Evaluation of the ability of indicator species ratios to determine the sensitivity of ozone to reductions in emissions of volatile organic compounds and oxides of nitrogen in northern California

Six indicator ratios were evaluated for their regulatory application in California using three-dimensional (3-D) photochemical transport modeling output for an ozone episode during 31 July–2 August 2000. The evaluation was based on four criteria with increasing usefulness for 8-h ozone controls. The four criteria can be briefly described as: (1) smooth functional behavior between ozone benefits/disbenefits and the magnitude of the indicator ratio, (2) narrow separation between beneficial and detrimental regimes for oxides of nitrogen (NOx) emissions controls, (3) narrow transition regime between where volatile organic compound controls are more beneficial than NOx controls and vice versa, and (4) invariance of these transition ranges in time and space. None of the indicator ratios met all criteria. We present results for five sub-regions in northern California. The transition regime for criteria (3) was found to be much wider than that for criteria (2) for all sub-regions. Hence, the indicator ratios may be of only limited usefulness in determining precursor limitations and might be more valuable in diagnosing if NOx controls are beneficial or detrimental.