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EPA updates AERMOD


On May 11, 2021 EPA issued a new version of AERMOD. In keeping with their penchant for “memorable” version numbers (remember version 12345?) EPA went the palindrome route with this one and christened it version 21112.


On May 24 EPA held a webinar to detail the changes—for the slides of that webinar head here: https://gaftp.epa.gov/Air/aqmg/SCRAM/webinars/AERMOD_21112/AERMOD_21112_Release_Webinar.pdf


As EPA hadn’t updated AERMOD since the pre-pandemic days of 2019 there were a number of changes to the model but it’s unlikely that any of these changes will affect your modeling analysis very much—it mostly consisted of some new alpha options for AERMOD and some bug fixes to AERMOD, AERMET, and AERSCREEN.


Recall that alpha options in the AERMOD system are non-regulatory options that allow the user to invoke research/experimental options such that the modeling community can have a chance to assess their usefulness/performance. In the hierarchy of model options, they’re the “youngest”—after alpha options come beta options which are those that have been vetted by the modeling community and are awaiting to be officially blessed as regulatory options. It’s the beta options that you can use with alternative model approval from the Model Clearinghouse.


Brief notes on the changes to AERMOD:


Building downwash


Two new alpha options were added for new building downwash approaches which were developed by the Air and Waste Management Association’s PRIME2 Subcommittee, bringing the total of alpha options for building downwash to five.


NO2 conversion


Two new alpha options were added for NO to NO2 conversion. The first limits the conversion rate based on travel time between the source and receptor (usually the conversion happens within minutes) and the second is based on equilibrium chemistry, adopted from the UK’s ADMS model. The use of either option will require background data (ozone for the time limit option and NOx for the equilibrium option).


Treatment of low winds

Two new alpha options were added to the treatment of low wind speeds, which brings the total number of low wind options up to five.


RLINE updates


RLINE is used to model emissions from mobile sources (think roads) in AERMOD. With this version of AERMOD there were some updates to the roadside barrier algorithms.


Multiple buoyant line groups


When the BLP model was incorporated into AERMOD all the code remained unchanged, and one of the remnants (and hassles) of BLP was that multiple buoyant lines had to be of similar characteristics—in other words, they had to be lined up in the same orientation. With this new version of AERMOD individual BUOYLINE sources can be grouped together in separate BLPGROUPs which have similar characteristics. This creates a lot of more flexibility in AERMOD because now you can have buoyant line sources oriented in an east/west fashion and those oriented in a north/south fashion all in the same AERMOD run.


Turbulence treatment options


Two new default options have been added which allow the user to ignore non-missing turbulence parameters in the profile data, one for all hours and one for stable hours only. There are also seven new non-default options for handling the turbulence parameters under various stability conditions (these latter options are mostly for testing purposes).


Deposition parameter default values


With this version of AERMOD the user can now set default deposition parameters for elemental mercury, divalent mercury, dioxins, polycyclic aromatic hydrocarbons, SO2, and NO2. Similarly, the user can set optional default particle deposition parameters for Method 2 (which is an alpha option) for arsenic, cadmium, lead, mercury, and polycyclic aromatic hydrocarbons.


All in all it’s unlikely your modeling analysis will be impacted much, if at all, by these changes. However, with every new version of AERMOD you’ll want to make sure that if you’re conducting modeling in a regulatory context (e.g., in support of a permit application) that you’re using this latest version.

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