Within the last 2 decades emissions of the chlorine-containing very short-lived substances (Cl-VSLSs) dichloromethane (CH2Cl2) and trichloromethane (chloroform, CHCl3) have increased significantly by about 8 % yr−1 (Hossaini et al., 2015) and 3.5 % yr−1 (Fang et al., 2018), respectively. With both Cl-VSLSs not being regulated by the Montreal Protocol on Substances that Deplete the Ozone Layer and its amendments and adjustments, their influence on stratospheric ozone depletion is currently an important topic of investigation. Owing to the sparseness of Cl-VSLS measurements in the stratosphere (e.g., Schauffler et al., 1993, 2003; Laube et al., 2008; Park et al., 2010; Adcock et al., 2021), the impact of changes in Cl-VSLS surface emissions on their distribution in the stratosphere has yet to be fully characterized on an observational basis. Particularly important is the identification of rapid and efficient transport pathways for Cl-VSLSs from their source regions into the stratosphere. In the present study we use airborne in situ measurements of CH2Cl2 and CHCl3 in the extratropical upper troposphere and lower stratosphere (Ex-UTLS) to analyze the impact of different source regions on the stratospheric chemical composition and to identify transport pathways of CH2Cl2 and CHCl3 into the stratosphere.
CH2Cl2 is almost exclusively emitted by anthropogenic sources with only about 10 % of its emission being of natural origin (Engel et al., 2018). Thereby CH2Cl2 mixing ratios in the troposphere at Northern Hemisphere (NH) midlatitudes are a factor of 3 larger than those in the Southern Hemisphere (Hossaini et al., 2017). Global CH2Cl2 emissions in 2017 are estimated to be about 1 Tg Cl yr−1, and almost 90 % of the global CH2Cl2 emission sources are located in Asia (Claxton et al., 2020). Other more localized studies estimate that about 10 % of global CH2Cl2 emissions originate in India (Say et al., 2019) and that 25 %-37 % (Feng et al., 2018) or even 45 % (Oram et al., 2017) of global CH2Cl2 emissions originate in China. Collected air samples from IAGOS-CARIBIC confirm particularly high emissions in the broad region of southern and eastern Asia (Leedham-Elvidge et al., 2015) as similarly shown for the north Indian subcontinent from air sampled during the StratoClim aircraft campaign in summer 2017 (Adcock et al., 2021). European and American CH2Cl2 sources in 2017 were estimated to contribute less than 10 % to global CH2Cl2 emissions (Claxton et al., 2020).
Based on ground-based measurements from the AGAGE network, Engel et al. (2018) estimate the global CHCl3 emissions in 2017 to be about 0.29 Tg Cl yr−1. Compared to CH2Cl2 the distribution of CHCl3 emission sources is rather unclear. On average globally, Engel et al. (2018) estimate CHCl3 emissions from anthropogenic sources to be as high as from biogenic sources. However, emission estimates of anthropogenic CHCl3 sources range between 60 % (Trudinger et al., 2004), 30 % (Worton et al., 2006), and 10 % (McCulloch, 2003) of the total emissions. While CH2Cl2 is believed to have no significant oceanic sources and is only temporarily taken up by the oceans to be re-released to the atmosphere later, a process that is not yet fully understood (Moore, 2004), CHCl3 is estimated to have about 50 % of its biogenic emission sources located in offshore seawater (Laturnus et al., 2002; McCulloch, 2003). The increase in global CHCl3 emissions during the last decade was traced back entirely to an increase in eastern Chinese CHCl3 emissions of most likely anthropogenic origin (Fang et al., 2018). In addition, Chinese CHCl3 emissions amount to almost 90 % of all East Asian CHCl3 emissions (Fang et al., 2018). Nevertheless, on a global scale CHCl3 has a significant fraction of biogenic emission sources in contrast to CH2Cl2, which is almost exclusively emitted by anthropogenic sources.
For CH2Cl2 Hossaini et al. (2019) suggest an average tropospheric lifetime of 168 d (about 6 months), and a stratospheric lifetime of 1-2 years (outside the poles) was estimated by Hossaini et al. (2017). The main atmospheric sink of both CH2Cl2 and CHCl3 is the reaction with hydroxyl radicals (OH) in the troposphere. Both species have similar reaction rates with OH, implying similar photochemical lifetimes for both Cl-VSLSs (Hsu and DeMore, 1994). Time series of background mixing ratios of both species are anticorrelated to the seasonal cycle of OH (Cox et al., 2003). In the NH, seasonal anthropogenic use of products releasing CHCl3 to the atmosphere (e.g., landfill and chlorination of water) has been observed to have a small local impact on the seasonality of CHCl3 (Gentner et al., 2010). In addition, the global distribution of OH shows significant regional differences (Spivakovsky et al., 2000; Hanisco et al., 2001; Lelieveld et al., 2016). Therefore, the photochemical lifetimes of CH2Cl2 and CHCl3 are also regionally different.
