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Measuring ozone the right way
Ground-level ozone affects our crop yields, with wheat being the most sensitive. Various indices can be used to express the ozone exposure of crops. The most common is the daytime average exceedances of 40 ppb ozone (AOT40). Another exposure index is the Phytotoxic Ozone Dose (POD), accounting for not only the atmospheric concentrations but also the stomatal uptake of ozone, since it includes environmental factors such as solar radiation, air humidity and temperature and sometimes also plant specific factors. The development of POD started 20 years ago in an effort to find a physiologically more relevant dose, with accumulated evidence for different plant types. While no exposure index will perfectly reflect the effective dose, the POD has been shown to be more accurately associated with O3 effects on vegetation. There are different models for POD depending on which environmental (or plant phenotype) factors are included to assess the stomatal uptake. A recent study (Pleijel et al. 2022) found that the variation in ozone sensitivity between experiments was reduced by a factor >6 when using a POD6 index compared to AOT40. It can therefore be stated that environmental variables, such as solar radiation, air humidity and temperature, can explain a substantial part of the variation in O3 response. The significance of choosing the right index is seen in an report by Eionet. When using AOT40 they found a significant decrease in the crop yield impact between 1990 and 2010. However when using POD there was no significant change.
Eionet Report - ETC/ACM 2018/15