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 (GPPd) 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 GPPd, 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.