Temporal development of total chlorine in the high-latitude stratosphere based on reference distributions of mean age derived from CO2 and SF6

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Abstract

[1] We present balloon-borne observations of CO2 and SF6 and derived vertical profiles of mean age for polar winter (inside vortex) and for midlatitude (nonwinter) conditions. For SF6-derived mean ages above 5 years a mesospheric SF6 sink may lead to an overestimation of mean age, while for younger mean ages (below 2 years) the seasonal cycle of CO2 may influence the mean age determination based on CO2. We suggest that SF6 be used as an age tracer in the lower part of the stratosphere (i.e., for low mean age), whereas CO2 will be the better age tracer for older air. The mean age distributions together with an estimate of the width of the age spectrum are used to estimate the stratospheric chlorine loading. On the basis of an emission scenario and the lifetimes of chlorine-containing compounds we estimate the future stratospheric chlorine loading. Inside the polar vortex, at an altitude of 20 km, we expect total chlorine to return to the values present in this region in 1980, when the ozone hole first appeared, around the year 2060. This estimate is based on the assumption that the general transport characteristics will not change over the period of the analysis and can thus be regarded as a best estimate based on stationary transport, the assumed set of lifetimes and emissions, and the mean age distribution derived from our measurements. As other factors influencing ozone may undergo temporal changes, this does not mean that ozone will necessarily recover at this time. While total chlorine is probably a good proxy for inorganic chlorine at altitudes above 20 km inside the polar vortex, the changing halocarbon mix may actually lead to an earlier return of inorganic chlorine to pre-ozone-hole values at lower altitudes.

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