Aug 19, 2015

US Environmental Protection AgencyLast week, BREEZE staff made the trip to U.S. EPA's offices in North Carolina for their 11th Modeling Conference to get the latest updates on upcoming changes to the AERMOD system, CALPUFF, and other issues. The highlights include: 

  • Relief is coming to U.S. AERMOD users dealing with overprediction of concentrations in low wind speed hours: U.S. EPA plans to make the LOWWIND3 and ADJ_U* options in AERMOD and AERMET regulatory default options in Spring 2016.
  • Similarly, U.S. AERMOD users dealing with NO2 issues may see reduced concentrations in the future, as U.S. EPA plans to make the ARM2 option in AERMOD a regulatory default option in Spring 2016.
  • Proposed revisions to Appendix W, U.S. EPA's primary air quality modeling guidance document, were discussed and comments are being accepted through October 27, 2015. 
  • U.S. EPA hopes to have the new version of Appendix W as a final, approved rule in Spring 2016. If it is delayed much beyond that, the 2016 U.S. presidential election cycle could disrupt the approval of the changes to Spring 2017 or beyond. 
  • Improvements in the AERMOD building downwash algorithm (BPIP-PRIME) are in early stages of planning, but should not be expected soon. 
  • U.S. EPA is considering moving CALPUFF from a preferred model to a recommended screening model for long-range transport. 
  • U.S. EPA has proposed a tiered approach to modeling of ozone and secondary PM2.5, but details were somewhat limited as to how each level will be defined.  

The biggest near-term news coming out of the conference centered on several of the BETA options in AERMOD, including the horizontal stack and capped stack source types, options designed to reduce overly-conservative low wind speed performance (LOWWIND 1, 2, and 3, ADJ_U*), and options designed to reduce overly conservative NO2/NOx ratios for NO2 modeling. All of these features were introduced as BETA options in AERMOD in recent years, meaning they can be used for U.S. regulatory purposes only if the user goes through the official “alternative model” approval process. 

EPA Proposed Changes to Appendix WWith the new release of U.S. EPA AERMOD and AERMET executables version 15181, many had hoped these beneficial features would be approved as standard regulatory default options in AERMOD and AERMET. At the Modeling Conference, U.S. EPA officials confirmed that they do intend for many of these BETA options to become regulatory default options that would not require special permission for use. However, the approval of these options is tied to the revised version of Appendix W. If Appendix W is approved as expected in Spring 2016, U.S. EPA plans to make the horizontal and capped point sources, LOWWIND3 and ADJ_U* low wind improvement options, and ARM2 NO2/NOx ratio option regulatory defaults for U.S. air quality modeling. These changes could yield significant improvement in the accuracy of AERMOD and significant reductions in modeled concentrations in some cases. 

While U.S. EPA plans to make these improvements standard options in AERMOD in Spring 2016, EPA officials cautioned that delays in the Appendix W rulemaking process could put things on hold. With the U.S. presidential election coming up in November 2016, if the rulemaking process goes on beyond the Spring, election politics could delay the revised Appendix W, and with it the ability to freely use the new model options, until after the election at the earliest. 

Also, view the public presentations that provided comment to the EPA on the proposed rulemaking.

Stay tuned for updates, and let us know if you have any questions about what we learned at the conference or any other air quality modeling issues! We'll be here and happy to assist in any way possible. Give us a call at +1 (972) 661-8881or email us at