Week 13: Stratospheric Chemistry II

Monday, 17^th and Monday, 24^th November 2008, lecture I (pdf), lecture 2 (pdf)

Overhead sources are
Atmospheric Chemistry and Global Change, Brasseur, Orlando, Tyndall, Eds., Oxford University Press, 1999
Chemistry of the Upper and Lower Atmosphere, Finlayson-Pitts and Pitts, Academic Press, 2000

… and several internet pages, such as http://www.theozonehole.com

Points/Topics to remember from this week’s classes:

· Besides O_x, NO_x, and HO_x, further stratospheric ozone loss is caused by active halogen (XO_x) chemistry, which can be described generically by

11 ·X + O₃ → XO· + O₂

12. XO· + O → X + O₂

… creating the same net reaction as 7+8 (last week). In fact, X can not just be Cl, Br, or I, but equally well H or NO to create e.g. reaction 7. Due to its abundance, the reactions with X=Cl provide the dominant active halogen ozone loss process in the stratosphere, most relevant towards the top of the ozone layer. The major sources of chlorine that cause this problem are, at this point in time, anthropogenically emitted chlorofluorocarbons (CFCs) that photolyze at short UV-C wavelengths not available in the lower stratosphere, by breaking the C-Cl bond. There are also natural sources of chlorine (and bromine, and iodine) to the stratosphere, the largest being CH₃Cl, methylchloride, emissions from the biosphere (ocean). A small amount of methylchloride reaches the stratosphere, where it currently represents only a relatively small “background” active chlorine, ClO_x, source compared to anthropogenic sources. Methylbromide is the equivalent natural source of bromine to the atmosphere, which, however, is currently dwarfed by a higher anthropogenic amount of this chemical, emitted as a cause of its use as a fumigant in agriculture (think of that the next time you buy conventionally produced strawberries …).

The effect active halogen from anthropogenic CFC emissions (from the then widespread use as propellants and refrigerants) has on (catalytic) ozone removal in the stratosphere was first pointed out by Mario Molina and Sherwood Rowland in a groundbreaking publication in Science in 1974. Along with Paul Crutzen, Molina and Rowland received the Nobel Price in Chemistry in 1995 for this discovery. They were unaware at the time, what major chemical (“ozone hole”), and then political (“The Montreal Protocol”) consequences their discovery would have.

· Similar to O-atom reactions with CH₄, H₂, etc., chlorine atoms also abstract hydrogen from these molecules and are thereby sequestered into HCl, which represents the major removal pathway of active chlorine from the stratosphere (and the other halogens). Another, but temporary sequestration process is reaction

13. ClO· + NO₂ (+M) → ClONO₂ (+M) (+ hν → Cl· + NO₃)

ClONO₂ can be photolyzed, but as its lifetime towards that process is relatively long, it accumulates over time, and plays a similar role for Cl as PAN for NO_x in the troposphere (‘temporary reservoir’). ClO_x can be “recycled” from ClONO₂ also via heterogeneous reactions on particles, which represent the only surfaces in the stratosphere.

I have summarized these and other reactions, as they are particularly relevant to disturbed stratospheric chemistry in the (Ant)arctic stratosphere (“ozone hole chemistry”), on a separate web page here.