ADEQ Upcoming Modeling Guidelines May Soften Modeling Costs



Some of the most challenging standards to review for purposes of regulatory air dispersion modeling evaluations are those associated with nitrogen dioxide (NO2) and the 1-hour averaging period. Regulatory agencies recognize this challenge and make available three "tiers" of potential modeling refinements. These refinements identify how much of the nitrogen oxides (NOx) emissions should be considered NO2 in the analysis. The Tier 3 refinement is considered the most complicated of these refinements but has the ability, in some cases, to produce the most favorable modeling results. This tier accounts for ambient conversion of NO to NO2 in the presence of ozone, namely the ozone titration mechanism. Therefore, in order for the algorithms to function properly, the user has to generate the hourly ozone concentrations to be used by the model. This can be a time and cost intensive effort since it requires the identification of representative ozone monitors, addressing any data gaps, processing the data, etc. The new ADEQ modeling guidelines include this analysis for various areas in Arizona allowing for a streamlined approach to Tier 3 1-hr NO2 modeling. Another area that may consume time and resources is the identification of a representative 1-hr NO2 background concentration to be added to model results to complete the regulatory analyses. Here again, the ADEQ has streamlined the level of effort for this analysis by developing these concentrations for various sites in Arizona. It is to be noted that the datasets for both of these parameters are already on the ADEQ website.