, Wilmington, North Carolina. Joseph A. Tikvart Memorandum, dated July 25, 1994. U.S. Environmental Protection Agency, Research Triangle Park, NC • Determined using wind tunnel modeling • How does EBD Improve Accuracy? Watch video What is EBD?
  20. 20. How to Use EBD for Regulatory Purposes?  Step 1: Develop a protocol outlining the EBD study  Step 2: Submit EBD protocol for approval to regulatory agency. Also need to involve Model Clearinghouse  Step 3: Perform wind tunnel testing  Step 4: Use building geometry from EBD study in AERMOD  Step 5: Submit final report for agency review and approval
  21. 21. Current Status Regulatory Status of EBD From October 24, 2011 Model Clearinghouse Review of EBD for AERMOD • “any EBD studies being considered should be discussed with the appropriate reviewing authority as early in the process as possible and that the Model Clearinghouse should also be engaged as early as possible.” • Memo stressed that these wind tunnel studies are source characterization studies not subject to alternative modeling requirements Other • EPA has acknowledged the limitation of BPIPPRM derived parameters for some cases1,2 1. Roger Brode’s (EPA) comments at 9th Modeling Conference 2. Roger Brode’s (EPA) comments at 10th Modeling Conference
  22. 22. Summary of Approved Projects • Studies conducted and approved using original guidance for ISC applications – Amoco Whiting Refinery, Region 5, 1990 – Public Service Electric & Gas, Region 2, 1993 – Cape Industries, Region 4, 1993 – Cambridge Electric Plant, Region 1, 1993 – District Energy, Region 5, 1993 – Hoechst Celanese Celco Plant, Region 3, 1994 – Pleasants Power, Region 3, 2002 • Studies conducted using original guidance for AERMOD/PRIME applications – Hawaiian Electric (Approved), Region 9, 1998 – Mirant Power Station (Approved), Region 3, 2006 – Cheswick Power Plant (Approved), Region 3, 2006 – Radback Energy (Protocol Approved), Region IX, 2010 – Chevron 1 (Approved), Region 4, 2012 – Chevron 2 (Approved), Region 4, 2013 – Chevron 3 (In process), Region 4, 2015
  23. 23. Monte Carlo Approach • Pioneered by the Manhattan Project scientists in 1940’s • Technique is widely used in science and industry • EPA has approved this technique for risk assessments • Used by EPA in the Guidance for 1-hour SO2 Nonattainment Area SIP Submissions (2014)
  24. 24. Emission Variability Processor • Assuming fixed peak 1‐hour emissions on a continuous basis will result in unrealistic modeled results • Better approach is to assume a prescribed distribution of emission rates • EMVAP assigns emission rates at random over numerous iterations • The resulting distribution from EMVAP yields a more representative approximation of actual impacts • Incorporate transient and variable emissions in modeling analysis • EMVAP uses this information to develop alternative ways to indicate modeled compliance using a range of emission rates instead of just one value
  25. 25. Background Concentrations
  26. 26. Siting of Ambient Monitors According to the Ambient Monitoring Guidelines for Prevention of Significant Deterioration (PSD): The existing monitoring data should be representative of three types of area: 1) The location(s) of maximum concentration increase from the proposed source or modification; 2) The location(s) of the maximum air pollutant concentration from existing sources; and 3) The location(s) of the maximum impact area, i.e., where the maximum pollutant concentration would hypothetically occur based on the combined effect of existing sources and the proposed source or modification. (EPA, 1987) U.S. EPA. (1987). “Ambient Monitoring Guidelines for Prevention of Significant Deterioration (PSD).”EPA‐450/4‐87‐007, Research Triangle Park, NC.
