Chamber Evaluation of Process Diagnostics and Photochemical Indicators

Historically photochemical mechanisms and air quality models have been evaluated primarily in terms of there ability to simulate observed O3 data. There is an increasing awareness that mechanism and models must also be evaluated in terms of their ability to simulate the fundamental chemical processes that control O3 formation and the sensitivity of O3 to emissions reductions (Arnold et al., 1998). Such evaluative methods include process diagnostics which are useful for characterizing O3 photochemistry (Jeffries and Tonnesen, 1994), and photochemical indicators which can be used to characterize O3 sensitivity to VOC and NOx (Milford et al., 1994; Sillman, 1995; Chang et al., 1997; Blanchard et al., 1999; Tonnesen and Dennis, 1999). Both of these approaches are currently being used in diagnostic model evaluations (e.g., Sillman et al., 1997a,b, 1998; Imre et al, 1999). There are considerable uncertainties, however, in the usefulness of these methods due to uncertainties in each of the many sink and source terms that can contribute to production or loss of trace species. For example, there are uncertainties of 20% or more in important gas phase reactions (Donahue et al, 1997; Gao et al., 1995, 1996), large uncertainties in heterogeneous chemistry (Dentener and Crutzen, 1994) and in emissions inventories, deposition rates and transport. Because of these uncertainties, it can be difficult to use ambient data to validate model simulations of chemical processes. Furthermore, the particular values of photochemical indicators that distinguish NOx sensitive or VOC sensitive conditions have only been derived from model simulations. To date there has been no empirical validation of the usefulness of these indicators. Here we propose that before process diagnostics and photochemical indicators can be used with confidence in model evaluations, their behavior must be characterized under the relatively well controlled conditions of chamber experiments. Each of these approaches is discussed below, and the measurements necessary to evaluate them in chamber experiments are listed.