Atmospheric chemistry

Day 2Stratospheric chemistry

O2 O(3P) + O(3P) Threshold = 242 nm

O2 O(3P) + O(1D) Threshold = 176 nm

UV absorption spectrum of O3 at 298 K

Small but significant absorption out to 350 nm (Huggins bands)

Hartley bands

Very strong absorption

Photolysis mainly yields O(1D) + O2, but as the stratosphere is very dry (H2O ~ 5 ppm), almost all of the O(1D) is collisionally relaxed to O(3P)

At the ground [O3] ~ 10-100 ppb, in the stratosphere [O3] ~ 5-10 ppm

O3 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?

TimescaleSlow (J is small)Fast < 100 secsFast ~ 1000 sSlow (activation barrier)

[O] < < [O3]

[O]x = [O3] + [O]

~ [O3]

Odd oxygen

( at least 99% of odd oxygen is O3 – below 50 km)

J1 = rate of O2 photolysis (s-1)

J3 = rate of O3 photolysis (s-1)

Graph shows the altitude dependence of the rate of photolysis of O3 and O2. Note how J1 is very small until higher altitudes

(1) The ratio J1/J3 increases rapidly with altitude, z

(2) As pressure exp (-z) then [O2]2 [M] decreases rapidly with z

z][][][ 2

24

2

3

13 MO

k

k

J

JO

This balance results in a layer of O3

Altitude/km

J1J3

J1

J3

The Chapman mechanism overpredicts O3 by a factor of 2.

Something else must be removing O3

(Or the production is too high, but this is very unlikely)

Alt

itu

de

/ km

HOW GOOD IS THE CHAPMAN MECHANSIM?

Catalytic ozone destructionThe loss of odd oxygen can be accelerated through catalytic cycles whose net result is the same as the (slow) 4th step in the Chapman cycle

Uncatalysed: O + O3 O2 + O2 k4

Catalysed: X + O3 XO + O2 k5

XO + O X + O2 k6

Net rxn: O + O3 O2 + O2

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 O3 to 2 molecules of O2 is much faster, and more ozone is destroyed.

Using the steady-state approximation for XO, R5=R6 and hence k5[X][O3] = k6[XO][O]

Rate (catalysed) / Rate (uncatalysed) = R5/R4 = k5[X][O3]/k4[O][O3]= k5[X]/k4[O]

Or Rate (catalysed) / Rate (uncatalysed) = R6/R4 = k6[XO][O]/k4[O][O3]=k6[XO]/k4[O3]

Note that rate coefficients for X+O3 (k5) and XO+O (k6) are much higher than for O + O3 (k4) So don’t need much X present to make a difference

k5 k6

k4

Altitude

z / km

Maximum in the O3 mixing ratio is about here

Fraction of odd oxygen loss

HOx

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 years 100 years

Why are values in the N hemisphere slightly higher?

“Do nothing” cycles

Ox is not destroyed

Reduces efficiency of O3 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 O3 before being removed to form HCl

Interactions between different catalytic cycles

Reservoir species limit the destruction of ozone

ClONO2 stores two catalytic agents – ClO and NO2

Effects of catalytic cycles are not additive due to coupling

Mechanism Ozone Column

(Dobson units)

Chapman only (C) 644

C + NOx 332

C + HOx 392

C + ClOx 300

C + NO x+ HOx + ClOx 376

Coupling to NO leads to null cycles for HOx and ClOx 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 + O3 BrO + O2

Cl + O3 ClO + O2

BrO + ClO Br + ClOO

ClOO Cl + O2

Net 2O3 3 O2

Br and Cl are regenerated, and cycle does not require O atoms, so can occur at lower altitude

Source of bromine : CH3Br (natural emissions from soil and used as a soil fumigant)

Halons (fire retardants)

Catalytic cycles are more efficient as HBr and BrONO2 (reservoirs for active Br) are more easily photolysed than HCl or ClONO2

But, there is less bromine than chlorine

Bromine is very important for O3 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 O3 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+BrOCl+Br+O2

Simultaneous measurements of ClO and O3 on the ER-2

Late August 1987 September 16th 1987

The “smoking gun” experiment – proved the theory was OK

Still dark over Antarctica Daylight returns

Simultaneous measurements of ClO and O3 on the ER-2

Late August 1987 September 16th 1987

The “smoking gun” experiment – proved the theory was OK

Still dark over Antarctica Daylight 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