Concentrations of ozone have approximately doubled over the past century. In polluted air, photochemical reactions may produce concentrations of ozone up to 100 ppbv or more, even in areas relatively remote from sources of industrial pollution. The large ozone concentrations observed in these episodes, which may persist for several days, are markers for the presence of intense photochemical activity in the atmosphere. Experiments designed to investigate the effects of ozone on plants rarely, if ever, reproduce the chemical conditions which are always associated with ozone episodes.
Exposure of plants to ozone is strongly influenced by topography and meterological conditions. Windy sites which are well coupled to the atmosphere experience only a small diurnal variation in ozone concentration, and peak concentrations during episodes are generally much larger than at sheltered sites.
Stomatal opening effectively controls the amount and rate at which ozone enters leaves. Factors which affect stomatal opening therefore determine the dose of ozone for a given concentration in air. Extrapolation of results from controlled experiments to the field must assess whether growing conditions, including stomatal conductance, are comparable between experiment and field.
The Critical Levels approach has shown that crop plants and natural vegetation are likely to be exposed to potentially damaging ozone concentrations in most industrialised countries.