Recent Trends in Stratospheric Chlorine From Very Short-Lived Substances.

Ryan Hossaini,Stephen A Montzka,Elliot Atlas,Dickon Young,Chris R Lunder,Jgor Arduini,Simon O'Doherty,Jeremy J Harrison,Amber A Leeson,Martin K Vollmer,Sandip S Dhomse,Anton M Fernando,Jens Mühle,Paul Krummel,Stefan Reimann,Michela Maione,Peter F Bernath,Martyn P Chipperfield,Wuhu Feng

doi:10.1029/2018jd029400

Abstract

Very short‐lived substances (VSLS), including dichloromethane (CH2Cl2), chloroform (CHCl3), perchloroethylene (C2Cl4), and 1,2‐dichloroethane (C2H4Cl2), are a stratospheric chlorine source and therefore contribute to ozone depletion. We quantify stratospheric chlorine trends from these VSLS (VSLCltot) using a chemical transport model and atmospheric measurements, including novel high‐altitude aircraft data from the NASA VIRGAS (2015) and POSIDON (2016) missions. We estimate VSLCltot increased from 69 (±14) parts per trillion (ppt) Cl in 2000 to 111 (±22) ppt Cl in 2017, with >80% delivered to the stratosphere through source gas injection, and the remainder from product gases. The modeled evolution of chlorine source gas injection agrees well with historical aircraft data, which corroborate reported surface CH2Cl2 increases since the mid‐2000s. The relative contribution of VSLS to total stratospheric chlorine increased from ~2% in 2000 to ~3.4% in 2017, reflecting both VSLS growth and decreases in long‐lived halocarbons. We derive a mean VSLCltot growth rate of 3.8 (±0.3) ppt Cl/year between 2004 and 2017, though year‐to‐year growth rates are variable and were small or negative in the period 2015–2017. Whether this is a transient effect, or longer‐term stabilization, requires monitoring. In the upper stratosphere, the modeled rate of HCl decline (2004–2017) is −5.2% per decade with VSLS included, in good agreement to ACE satellite data (−4.8% per decade), and 15% slower than a model simulation without VSLS. Thus, VSLS have offset a portion of stratospheric chlorine reductions since the mid‐2000s.

Highlights

The depletion of the global ozone layer by chlorine and bromine compounds is a well‐established and persistent environmental issue (e.g., Solomon, 1999)
The aim of this paper is to provide an up‐to‐date assessment of the stratospheric chlorine loading due to Cl‐Very short‐lived substances (VSLS) (CH2Cl2, CHCl3, C2Cl4, C2H4Cl2, and C2HCl3) and to examine trends over the 2000–2017 period
We find that VSLCltot is fairly insensitive to changes in tropospheric [OH]; increasing/decreasing [OH] by 25%

Summary

Introduction

The depletion of the global ozone layer by chlorine and bromine compounds is a well‐established and persistent environmental issue (e.g., Solomon, 1999). 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, 2018). Another example is the recent indication of increasing dichloromethane emissions since the early 2000s, based on a network of surface observations (Hossaini et al, 2015a; Hossaini et al, 2017) and measurements made in the upper troposphere (Leedham Elvidge et al, 2015; Oram et al, 2017). Dichloromethane (CH2Cl2), along with chloroform (CHCl3), perchloroethylene (C2Cl4), and 1,2‐dichloroethane (C2H4Cl2), 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, 2003)

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