Rice University researchers and collaborators used ice cores, like the one shown here from Antarctica, in combination with atmospheric chemistry models to establish an upper limit for the increase in ozone levels in the lower atmosphere since 1850. (Photo by Jeff Fitlow/Rice University)

Isotopic constraint on the twentieth-century increase in tropospheric ozone

Laurence Y. Yeung, Lee. T. Murray, Patricia Martinerie, Emmanuel Witrant, Huanting Hu, Asmita Banerjee, Anaïs Orsi & Jérôme Chappellaz

Nature 570 (2019) 224-227

Abstract

Tropospheric ozone (O₃) 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 O₃ concentrations; however, the magnitude of this increase is uncertain. Atmospheric chemistry models typically predict an increase in the tropospheric O₃ burden of between 25 and 50 per cent since 1900, whereas direct measurements made in the late nineteenth century indicate that surface O₃ mixing ratios increased by up to 300 per cent over that time period. However, the accuracy and diagnostic power of these measurements remains controversial. Here we use a record of the clumped-isotope composition of molecular oxygen (¹⁸O¹⁸O in O₂) trapped in polar firn and ice from 1590 to 2016 ad, as well as atmospheric chemistry model simulations, to constrain changes in tropospheric O₃ concentrations. We find that during the second half of the twentieth century, the proportion of ¹⁸O¹⁸O in O₂ decreased by 0.03 ± 0.02 parts per thousand (95 per cent confidence interval) below its 1590–1958 ad mean, which implies that tropospheric O₃ increased by less than 40 per cent during that time. These results corroborate model predictions of global-scale increases in surface pollution and vegetative stress caused by increasing anthropogenic emissions of O₃ precursors. We also estimate that the radiative forcing of tropospheric O₃ since 1850 ad is probably less than +0.4 watts per square metre, consistent with results from recent climate modelling studies.

DOI: 10.1038/s41586-019-1277-1