Appendix W Revision: The New Era of Air Quality Modeling – SO2 Modeling Updates

If you’ve visited ALL4’s website within the past few weeks (c’mon, don’t you visit daily?), you’ve likely seen a wealth of information regarding the U.S. Environmental Protection Agency (U.S. EPA) revisions to 40 CFR Part 51, Appendix W (also known has the Guideline on Air Quality Models).  The revisions were finally published in the Federal Register (FR) on January 17, 2017 and cover a variety of topics.  For an introductory look at the changes made to Appendix W, visit the blog published on January 17, 2017.  For SO₂, the revisions incorporated many less conservative air quality modeling methodologies that were provided in numerous guidance documents.  With the SO₂ Data Reporting Rule (DRR) and U.S. EPA emphasis on SO₂, improved air quality modeling techniques are important for regulated stakeholders.  These revisions could mean that SO₂ air quality modeling is no longer the fatal flaw when it comes down to evaluating the feasibility of a project.  For a look into what these changes mean to a previous project or future project, read on!

So what exactly changed as it pertains to SO₂ air quality modeling guidelines?

Air quality models for SO₂ are needed to address compliance with the National Ambient Air Quality Standards (NAAQS) and Prevention of Significant Deterioration (PSD) increments, development of state implementation plans (SIP), and for characterizing current air quality through the use of modeling.  Updates to Appendix W that impact SO₂ air quality modeling include the incorporation of ADJ_U* as a regulatory default option (as opposed to a beta option); the ability to represent emissions from local sources at the level at which the sources actually emitted, rather than at the level the source is permitted to emit; and the incorporation of background concentrations that are reflective of seasonal and diurnal patterns.

Let’s dive into some of the details.

Good news! The revisions to Appendix W are in your favor.

One of the major updates to Appendix W includes the incorporation of ADJ_U* as a regulatory option in AERMOD’s meteorological data processor (AERMET).   To put it simply, when the wind speed is low or calm (think a still summer evening) the dispersion algorithms in AERMOD have a tendency to overstate the modeled concentrations.  The ADJ_U* option adjusts the variables used to calculate the dispersion of a plume to avoid over-predicting SO₂ concentrations under calm conditions.  What could this mean for you?  Utilizing ADJ_U* could lower anticipated impacts of emissions units on overly-conservative 1-hour SO₂ concentration predictions, which in turn provides more wiggle-room for current and future permitting projects.

Another just-as-important update is that local sources will no longer be modeled at their potential-to-emit (PTE) emissions rate.  Instead, local sources will be modeled as average actual emissions over the last two normal years of that source’s operation.  Since sources seldom emit at the permitted levels, the use of actual emissions provides a more representative assessment of actual air quality.  The ability to model local sources at their actual emissions rates will reduce the concentration levels of local sources in NAAQS analyses.  Also remember when a third party includes your facility in an air quality modeling study, your facility will be represented more realistically.  As with ADJ_U*, this change will ultimately provide your facility with more flexibility in current and future air permitting projects.

Lastly, U.S. EPA made revisions to Appendix W to incorporate new language surrounding the selection of background concentrations during short-term modeling.  For short-term standards, the air quality monitoring data may be paired in a temporal manner (e.g., pairing by season and/or hour of day).  This approach may be taken to minimize daily variation of ambient concentrations with season or time of day.  The ability to utilize these methods allows a better representation and less conservative option for accounting for background concentrations.  In short, it helps you.

Have questions, need assistance, or just don’t have the time?  We can help!

ALL4 has your back!  We have plenty of dedicated staff at ALL4 who would love to dive into your facility’s existing, new, or proposed projects and offer a helping hand.  If you have any questions, feel free to contact our ALL4 team members.