Abstract
Abstract. In a changing climate, potential stratospheric circulation changes require long-term monitoring. Stratospheric trace gas measurements are often used as a proxy for stratospheric circulation changes via the mean age of air values derived from them. In this study, we investigated five potential age of air tracers – the perfluorocarbons CF4, C2F6 and C3F8 and the hydrofluorocarbons CHF3 (HFC-23) and HFC-125 – and compare them to the traditional tracer SF6 and a (relatively) shorter-lived species, HFC-227ea. A detailed uncertainty analysis was performed on mean ages derived from these new tracers to allow us to confidently compare their efficacy as age tracers to the existing tracer, SF6. Our results showed that uncertainties associated with the mean age derived from these new age tracers are similar to those derived from SF6, suggesting that these alternative compounds are suitable in this respect for use as age tracers. Independent verification of the suitability of these age tracers is provided by a comparison between samples analysed at the University of East Anglia and the Scripps Institution of Oceanography. All five tracers give younger mean ages than SF6, a discrepancy that increases with increasing mean age. Our findings qualitatively support recent work that suggests that the stratospheric lifetime of SF6 is significantly less than the previous estimate of 3200 years. The impact of these younger mean ages on three policy-relevant parameters – stratospheric lifetimes, fractional release factors (FRFs) and ozone depletion potentials – is investigated in combination with a recently improved methodology to calculate FRFs. Updates to previous estimations for these parameters are provided.
Highlights
The “mean age of air”, defined as the average time that an air parcel has spent in the stratosphere, is an important derived quantity used in several stratospheric research fields, often when direct physical or chemical measurements are scarce, not available or inadequate
We included HFC-125 as a potential age tracer as we believe its current estimated stratospheric lifetime of 351 years (SPARC, 2013; derived from model outputs) is potentially an underestimate based on preliminary mean age interpretations at University of East Anglia (UEA)
We have presented tropospheric trends and stratospheric measurements of seven trace gases and evaluated their capability to estimate stratospheric mean ages, which are useful proxies for stratospheric transit times
Summary
The “mean age of air” (mean AoA), defined as the average time that an air parcel has spent in the stratosphere, is an important derived quantity used in several stratospheric research fields, often when direct physical or chemical measurements are scarce, not available or inadequate. Air from the novel AirCore method has been used to calculate CO2-derived mean ages (Engel et al, 2017) and lightweight stratospheric bag samplers have been developed (Hooghiem et al, 2017) These technologies provide an excellent opportunity to increase the temporal and spatial coverage of stratospheric measurements in an affordable manner. We included HFC-125 as a potential age tracer as we believe its current estimated stratospheric lifetime of 351 years (SPARC, 2013; derived from model outputs) is potentially an underestimate based on preliminary mean age interpretations at UEA (finalised data included throughout this paper). All seven of the above-mentioned tracers currently fulfil the prerequisite of having well-constrained monotonically increasing growth rates in the troposphere
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