Bandozone destroying chemicals mysteriously hit record high. The 4th of April 2023, by Luke Western and Johannes Lauba, the conversation. Despite a global ban in place since 2010, atmospheric concentrations of five ozone-depleting chemicals have reached a record high. Glorofluoric carbons, or CFCs, are entirely man-made gases used in a variety of applications, including refrigeration, air conditioning, or as chemical solvents. They have been increasingly regulated by a series of international treaties since the 1980s. The 1987 Montreal Protocol, which has been unanimously ratified, restricted the release of CFCs to the atmosphere where they contribute to the destruction of the ozone layer, a region high up in the stratosphere which absorbs harmful ultraviolet UV radiation and protects light below. The goal of the Montreal Protocol was to induce a decline in the atmospheric CFC concentration through controlling, and increasingly restricting, the production of these chemicals. This has worked well for many ozone-depleting substances, which is why the ozone layer is slowly recovering. And so the recent increase in atmospheric concentrations of five CFCs is quite surprising. Our findings, while worrying, should be considered an early warning. The impact of all five CFCs on the recovery of the ozone layer is still small. Nevertheless, we do not fully understand where they are coming from, so this could change in the future, and we should not ignore the cumulative effect of these emissions on human health and the environment. The global picture, our team has been analyzing air samples from all over the world, focusing on so-called background sites that are far away from the sources of these CFCs, or in fact any industrial emissions. An example is the caprium of servetry on the remote west coast of Tasmania. This is the basis for our assessment of the threat these chemicals pose, as it reveals global trends in their atmospheric concentration. Our main findings for the period 2010-2020 were twofold. First, concentrations of CFC 13 and CFC 113 are continued the previously observed, and puzzling, increase. This in concentrations of CFC 113 are even accelerated around 2016. We found that three of the five CFCs, CFC 113A, CFC 114A, and CFC 115 can be produced during the manufacture of other chemicals, which is allowed under the Montreal Protocol, most notably hydroplorocarbons or HFCs. HFCs have replaced CFCs for many applications as an ozone-friendly alternative. However, like CFCs, they are greenhouse gases and their production is now being reduced in many countries under the 2016 Cagali amendment to the Montreal Protocol, which could reduce climate-related warming by 0.5 degrees Celsius. It's likely that the CFCs are leaking out during the production process, where they are either used as a feedstock, a chemical ingredient to make another chemical, or as a result of incomplete conversion of the feedstock to the target chemical. The production of HFCs really took off in developing countries after CFCs were banned in 2010, which is around the same time as the increase in emissions of these five CFCs. The production of HFCs is predicted to further increase over the next few years, which could result in increasing emissions of these CFCs. CFC 113A is used to make at least one hydrochlorolephymore HFO, which are alternatives to HFCs that don't keep the climate and may be used long into the future. Despite HFCs and HFOs being more benign alternatives to CFCs, there may still be some cost to ozone during their production if CFCs continue to leak into the atmosphere. We were unable to find a plausible source for the other two CFCs, CFC13 and CFC 112A. The fact that their emissions are increasing and we don't know why is a concern in itself. Time to revisit Montreal. The Montreal Protocol has been a huge success in mitigating emissions of ozone depleting substances. Total CFC emissions are now only around 5% of their peak in the late 1980s. Yet an increase in the atmospheric abundance of some CFCs is still at odds with the treaty's goals, and their elimination, by clogging leaks in industrial processes, could present an easy win to reduce these country-sized emissions of ozone depleting and climate warming gases. It will take careful consideration by country signed up to the Protocol to find the necessary controls for quashing these trend-bucking emissions. In the meantime, we will continue to use our eyes in the sky to monitor the progress of a whole host of earth-damaging gases. Luke Weston, Research Associate in Atmospheric Science, University of Bristol and Johannes Lauba, Honorary Lecturer, Center for Ocean and Atmospheric Sciences, University of East Anglia.