A refined method for calculating equivalent effective stratospheric chlorine

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 TOM-CAT 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.

Global warming potential estimates for the C<sub>1</sub>–C<sub>3</sub> hydrochlorofluorocarbons (HCFCs) included in the Kigali Amendment to the Montreal Protocol

“…The fractional release factor and global lifetime for CFC-11 were taken from the WMO (2014) ozone assessment report to be 0.47 and 52 years, respectively. Note that a new method to calculate FRF has been suggested by Ostermöller et al (2017a, b), which has been applied by Leedham Elvdige et al (2018) and Engel et al (2018). Overall, there is good agreement between the new method and the empirical parameterization applied in this work.…”

Temporal Evolution of the Bromine Alpha Factor and Equivalent Effective Stratospheric Chlorine in Future Climate Scenarios

“…EESC values are presented in Table 3 for historical and future chemistry-climate scenarios. In all cases, EESC compuations were informed by the time-independent fractional release factors provided in Table 1 of Engel et al (2018). These EESC calculations are visualized in Figure 5.…”

“…This quantity expresses the ozone-depleting power of a parcel of well-mixed stratospheric trace gases as a function of mean stratospheric age of the parcel, Γ, and the trace gas background of the stratosphere at time t (Daniel et al, 1995;Newman et al, 2007). Equation 3 provides the most recently suggested formulation of EESC, in which f i (Γ) is the time-independent fractional release factor for species i for a parcel of air with mean age Γ, which contains n i,Cl chlorine atoms and n i,Br bromine atoms, scaled by α Br (t, Γ), where it is assumed that Γ can serve as a proxy for ρ (Ostermöller et al, 2017;Engel et al, 2018). Inside the integral, the mixing ratio of species i is computed for each element comprising the age spectrum and normalized to the contribution of that element to the age spectrum.…”

“…(9)) relative to the benchmark chemoclimatic state. Note again that ρ has been substituted with Γ per Engel et al (2018).…”

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, 2010;Chipperfield et al, 2017;Li et al, 2009;Oman & Douglass, 2014;Steinbrecht et al, 2018;Stone et al, 2018;World Meteorological Organization, 2014) due to the Montreal Protocol, recent modeling and observational studies show that additional halogen sources could slow this recovery (Engel et al, 2018;Hossaini et al, 2016;Yang et al, 2014). In particular, convectively transported halogenated very short lived substances of chlorine (Hossaini, Chipperfield, Montzka, et al, 2015;Hossaini, Chipperfield, Saiz-Lopez, et al, 2015;Hossaini et al, 2017;Laube et al, 2008;Oram et al, 2017), bromine (Aschmann et al, 2009;Dessens et al, 2009;Hossaini, Chipperfield, Montzka, et al, 2015;Liang et al, 2014;Salawitch et al, 2005;Wales et al, 2018;Yang et al, 2014), and iodine (Hossaini, Chipperfield, Saiz-Lopez, et al, 2015;Saiz-Lopez et al, 2015;Youn et al, 2010) have been shown to be important sources of stratospheric halogens.…”