Editing IPCC:AR6/WGI/Chapter-6 (section)

=== 6.3.4 Short-lived Halogenated Species ===

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Halogenated species are emitted in the atmosphere in the form of the synthetically produced chloroﬂuorocarbons (CFCs), halons, hydrochloroﬂuorocarbons (HCFCs), hydroﬂuorocarbons (HFCs) and others. Their historical global abundances are provided in [[IPCC:Wg1:Chapter:Annex-iii|Annex III]] and discussed in [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] ( [[IPCC:Wg1:Chapter:Chapter-2#2.2.4|Section 2.2.4]] and Table 2.3). In summary, for the period 2011–2019, the abundance of total chlorine from HCFCs has continued to increase in the atmosphere with decreased growth rates; total tropospheric bromine from halons and methyl bromide continued to decrease while abundances of most currently measured HFCs increased significantly, consistent with expectations based on the ongoing transition away from the use of ODSs. Here, emphasis is given on the very short-lived halogenated species (VSLSs). The trends for these species were not discussed in IPCC AR5.

VSLSs are halogenated substances with atmospheric lifetimes less than half a year. While longer-lived ODSs account for most of the present-day stratospheric halogen loading, there is ''robust evidence'' that VSLSs contribute to stratospheric bromine and chlorine ( Carpenter et al. , 2014; Elvidge et al. , 2015a; Hossaini et al. , 2015 ), thus also contributing to stratospheric ozone depletion.

Of the atmospheric VSLSs, brominated and iodinated species are predominantly of oceanic origin, while chlorinated species have significant additional anthropogenic sources ( [[#Carpenter--2014|Carpenter et al., 2014]] ; [[#Hossaini--2015|Hossaini et al., 2015]] ). Global mean chlorine from the VSLSs has increased in the troposphere from about 91 ppt in 2012 to about 110 ppt in 2016 ( [[#Engel--2018|Engel et al., 2018]] ). This increase is mostly due to dichloromethane (CH <sub>2</sub> cl <sub>2</sub> ), a species that has predominantly anthropogenic sources reflected by three-times higher concentrations in the Northern Hemisphere than in the Southern Hemisphere ( [[#Hossaini--2017|Hossaini et al., 2017]] ). The upward dichloromethane trend is corroborated by upper-tropospheric aircraft data over the period 1998–2014 ( [[#Elvidge--2015b|Elvidge et al., 2015b]] ; [[#Oram--2017|Oram et al., 2017]] ). The observations from the surface networks show that the abundance of dichloromethane continued to increase until 2019 (Annex III), although the accuracy of global abundance of VSLSs is limited by the scarce coverage by networks. No long-term changes of the bromine-containing VSLSs have been observed ( [[#Engel--2018|Engel et al., 2018]] ).

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