On the indicator-based approach to assess ozone sensitivities to reductions in VOC and NOx emissions

Previous model studies suggested that indicator species could be used to assess the sensitivities of ozone to reductions in precursors emissions and proposed threshold criteria to distinguish VOC- and NOx-sensitive regimes. Other studies have assumed the universality of these threshold criteria and have compared local observed indicator species with previously established criteria to assess ozone sensitivities. In this study, the results of SARMAP Air Quality Model (SAQM) simulations in the San Joaquin Valley, California, USA, are used to investigate the applicability of indicator species to assess ozone sensitivities. Threshold criteria of indicator species (NOy, O₃/NOz, HCHO/NOy, and H₂O₂/HNO₃) that distinguish various ozone sensitivities are derived. The use of indicator species along with threshold criteria is found to be effective for identifying VOC- or NOx-sensitive regimes. However, SAQM-derived threshold criteria for this episode are found to differ from threshold criteria proposed by previous studies using different models and under different conditions. Such differences suggest that threshold criteria are likely to be dependent on local environmental conditions, including emission patterns and rates. Therefore, local observed indicator species can be used to assess ozone sensitivities only if appropriate threshold criteria have been derived for the local conditions.