The Gas Ratio as a diagnostic indicator of possible nonlinear interactions between sulfate and total-nitrate fine particle reductions: characterizing its performance as an indicator

Most diagnostic indicators developed to date involve ratios or the relationship of one specie to another, as in a slope. The Gas Ratio, the ratio of free ammonia to total-nitrate, developed for inorganic fine particles is no exception. Diagnostic indicators are typically developed through a good conceptual understanding of the functioning of the chemical and/or physical system supported by sensitivity analyses with box models. When the diagnostic indicators are used in a 3-D air quality model to define the state of the chemical system relative to the ozone response for an emission reduction, it has been found that a range of indicator values will be associated with the same relative or absolute change of ozone levels. That is, the indication of the transition from one state to another is fuzzy. In this work, it is shown that the Gas Ratio is similarly fuzzy when used to characterize a 3-D air quality model's nonlinear response to changes in emissions. The nonlinear response examined, in particular, is the increase in aerosol nitrate associated with a decrease in aerosol sulfate. This work also shows that the standard conceptual model of the association between system state and the nonlinear response in aerosol nitrate differs from the response extracted from the 3-D model. In addition, this work shows that the Gas Ratio, as a ratio, does not provide a unique association between it and the nonlinear aerosol response. One member of the ratio, the free ammonia, is a more important variable vis a vis determination of the nonlinear system response. Nonetheless, the Gas Ratio serves as a valuable intellectual construct to help guide diagnostic evaluation of the inorganic fine particle system representation in 3-D air quality models.