A review study on ozone phytotoxicity metrics for setting critical levels in Asia

Ground-level ozone (O3) can be a menace for vegetation, especially in Asia where O3 levels have been dramatically increased over the past decades. To ensure food security and maintain forest ecosystem services, such as nutrient cycling, carbon sequestration and functional diversity of soil biota, in the over-populated Asia, environmental standards are needed. To set proper standards, dose-response relationships should be established from which critical levels are derived. The predictor of the response in the dose-response relationship is an O3 metric that indicates the dose level to which the plant has been exposed. This study aimed to review the relevant scientific literature and summarize the O3 metrics used worldwide to provide insights for Asia. A variety of O3 metrics have been used, for which we discuss their strengths and weaknesses. The most widely used metrics are based only on O3 levels. Such metrics have been adopted by several regulatory agencies in the global. However, they are biologically irrelevant because they ignore the plant physiological capacity. Adopting AOT40 (O3 mixing ratios Accumulated Over the Threshold of 40 nmol mol-1) as the default index for setting critical levels in Asia would be a poor policy with severe consequences at national and Pan-Asian level. Asian studies should focus on flux-based O3 metrics to provide relevant bases for developing proper standards. However, given the technical requirements in calculating flux-based O3 metrics, which can be an important limitation in developing countries, no-threshold cumulative exposure indices like AOT0 should always accompany flux-based indices. © 2010 Asian Journal of Atmospheric Environment.