Ozone response to precursor controls: comparison of data analysis methods with the predictions of photochemical air quality simulation models

Comparisons were made between the predictions of six photochemical air quality simulation models (PAQSMs) and three indicators of ozone response to emission reductions: the ratios of O3/NOz and O3/NOy and the extent of reaction. The values of the two indicator ratios and the extent of reaction were computed from the model-predicted mixing ratios of ozone and oxidized nitrogen species and were compared to the changes in peak 1 and 8 h ozone mixing ratios predicted by the PAQSMs. The ozone changes were determined from the ozone levels predicted for base-case emission levels and for reduced emissions of volatile organic compounds (VOCs) and oxides of nitrogen (NOx). For all simulations, the model-predicted responses of peak 1 and 8 h ozone mixing ratios to VOC or NOx emission reductions were correlated with the base-case extent of reaction and ratios of O3/NOz and O3/NOy. Peak ozone values increased following NOx control in 95% (median over all simulations) of the high-ozone (>80 ppbv hourly mixing ratio in the base-case) grid cells having mean afternoon O3/NOz ratios less than 5 : 1, O3/NOy less than 4 : 1, or extent less than 0.6. Peak ozone levels decreased in response to NOx reductions in 95% (median over all simulations) of the grid cells having peak hourly ozone mixing ratios greater than 80 ppbv and where mean afternoon O3/NOz exceeded 10 : 1, O3/NOy was greater than 8 : 1, or extent exceeded 0.8. Ozone responses varied in grid cells where O3/NOz was between 5 : 1 and 10 : 1, O3/NOy was between 4 : 1 and 8 : 1, or extent was between 0.6 and 0.8. The responses in such grid cells were affected by ozone responses in upwind grid cells and by the changes in ozone levels along the upwind boundaries of the modeling domains.