PM2.5 Modeling Guidance Overview and Case Studies for Secondary Formation of PM2.5 from Precursors. Extended Abstract No

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1 PM2.5 Modeling Guidance Overview and Case Studies for Secondary Formation of PM2.5 from Precursors Extended Abstract No Prepared By: Anna Henolson, PE, CM Managing Consultant Justin Fickas, PE, CM Managing Consultant TRINITY CONSULTANTS Merit Drive Suite 900 Dallas, TX (972) trinityconsultants.com June 2014 Environmental solutions delivered uncommonly well

2 PM 2.5 Modeling Guidance Overview and Case Studies for Secondary Formation of PM 2.5 from Precursors Extended Abstract No Anna Henolson, P.E., C.M. Trinity Consultants, nd Ave South, Suite 610, Kent, WA Justin Fickas, P.E., C.M. Trinity Consultants, 53 Perimeter Center East, Suite 230, Atlanta, GA INTRODUCTION The United States Environmental Protection Agency (U.S. EPA) issued draft guidance in March 2013 describing the recommended methods for demonstrating compliance with the National Ambient Air Quality Standard (NAAQS) for primary and secondary particulate matter less than 2.5 micrometers in diameter (PM2.5). 1 It is possible that the U.S. EPA will issue final guidance, or, more likely, reissue revised draft guidance in the near future. In the absence of concrete information regarding the content of new guidance, this paper focuses on the March 2013 draft guidance and how it has or has not been implemented by state permitting authorities. Before the March 2013 PM2.5 guidance was issued, only in rare cases were applicants required to assess the increase in PM2.5 concentrations due to formation of nitrates and sulfates from NOX and SO2 emissions ( secondary PM ). The directly emitted pollutants that form secondary PM (NOX and SO2) are referred to as precursors. With the issuance of the draft guidance, many states are now requiring assessment of secondary PM if NOX or SO2 are greater than the Significant Emission Rates (SERs) established under EPA s Prevention of Significant Deterioration (PSD) program for major new source review (40 tons per year for NOX and SO2). However, due to the draft nature of the guidance document, at this time there is still significant uncertainty in how secondary PM emissions should be evaluated through modeling, and inconsistency in how permitting authorities are requiring applicants to evaluate modeling impacts related to secondary PM. For example, some permitting authorities (e.g., Virginia DEQ, Idaho DEQ) would currently require a hybrid qualitative/quantitative approach for evaluation of secondary PM, while others (e.g., ADEM, Georgia EPD) would currently recommend continued use of the March 2010 Page memo regarding PM2.5 modeling demonstrations in lieu of a direct modeling assessment of secondary PM impacts, while still others (Oregon DEQ) use codified interpollutant offset ratios. The EPA guidance prescribes different methods for demonstrating that a given project does not cause or contribute to a violation of the PM2.5 NAAQS, depending on under which of the four cases a project falls: 1

3 Case 1: PM2.5 < 10 tpy and precursors (NOX and SO2) < 40 tpy, no demonstration is required. Case 2: PM2.5 > 10 tpy and precursors (NOX and SO2) < 40 tpy, traditional primary/direct PM2.5 modeling demonstration is required. Case 3: PM2.5 > 10 tpy and precursors (NOX and SO2) > 40 tpy, primary/direct PM2.5 and must account for secondary PM2.5 formation. Case 4: PM2.5 < 10 tpy and precursors (NOX and SO2) > 40 tpy, only account for secondary PM2.5 formation (primary/direct PM2.5 demonstration not required). For cases 3 and 4, the secondary PM2.5 formation assessment takes the form of three complexity levels: Qualitative, Hybrid Qualitative/Quantitative, and Quantitative SECONDARY PM2.5 FORMATION CASE STUDIES An overview of recent demonstrations under Case 3 and Case 4 requiring secondary PM2.5 assessments submitted to-date provides perspective on the quantity and variety of recent assessment methods. A comprehensive overview also allows the evaluation of the level to which EPA s assertion that only a handful of situations would require explicit photochemical grid modeling has held true. For the hybrid qualitative/quantitative assessments, several case studies are summarized in more detail to compare the various secondary PM2.5 assessment methods, evaluating the tradeoff between prediction accuracy and model resource requirements. Methods PM2.5 NAAQS demonstration assessments or protocols submitted from March 2013 through February 2014 are obtained through querying State and EPA regional offices for PM2.5 modeling protocols and PSD application reports for Case 3 and Case 4 projects. Additionally, analyses conducted by the author s colleagues at Trinity Consultants are included in greater detail. Results The majority of secondary PM2.5 modeling demonstrations reviewed that were prepared subsequent to the March 2013 draft guidance followed a hybrid qualitative/quantitative approach; however, the qualitative approach was applied almost as frequently; and many applications (not all of which are listed below) apply the High First High PM2.5 model results consistent with the 2010 Page memo, and include only a brief discussion of secondary PM2.5 or omit the discussion all together. Table 1 below presents a summary of the March 2013 February 2014 PM2.5 NAAQS compliance demonstrations identified under Case 3 and Case 4 requiring secondary PM2.5 assessments. 2

