An update to the ambient ratio method for 1-h NO2 air quality standards dispersion modeling. (February 2015) - Podrez, Mark
Abstract: Nitrogen oxide (NO X ) gases are typically emitted by fuel combustion sources in the form of nitric oxide (NO), which then reacts with ozone and other oxidants in the atmosphere to convert a portion of the NO to nitrogen dioxide (NO2 ). EPA has promulgated a 1-h average National Ambient Air Quality Standard (NAAQS) for NO2, and major sources of NO X emissions must estimate their NO2 air quality impacts as part of EPA's air quality permitting programs. The AERMOD dispersion model has been developed by EPA for these air quality impact analyses, and AERMOD contains three different NO to NO2 conversion methods for estimating the ambient concentrations of NO2 . This paper describes a refinement to one of the methods, the Ambient Ratio Method version 2 (ARM2). ARM2 is an empirical approach that uses a variable conversion factor, based on an analysis of ambient air measurements of NO and NO2, to estimate the portion of the AERMOD predicted air concentration of total NO X species that is in the form of NO2 . The performance of ARM2 has been evaluated and found to compare well to actual ambient measurements and to other more complex EPA conversion methods. EPA has included ARM2 as a "beta-testing" option in AERMOD version 14134, and provided guidance on the use of ARM2 for regulatory modeling analyses in a September 2014 memorandum. This paper also discusses this recent EPA guidance. Graphical abstract: Highlights: Ambient NO X monitoring data was used to determine the conversion of NO to NO2 gases. A conversion equation was developed and incorporated into EPA's AERMOD model. Performance testing of this method was conducted. The results indicate that the method conservatively estimates ambient NO2 levels. EPA has approved ARM2 subject to certain limitations as discussed in this paper.