Comparison of Source Apportionment and Source Sensitivity of Ozone in a Three-Dimensional Air Quality Model

The ozone source apportionment technology (OSAT) estimates the contributions of different sources to ozone concentrations using a set of tracers for NO_x, total VOCs, and ozone and an indicator that ascribes instantaneous ozone production to NO_x or VOCs. These source contributions were compared to first-order sensitivities obtained by the decoupled direct method (DDM) for a three-dimensional simulation of an ozone episode in the Lake Michigan region. The cut-point for the OSAT indicator between VOC- and NO_x-sensitive ozone production agrees well with the DDM sensitivities to VOC and NO_x. In a ranking of the most important contributors to ozone concentrations >80 ppb, the OSAT and DDM results agreed on four of the top five contributors on average. The spatial distributions of the sensitivities and source contributions are similar, and the OSAT and DDM results for ozone >80 ppb correlate well. However, the source contributions ascribe substantially less relative importance to anthropogenic emissions and greater relative importance to the boundary concentrations than do the sensitivities. In regions where NO_x inhibits ozone formation and the sensitivity is negative, the source contribution is small and positive. For the same subdivision of the emissions, the OSAT is 14 times faster than the DDM, but the DDM has greater flexibility in defining which emissions to include and generates results for species other than ozone. The first-order sensitivities explain, on average, 70% of the ozone concentrations.