Diagnostic evaluation of numerical air quality models with specialized ambient observations: testing the Community Multiscale Air Quality modeling system (CMAQ) at selected SOS 95 ground sites

Three probes for diagnosing photochemical dynamics are presented and applied to specialized ambient surface-level observations and to a numerical photochemical model to better understand rates of production and other process information in the atmosphere and in the model. However, care must be taken to ensure that rate and process information is not confounded by inappropriate averaging over these diurnally changing photochemical dynamics. One probe, the [O3] response surface probe [O3]/[NOX], is used here as a chemical filter to select NOX-limited hours in the observations and the simulations. Other probes used here are the fraction NOZ/NOY, a measure of chemical aging, and a measure of the production efficiency of O3 per NOX converted, [O3] to [NOZ]. The key ambient measurements for all three probes are accurate [NO2] and a reliable estimate of total NOY. Good agreement is shown between models and observations in cases where local photochemical production dominates and where model emissions inputs are thought to be mostly complete. We interpret this agreement to mean that the photochemical processing in CMAQ is substantially similar to that in the atmosphere. More importantly, we see that the three probes provide consistent information about photochemical processing, especially when used together.