A refined method for calculating equivalent effective stratospheric chlorine
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...
Main Authors: | , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2018-01-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/18/601/2018/acp-18-601-2018.pdf |
Summary: | 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
<q>release time distribution</q>. 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. |
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ISSN: | 1680-7316 1680-7324 |