UV absorption spectrum of O 3 at 298 K Small but significant absorption out to 350 nm (Huggins bands) Hartley bands Very strong absorption Photolysis mainly yields O( 1 D) + O 2, but as the stratosphere is very dry (H 2 O ~ 5 ppm), almost all of the O( 1 D) is collisionally relaxed to O( 3 P)
At the ground [O 3 ] ~ 10-100 ppb, in the stratosphere [O 3 ] ~ 5-10 ppm O 3 altitude profile measured from satellite
Total column amount of ozone measured by the Total Ozone Mapping Spectrometer (TOMS) instrument as a function of latitude and season Can we account for the distribution of ozone?
Timescale Slow (J is small) Fast < 100 secs Fast ~ 1000 s Slow (activation barrier)
[O] < < [O 3 ] [O] x = [O 3 ] + [O] ~ [O 3 ] Odd oxygen ( at least 99% of odd oxygen is O 3 – below 50 km)
J 1 = rate of O 2 photolysis (s -1 ) J 3 = rate of O 3 photolysis (s -1 ) Graph shows the altitude dependence of the rate of photolysis of O 3 and O 2. Note how J 1 is very small until higher altitudes (1)The ratio J 1 /J 3 increases rapidly with altitude, z (2)As pressure exp (-z) then [O 2 ] 2 [M] decreases rapidly with z z This balance results in a layer of O 3 Altitude/km J1J1 J3J3 J1J1 J3J3
The Chapman mechanism overpredicts O 3 by a factor of 2. Something else must be removing O 3 (Or the production is too high, but this is very unlikely) Altitude / km HOW GOOD IS THE CHAPMAN MECHANSIM?
Catalytic ozone destruction The loss of odd oxygen can be accelerated through catalytic cycles whose net result is the same as the (slow) 4 th step in the Chapman cycle Uncatalysed: O + O 3 O 2 + O 2 k 4 Catalysed: X + O 3 XO + O 2 k 5 XO + O X + O 2 k 6 Net rxn: O + O 3 O 2 + O 2 X is a catalyst and is reformed X = OH, Cl, NO, Br (and H at higher altitudes) Reaction (4) has a significant barrier and so is slow at stratospheric temperatures Reactions (5) and (6) are fast, and hence the conversion of O and O 3 to 2 molecules of O 2 is much faster, and more ozone is destroyed. Using the steady-state approximation for XO, R 5 =R 6 and hence k 5 [X][O 3 ] = k 6 [XO][O] Rate (catalysed) / Rate (uncatalysed) = R 5 /R 4 = k 5 [X][O 3 ]/k 4 [O][O 3 ]= k 5 [X]/k 4 [O] Or Rate (catalysed) / Rate (uncatalysed) = R 6 /R 4 = k 6 [XO][O]/k 4 [O][O 3 ]=k 6 [XO]/k 4 [O 3 ]
Note that rate coefficients for X+O 3 (k 5 ) and XO+O (k 6 ) are much higher than for O + O 3 (k 4 ) So don’t need much X present to make a difference k5k5 k6k6 k4k4
Altitude z / km Maximum in the O 3 mixing ratio is about here Fraction of odd oxygen loss HO x
Data from NOAA CMDL Ozone depleting gases measured using a gas chromatograph with an electron capture detector (invented by Jim Lovelock) These are ground-based measurements. The maximum in the stratosphere is reached about 5 years later 45 years100 years Why are values in the N hemisphere slightly higher?
“Do nothing” cycles O x is not destroyed Reduces efficiency of O 3 destruction Removal of the catalyst X. Reservoir is unreactive and relatively stable to photolysis. X can be regenerated from the reservoir, but only slowly. [X] is reduced by these cycles. For Cl atom, destroys 100,000 molecules of O 3 before being removed to form HCl
Interactions between different catalytic cycles Reservoir species limit the destruction of ozone ClONO 2 stores two catalytic agents – ClO and NO 2
Effects of catalytic cycles are not additive due to coupling MechanismOzone Column (Dobson units) Chapman only (C)644 C + NO x 332 C + HO x 392 C + ClO x 300 C + NO x + HO x + ClO x 376 Coupling to NO leads to null cycles for HO x and ClO x cycles Increase of Cl and NO concentrations in the atmosphere has less effect than if Cl or NO concentrations were increased separately (because ClOx and NOx cycles couple, hence lowering [X])
Bromine cycle Br + O 3 BrO + O 2 Cl + O 3 ClO + O 2 BrO + ClO Br + ClOO ClOO Cl + O 2 Net 2O 3 3 O 2 Br and Cl are regenerated, and cycle does not require O atoms, so can occur at lower altitude Source of bromine : CH 3 Br (natural emissions from soil and used as a soil fumigant) Halons (fire retardants) Catalytic cycles are more efficient as HBr and BrONO 2 (reservoirs for active Br) are more easily photolysed than HCl or ClONO 2 But, there is less bromine than chlorine Bromine is very important for O 3 destruction in the Antarctic stratosphere where [O] is low
TOMS (on Nimbus 7 satellite) o Dobson spectrophotometer October ozone column, Halley Bay, Antarctica
October 2000 “For the Second time in less than a week dangerous levels of UV rays bombard Chile and Argentina, The public should avoid going outside during the peak hours of 11:00 a.m. and 3:00 p.m. to avoid exposure to the UV rays” Ushaia, Argentina The most southerly city in the world
At 15 km, all the ozone disappears in less than 2 months This cannot be explained using gas- phase chemistry alone US Base in Antarctica