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

Abstract Surface ozone (O 3 ) is detrimental to plant health. Traditional exposure indexes, such as accumulated hourly O 3 concentrations over a threshold of 40 ppb (AOT40), are easy to be derived and widely used to assess O 3 damage effects on vegetation. However, the regulation of environmental stresses on O 3 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 (O 3 RH) representing O 3 damage effects on vegetation, which can be simply derived using ground-level O 3 and relative humidity (RH). Compared with O 3 damages to gross primary productivity (GPP d ) derived from a process-based scheme over May to October in 2015–2018, the O 3 RH 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 O 3 RH 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 O 3 RH reveal relatively stable trend of O 3 vegetation damages in eastern U.S. and western Europe, despite the long-term reductions in local O 3 pollution levels. Our study suggests the substitution of traditional exposure-based indexes such as AOT40 with O 3 RH for more reasonable assessments of O 3 ecological effects.