The stratosphere is the region of the atmosphere extending from ~10-50 km above Earth’s surface. It is home to the ozone layer, which absorbs harmful solar ultraviolet radiation, allowing life on Earth to flourish. But not so long ago the ozone layer was in crisis: in the 1980’s scientists observed that stratospheric ozone levels were dropping at an alarming rate. The greatest ozone decrease occurred over Antarctica, resulting in the well-known “ozone hole”, but significant ozone decreases were also observed throughout the rest of the stratosphere. The main culprits causing the ozone destruction were human-produced chlorine-containing chemicals like chlorofluorocarbons (CFCs), which were used in refrigerators, air conditioners, and spray cans.  

Governments across the world realized the importance of protecting the ozone layer and came together in 1987 to enact the Montreal Protocol, an international treaty to control the production and emission of CFCs, along with a few other chemicals. The quick adoption of the Montreal Protocol is still one of the major environmental success stories: people across the world came together to save the planet. Recent observations show that ozone throughout much of the stratosphere is recovering.  

Problem solved? Almost. To determine if atmospheric chlorine concentrations are decreasing as expected, we rely on long-term measurements from both ground-based and satellite instruments. The ground-based observations, which are primarily from the National Oceanic and Atmospheric Administration (NOAA) and the Advanced Global Atmospheric Gases Experiment (AGAGE) networks, provide information on how much of each chlorine-containing gas is being emitted into the atmosphere. Meanwhile, satellite observations, particularly those from limb sounders like the Atmospheric Chemistry Experiment - Fourier Transform Spectrometer (ACE-FTS), tell us how much chlorine there is in the stratosphere. Ideally these numbers would be consistent, but they are not so easy to compare.  

What we did 

To compare the ground-based measurements of chemicals regulated by the Montreal Protocol to stratospheric chlorine abundances it is necessary to account for the circulation of the atmosphere as this controls how long it will take for a molecule emitted at the Earth to reach a given location in the stratosphere. Our study was the first to do this in a quantitative way, and we found that the decrease in the amount of total stratospheric chlorine is smaller than the decrease in emissions of chlorine-containing gases!  

What is causing the discrepancy? 

While the Montreal Protocol and its subsequent amendments successfully reduced emissions of what are called long-lived ozone depleting substances (long-lived ODSs), they neglected a different type of chemical: chlorinated very short-lived substances (Cl-VSLSs). Cl-VSLSs are also largely produced by human activities, and recent research shows that their abundance in the atmosphere has increased by about 2.5 parts per trillion each year (Laube et al. 2022). 

Cl-VSLSs are called “short-lived” because they do not last long in the atmosphere before they react with other molecules and get converted to other chlorine-containing gases. This means that they are not in their original form by the time they reach the stratosphere and so cannot be directly measured. But they do still end up contributing to the amount of total stratospheric chlorine. This is why the ground-based observations of long-lived ODSs are inconsistent with the satellite observations of stratospheric chlorine: we found that Cl-VSLSs explain nearly all of the difference. This means that it will take longer for the ozone layer to recover than expected based solely on reductions in emissions of long-lived ODSs. 

Looking forward 

The state of the atmosphere affects every aspect of life on Earth, so it is very important to monitor the atmosphere so that we know what is going on! Without long-term measurements from both space and the ground, this study would not have been possible. But more importantly, we would not have noticed that the ozone layer was being destroyed and been able to act to solve the problem. Lately the stratosphere has been changing in new and unexpected ways due to wildfires (e.g. Solomon et al., 2022) and volcanic eruptions (e.g. Khaykin et al., 2022). It is impossible to figure out how these events impact the atmosphere without long-term observations. Unfortunately, many instruments are at the end of their lifetimes and new missions meant to replace them are at risk of being cancelled. We hope that governments across the world can come together, just like they did to protect the ozone layer, and continue funding instruments that measure atmospheric composition. 

References 

Khaykin, S., Podglajen, A., Ploeger, F. et al. Global perturbation of stratospheric water and aerosol burden by Hunga eruption. Commun Earth Environ 3, 316 (2022). https://doi.org/10.1038/s43247-022-00652-x

Laube, J. et al. Update on Ozone-Depleting Substances (ODSs) and Other Gases of Interest to the Montreal Protocol. In Scientific Assessment of Ozone Depletion: 2022, GAW Report No. 278, chap. 1, 53–113 (WMO, Geneva, Switzerland, 2022) 

Solomon, K. Dube, K. Stone, P. Yu, D. Kinnison, O.B. Toon, S.E. Strahan, K.H. Rosenlof, R. Portmann, S. Davis, W. Randel, P. Bernath, C. Boone, C.G. Bardeen, A. Bourassa, D. Zawada & D. Degenstein,  On the stratospheric chemistry of midlatitude wildfire smoke, Proc. Natl. Acad. Sci. U.S.A. 119 (10) e2117325119, https://doi.org/10.1073/pnas.2117325119 (2022).