Abstract
Efficient transport pathways for ozone depleting very short-lived substances (VSLS) from their source regions into the stratosphere are a matter of current scientific debate, however they have yet to be fully identified on an observational basis. Understanding the increasing impact of chlorine containing VSLS (Cl-VSLS) on stratospheric ozone depletion is important in order to validate and improve model simulations and future predictions. We report on the first transport study using airborne in situ measurements of the Cl-VSLS dichloromethane (CH2Cl2) and trichloromethane (chloroform, CHCl3) to derive a detailed description of the two most efficient and fast transport pathways from (sub-)tropical source regions into the extratropical lower stratosphere (Ex-LS) in northern hemisphere (NH) late summer. The Cl-VSLS measurements were obtained in the upper troposphere and lower stratosphere (UTLS) above Western Europe and the mid latitude Atlantic Ocean in the frame of the WISE (Wave-driven ISentropic Exchange) aircraft campaign in autumn 2017 and are combined with the results from a three-dimensional simulation of a Lagrangian transport model as well as back-trajectory calculations. Compared to background measurements of similar age we find up to 150 % enhanced CH2Cl2 and up to 100 % enhanced CHCl3 mixing ratios in the Ex-LS. We link the measurements of enhanced mixing ratios to emissions in the region of southern and eastern Asia. Transport from this area to the Ex-LS at potential temperatures in the range of 370–400 K takes about 5–10 weeks via the Asian summer monsoon anticyclone (ASMA). Our measurements suggest anthropogenic sources to be the cause of these strongly elevated Cl-VSLS concentrations observed at the top of the lowermost stratosphere (LMS). A faster transport pathway into the Ex-LS is derived from particularly low CH2Cl2 and CHCl3 mixing ratios in the UTLS. These low mixing ratios reflect weak emission sources and a local seasonal minimum of both species in the boundary layer of Central America and the tropical Atlantic. We show that air masses uplifted by hurricanes, the North American monsoon, and general convection above Central America into the tropical tropopause layer to potential temperatures of about 360–370 K are transported isentropically within 1–5 weeks into the Ex-LS. This transport pathway linked to the North American monsoon mainly impacts the middle and lower part of the LMS with particularly low CH2Cl2 and CHCl3 mixing ratios. In a case study, we specifically analyze air samples directly linked to the uplift by the category 5 hurricane Maria that occurred during October 2017 above the Atlantic Ocean. Regionally differing CHCl3 : CH2Cl2 emission ratios derived from our UTLS measurements suggest a clear similarity between CHCl3 and CH2Cl2 when emitted by anthropogenic sources and differences between the two species mainly caused by additional, likely biogenic, CHCl3 sources. Overall, the transport of strongly enhanced CH2Cl2 and CHCl3 mixing ratios from southern and eastern Asia via the ASMA is the main factor for increasing the chlorine loading from the analyzed VSLS in the Ex-LS during NH late summer. Thus, further increases in Asian CH2Cl2 and CHCl3 emissions, as frequently reported in recent years, will further increase the impact of Cl-VSLS on stratospheric ozone depletion.
Highlights
Within the last 2 decades emissions of the chlorinecontaining very short-lived substances (Cl-VSLSs) dichloromethane (CH2Cl2) and trichloromethane have increased significantly by about 8 % yr−1 (Hossaini et al, 2015) and 3.5 % yr−1 (Fang et al, 2018), respectively
The analysis presented in this paper is mainly based on the CH2Cl2–N2O relationship observed during WISE (Fig. 3)
Here we focus on the transport since convection by Hurricane Maria to derive impacts on the air parcels induced by processes in the upper troposphere and lower stratosphere (UTLS) region.) Those air samples related to short transport times contain the lowest CH2Cl2 mixing ratios at N2O > 325 ppb measured during WISE (Fig. 10a)
Summary
Within the last 2 decades emissions of the chlorinecontaining very short-lived substances (Cl-VSLSs) dichloromethane (CH2Cl2) and trichloromethane (chloroform, CHCl3) have increased significantly by about 8 % yr−1 (Hossaini et al, 2015) and 3.5 % yr−1 (Fang et al, 2018), respectively. With both Cl-VSLSs not being regulated by the Montreal Protocol on Substances that Deplete the Ozone Layer and its amendments and adjustments, their influence on stratospheric ozone depletion is currently an important topic of investigation. Global CH2Cl2 emissions in 2017 are estimated to be about 1 Tg Cl yr−1, and almost 90 % of the global CH2Cl2 emission sources are located in Asia (Claxton et al, 2020). European and American CH2Cl2 sources in 2017 were estimated to contribute less than 10 % to global CH2Cl2 emissions (Claxton et al, 2020)
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