Ozone Hole Chemistry


Monday, 24th November 2008; lecture I (pdf), lecture II (pdf)

 

  Points/Topics to remember about the “ozone hole”:

  • The “Ozone Hole” is a periodic, localized phenomenon over the polar regions that is caused by a combination of dynamical isolation, very cold air masses, and subsequent effective, heterogeneous and homogeneous air chemistry that leads to ozone destruction.
  • With the onset of the (Ant)arctic winter, the temperature in the stratosphere gradually falls below -80 °C, when so-called polar stratospheric clouds (PSCs) begin to form. The process starts with H2SO4 taking up water to form small particles, which can eventually crystallize to sulfuric acid tetrahydrate (SAT). As the temperature drops, these particles take up more and more water and also HNO3. The ternary system H2SO4×H2O×HNO3 stays liquid at even lower temperatures. Though nitric acid trihydrate (HNO3×3H2O, NAT) may crystallize, the continued uptake of more water eventually lets the particle grow to a size (>10 µm) where it starts to gravitationally settle (“stratospheric rain”). Once the pole is completely in the dark, this process permanently removes NOy from the gas phase, and also dries the stratosphere. During their growth and settling to lower elevations in the stratosphere, the particles also take up HCl and HBr, two important halogen reservoirs. As PSC particles settle out and eventually get removed from the stratosphere, this process has been called denitrification. Further denitrification is caused by the heterogeneous reactions (g=gas phase; l=liquid phase; s=solid phase)

14.                       N2O5 (g) + H2O (l,s)  →  2 HNO3 (l,s)

15.                       N2O5 (g) + HCl (l,s)  →  HNO3 (l,s) + ClNO2 (g)

With photolysis absent from the air mass, no new NOx is formed from N2O oxidation (reaction 6), and no radical chemistry consumes ozone. At the same time, a stable vortex has been established that effectively isolates the polar stratospheric air mass from lower latitudes.

·       During the polar night, several heterogeneous reactions on PSCs recycle active chlorine and bromine previously sequestered in HCl or HBr back into the gas phase. The most important reactions are

16.                       ClONO2 (g) + H2O (s)  →  HNO3 (s) + HOCl (g)

17.                       ClONO2 (g) + HCl (s)  →  HNO3 (s) + Cl2 (g)

18.                       HOCl (g) + HBr/HCl (s)  →  H2O (s) + BrCl or Cl2 (g)  and

19.                       HOBr (g) + HCl (s)  →  H2O (s) + BrCl (g) 

·       With light returning to the stratosphere in springtime, an “explosion” of active halogen chemistry starting with the photolysis of Cl2 (BrCl) and HOCl occurs. As the polar vortex is intact at this time, catalytic ozone removal via reactions 11+12 proceeds rapidly without ozone replacement from lower latitudes, and without effective sequestration of chlorine in ClONO2 due to the absence of NOx. ClO mixing ratios increase by 10-100 across the vortex edge. At high ClO, its self-reaction becomes important:

20.                       ClO· + ClO·  (+M)  →  ClOOCl  (+M)

… removing active chlorine. However, catalytic ozone removal via ClOx is kept efficient via recycling of Cl· in

21.                       ClOOCl + hν  →  ClOO· + Cl·

22.                       ClOO· + M  →  Cl· + O2 + M

ClO (and BrO) can be sensed remotely from space and show clearly elevated mixing ratios over the poles, areas where denitrification and subsequent photolysis of heterogeneously recycled active chlorine (bromine) has taken place in the stratosphere. Compared to chlorine, bromine abundances are much smaller, but active bromine is proportionally more effective in catalytic ozone removal than chlorine. Model results indicate a significant and growing role of bromine chemistry in the stratosphere.

·       The “ozone hole” disappears with the breakup of the polar vortex and the evaporation (or settling and removal) of the PSCs in late spring. Ozone is replenished from lower latitudes and chlorine is again sequestered. As a side-effect, a short-term decline in lower latitude ozone occurs as it is “flowing into the hole”.

 

·       Two Myths:

o      “The ‘ozone hole’ is named such because the ozone layer is completely destroyed”. Wrong: Ozone is only destroyed in the polar vortex at elevations where the input of active chlorine was highest as a result of PSC abundance. Remember that vertical mixing in the stratosphere is extremely slow, wherefore the chemistry tends to be in SS. Ozone in the upper stratosphere and at its bottom is therefore usually not completely removed, and a column amount of roughly 100 DU ozone remains (the “hole” edge is defined by an amount of lower than 220 DU).

o      “The ‘ozone hole’ is actually not of concern, because it forms only over Antarctica”. Wrong: Because PSCs do not form as often in the Arctic stratosphere due to higher temperatures compared to the Antarctic, and the Arctic polar vortex is not as stable as the Antarctic one, an “ozone hole” comparable in size and depth to that over Antarctica has not yet been observed in the Northern Hemisphere. However, since 1999, large ozone losses in the arctic vortex (and the resulting increases of UV-B radiation at the surface) were observed consistent with our understanding of “ozone hole” chemistry. In addition, global (tropospheric) warming is expected to lead to stratospheric cooling. As PSC formation is likely going to be enhanced in a cooler stratosphere, increased ozone losses also in the Arctic are expected to occur in the following decades (observations indicate that this has started already). Furthermore, as model calculations show, gas phase chemistry of active chlorine species is likely related to the slow global decrease in column ozone observed since the 1970s.

 

·       Note that stratospheric, catalytic ozone removal via 11+12 is highly efficient, more so when X=Br than with X=Cl. If more halons (the equivalent of CFCs with Bromine instead of Chlorine; uses include fire extinguisher fillings and flame retardants) had been or would be emitted, even higher ozone losses in the stratosphere would be observed. This is largely a result of the much shorter lifetime of BrONO2 as compared to ClONO2 towards photolysis, and inefficient sequestration of Bromine in HBr. It is therefore of large concern (and has partially been addressed through Montreal Protocol amendments) that anthropogenic emissions of brominated, volatile species are still high or even on the rise (example: methylbromide has a high ODP).

·       The “ozone hole” is likely to disappear some time in the 2060s or 70s based on decreasing tropospheric halogen levels as a result of the Montreal Protocol. However, continued CFC proliferation and increasing HCFC and bromine species emissions may delay the recovery until the end of this century

 

·       A comparative ozone loss to the Antarctic has not yet occurred in the Arctic stratosphere for two related reasons: 1) the arctic polar vortex is less stable during Arctic winter, partially due to an uneven distribution of land and ocean area in the Arctic compared to the Antarctic, and 2) temperatures often do not drop below -80°C inside the vortex for long enough periods to develop PSCs that affect ozone depleting active chlorine formation

·       Nevertheless, dramatic ozone loss has occurred also in the arctic stratosphere (e.g. in 1999) when conditions were right, namely the (more localized) occurrence of low temperatures for PSC formation and subsequent denitrification and catalytical active chlorine formation. Should stratospheric temperatures drop as a result of global (tropospheric) warming, the occurrence of a NH ozone hole becomes more likely, potentially affecting the biosphere alongside hundreds of millions of people in Europe, Russia, and Canada

 

Mandatory “browsing” this week: http:///www.theozonehole.com