Tropospheric ozone (O3) is key component of air pollution and an important anthropogenic greenhouse gas. During the 20th century, proliferation of the internal combustion engine, rapid industrialization, and land-use change led to a global-scale increase in its concentrations, but the magnitude of this increase is not known. Atmospheric chemistry models typically predict an increase in the tropospheric O3 burden of less than 50% since 1900,1,2 while direct measurements made in the late 19th century imply that surface O3 mixing ratios increased by much more, up to 300%3-5. Improvements in model chemistry have reduced disparities somewhat6,7, but the accuracy and diagnostic power of the measurements remains controversial8. Here, we place limits on the 20th-century increase in tropospheric O3 using a record of the clumped-isotope composition of molecular oxygen (18O18O in O2) trapped in polar firn and ice. We find that proportions of 18O18O in O2 decreased by 0.03 ± 0.02‰ below their 1590 – 1958 C.E. mean during the second half of the 20th century, implying that tropospheric O3 increased less than 40% 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 precursors1,2,9. The radiative forcing due to tropospheric O3 since 1850 C.E. is likely less than +0.4 W m-2, placing limits on the impact of air-quality regulation as a global warming mitigation strategy.
Authors : Laurence Y. Yeung, Lee. T. Murray, Patricia Martinerie, Emmanuel Witrant, Huanting Hu, Asmita Banerjee, Anaïs Orsi, and Jérôme Chappellaz.
Ref. : Nature, vol. 570, no 7760, p. 224, 2019
