Atmospheric chemistry Day 2 Stratospheric chemistry

O 2  O( 3 P) + O( 3 P) Threshold = 242 nm O 2  O( 3 P) + O( 1 D) Threshold = 176 nm

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 ] ~ ppb, in the stratosphere [O 3 ] ~ 5-10 ppm O 3 altitude profile measured from satellite

Integrated column - Dobson unit

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

What are the sources of X ?

CFC’s are not destroyed in the troposphere. They are only removed by photolysis once they reach the stratosphere.

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

Total Ozone Mapping Spectrometer (TOMS) Monthly October averages for ozone, 1979, 1982, 1984, 1989, 1997, 2001 Dobson units (total O 3 column)

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

Steps leading to ozone depletion within the Antarctic vortex ClO+BrO  Cl+Br+O 2

Simultaneous measurements of ClO and O 3 on the ER-2 Late August 1987September 16 th 1987 The “smoking gun” experiment – proved the theory was OK Still dark over AntarcticaDaylight returns

Simultaneous measurements of ClO and O 3 on the ER-2 Late August 1987September 16 th 1987 The “smoking gun” experiment – proved the theory was OK Still dark over AntarcticaDaylight returns

Ozone loss does appear in the Arctic, but not as dramatic Above Spitzbergen Some years see significant depletion, some years not, and always much less than over Antarctica