  27. 27. Wildfires in 2015 NASA’s Earth Observatory
  28. 28. 24-hr PM2.5 Santa Fe, NM Airport Background Concentration and Methods to Establish Background Concentrations in Modeling. Presented at the Guideline on Air Quality Models: The Path Forward. Raleigh, NC, 2013. Bruce Nicholson
  29. 29. Probability of Two Unusual Events Happening at the Same Time
  30. 30. Combining 99th Percentile Pre and Bkg (1-hr SO2) 99th percentile is 1st rank out of 100 days = 0.01 P(Pre ∩ Bkg) = P(Pre) * P(Bkg) = (1-0.99) * (1-0.99) = (0.01) * (0.01) = 0.0001 = 1 / 10,000 days Equivalent to one exceedance every 27 years! = 99.99th percentile of the combined distribution
  31. 31. Proposed Approach to Combine Modeled and Monitored Values • Combining the 99th %(for 1-hr SO2) monitored concentration with the 99th % predicted concentration is too conservative. • A more reasonable approach is to use a monitored value closer to the main distribution (i.e., the median). Evaluation of the SO2 and NOX offset ratio method to account for secondary PM2.5 formation Sergio A. Guerra, Shannon R. Olsen, Jared J. Anderson Journal of the Air & Waste Management Association Vol. 64, Iss. 3, 2014
  32. 32. Combining 99th Pre and 50th Bkg 50th Percentile is 50th rank out of 100 days = 0.50 P(Pre ∩ Bkg) = P(Pre) * P(Bkg) = (1-0.99) * (1-0.50) = (0.01) * (0.50) = 0.005 = 1 / 200 days Equivalent to 1.8 exceedances every year = 99.5th percentile of the combined distribution Evaluation of the SO2 and NOX offset ratio method to account for secondary PM2.5 formation Sergio A. Guerra, Shannon R. Olsen, Jared J. Anderson Journal of the Air & Waste Management Association Vol. 64, Iss. 3, 2014
  33. 33. Advanced Model Input Analysis Solutions • Emission Variability Processor (EMVAP) • Evaluation of background concentrations EM Magazine, December 2014 Guerra, S.A. “Innovative Dispersion Modeling Practices to Achieve a Reasonable Level of Conservatism in AERMOD Modeling Demonstrations.” EM Magazine, December 2014.
  34. 34. Case Study: Three Cases Evaluated 1. Using AERMOD by assuming a constant maximum emission rate (current modeling practice) 2. Using AERMOD by assuming a variable emission rate 3. Using EMVAP to account for emission variability
  35. 35.
  36. 36. Three Cases Used to Model Power Plant Input parameter Case 1 Case 2 Case 3 Description of Dispersion Modeling Current Modeling Practices AERMOD with hourly emission EMVAP (500 iterations) SO2 Emission rate (g/s) 478.7 Actual hourly emission rates from CEMS data Bin1: 478.7 (5.0% time) Bin 2: 228.7 (95% time) Stack height (m) 122 Exit temperature (degrees K) 416 Diameter (m) 5.2 Exit velocity (m/s) 23
  37. 37. Results of 1-hour SO2 Concentrations Case 1 (µg/m3) Case 2 (µg/m3) Case 3 (µg/m3) Dispersion Modeling Current Modeling Practices AERMOD with hourly emission EMVAP (500 iterations) H4H 229.9 78.6 179.3 Percent of NAAQS 117% 40% 92%
  38. 38. St. Paul Park 436 Ambient Monitor
  39. 39. Positively Skewed Distribution
  40. 40. Histogram of 1-hr SO2 Observations Innovative Dispersion Modeling Practices to Achieve a Reasonable Level of Conservatism in AERMOD Modeling Demonstrations. Sergio A. Guerra EM Magazine, December 2014.
  41. 41. Concentrations at Different Percentiles St. Paul Park 436 monitor (2011-2013) Percentile µg/m3 50th 2.6 60th 3.5 70th 5.2 80th 6.1 90th 9.6 95th 12.9 98th 20.1 99th 25.6 99.9th 69.5 99.99th 84.7 Max. 86.4
  42. 42. Case 3 with Three Different Backgrounds Case 3 with Max. Bkg (µg/m3) Case 3 with 99th % Bkg (µg/m3) Case 3 with 50th % Bkg (µg/m3) H4H 179.3 179.3 179.3 Background 86.4 25.6 2.6 Total 265.7 204.9 181.9 Percent of NAAQS 135.6% 104.5% 92.8%
  43. 43. Conclusion Current regulatory practices in dispersion modeling lead to unrealistically high predicted concentrations. • Source characterization techniques such as wind tunnel generated building dimensions can mitigate downwash overpredictions. • Probabilistic methods to account for emission variability can help achieve more realistic concentrations. • Use of 50th % monitored concentration is statistically conservative when pairing it with the 99th % predicted concentration.
  44. 44. Conclusion These Advanced Modeling Techniques are: • Protective of the NAAQS, • Provide a reasonable level of conservatism, • In harmony with probabilistic nature of 1-hr standards
  45. 45. Thank You! Ron Petersen, PhD, CCM Sergio Guerra, PhD Cell: 970 690 1344 Cell: 612 584 9595 CPP, Inc. 2400 Midpoint Drive, Suite 190 Fort Collins, CO 80525 @CPPWindExperts