A multi-parent assignment method for analyzing atmospheric chemistry mechanisms

A new chemical mechanism analysis tool known as multi-parent assignment (MPA) is used to explore reactant contributions to ozone formation in the SAPRC-99 atmospheric chemistry mechanism. Mathematical details of MPA, which extends an earlier method by accounting for multiple reactants, are presented. MPA uses reaction rate information and mechanism stoichiometry to trace reaction pathways within a mechanism from products back to parent reactants. The net contribution of different reactants to products, such as O3 or OH, is then determined. Three formulas for assigning products among multiple reactants in a reaction are defined and compared. MPA analysis of an urban ozone formation scenario at three different NOx levels shows that all individual reactants contribute in some degree to ozone formation. NO and NO2 contributions vary greatly depending on VOC/NOx ratio and on how parent reactants are weighted in individual reactions. The most productive hydrocarbons on a ppm O3 per ppmC basis are generally aldehydes, followed by alkenes and higher aromatics, and then alkanes and lower aromatics. Reactant contributions to ozone are shown to vary with time, and shifts in limiting reactant behavior are also observed.