The Ever-Evolving Science of Air Quality Modeling

One of the reasons I’ve enjoyed air quality modeling during my 16-year career with ALL4 is because it’s an ever-evolving science.  The air quality modeling that I was learning and conducting 16 years ago is vastly different than the air quality modeling associated with my current and future modeling projects. This seems to marry well with what I believe was the most important lesson I learned in college.  That is, I learned how to learn.  This has been critical to me in learning the evolving science and the associated regulations related to air quality modeling.

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By design, the framework that governs air quality modeling, 40 CFR Part 51 Appendix W – Guideline on Air Quality Models (The Guideline) is setup to incorporate the current state-of-the-science.  This was evident from the beginning with the 1977 Clean Air Act (CAA) that legislated a requirement [42 U.S.C §7620(a)] to conduct a conference on air quality modeling “Not later than six months after August 7, 1977, and at least every three years thereafter.”  The purpose of the triennial “Conference on Air Quality Models,” as it has come to be known, is to provide an opportunity for State and Local air pollution agencies, scientific bodies, and the regulated community to participate and contribute to the current state-of-the-science as it relates to air quality modeling.  I’ve had the opportunity to participate and attend the 8^th, 9^th, 10^th, and 11^th U.S. EPA hosted Conference on Air Quality Models.  A parallel technical program for air quality modelers has been organized by the Air and Waste Management Association (AWMA).  AWMA has hosted several air quality modeling specific conferences and in March 2019 I attended one such conference titled the “Guideline on Air Quality Models: Planning Ahead.”  U.S. EPA also attended the March 2019 AWMA conference and tentatively scheduled the 12^th U.S. EPA Conference on Air Quality Models for October 1-3, 2019 in Research Triangle Park, NC.

The AWMA conference was a great venue to hear about the current state-of-the-science and what other consultants and scientific bodies are currently working on.  It was also a great venue to hear from the U.S. EPA Office of Air Quality Planning and Standards (OAQPS) Air Quality Modeling group about their priorities and future plans.  The rest of this article summarizes some of the key updates from the AWMA conference and other recent actions with a focus on how it might affect your next project that involves air quality modeling.

Revised Policy on Ambient Air

U.S. EPA  provided background about items addressed in the November 2018 draft “Revised Policy on Exclusion from Ambient Air” (Ambient Air Policy).  Many in the regulated community didn’t think the policy went far enough in redefining “ambient air” (i.e., the area where the general public can be exposed to air pollutants) because it didn’t address the duration component associated with ambient air for individual pollutants.  Specifically, some National Ambient Air Quality Standards (NAAQS) have only long-term averaging periods (i.e., 24-hour or annual) and the period of exposure should have been addressed for locations where it would be highly improbable for the public to reside for long term durations.  U.S. EPA’s response was that the items addressed in the Revised Ambient Air Policy were as far as they could go without rulemaking and based on previous experience without risking significant legal challenges.  The takeaway for your next project is that the revised Ambient Air Policy replaces “a fence or other physical barriers,” with; “Measures, which may include physical barriers, that are effective in deterring or precluding access to the land by the general public.”  The Revised Ambient Air Policy identified “measures” as:

• Video surveillance and monitoring
• Clear signage
• Routine security patrols
• Drones
• Potential future technologies

While some facilities may already rely on these measures, the policy provides a backstop if your next project were to get challenged, provides consistency across state and U.S. EPA regions, and may provide consistency between Prevention of Significant Deterioration (PSD) permitting and State rule air quality modeling.

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Update on Modeled Emissions Rates for Precursors (MERPs)

During the AWMA conference U.S. EPA gave a preview to updates to be incorporated into a final version of the “Guidance on the Development of Modeled Emissions Rates for Precursors (MERPs) as a Tier 1 Demonstration Tool for Ozone and PM_2.5 under the PSD Permitting Program” and also indicated that the final version would be published by the end of April 2019.  Low and behold, U.S. EPA kept their word and released the final version of the guidance on April 30, 2019.  If MERP is a new acronym to you check out ALL4’s blog post on MERPs when the draft guidance was published in December 2016.

As a quick refresher, MERPs are essentially emissions thresholds that can be utilized as part of a PSD permit to demonstrate that proposed increases in fine particulate (PM_2.5) precursor emissions [sulfur dioxide (SO₂) and nitrogen oxides (NO_X)] and ozone precursor emissions [volatile organic compounds (VOCs) and NO_X] from your project will be below ozone and PM_2.5 significant impact levels (SILs) and therefore won’t have the potential to cause or contribute to a violation of the NAAQS.  The MERPs are based on photochemical modeling conducted across the U.S. and are regionally representative.

A MERP analysis is considered a Tier I demonstration and if your project-related precursor emissions are greater than the MERPs, a Tier II demonstration would be required involving the use of a photochemical grid model.  However, before you get too worried about conducting photochemical modeling, I was also interested to hear, that since the draft 2016 MERP guidance has been released there have been no submitted permit applications utilizing a Tier II demonstration and that all applications triggering the requirement to evaluate precursor impacts have successfully relied on a Tier I demonstration.