4 Table 1. Overview Secondary PM2.5 Compliance Demonstrations. Project Description Case Approach Status Virginia 1 Case 3 Hybrid Qualitative/Quantitative Georgia EPD Offset Ratios North Case 3 Qualitative Application Under Review Carolina 1 North Case 3 Qualitative Application Under Review Carolina 2 Michigan Generating Station Case 3 Hybrid Qualitative/Quantitative CSAPR Modeling Ratios Comment Texas Generating Station Case 3 Hybrid Qualitative/Quantitative CSAPR Modeling Ratios Application Under Review, Method approved by TCEQ Texas LNG Plant Case 3 Qualitative Undergoing TX Contested Case Process Louisiana 1 Case 3 Qualitative Alaska Oil/Gas Case 3 NA relied on March 2010 Page Idaho Fertilizer Plant Case 3 Hybrid Qualitative/Quantitative Applied monitoring-based ratios Draft Permit Issued, No EPA to-date Oregon 1 Case 3 Simplified Hybrid Qualitative/Quantitative Used codified offset ratios Washington 1 Case 3 Hybrid Qualitative/Quantitative Permit Issued Applied monitoring-based ratios Georgia 1 Case 3 NA relied on March 2010 Page South Carolina 1 Case 3 Qualitative Comment Georgia 2 Case 3 NA relied on March 2010 Page Modeling Protocol Reviewed, Application in Alabama 1 Case 3 NA relied on March 2010 Page Progress Modeling Protocol Reviewed, Application in Progress Detailed Qualitative Example For two separate PSD projects in North Carolina, a qualitative-only approach was applied. The two projects included similar points explaining why secondary PM2.5 formation would not cause or contribute to a violation of the NAAQS: 3

5 Maximum primary PM2.5 impacts were driven by fugitive/ambient releases, not the combustion sources. As such, the location of the maximum primary PM2.5 concentration was along the fenceline, and the maximum concentration from the combustion source would occur at a different time and location as the overall PM2.5 maximum. Secondary PM2.5 would be a much smaller contributor that primary. To address any concern that secondary PM2.5 could impact NAAQS attainment further downwind of the source (away from primary PM2.5 impacts from the source), discussion was also included emphasizing ample margin between the total modeled PM2.5 with background result and the NAAQS. Detailed Hybrid Qualitative/Quantitative Examples All but the smallest electric power generating facilities would fall under Case 3, due to primary PM2.5 and NOX precursor emissions from fuel combustion, and in some cases, also include SO2 precursors above the 40 tpy threshold for higher sulfur fuel (e.g., coal). For the Texas Generating Station example, the facility emits over 10 tpy PM2.5 and over 40 tpy NOX, resulting in the need to evaluate impacts due to nitrate formation. The applicant applied a hybrid qualitative/quantitative approach using the evidence summarized in the following list. Maximum primary and secondary concentrations are not expected to occur at the same time or location, because the maximum primary impacts occur within 2 kilometer and secondary impacts within this distance are expected to be minimal. Warm ambient temperatures in the region minimize impact of nitrates as PM2.5, because almost all nitrate at local temperatures is in vapor form. Screening estimates of nitrate concentrations were made, by applying a monitoringbased PM2.5 nitrate to NOX ratio to modeled NOX concentrations at a conservative distance of 5 km (the distance the plume is expected to travel in one hour over 90% of the time). As an alternative and potentially simpler approach to the Texas example, the Virginia DEQ recommends the use of offset ratios to evaluate secondary PM impacts, based on research done into appropriate offset ratios for NOx and SO2 as developed by Dr. James Boylan of the Georgia EPD. Dr. Boylan provided a presentation on his research at the 2013 CMAS conference in Chapel Hill, NC on October 30, A permitting evaluation has been done for a source within Virginia using the hybrid qualitative/quantitative approach using offset ratios. Other states (e.g., Minnesota, Washington/Oregon/Idaho under NW Airquest Consortium) are also pursuing development of area-specific offset ratios. The notion of use of offset ratios to evaluate secondary PM modeled impacts is simple in concept, but could pose challenges in execution. The offset ratio concept involves first the state or other local authority establishing an appropriate area-specific offset ratio for NOx and SO2, and then those ratios would be applied to use in a standard modeling assessment using an EPA approved model (i.e., AERMOD) as part of the standard modeling methodologies in use. For example, if an offset ratio of 30:1 was found to be appropriate for SO2, for every 30 tons/yr of SO2 emissions from a source 1 tpy of PM2.5 emissions would be added to the source to be modeled, and this would account for the impacts of the precursor pollutant (SO2) on PM2.5 modeled impacts. An alternative approach would involve scaling of the actual precursor results provided in the model output (i.e. AERMOD) to account for secondary PM impacts. 4