In the tropical tropopause layer (TTL) the lifetime of both CH2Cl2 and CHCl3 is estimated to be about 6-10 months, being long enough for both Cl-VSLSs to enter the stratosphere under normal dynamic conditions (Park et al., 2010). For the level of zero radiative heating, Hossaini et al. (2015) simulated an increase in average CH2Cl2 mixing ratios of about 83 % between 2005 and 2013. Hossaini et al. (2019) estimate an increase in total stratospheric chlorine from Cl-VSLSs from about 69 ppt in 2000 to about 111 ppt in 2017, of which >80 % enters the stratosphere as source gases and the rest as product gases of Cl-VSLSs. Hossaini et al. (2019) further state that CH2Cl2 and CHCl3 contribute to this increase with about 68 % and 19 %, respectively. However, due to high Asian emissions and efficient transport into the stratosphere via the Asian summer monsoon (ASM), the estimation of stratospheric chlorine from Cl-VSLSs could even be underestimated by 8 %-26 % (Adcock et al., 2021).
Between June and September the ASM is a widespread convective system located above the Indian subcontinent, East Asia, and Southeast Asia (e.g., Yihui and Chan, 2005). The ASM provides fast vertical transport of surface air into the large-scale ASM anticyclone (ASMA) above, which spans from the upper troposphere at about 360 K potential temperature to the lower stratosphere at about 450 K potential temperature (e.g., Park et al., 2007, 2009; Bergman et al., 2013; Vogel et al., 2019). Within the ASMA air masses are somewhat confined and separated from the surrounding upper troposphere and lower stratosphere (UTLS) air by a strong gradient of potential vorticity (e.g., Ploeger et al., 2015). Several studies have shown that these air masses are transported further vertically into the tropical pipe or break out of the ASMA to enter the extratropical lowermost stratosphere (LMS) quasi-horizontally by Rossby wave breaking events (e.g., Popovic and Plumb, 2001; Garny and Randel, 2016; Vogel et al., 2014, 2016). Thus, the ASM has a strong impact on the chemical composition of the stratosphere in boreal summer (e.g., Randel et al., 2010; Randel and Jensen, 2013; Vogel et al., 2015; Santee et al., 2017).
The most efficient transport pathway for Cl-VSLSs into the stratosphere is suggested to be via the ASMA. This is why Cl-VSLS emissions from the region of continental Asia are suggested to have the highest ozone depletion potential (ODP) compared to emissions from other source regions (Claxton et al., 2019). Projecting different past CH2Cl2 emission rates, Hossaini et al. (2017) predict a possibly significant delay to the recovery date of stratospheric ozone ranging from a few years up to no recovery at all compared to estimations including only long-lived chlorinated species. However, the estimated impact of Cl-VSLSs on stratospheric ozone trends is small compared to that of long-lived chlorinated species or even the impact of meteorology or the 11-year solar cycle (Chipperfield et al., 2018). Nevertheless, with the expected decrease in long-lived chlorinated trace gases during the next decades due to the Montreal Protocol and its amendments and adjustments the relative importance of Cl-VSLSs in stratospheric ozone depletion will further increase.
Observational evidence for Cl-VSLSs being transported into the stratosphere is extremely rare (e.g., Schauffler et al., 1993; Woodbridge et al., 1995; Schauffler et al., 2003; Laube et al., 2008; Park et al., 2010; Adcock et al., 2021). Transport pathways into the stratosphere for VSLSs have been derived from observations of brominated VSLSs (Br-VSLSs; e.g., Sturges et al., 2000; Ashfold et al., 2012; Wales et al., 2018; Filus et al., 2020; Keber et al., 2020; Rotermund et al., 2021) or modeled specifically for Br-VSLS (e.g., Levine et al., 2007; Aschmann et al., 2009; Ashfold et al., 2012; Liang et al., 2014) which have mainly natural emission sources (Engel et al., 2018). However, the only Br-VSLS with a photochemical lifetime comparable to those of CH2Cl2 and CHCl3 is CH2Br2 (150 d; WMO, 2018), which is mostly emitted by the oceans and, consequently, is differently distributed in the troposphere than the mainly anthropogenically (land-based) emitted CH2Cl2 and most Cl-VSLSs (e.g., Engel et al., 2018). Thus, transport studies of Br-VSLSs focus on transport into the stratosphere from likely different source regions than those of Cl-VSLS, and their results might not necessarily be directly applicable to the transport into the stratosphere of CH2Cl2 and CHCl3. In addition, in order to specifically study transport into the stratosphere via the ASM, it is beneficial to observe VSLSs with their strongest sources being located in the core region of the ASM. This is the case for CH2Cl2, while most Asian Br-VSLS sources are located only in adjacent regions of the ASM.
In the present paper we use in situ measurements of CH2Cl2 and CHCl3 to identify two efficient transport pathways from the boundary layer into the extratropical lower stratosphere (Ex-LS). In addition we provide observational evidence for different impacts on the stratospheric chemical composition depending on the transport pathway the two Cl-VSLSs take to enter the Ex-LS in the NH late summer. A study by Rotermund et al. (2021) employed similar methods to identify source regions and the impact on the Ex-LS of Br-VSLS using measurements from the same aircraft campaign as the measurements used in the present paper are taken from and is compared to our results in Sect. 4.
