A growing threat to the ozone layer from short-lived anthropogenic chlorocarbons

Abstract

<p><strong>Abstract.</strong> Large and effective reductions in emissions of long-lived ozone-depleting substance (ODS) are being achieved through the Montreal Protocol, the effectiveness of which can be seen in the declining atmospheric abundances of many ODS. An important remaining uncertainty concerns the role of very short lived substances (VSLS) which, owing to their relatively short atmospheric lifetimes (less than 6 months), are not regulated under the Montreal Protocol. Recent studies have found an unexplained increase in the global tropospheric abundance of one VSLS, dichloromethane (CH<sub>2</sub>Cl<sub>2</sub>), which has increased by around 60 % over the past decade. Here we report dramatic enhancements of several chlorine-containing VSLS, including CH<sub>2</sub>Cl<sub>2</sub> and CH<sub>2</sub>ClCH<sub>2</sub>Cl (1,2-dichloroethane), observed in surface and upper tropospheric air in East and South East Asia. Surface observations were an order of magnitude higher than previously reported in the marine boundary layer, whilst upper tropospheric data were up to 3 times higher than expected. In addition we provide further evidence of an atmospheric transport mechanism whereby substantial amounts of industrial pollution from East Asia, including these chlorinated VSLS, can rapidly, and regularly, be transported to tropical regions of the western Pacific and subsequently uplifted to the tropical upper troposphere. This latter region is a major provider for air entering the stratosphere and so this mechanism, in conjunction with increasing emissions of Cl-VSLS from East Asia, could potentially slow the expected recovery of stratospheric ozone.</p>

Recent Trends in Stratospheric Chlorine From Very Short‐Lived Substances

“…The observed SGI from the most recent campaign, POSIDON in 2016, was ~120 (±27) ppt Cl, also in good agreement to our campaign‐sampled model (118 ± 6 ppt Cl). In principle, the relatively large chlorine SGI from the POSIDON mission, conducted in the West Pacific, could have been influenced by proximity to major VSLS source regions (e.g., Oram et al, ). However, we note that the campaign‐sampled model output—with no zonal variability in VSLS loading at the surface—agrees well with the measurement data.…”

“…Notably, for example, long‐term surface measurements show that emissions of CFC‐11 have likely increased since 2012, despite its reported production being near zero (Montzka et al, ). Another example is the recent indication of increasing dichloromethane emissions since the early 2000s, based on a network of surface observations (Hossaini et al, ; Hossaini et al, ) and measurements made in the upper troposphere (Leedham Elvidge et al, ; Oram et al, ). Dichloromethane (CH 2 Cl 2 ), along with chloroform (CHCl 3 ), perchloroethylene (C 2 Cl 4 ), and 1,2‐dichloroethane (C 2 H 4 Cl 2 ), among others, are so‐called Very Short‐Lived Substances (VSLS)—compounds with mean surface lifetimes typically less than 6 months (e.g., Ko et al, ).…”

“…For example, CH 2 Cl 2 is widely used as a solvent and in the production of foam agents, among other applications (Feng et al, ). A substantial portion of the estimated present‐day global CH 2 Cl 2 emission rate of ~1 Tg year (Hossaini et al, ) is expected to occur in Asia (Oram et al, ). While no long‐term C 2 H 4 Cl 2 surface record exists, observed interhemispheric gradients suggest that it too has anthropogenic sources (Hossaini et al, ).…”

On the Regional and Seasonal Ozone Depletion Potential of Chlorinated Very Short‐Lived Substances

“…Although the influence of CH 2 Cl 2 on ozone has been modest in the recent past (Chipperfield et al, ), if sustained CH 2 Cl 2 growth continues in coming decades, ozone layer recovery could be delayed (Hossaini et al, ). A substantial portion of CH 2 Cl 2 emissions, estimated globally at ~0.8 Tg/year in 2012 (Carpenter et al, ), is believed to occur in Asia (Oram et al, ). CH 2 Cl 2 emissions from China, for example, are thought to have increased by a factor of ~3 between 2005 and 2016, with further increases projected until 2030 (Feng et al, ).…”

Modeling the Effect of Potential Nitric Acid Removal During Convective Injection of Water Vapor Over the Central United States on the Chemical Composition of the Lower Stratosphere

“…While stratospheric ozone recovery from the slow removal of inorganic chlorine from the stratosphere is expected to occur within this century driven largely by a decrease in ozone depleting substances (Austin et al, ; Chipperfield et al, ; Li et al, ; Oman & Douglass, ; Steinbrecht et al, ; Stone et al, ; World Meteorological Organization, ) due to the Montreal Protocol, recent modeling and observational studies show that additional halogen sources could slow this recovery (Engel et al, ; Hossaini et al, ; Yang et al, ). In particular, convectively transported halogenated very short lived substances of chlorine (Hossaini, Chipperfield, Montzka, et al, ; Hossaini, Chipperfield, Saiz‐Lopez, et al, ; Hossaini et al, ; Laube et al, ; Oram et al, ), bromine (Aschmann et al, ; Dessens et al, ; Hossaini, Chipperfield, Montzka, et al, ; Liang et al, ; Salawitch et al, ; Wales et al, ; Yang et al, ), and iodine (Hossaini, Chipperfield, Saiz‐Lopez, et al, ; Saiz‐Lopez et al, ; Youn et al, ) have been shown to be important sources of stratospheric halogens. Further, both emissions of these halogenated very short lived substance (Ziska et al, ) and their transport to the stratosphere (Falk et al, ; Hossaini et al, ) are expected to increase with climate change resulting in a greater impact on stratospheric ozone (Fernandez et al, ; Tegtmeier et al, ).…”