Can atmospheric chemistry deposition schemes reliably simulate stomatal ozone flux across global land covers and climates?

Short Summary: The risk of ozone pollution to plants is estimated based on the flux through the plant pores which still has uncertainties. In this study, we estimate this quantity with nine models at different land types worldwide, driven by measurement data. The models mostly estimated reasonable summertime ozone flux to plants. The model results varied by land cover, mainly related to the a lack of moisture in the soil. This work is an important step for assessing the ozone impact on vegetation.

Abstract. Over the past few decades, ozone risk assessments for vegetation have evolved two methods based on stomatal O₃ flux. However, substantial uncertainties remain in accurately simulating these fluxes. Here, we investigate stomatal O₃ fluxes across various land cover types worldwide simulated by six established deposition models. Hourly O₃ concentration and meteorological data at nine sites were extracted from the Tropospheric Ozone Assessment Report (TOAR) database, a comprehensive global collection of measurements, for the model simulations. 

The models estimated reasonable O₃ deposition (0.5–0.8 cm s⁻¹ in summer), which is mostly in agreement with the literature. Simulations of canopy conductance showed differences that varied by land cover type with correlation coefficients of 0.75, 0.80, and 0.85 for forests, crops, and grasslands among the models. Differences between models were primarily influenced by soil moisture and vapour pressure deficit, depending on each model's specific structure. Across models, the range of O₃ damage simulations at each site was most consistent for crops (6 to 11 mmol O₃ m⁻²), followed by forests (3 to 19.5 mmol O₃ m⁻²) and grasslands (7 to 33 mmol O₃ m⁻²). The median estimate across models aligns well with the literature at the sites most vulnerable to O₃ damage. 

Overall, this study represents a critical first step in developing and evaluating tools for broad-scale assessment of O₃ impacts on vegetation within the framework of TOAR phase II.