A humidity-based exposure index representing ozone damage effects on vegetation

Surface ozone (O₃) is detrimental to plant health. Traditional exposure indexes, such as accumulated hourly O₃ concentrations over a threshold of 40 ppb (AOT40), are easy to be derived and widely used to assess O₃ damage effects on vegetation. However, the regulation of environmental stresses on O₃ stomatal uptake is ignored. In comparison, the dose-based indexes are much more reasonable but require complex parameterization that hinders further applications. Here, we propose a new humidity-based index (O₃RH) representing O₃ damage effects on vegetation, which can be simply derived using ground-level O₃ and relative humidity (RH). Compared with O₃ damages to gross primary productivity (GPP_d) derived from a process-based scheme over May to October in 2015–2018, the O₃RH index shows spatial correlations of 0.59 in China, 0.62 in U.S., and 0.58 (P< 0.01) in Europe, much higher than the correlations of 0.16, −0.22, and 0.24 (P< 0.01) for AOT40. Meanwhile, the O₃RH index shows temporal correlations of 0.73 in China, 0.82 in U.S, and 0.81 (P< 0.01) in Europe with GPP_d, again higher than the correlations of 0.50, 0.67, and 0.79 (P< 0.01) for AOT40. Analyses of O₃RH reveal relatively stable trend of O₃ vegetation damages in eastern U.S. and western Europe, despite the long-term reductions in local O₃ pollution levels. Our study suggests the substitution of traditional exposure-based indexes such as AOT40 with O₃RH for more reasonable assessments of O₃ ecological effects. (letter)