6 However, as indicated from Dr. Boylan s research, implementation of offset ratios could be difficult. This difficulty is due to the fact that the true offset ratios from a source could be dependent on factors such as the stack parameters (i.e., stack height), season of the year, and distance from the source in question. These multiple variables could make application of consistent methods for applying project-specific offset ratios for NOx and SO2 challenging to find the appropriate balance between protective (conservative) simple offset ratios versus more refined ratios representing the specific source characteristics, time periods, and distances of concern. Conversely such complexity could mean that EPA will recommend in the future use of a single, highly conservative and restrictive offset ratio for NOx and SO2 in order to account for the variability observed. The tiered approach outlined by Dr. Boylan could provide a workable solution similar to the tiered approach for conversion of NO to NO2: start with the most simple, most conservative ratio and move toward more refined techniques as warranted for the specific assessment needs. Detailed Quantitative Example No full scale quantitative assessments using photochemical modeling for a full assessment of secondary PM impacts, since issuance of the draft 2013 guidance document, were identified. In fact, only one photochemical modeling evaluation for criteria pollutants as part of a PSD permitting exercise (for NAAQS compliance) was identified, and this assessment pre-dated by over 3 years the draft 2013 guidance document. 3 In this instance, the permitting authority (Georgia EPD) utilized the CAMx model to evaluate secondary PM2.5 impacts from precursor emissions (i.e., SO2) associated with a proposed coal fired power plant. The lack of many examples of full scale quantitative assessments through use of photochemical models is not surprising. The draft 2013 guidance document refers to such assessments as rare cases. With a lack of frequent use of such modeling techniques as part of the regulatory permitting process imposed on permittees, there is a lack of direct guidance on modeling techniques associated with photochemical models for this application (single source secondary PM2.5 assessment). There is also a lack of extensive experience in use of such models generally, and particularly in permitting by both the regulated community and the permitting authorities responsible for maintaining and attaining the NAAQS. Discussion Permitting Implications of Secondary PM2.5 Modeling Requirements There are a great many implications when considering secondary PM in modeling exercises. Those implications will depend heavily on the specific requirements imposed by the regulatory authority regarding secondary PM modeling evaluations. Such implications would include;\; State construction permit (minor permit) authorization requirements. Some regulatory authorities (e.g., South Carolina DHEC) require an evaluation of compliance with State ambient air quality standards (which mirror the NAAQS) as part of even minor source construction permitting. Will permitting authorities require evaluation of secondary PM even as part of minor source (non-psd) permitting? 5

7 Compliance with the annual PM2.5 NAAQS. On January 15 th, 2013 EPA published in the Federal Register the revision of the annual PM2.5 NAAQS from 15 µg/m 3 to 12 µg/m 3. Due to the high background concentrations of annual PM2.5 in many areas of the United States, the 2013 revision of the annual PM2.5 standard has already made compliance modeling demonstrations for the annual PM2.5 NAAQS much more difficult in certain parts of the country. With an additional requirement to evaluate modeling impacts from secondary PM emissions, compliance demonstration with the PM2.5 NAAQS will become increasingly difficult. Inconsistency among the regulated community. As discussed above, there is already a significant amount of disparity amongst permitting authorities in how evaluation of secondary PM should currently be addressed. This discord could lead to permit applicants being required to conduct last-minute detailed secondary PM2.5 assessments (resulting in permitting delays), due to s from the public or EPA after the permitting authority had already provided direction to use a simpler approach. Whether this inconsistency will decrease with reissuance of a revised guidance document regarding secondary PM2.5 modeling evaluations, is unknown. Reissuance of the guidance document has the potential to create even greater inconsistency among States/Regions in how secondary PM should be evaluated, leading to increased confusion among the regulated community. ACKNOWLEDGEMENTS The author wishes to extend acknowledgments to the many colleagues who provide examples and background information: Angie Wanger, Anna Unruh, George Schewe, Jonathan Hill, Michael Meister, Tony Schroeder, and Will Backus. REFERENCES 1. EPA-454/D Boylan, James; Byeong-Uk Kim. Development and Application of PM2.5 Interpollutant Trading Ratios to Account for PM2.5 Secondary Formation in Georgia. Presented at the 2013 CMAS Conference, Chapel Hill, NC, October 30, Kim, Byeong-Uk. CAMx Photochemical Modeling Review: Plant Washington, Washington County. Memorandum thru Jim Boylan to Purva Prabhu, August 12, See /Appendixe.pdf (Accessed Apr 29, 2014). KEYWORDS PM2.5, secondary particulate matter, dispersion modeling 6