Evaluation of an explicit NO chemistry method in AERMOD.

Unlabelled: An explicit NO chemistry method has been implemented in AERMOD version 15181, ADMSM. The scheme has been evaluated by comparison with the methodologies currently recommended by the U.S. EPA for Tier 3 NO calculations, that is, OLM and PVMRM2. Four data sets have been used for NO chemistry method evaluation. Overall, ADMSM-modeled NO concentrations show the most consistency with the AERMOD calculations of NO and the highest Index of Agreement; they are also on average lower than those of both OLM and PVMRM2. OLM shows little consistency with modeled NO concentrations and markedly overpredicts NO. PVMRM2 shows performance closer to that of ADMSM than OLM; however, its behavior is inconsistent with modeled NO in some cases and it has less good statistics for NO. The trend in model performance can be explained by examining the features particular to each chemistry method: OLM can be considered as a screening model as it calculates the upper bound of conversion from NO to NO possible with the background O concentration; PVMRM2 includes a much-improved estimate of in-plume O but is otherwise similar to OLM, assuming instantaneous reaction of NO with O; and ADMSM allows for the rate of this reaction and also the photolysis of NO. Evaluation with additional data sets is needed to further clarify the relative performance of ADMSM and PVMRM2.

Implications: Extensive evaluation of the current AERMOD Tier 3 chemistry methods OLM and PVMRM2, alongside a new scheme that explicitly calculates the oxidation of NO by O and the reverse photolytic reaction, shows that OLM consistently overpredicts NO concentrations. PVMRM2 performs well in general, but there are some cases where this method overpredicts NO. The new explicit NO chemistry scheme, ADMSM, predicts NO concentrations that are more consistent with both the modeled NO concentrations and the observations.