Changes from the 2016 draft guidance to the April 30, 2019 final version included:

1. Additional hypothetical single source modeled sources,
2. Additional details on how to use existing modeling for NAAQS demonstration, and
3. Additional details on considering secondary PM_2.5 for PM_2.5 PSD increment demonstrations.

The changes related to the additional hypothetical single sources are good news for those projects that may require further refinement from use of the most conservative illustrative MERPs which are geographically categorized by eastern, central, and western regions.  Specifically, more geographically and stack height specific MERPs can be developed for your facility.  It appears that the approach of developing a more geographically and stack height site-specific MERP value is what has enabled everyone to avoid a Tier II demonstration to date.  The Georgia Environmental Protection Division (GA EPD) Air Quality Division seems to be leading the way with state-specific guidance that outlines how to develop refined MERP values.  In addition, GA EPD has even conducted additional photochemical modeling to add their own state-specific facilities.  The Kentucky Department of Environmental Protection (KYDEP) and the Texas Commission on Environmental Quality (TCEQ) have also developed similar MERP guidance on refining MERP values for your site.

Clarification on Local Source Emissions Rates used for NAAQS Modeling Demonstrations

During the AWMA townhall event, U.S. EPA representatives sought to provide clarity on a misinterpretation many may have been making with respect to the 2017 Guideline amendments.  The potential misinterpretation centers around amendments made to 40 CFR Part 51 Appendix W Section 8.2.2 Table 8-2 that summarizes the modeled emissions inputs for NAAQS compliance demonstrations as part of PSD permits.  Specifically, the 2017 amendments updated language to allow use of average actual emissions for nearby sources when adjusting for the operating level.  This adjustment has been misinterpreted by many in the regulated community to be synonymous with modeling actual reported emissions from the most recent two years when modeling local sources as part of a NAAQS compliance demonstration.  However, the slight nuance is that the two most recent years of actual emissions can be used to develop an operating level [i.e., Million British thermal units (MMBtu)/hr or lb throughput/hr] that can be multiplied by the maximum allowable emissions limit or federally enforceable permit limit (i.e., lb/MMBtu or lb/throughput) to determine the required emissions rate.  In some cases, this actual operating level emissions rate may be the same as the actual reported emissions but not in all cases.  Adding to the confusion is that some State modeling requirements do allow the use of actual emissions when modeling local sources as part of state modeling requirements (not PSD modeling).

While it appears that some State agencies have been misinterpreting this slight nuance for PSD permitting, I suggest taking a closer look at it during your next project as it may leave your project vulnerable to increased scrutiny during the public review process.  This particular clarification to NAAQS air quality modeling guidance was developed to provide for a more refined approach for addressing local sources, and while it still does, it may not be as “refined” as some would have liked.

Update on the 12^th Conference On Air Quality Models

During the AWMA conference, U.S. EPA representatives gave a preview of the focus for their 12^th Conference on Air Quality Models.  U.S. EPA indicated that the recent conferences were structured to establish the even-numbered conferences (i.e., the 8^th, 10^th, and 12^th conferences) as planning and development conferences and the odd-numbered conferences as official rulemaking conferences, also acting as the official public hearing for Guideline rulemaking.  Therefore, the 12^th conference will follow this format, and the priority will be on planning and development for potential rulemaking at the 13^th conference.  The planning and development conferences allow for more public involvement as they do not include official public hearings.  In advance of the October 2019 conference, expect stakeholder groups (and me) to be asking what air quality modeling issues are important for your facility and need to be addressed.

U.S. EPA also outlined several AERMOD alpha options that they hoped to release before the conference so that the regulated community has a chance to evaluate and provide feedback during the conference.  An alpha option within AERMOD is an option that is experimental and not for regulatory use.  Once proper evaluation and peer review (as specifically defined in Section 3.2 of the Guideline) has been completed U.S. EPA can “graduate” the option to a beta option.  A beta option can then be used for regulatory applications with an alternative model approval (also specifically defined in Section 3.2 of the Guideline).  Rulemaking is then necessary to “graduate” from an AERMOD beta option to an AERMOD default option.  The alpha options that stakeholders and U.S. EPA are working on address issues related to:

1. Building Downwash,
2. Low Wind Conditions,
3. Nitrogen Dioxide (NO₂) Enhancement,
4. Mobile Sources,
5. Overwater Modeling, and
6. Saturated Plumes/Plume Rise.

If you’re currently dealing with a project that involves air quality modeling and are having issues with any of these topics, reach out to me at (610) 422-1118 or at ddix@all4inc.com to learn more about how these options might help.  There may be the opportunity to work together to move the science forward so that we can help some of these alpha options graduate to beta and default options for use in your next project.

Learn More About the Author – Dan Dix