U.S. EPA Releases 1-Hour NO2 NAAQS Modeling Guidance

The U.S. EPA Air Quality Modeling Group (AQMG) recently released a new guidance document for NO2 modeling.   The document outlines procedures that can be used to calculate the NO2 design values for comparison to the new 1-hour NO2 National Ambient Air Quality Standard (NAAQS).  The new 1-hour NO2 standard of 100 ppb goes into effect on April 12, 2010.  This 1-hour NO2 standard is based on the 3-year average of the 98th-percentile of the annual distribution of daily maximum 1-hour concentrations.

Right now, AERMOD, U.S. EPA’s current regulatory air quality dispersion model, can only calculate 1-hour averages.  Therefore, the guidance document describes procedures on how to manually post-process the AERMOD results to calculate the 3-year average of the 98th-percentile of the annual distribution of daily maximum 1-hour concentrations.  Additionally, the document states that AQMG is currently working on a post-processor that will automatically post-process AERMOD output files to calculate the 3-year average of the 98th-percentile of the annual distribution of daily maximum 1-hour concentrations.  This post processor should be released in the near future and will be found here.

The guidance document also outlines procedures to use when modeling with National Weather Service (NWS) data.  Since Appendix W of 40 CFR Part 51 requires the use of five (5) years of NWS meteorological data for air quality modeling evaluations, the document states that the 5-year average of the 98th-percentile of the annual distribution of daily maximum 1-hour concentrations should be calculated and compared to the new 1-hour NO2 NAAQS standard.  Specifically, the document states that “while the 1-hour NAAQS for NO2 is defined in terms of the 3-year average for monitored design value to determine attainment of the NAAQS, this definition does not preempt the Appendix W requirement for the use of 5 years of NWS data, and the 5-year average serves as an unbiased estimate of the 3-year average for purposes of modeling demonstration with the NAAQS.”

At this time, AQMG has not outlined procedures to follow when using site specific meteorological data.  When using site-specific meteorological data for air quality modeling evaluations, Appendix W of 40 CFR Part 51 only requires one (1) year of on-site meteorological data to be used.  As mentioned previously, AQMG is recommending following Appendix W of 40 CFR Part 51 guidance as an appropriate substitute for the proposed NAAQS when using NWS data.  Therefore, the use of a one (1) year average for onsite data could presumably be utilized as outlined in Appendix W of 40 CFR Part 51.  The NO2 modeling guidance document can be found here.