Investigating stratospheric changes between 2009 and 2018 with halogenated trace gas data from aircraft, AirCores, and a global model focusing on CFC-11

We present new observations of trace gases in the stratosphere based on a cost-effective sampling technique that can access much higher altitudes than aircraft. The further development of this method now provides detection of species with abundances in the parts per trillion (ppt) range and below. We obtain mixing ratios for six gases (CFC-11, CFC-12, HCFC-22, H-1211, H-1301, and SF6), all of which are important for understanding stratospheric ozone depletion and circulation. After demonstrating the quality of the data through comparisons with ground-based records and aircraft-based observations, we combine them with the latter to demonstrate its potential. We first compare the data with results from a global model driven by three widely used meteorological reanalyses. Secondly, we focus on CFC11 as recent evidence has indicated renewed atmospheric emissions of that species relevant on a global scale. Because the stratosphere represents the main sink region for CFC-11, potential changes in stratospheric circulation and troposphere–stratosphere exchange fluxes have been identified as the largest source of uncertainty for the accurate quantification of such emissions. Our observations span over a decade (up until 2018) and therefore cover the period of the slowdown of CFC-11 global mixing ratio decreases measured at the Earth’s surface. The spatial and temporal coverage of the observations is insufficient for a global quantitative analysis, but we do find some trends that are in contrast with expectations, indicating that the stratosphere may have contributed to the slower concentration decline in recent years. Further investigating the reanalysis-driven model data, we find that the dynamical changes in the stratosphere required to explain the apparent change in tropospheric CFC-11 emissions after 2013 are possible but with a very high uncertainty range. This is partly caused by the high variability of mass flux from the stratosphere to the troposphere, especially at Published by Copernicus Publications on behalf of the European Geosciences Union. 9772 J. C. Laube et al.: Investigating stratospheric changes between 2009 and 2018 timescales of a few years, and partly by large differences between runs driven by different reanalysis products, none of which agree with our observations well enough for such a quantitative analysis.


.1 Air sampling
Research and passenger aircraft sampling and analysis for mixing ratios of halogenated trace gases have been described in previous papers (Kaiser et  used, all of which focused on maximising the amount of air collected in the stratosphere. A special version was especially developed for maximising the altitude resolution of the air collected in that region. For that purpose the part of the AirCore that samples tropospheric air and normally consists of ¼" tubing was replaced with ½" tubing, i.e. increasing the internal volume collected into the ⅛" part. All sampled air was dried using a Mg(ClO4)2-filled trap at the AirCore inlet. More details for these flights are given in Table S1. and improved (ISS, see text for details) sub-samplers after recovery. In all cases sub-sampler air was later analysed for ppt-level trace gases using the GC-MS system described in the Methods section. the Picarro analyser, a pre-determined lag time between the internal cell and the sub-sampler was used to determine the start of the AirCore profile and therefore the sub-sampling process.
The second method is based on a 32-port 1/8" valve (from VICI, Switzerland), with a common in-and outlet. We attached loops of ¼" stainless steel to each pair of ports, resulting in 15 loops with an internal volume of about 20 ml each (two ports were blanked for a default position that can be exposed to lab air when connecting the AirCore). All metal surfaces were Silco-1000-treated, which is an established technique to increase the inertness of such surfaces. These new samplers were evacuated directly after connecting the AirCore (having recovered it after a flight) and then the stratospheric end of the AirCore was opened to each loop successively. The tropospheric end of the AirCore was opened to room air to ensure that each loop was filled to the same pressure (and the latter was monitored with a pressure sensor). As the tropospheric part of the AirCore was not sampled and diffusion through the long thin ⅛" tubing of the AirCore is very slow, there was no contamination with lab air.
For both methods diffusion inside the AirCore is negligible over the period of 2-6 hours in between landing and sub-sampling. contamination would look like, as for two AirCore flights (highlighted in red in Figure S3) we did indeed find slightly enhanced mixing ratios resulting in a weaker correlation. As can be seen in Figures 2, S1, S2, and S4, HCFC-22 is the only species that is contaminated for those flights. CFC-11 trends at different mean ages as presented in the main manuscript were also recalculated without using these two flights, but changes to the slopes were much smaller (6-16 times) than the one σ uncertainties of those slopes. We conclude that even if there were In addition, if a contamination occurs, one should also observe enhancements of mixing ratios near the tropopause that are well above the northern hemispheric background values at that time. Figures S5 and S6 show these enhancements for five of the gases (excluding H-1301 due to the lack of an available northern hemispheric time series and the limited measurements precisions). Only small enhancements are observed, both from aircraft as well as from

S1.1.2 Ensuring contamination-free sampling and measurements
AirCores and there are almost no mixing ratios enhanced above the northern hemispheric background by more than two measurement standard deviations, which gives further confidence in the data.
Finally, all AirCores, sub-samplers and connections were leak-checked before each flight/filling. AirCores were conditioned and filled with nearly trace gas-free fill gas (either air or nitrogen; low ppt-level mixing ratios observed for some gases) before each flight. Since the uppermost one or two samples still contain a portion of this fill gas and correcting for that is complicated and introduces uncertainties, these samples were excluded for the analysis in this work. Sub-samplers were conditioned with either fill gas (Finland flights) or ultra-clean Helium (UK flights) before filling them with AirCore air. Storage tests of unpolluted air in AirCores and sub-samplers showed, within one σ uncertainties, no effect on any of the six gases reported here after several weeks.     to S12 visualise these results and confirm that model data from all three reanalyses consistently produces more negative trends for both gases than observation-based data at mean ages of one to three years, i.e. in the lower part of the stratosphere, where most of its mass resides. In addition, the three runs seem to overestimate the mixing ratios of CFC-11 and CFC-12 present in the lower stratosphere (one and two years AoA), while underestimating it at an AoA of 4 years -apart from MERRA-2 which consistently overestimates at all AoAs and has the highest offsets to observations. Figure S7. The same as in Figure 3 but for CFC-12. Figure S8. The same as in Figure 3 but for CFC-12 and at AoAs of one and three years. Figure S9. The same as in Figure 3 but for CLaMS-JRA-55. Figure S10. The same as in Figure 3 but for CLaMS-JRA-55 and at AoAs of one and three years. Figure S11. The same as in Figure 3 but for CLaMS-MERRA-2. Figure S12. The same as in Figure 3 but for CLaMS-MERRA-2 and at AoAs of one and three years.

S1.3 Further details on the CLaMS model runs
The