Evaluation of the Ability of Indicator Ratios to Determine Ozone Sensitivity to Precursor Emissions in Northern California

A successful ozone (O3) abatement program depends on an understanding of the factors that contribute to ambient ozone concentrations during ozone episodes. To reduce ambient O3 concentrations, anthropogenic emissions of oxides of nitrogen (NOx), volatile organic compounds (VOC), or both must be reduced depending on whether O3 production is limited by the availability of NOx and/or VOC. In the past, a number of observation-based analysis methods, calculated from hourly mixing ratios of O3, nitric oxide (NO), NOx, all oxidized products of nitrogen (NOy), nitric acid (HNO3), hydrogen peroxide (H2O2), and organic peroxides, have been used to diagnose if photochemical ozone production was limited by the availability of NOx or VOC. We have conducted a critical evaluation of observation-based analysis methods (also known as indicator ratios) using photochemical modeling results for central California.

The critical evaluation was conducted in four steps. First, we calculated indicator ratios based on hourly-averaged outputs from a photochemical transport model (CAMx) for the Central California Ozone Study (CCOS) domain for the duration of July 31 to August 2, 2000. Second, the consistency among calculated indicator ratios was examined and causes for the correlation or deviation were analyzed. Third, we compared the conclusions of indicator ratios with simulated ozone responses to NOx and VOC controls. Finally, we analyzed the soundness of the theoretical basis of the commonly used indicator ratios, including the Extent of Reaction of the Smog Production Algorithm.

Based on our analysis, we find the ratio of total peroxides to nitric acid, though not readily available from ambient measurements even during extensive field studies, to be the best among indicator ratios included in this study. We also found the Extent of Reaction of the Smog Production Algorithm to correlate with the [(NOy-NOx)/NOy]3/2 ratio, which is an indicator of the age of the air mass rather than the sensitivity of ozone production to its precursors. This correlation, together with the results of an analysis to derive the definition of the Extent of Reaction from fundamental chemical reactions, suggests that the determination of precursor limitations on ozone mixing ratios at individual monitoring stations is beyond the capabilities of the Smog Algorithm in its current form.