Stratospheric Ozone Chemistry Flashcards
Absorption of UV light by O2
Produces ground state (3P) and excited state (1D) O which is rapidly relaxed by collision with third body
Processes in Chapman mechanism
O2 = O + O
O + O2 + M = O3
O + O3 = O2 + O2
O3 = O2 + O
Steady state of O3
Find steps
Rate laws
Equations for rate of change of conc of odd O
Set both = 0
Add together and rearrange for SS O3
Discrepancies between Chapman and actual
Chapman model ignores presence of trace gases
Catalytic loss of O3 cycles
HOx cycle, NOx cycle and HalOx cycle
HOx cycle
H/OH produces by reactions involving O (1D) and removed O3
NOx cycle
NO produced in stratosphere by reaction of O (1D) with N2O and can remove O3
HalOx cycle
Cl/Br produced in clean stratosphere by photodissociation of naturally occurring alkyl halides and can remove O3
Reservoir species
HNO3/ClONO2, stable species that couple catalytic loss cycles and (HOx with NOx and HalOX with NOx) and reduce their effect by trapping the reactive catalytic species
PSCs
Polar stratospheric clouds, form in stratosphere when local temp falls below 200K, two types
Type 1 PSC
Smaller than 1μm in diameter, hydrates of HNO3 around sulfate aerosol core
Type 2 PSC
Larger than 1μm in diameter, water ice on type 1 PSC core, found when temp drops below 195K ie fpt of water
Why is O3 loss greater with PSC present
Ice surface provides catalytic surface to assist reaction that generates Cl2 and HNO3
Consequences of reactions on PSC surface
Cl2 accumulates in stratosphere in cold winter
NOx trapped on PSC as solid nitric acid hydrates
PSC size increases = fall out of cold winter stratosphere = depletion of NOx = less NOX able to form reservoir molecules
Return of Sun in spring photodissociates accumulated Cl2
