Abstract
Abstract. Balloon-borne observations of ozone from the South Pole Station have been reported to reach ozone mixing ratios below the detection limit of about 10 ppbv at the 70 hPa level by late September. After reaching a minimum, ozone mixing ratios increase to above 1 ppmv on the 70 hPa level by late December. While the basic mechanisms causing the ozone hole have been known for more than 20 yr, the detailed chemical processes determining how low the local concentration can fall, and how it recovers from the minimum have not been explored so far. Both of these aspects are investigated here by analysing results from the Chemical Lagrangian Model of the Stratosphere (CLaMS). As ozone falls below about 0.5 ppmv, a balance is maintained by gas phase production of both HCl and HOCl followed by heterogeneous reaction between these two compounds in these simulations. Thereafter, a very rapid, irreversible chlorine deactivation into HCl can occur, either when ozone drops to values low enough for gas phase HCl production to exceed chlorine activation processes or when temperatures increase above the polar stratospheric cloud (PSC) threshold. As a consequence, the timing and mixing ratio of the minimum ozone depends sensitively on model parameters, including the ozone initialisation. The subsequent ozone increase between October and December is linked mainly to photochemical ozone production, caused by oxygen photolysis and by the oxidation of carbon monoxide and methane.
Highlights
Since the discovery of the ozone hole (Farman et al, 1985), the chemical mechanisms causing ozone depletion have been clarified in increasing detail (e.g., Solomon, 1999; WMO, 2011)
The Chemical Lagrangian Model of the Stratosphere (CLaMS) 3-D simulations for the ozone hole in the year 2003 were described in detail in a dissertation for a university diploma (Walter, 2005), which focused on the time period up to November 2003
For investigations of the chemical processes around the ozone minimum in late September, a trajectory was chosen that exactly intersects an observation from the South Pole Station of about 10 ppbv ozone at the pressure level of 70 hPa on 24 September 2003
Summary
Since the discovery of the ozone hole (Farman et al, 1985), the chemical mechanisms causing ozone depletion have been clarified in increasing detail (e.g., Solomon, 1999; WMO, 2011). These mechanisms lead to an almost complete destruction of ozone at certain altitudes (≈15–20 km) in the Antarctic stratosphere, as observed in measurements by ozone sondes over four decades (Solomon et al, 2005). There are some observations that do not display ozone depletion over the South Pole Station, e.g. between late September and early November.
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