Abstract:Ground-level ozone (O3) is a major pollutant with adverse effects on plant growth and yield. The impact on plants is generally studied by controlling the O3 concentration using the open top chamber (OTC) method. The effects of O3 on ecosystems are studied by combining assessment models and indices, calculated using O3 concentration or flux measurements over the vegetation. As these effects are related to the level of O3 entering into the plant's gas exchange, compared with concentration-based indices,the O3 stomatal flux-based indices are considered the better standards for evaluating the influence of O3 on ecosystems. In China, the rapid industrialization and urbanization has resulted in elevated O3 concentration which is threatening crop production and yields. Studies are required to elucidate the O3 risk-assessment indices, including consideration for the status of the plant. In this paper, we analyze the advantages and disadvantages of two kinds of assessment indices. We then introduce several O3 flux observation and stomatal uptake estimation methods at ecosystem scale. We also review the progress in O3 flux observations, stomatal uptake estimations and risk assessment across different ecosystems. Additionally, we present case studies and the future prospects of O3 research in China. Concentration-based indices are easily observed and calculated, but these types of indices are often lacking sufficient, robust experimental design and data. In contrast, O3 flux-based indices are difficult to obtain, although they incorporate the status of the ecosystem. The method used for measuring O3 flux is mainly the eddy covariance technique, generally seen as the best modern technique. However, this method is inferior compared with CO2/H2O flux measurements. The lack of a fast-response O3 analyzer is the main limiting factor. To estimate O3 stomatal uptake, one of the well-established methods uses a resistance model for partitioning the total O3 flux. Stomatal and non-stomatal resistance can be estimated by parameterizing or converting CO2/H2O resistance, which can be estimated using the empirical methods or the Penman-Monteith equation. O3 deposition velocity (Vd) is a better variable to compare O3 deposition characteristics across different ecosystems and the Vd value may be affected by the underlying surface status and atmospheric conditions. The Vd on abiological surfaces (0.1 cm/s or less) is much lower than on the surface of the vegetation. Generally, Vd over forest ecosystems (1 cm/s) is larger compared with that over grassland and cropland ecosystems (0.5 cm/s). Radiation and atmospheric humidity are the main factors controlling O3 deposition. Fractions of stomatal uptake over different locations and ecosystems vary considerably with leaf size, stomatal aperture, canopy structure and the physiology of plants. For ecosystem risk assessment, the performance of flux-based indices was better compared with concentration-based indices over various ecosystems. In China, most research focuses on the effects of different O3 concentrations on crop growth and yield using the OTC method, obtaining some significant results. There are relatively few studies that have investigated the effects of O3 concentration and flux at the ecosystem level. Therefore, relevant research on ecosystem O3 risk assessment is urgently required in future.
