Isotopic constraint on the twentieth-century increase in tropospheric ozone.

Tropospheric ozone (O) is a key component of air pollution and an important anthropogenic greenhouse gas. During the twentieth century, the proliferation of the internal combustion engine, rapid industrialization and land-use change led to a global-scale increase in O concentrations; however, the magnitude of this increase is uncertain. Atmospheric chemistry models typically predict an increase in the tropospheric O burden of between 25 and 50 per cent since 1900, whereas direct measurements made in the late nineteenth century indicate that surface O mixing ratios increased by up to 300 per cent over that time period.