Ohio EPA Issues Revised Air Dispersion Modeling Guidance

On July 22, 2014, the Ohio Environmental Protection Agency (Ohio EPA) issued a final revised version of Engineering Guide #69, which provides guidance for the use of air dispersion models for regulatory applications in Ohio. The revised guidance supersedes the first issuance of Engineering Guide #69 dated July 1, 2003, and includes several changes and additions to the first issuance of the guide. A summary of the significant revisions is as follows:

  • Table 3 now includes an Ohio Significant Emission Rate (SER) of 10 tons per year (tpy) for PM2.5. Additionally, the SERs for sulfur dioxide (SO2) and nitrogen dioxide (NO2) have been increased from 25 tpy to 40 tpy, while the SER for PM10 has been increased from 10 tpy to 15 tpy. Any project with emissions exceeding these SERs requires air dispersion modeling.
  • Table 3 now includes a Prevention of Significant Deterioration (PSD) SER of 40 tpy for NOX as a precursor of Ozone. The revised Engineering Guide #69 clarifies that while Ohio EPA does not require modeling for Ozone, if NOX and/or VOC emissions exceed 40 tpy, a qualitative analysis is required to show that increases in NOX and/or VOC will not cause or contribute to an ozone exceedance.
  • The PM2.5 PSD monitoring de minimis concentration is changed to 0 µg/m3 to align with the January 2013 Court ruling.
  • Table 3 of Engineering Guide #69 now includes 1-hour Ohio Acceptable Incremental Impacts (AIIs) for SO2 and NO2 of 196 and 188 µg/m3, respectively. Trinity has confirmed with Ohio EPA's modeling group that modeling of 1-hour SO2 and NO2 in support of minor source permit applications is to be performed in accordance with U.S. EPA's 2010 clarification memos regarding the applicability of the Appendix W modeling guidance for the 1-hour SO2 and NO2 standards. To view these memos, click here.
  • For state-only netting modeling evaluations, the allowable to allowable difference remains acceptable, but must now be agreed upon in a modeling protocol. Modeling protocols are required for all Nonattainment New Source Review (NANSR) and PSD modeling analyses, as well as any modeling analyses that include non-default AERMOD options.
  • AERSCREEN is now required for all PSD screening analyses; however, SCREEN3 is still acceptable for state-only modeling analysis.
  • When creating receptor grids using the AERMAP preprocessor, National Elevation Dataset (NED) data with a maximum resolution of one arc second must now be used. Digital Elevation Model (DEM) files are no longer acceptable for determining elevation of receptors and other model objects.
  • The revised guidance now advises use of 50 meter grid spacing for receptors placed on the fenceline and "hotspots", as opposed to the previously recommended 100 meter spacing.
  • Information from U.S. EPA's PM2.5 Permit Modeling Guidance document (available here) on how to model both primary and secondary PM2.5 for NNSR and PSD permitting is included.

In addition to the revisions described above, several grammatical changes and changes to website links and references were also made. To view the revised Engineering Guide #69 in its entirety, click here. Additionally, Ohio EPA's responsive summary to comments received on the draft revisions can be found here.

For more information, please contact Mike Burr at 614-433-0733 or mburr@trinityconsultants.com.