strat - Stratospheric Ozone Chemistry Stratosphere: 15-50...

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1 Stratospheric Ozone Chemistry Stratosphere: 15-50 km depending on latitude O 3 partial pressure max = 35 km O 3 number density max = 22 km (Fig. 1-1) "ozone layer" is defined by maximum ozone number density and occurs between 15-35 km (lower- middle stratosphere) 1. Ozone formation: UV-C O 2 –––––––> 2O O + O 2 ––––> O 3 + heat (spontaneous, rxn = –105 kJ mol -1 ) ––> maximum ozone density = f(UV-C, [O 2 ]) ( [ O 2 ] = number density of O 2 ) 2. Non-catalytic ozone destruction: UV-B,C O 3 –––––––––> O 2 + O* 3. Catalytic ozone destruction (Mechanism I): catalyst : chemical species that accelerates a reaction rate, but is not consumed by it X· + O 3 –––––––> XO· + O 2 XO· + O –––––––> X· + O 2 ––––––––––––––––––––––––––––––– net O 3 + O –––––––> 2O 2 requires atomic oxygen (O) to regenerate catalyst Æ slow in lower stratosphere where n O is low Æ faster in middle and upper stratosphere where n O is higher
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2 • ozone destroying catalysts are free radicals , species with an odd number of electrons: Cl· (atomic chlorine) 17 electrons OH· (hydroxyl radical) 9 electrons NO· (nitric oxide) 15 electrons HOO· (hydroperoxy radical) 17 electrons O (atomic oxygen) 8 electrons (NOT a free radical) Natural ozone destroying catalysts a. NO· (dominant in middle stratosphere) source: tropospheric N 2 O from denitrification and nitrification
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strat - Stratospheric Ozone Chemistry Stratosphere: 15-50...

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