They identify new chemical mechanisms that hinder the recovery of the ozone layer

A report by the United Nations (UN) indicated last January that the ozone layer will recover in the next 40 years. In part, this is the result of the Montreal Protocol, the largest agreement to protect ozone by banning 96 chemicals. However, a new study shows how short-lived halogens are still unregulated compounds that prevent this positive trend in the lower tropical stratosphere (between 15 and 24 km altitude).

Specifically, these compounds are responsible for approximately a quarter of the total ozone loss in the tropical lower stratosphere.

The ozone loss trend in the tropical lower stratosphere has been mainly attributed to the acceleration of atmospheric circulation in response to global warming. However, the contribution of short-lived halogens had not been considered relevant until now. These compounds constitute an important source of chlorine, bromine and iodine injected into the atmosphere, highly reactive with ozone. Despite their short life span, less than six months, they can be rapidly transported by strong tropical convection and reach lower layers of the stratosphere.

Short-lived halogens are naturally emitted from the ocean surface and by anthropogenic sources associated with industrial activity. Simulations of atmospheric dynamics and chemistry to assess the effects of these short-lived compounds reveal the importance of concerted action between species of natural origin and of anthropogenic origin on ozone through cross-reactions. “This opens up the possibility of mitigating the ozone loss projected towards the end of the century by up to 30% by applying controls only to emissions of industrial origin,” says Julián Villamayor, co-author of the new study and researcher at the Blas Carrera Institute of Physical Chemistry ( IQF), dependent on the Higher Council for Scientific Research (CSIC) in Spain.

Stratospheric ozone levels, in this case centered in the Arctic, one of the two traditionally most problematic areas in this regard. The map shows in blue and turquoise one of the lowest values ​​on Earth for that region, 205 Dobson units, recorded on March 12, 2020. The color scale indicates the number of Dobson units in other areas as well. (Image: NASA’s Goddard Space Flight Center)

Unlike the well-known chlorofluorocarbons, less reactive halogens identified as major ozone depleters, industrial production of short-lived halogens is unregulated. Dichloromethane and chloroform are among the most abundant species considered in this work, which suggests the need to include these compounds in the regulatory treaty of the Montreal Protocol.

The coordinator of this study Alfonso Saiz-López (IQF) adds: “These results show the importance of considering a complete scheme of halogen chemistry in conventional climate models to obtain an accurate assessment of the evolution of the ozone layer.”

The study is titled “Very short-lived halogens amplify ozone depletion trends in the tropical lower stratosphere”. And it has been published in the academic journal Nature Climate Change. (Source: Alejandro Parrilla García / CSIC)