In March 2011, the U.S. EPA released AERSCREEN, a screening model based on the AERMOD dispersion algorithms, which is expected generally to yield more realistic concentrations than the existing SCREEN3 model, while maintaining conservatism over more refined analyses. On April 11, 2011, the U.S. EPA issued a clarification memo stating that AERSCREEN was intended to replace the SCREEN3 model, a change that is allowed, without formal rulemaking, in the Guideline on Air Quality Models (Appendix W of 40 CFR Part 51). AERSCREEN is a much more involved screening tool than SCREEN3 and its predictions have shown significant variability across different scenarios. As such, special care should be given to model setup and results interpretation from all AERSCREEN analyses.
In April 2011, the U.S. EPA released an update to the AERMOD modeling system (version 11103). The main updates to the model were to incorporate more advanced post-processing routines required by the new 1-hour NO2 and SO2 NAAQS. The meteorological preprocessor for AERMOD, AERMET, was also enhanced significantly to incorporate minute-by-minute wind observations rather than a simple measurement just before the hour. This AERMET update will eventually require analysts to reprocess AERMOD meteorological datasets in order to make them compatible with the newest model code. This change in processing approach will reduce the number of calms and/or missing data observations present in AERMOD-ready datasets, which will in turn have an impact on concentrations predictions from the model. States that provide model-ready datasets (e.g. North and South Carolina) anticipate that it could be a few months or more before they are able to reprocess the data using the new approach. As such, the new version of AERMOD may be used with the older version meteorological data in the interim.