Abstract
Abstract. Chlorine and bromine atoms lead to catalytic depletion of ozone in the stratosphere. Therefore the use and production of ozone-depleting substances (ODSs) containing chlorine and bromine is regulated by the Montreal Protocol to protect the ozone layer. Equivalent effective stratospheric chlorine (EESC) has been adopted as an appropriate metric to describe the combined effects of chlorine and bromine released from halocarbons on stratospheric ozone. Here we revisit the concept of calculating EESC. We derive a refined formulation of EESC based on an advanced concept of ODS propagation into the stratosphere and reactive halogen release. A new transit time distribution is introduced in which the age spectrum for an inert tracer is weighted with the release function for inorganic halogen from the source gases. This distribution is termed the release time distribution. We show that a much better agreement with inorganic halogen loading from the chemistry transport model TOMCAT is achieved compared with using the current formulation. The refined formulation shows EESC levels in the year 1980 for the mid-latitude lower stratosphere, which are significantly lower than previously calculated. The year 1980 is commonly used as a benchmark to which EESC must return in order to reach significant progress towards halogen and ozone recovery. Assuming that – under otherwise unchanged conditions – the EESC value must return to the same level in order for ozone to fully recover, we show that it will take more than 10 years longer than estimated in this region of the stratosphere with the current method for calculation of EESC. We also present a range of sensitivity studies to investigate the effect of changes and uncertainties in the fractional release factors and in the assumptions on the shape of the release time distributions. We further discuss the value of EESC as a proxy for future evolution of inorganic halogen loading under changing atmospheric dynamics using simulations from the EMAC model. We show that while the expected changes in stratospheric transport lead to significant differences between EESC and modelled inorganic halogen loading at constant mean age, EESC is a reasonable proxy for modelled inorganic halogen on a constant pressure level.
Highlights
It is well established that chlorine and bromine atoms in the stratosphere enhance ozone loss via catalytic reaction chains (Stolarski and Cicerone, 1974; Solomon, 1999; Molina and Rowland, 1974; Wofsy et al, 1975)
We have shown that for the calculation of the propagation of chlorine and bromine source gases with photochemical loss, different transit time distributions must be used to calculate the amount of organic or inorganic chlorine present at a given mean age level
factor f (FRF), which are independent of the tropospheric trends (Ostermöller et al, 2017), must be used to correctly describe the fraction that has been transferred from the organic source gas to the inorganic form and can influence ozone chemistry
Summary
It is well established that chlorine and bromine atoms in the stratosphere enhance ozone loss via catalytic reaction chains (Stolarski and Cicerone, 1974; Solomon, 1999; Molina and Rowland, 1974; Wofsy et al, 1975). The fractional chemical loss can be expressed in a very generalized way as (1 − f t , p ), where f t , p is a fractional release function, which is specific for each trace gas and will depend on the time the air parcel has spent in the stratosphere t and on the path p it has take, primarily the maximum path height during transport (Hall, 2000). CFC-11 is one of the most important chlorine source gases in the stratosphere As EESC is a proxy for inorganic halogen, we derive a new formulation of EESC which takes into account this interaction between chemistry and transport in an improved way
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