Location of large mystery source of banned ozone depleting substance uncovered
From the University of Bristol
The compound, carbon tetrachloride, contributes to the destruction of the Earth’s ozone layer, which protects us from harmful ultraviolet radiation.
As a result, the production of carbon tetrachloride has been banned throughout the world since 2010 for uses that will result in its release to the atmosphere. However, recent studies have shown that global emissions have not declined as expected, with about 40,000 tonnes still being emitted each year.
The origin of these emissions has puzzled researchers for many years.
Alongside collaborators from South Korea, Switzerland, Australia and the USA, researchers at the University of Bristol aimed to quantify emissions from eastern Asia.
To do this, they used ground-based and airborne atmospheric concentration data from near the Korean peninsula and two models that simulate the transport of gases through the atmosphere.
Their results, published in the journal Geophysical Research Letters, show that around half of the ‘missing’ global emissions of carbon tetrachloride originated from eastern China between 2009 and 2016.
Lead author, Dr Mark Lunt, from the University of Bristol’s School of Chemistry, said: “Our results show that emissions of carbon tetrachloride from the eastern Asia region account for a large proportion of global emissions and are significantly larger than some previous studies have suggested.
“Not only that, but despite the phase out of carbon tetrachloride production for emissive use in 2010, we found no evidence for a subsequent decrease in emissions.”
In fact, emissions from certain regions may have increased slightly since 2010. The results from the study show the emergence of a new source of emissions from the Shandong province of China after 2012.
Whilst the results of this and earlier studies in Europe and the USA now explain a large part of the global distribution of carbon tetrachloride emissions, there are still large gaps in our knowledge. Furthermore, recent reports have suggested that very large amounts of this gas may be emitted inadvertently during the production of other chemicals such as chlorine.
Dr Matt Rigby, Reader in Atmospheric Chemistry at the University of Bristol and co-author, said: “Our work shows the location of carbon tetrachloride emissions. However, we don’t yet know the processes or industries that are responsible. This is important because we don’t know if it is being produced intentionally or inadvertently.”
He also added: “There are areas of the world such as India, South America and other parts of Asia, where emissions of ozone-depleting gases may be ongoing, but detailed atmospheric measurements are lacking.”
It is hoped that this work can now be used by scientists and regulators to identify the cause of these emissions from eastern Asia. Ultimately, if these emissions can be avoided, it would hasten the recovery of the stratospheric ozone layer.
Dr Lunt said: “Studies such as this show the importance of continued monitoring of ozone-depleting gases. There is a temptation to see ozone depletion as a problem that has been solved. But the monitoring of man-made ozone-depleting gases in the atmosphere is essential to ensure the continued success of the phase-out of these compounds.”
Continued Emissions of the Ozone‐Depleting Substance Carbon Tetrachloride From Eastern Asia
Carbon tetrachloride (CCl4) is an ozone‐depleting substance, accounting for about 10% of the chlorine in the troposphere. Under the terms of the Montreal Protocol, its production for dispersive uses was banned from 2010. In this work we show that, despite the controls on production being introduced, CCl4 emissions from the eastern part of China did not decline between 2009 and 2016. This finding is in contrast to a recent bottom‐up estimate, which predicted a significant decrease in emissions after the introduction of production controls. We find eastern Asian emissions of CCl4 to be 16 (9–24) Gg/year on average between 2009 and 2016, with the primary source regions being in eastern China. The spatial distribution of emissions that we derive suggests that the source distribution of CCl4 in China changed during the 8‐year study period, indicating a new source or sources of emissions from China’s Shandong province after 2012.
Plain Language Summary
Carbon tetrachloride is one of several man‐made gases that contribute to the depletion of the ozone layer high in the atmosphere. Because of this, restrictions were introduced on the use of this ozone‐depleting substance, with the expectation that production should by now be close to 0. However, the slower than expected rate of decline of carbon tetrachloride in the atmosphere shows this is not the case, and a large portion of global emissions are unaccounted for. In this study we use atmospheric measurements of carbon tetrachloride from a site in East Asia to identify the magnitude and location of emissions from this region between 2009 and 2016. We find that there are significant ongoing emissions from eastern China and that these account for a large part of the missing emissions from global estimates. The presence of continued sources of this important ozone‐depleting substance indicates that more could be done to speed up the recovery of the ozone layer.
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