Scientific Background about Ozone Depletion

Polar Stratospheric Cloud (PSC) as observed at Kiruna on January 26, 2000 [photo courtesy by F. Hase].
 

At temperatures below about 193 K polar stratospheric clouds (PSC) are formed, and heterogeneous reactions take place on the surface of the corresponding particles (s means solid in PSC or liquid in ternary solution of H2SO4, HNO3 and H2O; g means gaseous).

ClONO2 (g) +  H2O (s)      =>    HOCl (g) +  HNO3 (s)                                           (1)

ClONO2 (g) +  HCl (s)       =>    Cl2  (g)    +   HNO3 (s)                                          (2)

HOCl (g)      +  HCl (s)       =>    Cl2  (g)    +   H2O (s)                                            (3)

Active chlorine compounds like HOCl and Cl2 are released and are photolysed by solar radiation forming Cl which destroys ozone in a catalytic cycle:

          Cl    +  O3                  =>    ClO   +   O2                                                       (4)

          ClO +  O                    =>    Cl      +   O2                                                      (5)

After stratospheric warming and disappearance of the PSC and liquid ternary solution particles the reservoir gases recover by the following reactions

ClO + NO2 + M                  =>    ClONO2  +  M                                                     (7)

Cl    + CH4                          =>    HCl         +  CH3                                                (8)

Reaction (8) is rather slow as compared to (7) resulting in an excess of ClONO2 as compared to HCl at the end of the winter . According to reaction (7), the recovery of ClONO2 is controlled not only by chlorine, but also by NO2, which is formed by photolysis of HNO3 via

HNO3 (g)  +  hν                  =>   NO2        +  OH                                                    (9).
 

As PSC are formed only at low temperatures chlorine activation processes are running mainly in the Arctic and, even stronger, in the Antarctic winter. Therefore FTIR measurements are performed at Kiruna (North of Sweden) to study the following:

• Small column amounts of the reservoir gases ClONO2 and HCl indicate chlorine activation.
• The partitioning of chlorine into its reservoir gases ClONO2 and HCl. The ratio of ClONO2 to HCl is changed by activation/deactivation processes during the Arctic winter. This is important because ClONO2 might be photolysed in sun-lit areas producing again active chlorine compounds. Due to the transport of ClONO2 it may be responsible for the ozone loss outside the polar vortex at mid-latitudes.
• Since HNO3 can be incorporated into PSCs the amount of gaseous HNO3 as measured by FTIR is a measure of sequestration of HNO3. In order to reduce the influence of dynamic effects on data which may cover chemical or physical effects HNO3 (as well as the chlorine compounds) are related to the dynamic tracer HF in scientific investigations.
• Long-term observations allow to look for trends of chlorine and fluorine containing gases as well as of greenhouse gases.

Atmospheric Trace Constituents and Remote Sensing  Atmosphärische Spurenstoffe und Fernerkundung