Ozone process insights from field experiments – Part III: extent of reaction and ozone formation

The analysis of ambient data offers a means of developing a qualitative understanding of the sensitivity of ozone formation at specific times and places to changes in VOC and NOx concentrations. The Integrated Empirical Rate (IER) model (Johnson, 1984, Proceedings of the Eighth International Clean Air Conference, Melbourne, Australia, pp. 715–731) and two revisions known as the Smog Production (SP) algorithm (Blanchard et al., 1999, Atmospheric Environment 33, 369–381; Chang et al., 1997, Atmospheric Environment 31, 2787–2794) are reviewed. Applied to ambient data, the algorithm requires measurements of ozone, NO, and either NOx or NOy and computes a quantity known as the extent of reaction. The extent of reaction is shown to be related to photochemical age and serves as an indicator of the sensitivity of instantaneous ozone production to changes in VOC or NOx concentrations. Extent of reaction alone is insufficient as an indicator of the sensitivity of ozone concentration to a complex upwind history of emission changes. Consideration of daily, hourly, and spatial patterns of extent of reaction is needed to interpret applications of the SP algorithm.