Dispersion Modeling Challenges for Air Permitting Justin Fickas Christine Haman Jake Stewart

Overview Many new challenges hindering our ability to find modeling solutions for PSD projects 2. Building Downwash Issues 1. Recently Revised NAAQS 3. PM 2.5 Proposed Annual NAAQS

Building Downwash Issues Changes to the way AERMOD handles building downwash have altered model results from the version to now

Building Downwash Issues ˃ Downwash calculated based on the “EPA Formula Height” H GEP = H b + 1.5L H b = building height above stack base L = lesser of building height and projected building width ˃ Good Engineering Practice (GEP) Height  Equal to the greater of the following two values: EPA Formula Height or 65 meters

Building Downwash Issues Building downwash effects were turned off if stack height was greater than or equal to EPA formula height Even though downwash was off modelers were still allowed to take credit for the “true” height Version 09292

Building Downwash Issues Subroutine WAKFLG was modified: No longer ignore potential downwash effects for stack heights that equal or exceed the EPA formula height (Not the same as GEP Height!) PRIME downwash algorithm: Determines when and how to apply downwash Versions and Later

Building Downwash Issues 1. Intent is to remove discontinuity in AERMOD building downwash treatment 2. Discontinuity did not exist prior to use of PRIME 3. Brings downwash back in play for sources that otherwise would have been “exempt”

Downwash Example One Setup: 104 m 56 m m H GEP = H b + 1.5L H GEP = m + 1.5(24.38 m) = 61 m

Downwash Example One ˃ Experiment varying stack height slightly below and above H GEP (H GEP = 61 m) ˃ Modeled identical emission rate ˃ Stack height above EPA equation is now subject to downwash and results are not as favorable Modeled Stack Height (m) Version hour Result (µg/m 3 ) Version hour Result (µg/m 3 )

Downwash Example Two Setup: Variable Heights H GEP = H b + 1.5L Stack H GEP = m GEP Height = 65 m

Downwash Example Two H GEP

Building Downwash Issues Conclusions:  Complexity of the downwash algorithms and the direction-dependent nature of downwash effects are such that the magnitude of the differences between different versions of AERMOD vary by scenario  Change to the treatment of downwash with respect to stack heights in the currently approved version of AERMOD has the potential to cause large increases in model-predicted concentrations

PM 2.5 Proposed NAAQS Revision Published in the Federal Register on 29 June Reduce the current annual standard from 15 µg/m 3 to a value between 12 and 13 µg/m 3 2. Aims to “provide increased protection against health effects associated with long- and short-term exposures”

PM 2.5 Proposed NAAQS Revision Challenges:  PM 2.5 standards already some of the most difficult standards to comply with  AERMINUTE data has increased difficultly in modeling compliance for low dispersion PM sources  Background concentrations typically leave little to no room for companies to implement projects for PM 2.5

PM 2.5 Proposed NAAQS Revision Conclusions:  Facilities in many cases will have to reduce fugitive impacts in order to demonstrate modeled compliance with the new standard  Compliance “gap” between background monitor values and the standard will shrink in some areas, and altogether disappear in others  Projects that have previously passed the NAAQS may no longer be able to expand (and trigger PSD for PM 2.5 ) without taking restrictions

Questions? Justin Fickas 53 Perimeter Center East Suite 230 Atlanta, GA Office: (678) Cell: (678) Fax: (678)

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