The mystery of ozone rests

Before the 1950s, direct observations of the composition of the atmosphere were extremely limited. The air trapped in pockets of snow and ice has made it possible to reverse the observation of certain gases for hundreds of years. But some gases that affect climate and air quality, such as ozone (O₃), are not stable in ice or snow, which limits their records in recent decades. Write in Nature, Yeung et al.¹ report that isotopic observations of oxygen (O₂) Molecules trapped in polar snow and ice can be a new constraint for estimating levels of ozone in the troposphere (the lowest 12 kilometers of the atmosphere) over the last 150 years. This greatly expands our knowledge of the concentration of this key atmospheric gas and could finally solve a problem that has been of concern to atmospheric chemists for decades.

Advances in atmospheric science are often made by taking advantage of "opportunity experiences" resulting from natural and anthropogenic changes in the atmosphere. It is therefore crucial to measure the atmosphere in the long term. The modern era of such measurements began in the late 1950s, with observations of carbon dioxide levels² from the Mauna Loa Observatory in Hawaii. Long-term measurements of other atmospheric constituents began shortly thereafter. It was not until the 1990s that the observation networks developed sufficiently to provide a global perspective of a wide range of air components.³.

However, the impact of the changing composition of the atmosphere depends both on current pollutant concentrations and their concentrations prior to the onset of change. Many atmospheric changes (climate change being the most notable example) are associated with the industrial revolution. Therefore, to understand the magnitude of these effects, we must understand the composition of the atmosphere over the last 100 to 200 years.

Changes in the concentrations of certain air components, such as CO₂ and methane, are very well known thanks to ice archives and modern observations. However, since ice data provide no constraint on estimates of changes in ozone levels, computer models of chemistry and atmospheric physics are used. These models calculate an increase of about 40% in tropospheric ozone levels between 1850 and present.⁴. The Intergovernmental Panel on Climate Change uses these estimates to assess the relative importance of ozone as a greenhouse gas, relative to CO 2.₂ and methane, and thus develop a climate policy⁵.

The use of computer estimates of ozone would not be a concern if an unlikely set of nineteenth-century ozone measurements by scientists at several sites around the world was not available ( figure 1). These measurements were made using various techniques that have since been evaluated by scientists today. They calibrated the data to locate the results on modern scales and determined that other pollutants, such as sulfur dioxide, might have interfered with the measurements.^6,7. It was, however, difficult to assess the extent of such interference. Surprisingly, the data suggested that ozone concentrations in the 19th century were very low and had increased by about 300% during the industrial period – a much larger change than that calculated by the models.⁸.

At first glance, these results have huge implications. Given that the current ozone concentrations calculated by the models are approximately correct⁹If we can not simulate concentrations before the industrial period, it seems that our understanding of atmospheric chemistry is a fundamental problem. In this case, how can we trust the predictions of the models concerning the composition of the future atmosphere and thus formulate a climate policy? If ozone concentrations had increased much more than previously thought, the role of gas in climate change would be greater than it was assumed, and efforts to reduce the levels ozone would have a greater potential for reducing global warming.

It seemed improbable to most atmospheric chemists that computer models could be so fake. The most likely explanation was that sulfur dioxide and other chemical reducers produced by burning coal during the industrial revolution interfered with the observation techniques used at that time. But the absence of an independent observation constraint on historical ozone levels has made nineteenth-century observations upset atmospheric chemists over the past few decades .

Although ozone molecules do not stay trapped in ice and snow, oxygen molecules do. Yeung et al. so measured the amounts of the common oxygen-16 isotope and the oxygen-18 isotope, much less common, in the oxygen molecules trapped in polar ice and snow. The production of ozone in the atmosphere alters the proportion of these isotopes in oxygen molecules in the atmosphere. The isotopic recording of trapped oxygen molecules thus contains a history of ozone concentrations over the past 150 years.

Yeung and colleagues' analysis shows that the increase in tropospheric ozone over this period was about 40%, well below the increases reported by nineteenth-century observations and consistent with provided by the models. It therefore seems likely that the interference caused by sulfur dioxide and other gases did artificially reduce the ozone concentrations recorded during historical measurements.

The persistent doubts of the atmospheric chemists can probably now be put aside. What may be disappointing for some, our understanding of the chemistry of the atmosphere does not seem to be a fundamental problem, which means that the management of ozone to reduce climate change is limited: radiative forcing (global warming) caused by tropospheric ozone. is only about 22% of that caused by CO₂^ten. However, efforts to reduce tropospheric ozone concentrations should not stop. Every small gesture contributes to the fight against climate change and reductions would avoid some of the estimated 1 million deaths each year due to ground level ozone.¹¹.

Other mysteries remain in the chemistry of the atmosphere, including the sources of organic aerosols in the atmosphere, the chemistry of halogen-containing molecules, and how natural emissions interact with those associated with human activities. But Victorian scientists do not have much to say about them.