On January 17, 2017 the final revisions to the Guideline on Air Quality Models (Appendix W) were published in the Federal Register. The effective date of the rule was subsequently delayed but became effective on May 22, 2017. The technical support materials and final rule information can be found on the SCRAM website.
One major change to the Guidelines was the incorporation of an algorithm known as Adjust-u* as a regulatory default option in AERMET, the meteorological pre-processor for the AERMOD. The algorithm adjusts surface friction velocity in stable, low wind conditions. Without this adjustment, the surface friction velocity can be underestimated leading to overestimated model concentrations.
The recently released Version C meteorological data incorporated all updates to AERMET but did not include application of this algorithm. The sensitivity of AERMOD to meteorological data processed using this algorithm has since been tested.
The predicted changes in model concentration are consistent with the published intent and formulation of the algorithm. Significant reductions in concentrations will likely be seen from emissions releases occurring in complex terrain, emissions releases from ground-level sources, and emissions being modeled against ambient air quality standards with short term averaging periods. The concentrations for some sources slightly above ground-level may increase by a small amount. Results in flat terrain remain largely unchanged regardless of source characteristics.
Applicants should begin using the latest version of meteorological data (Version D) immediately. The previous version of the meteorological data (Version C) can be used in cases where it has been approved in a dispersion modeling protocol.
The revised meteorological data files are available under the Dispersion Modeling page on the Department’s website.
Please contact Alyssa Fizel at alyssa.fizel@dnr.iowa.gov or by phone at (515) 725-9566, with any questions or comments on this update.