While it’s well established that industrialization and other human activity have increased tropospheric ozone (O3), the magnitude of that increase has been debated among researchers. Direct measurements from the late 19th century showed up to a 300% increase in surface ozone mixing ratios. However, atmospheric chemistry modeling studies from recent years estimate that tropospheric ozone levels rose substantially less (25%–50% ) since 1900.
Controversy surrounds “the accuracy and diagnostic power of these [historical] measurements,” researchers wrote in a recent study published in Nature. However, there is a compelling need to verify the accuracy of the modeling results because they are used to understand past ozone increases and make predictions informing future climate research or even policy.
Yet assessing the nature of preindustrial changes to ozone concentrations is a sizable challenge. “Concentrations of O3 precursors in the preindustrial atmosphere are not known well (with the exception of CH4 [methane]), resulting in poor constraints on past tropospheric O3,” the researchers wrote in the study, adding, “the historical latitudinal distribution of O3 precursors is also poorly known.”
When the researchers turned to ice cores and firn for more information about historical tropospheric ozone values, they knew they wouldn’t find ozone itself. Because ozone is very reactive, “the molecule isn’t preserved in ice cores,” said Laurence Yeung, a geochemist at Rice University and lead author of the study. Instead, they used a proxy: clumped oxygen isotopes.
Constraining Ozone Concentrations
Unlike ozone molecules, oxygen molecules get trapped in ice and snow, Mathew Evans noted in a Nature News and Views article about the research. Evans, an atmospheric chemist at the University of York in the United Kingdom, wasn’t involved with the research. “Yeung et al. therefore measured the amounts of the common oxygen-16 isotope and of the much less common oxygen-18 isotope in oxygen molecules trapped in polar ice and snow,” Evans wrote.
However, the decision to use the oxygen isotopes as a proxy was motivated by more than just their persistence in ice and snow. This proxy lacked downsides present with other substances. For instance, some proxies work only on a regional level, whereas others are subject to alterations and other effects after they are deposited in the ice and snow, Yeung and his collaborators noted in the study.
Between 1850 and 2005, the tropospheric concentration of ozone likely increased by less than 40%, “primarily near the surface,” with most of the increase happening between 1950 and 1980, Yeung and his collaborators concluded. They noted that the findings don’t invalidate the 19th century observations, despite the likelihood that they don’t accurately represent tropospheric ozone levels of the time.
When the results were in, “I was really happy that the [recent] models are doing really well and that we have a certain amount of confidence in atmospheric chemistry,” Yeung said.
As for the historical direct measurements, it “seems likely that interference from sulfur dioxide and other gases had indeed artificially lowered the ozone concentrations recorded in the historical measurements,” Evans wrote in his statement.
The study itself is “very novel,” and the researchers were “very careful about assessing the different factors that can impact the clustered isotope O2,” said Becky Alexander, an atmospheric scientist at the University of Washington in Seattle. However, “no proxy is perfect,” she said, noting that more studies are needed to explore whether researchers obtain similar results if other proxies for ozone are used.
Informing Climate Policies
The results of the study provide evidence in support of the models used by the Intergovernmental Panel on Climate Change to compare the effects of ozone, carbon dioxide, and methane as greenhouse gases and develop climate policies, according to Yeung and Evans.
The researchers determined the human impact on the radiative forcing of tropospheric ozone, which is about 22% of the radiative forcing from carbon dioxide. This places a limit on the prospective impacts that ozone mitigation efforts could have on climate change, Yeung said.
Still, the benefits of limiting ozone extend beyond potential impacts of climate change, as ozone impacts human health, noted Alexander, Evans, and Yeung.
“Efforts to reduce tropospheric ozone concentrations shouldn’t stop. Every little [bit] helps in the fight against climate change, and reductions would help to prevent some of the approximately one million deaths estimated to be caused by tropospheric ozone each year,” Evans wrote.
—Rachel Crowell (@writesRCrowell), Science Journalist