Large amounts of ozone — around 50% more than predicted by the world’s state-of-the-art climate models — are being destroyed in the lower atmosphere over the tropical Atlantic Ocean. A team of scientists from the National Center for Atmospheric Science and the Universities of York and Leeds made the discovery, which is significant because ozone in the lower atmosphere acts as a greenhouse gas and its destruction also leads to the removal of the third most abundant greenhouse gas; methane.
Ozone, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope, breaking down with a half life of roughly 30 minutes in the lower atmosphere, to normal dioxygen. Ozone is formed from dioxygen by the action of ultraviolet light and also atmospheric electrical discharges, and is present in low concentrations throughout the Earth’s atmosphere. In total, ozone makes up only 0.6 parts per million of the atmosphere.
Ozone destruction has been known as two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth’s stratosphere (the ozone layer), and a much larger springtime decrease in stratospheric ozone over Earth’s polar regions.
The most important part of the ozone destruction process in both is the catalytic destruction of ozone by atomic halogens. The main source of these halogen atoms in the stratosphere is photodissociation of man-made halocarbon refrigerants (CFCs, freons, halons).
A team of scientists from the National Center for Atmospheric Science and the Universities of York and Leeds has made a new discovery, which is significant because ozone in the lower atmosphere acts as a greenhouse gas and its destruction also leads to the removal of the third most abundant greenhouse gas; methane.
The findings, published in the scientific journal, Nature, come after the team analysed measurements from the new Cape Verde Atmospheric Observatory, recently set up by British, German and Cape Verdean scientists on the island of São Vicente in the tropical Atlantic.
Scientists flew a research aircraft into the atmosphere to make ozone measurements at different heights and more widely across the tropical Atlantic. The results mirrored those made at the Observatory, indicating major ozone loss in this remote area.
Instruments developed at the University of Leeds and stationed at the Observatory detected the presence of the chemicals bromine and iodine oxide over the ocean for this region. These chemicals, produced by sea spray and emissions from phytoplankton (microscopic plants in the ocean), attack the ozone, breaking it down. As the ozone is destroyed, a chemical is produced that attacks and destroys the greenhouse gas methane.
Until now, it has been impossible to monitor the atmosphere of this remote region over time because of its physical inaccessibility. Using this new chemistry in climate models will provide more accurate estimates of ozone and methane in the atmosphere and improve future climate predictions.
Professor Alastair Lewis, Director of Atmospheric Composition at the National Centre for Atmospheric Science and a lead scientist in this study, said: “At the moment this is a good news story – more ozone and methane being destroyed than we previously thought – but the tropical Atlantic cannot be taken for granted as a permanent ‘sink’ for ozone. The composition of the atmosphere is in fine balance here – it will only take a small increase in nitrogen oxides from fossil fuel combustion, carried here from Europe, West Africa or North America on the trade winds, to tip the balance from a sink to a source of ozone.”
John Plane, Professor of Atmospheric Chemistry at the University of Leeds, said: “This study provides a sharp reminder that to understand how the atmosphere really works, measurement and experiment are irreplaceable. The production of iodine and bromine mid-ocean implies that destruction of ozone over the oceans could be global”.
For further information see Ozone Destruction.
Article by Andy Soos, appearing courtesy Environmental News Network.