Ozone

What are the wavelengths of harmful UV radiation?

UVB: 280-320 nm

UVC: <280 nm

Why is ozone particularly generated in the tropics?

More sunlight

More lightning

Give the 4 stages of the Chapman cycle

Photolysis (1)

Recombination

Photolysis (2)

Bimolecular reaction

Give the equn of photolysis (1)

O₂ + hv→ O + O

hv: <240 nm

Give the equn of recombination

O + O₂ + M→ O₃ + M

Give the equn of photolysis (2)

O₃ + hv→ O₂ + O

hv: <310 nm

Give the equn of the biomolecular reaction

O₃ + O→ 2O₂

Why is recombination non-arrhenius?

pressure dependant

Why was there a discrepancy between calculations based off the chapman mechanism and observations?

Calculations overestimated ozone conc

sinks too low, in reality: catalytic cycle

Why are catalytic cycles important in ozone depletion?

trace amounts can destroy large amounts of ozone as they are not used up in a reaction so can go on to react many times before they are destroyed

Give a more accurate version of the last step of the chapman cycle

X + O₂→ XO+ O₂

XO+ O→ X + O₂

where X = H, OH, NO, Cl, Br

What is the range of wavelengths that CFCs absorb to release Cl.?

180-230 nm

Where does photolysis of CFCs take place?

suitable wavelengths only found in stratosphere

What is the ozone column?

The total integrated amount of ozone from the top of the atmosphere to the bottom over 1cm3

in Dobson units- 10^5 ozone molecules per cm3

What was early evidence of ozone depletion?

upward facing observations of ozone column in antarctica showed decline

What were the inital expectations of ozone depletion?

uniform, gradual depletion throughout stratosphere

What was surprising about ozone depletion?

strong, localised depletion

Why was alarm not initially raised?

potentially in line with interannual variation

How did ozone depletion vary with altitude?

Little to no ozone between 14-20 km where there had previously been a peak

other regions unaffected

Why did ozone depletion take place in the antarctic?

colder in winter than the arctic (-80 deg), leads to polar stratospheric cloud formation

polar vortex forms during the dark winter months which isolates the air from the rest of the atm

What are polar stratospheric clouds?

rare due to low mol fraction of water in stratosphere

only form in v low temperatures where water and nitric acid condense on sulf. acid containing particles

correspond to the altitude at which ozone depletion takes place

What are the breakdown products of CFCs?

HCl

ClONO2

which descends into polar vortex and serve as reservoirs

Describe ozone depletion in PSCs (words)

HCl + CLoNO2 adsorbed onto PSC surfaces react to form Cl2

Cl2 released into atm due to higher vapour pressure and builds up over polar winter

Cl2 photolyses in spring/summer to give Cl radicals

Cl radicals take part in catalytic cycles to deplete ozone

Describe ozone depletion in PSCs (equns)

HCl + ClONO₂→ HNO₃ + Cl₂

Cl₂ + hv→ Cl + Cl

Cl + O₃→ ClO + O₂

ClO + O→ Cl + O₂ OR ClO + ClO + M ⇌ ClOOCl + M (lower strt)

ClOOCl + hv→ Cl + Cl + O₂ (lower strt: cold temp, high presh)

Why is ozone depletion important?

Changes optical depth of column in UV

More UV reaches surface: skin cancer, eye cataracts, damage to plants and phytoplankton, photo oxidation of building materials

What about an arctic ozone hole?

arctic stratosphere becoming colder and more humid

suitable conditions for PSC

could lead to arctic ozone hole

What were the steps of solving the problem?

Vienna convention (1985) set out need to protect ozone layer

Montreal protocol (1987-1989): technology fund allows transition away from CFCs. Initial obligation to reduce production + consumption of CFCs by 50% by 1999. Amendment in 1990+1992 to bring forward pace of phase-out

What were the unintended consequences of CFC replacements?

excellent greenhouse gases- absorbed in a previously untouched region of IR

changed radiation budget- large greenhouse effect

What was the purpose of the Kigali amendment?

legally binding reduction of HFCs (replacement CFCs) by 80% by 2047