Ana - Lecture 1: Ozone

Question 1

What we know about ozone

Answer 1

• highly reactive and oxidising
• present in stratosphere and troposphere
• air pollutant (contributes to photochemical smog)
• strong oxidant, disinfecting power = ozonation
• toxic at high conc

Question 2

The ozone layer

Answer 2

In the troposphere, due to temp diff, the warmer air goes up and results in turbulent mixing. Most weather related events happen in this layer.
Tropopause: layer between the troposphere and the stratosphere, where you have temp inversion. Stratosphere (where ozone layer is) is very stable and is hotter than the troposphere due to the heating of ozone by the sun’s radiation. Only molecules with a long atmospheric lifetime will diffuse across the tropopause and once in the stratosphere they will stay there a long time. O3 in the ozone layer is of order 10 ppm.

Question 3

Benefits of the ozone layer

• Ozone hole
• What is Dobson unit?

Answer 3

Absorbs 97-99% of the sun’s medium frequency UV lights and hence protects living organisms from the harmful effects of UV light (UVB and UVC).
In 1985 it was first noticed that measurements of ozone conc were decreasing, mainly in the spring.
Dobson unit: a measure of the amount of ozone in the atmosphere above any given area
1 DU = 2.7 x 10^16 ozone molecules / cm^2
world average = 300 DU

Question 4

Natural ozone production and destruction

Answer 4

Ultraviolet radiation from the sun strikes a diatomic oxygen molecule and splits it into two oxygen atoms. The free oxygen atoms react with diatomic oxygen molecules to form ozone.
Ozone absorbs UV in mid/low range. This solar energy breaks apart the ozone molecules into diatomic oxygen molecules and oxygen atoms. The free oxygen atoms can react with an ozone molecules and form two molecules of diatomic oxygen.

Question 5

CFCs

Answer 5

Replaced early 20th century refrigerants such as ammonia, methyl chloride (CH3Cl), and sulfur dioxide (SO2). Class of organic compounds containing atoms of C, Cl and F. Are inert, stable, non-flammable and non-toxic.

Question 6

Negatives of CFCs

Answer 6

Ozone is being continually created and destroyed by free radical reactions in presence of UV. Free radicals of some species such as Cl, NO, Br, OH, H, also catalyse the destruction reaction but do not contribute to the formation. Happens in particular in spring (during Antarctic spring) as the isolation of cold, winter air at poles (polar vortex) leads to the formation of polar stratospheric ice clouds. Then, heterogeneous reactions at ice crystal surfaces break down ozone depleting e.g into Cl2 and Br2. Cl2 and Br2 are broken down by summer UV into Cl* and Br* and these free radicals catalyse ozone depletion. Due to stability and long life-span of CFCs into the troposphere, they can slowly migrate across the tropopause and enter the stratosphere where they suddenly become reactive.

Question 7

The ozone solution

Answer 7

Limit/phase out substances harmful to the ozone layer.

• 1985 Vienna (framework) convention on protection of ozone layer
• 1987 Montreal Protocol limited CFC production and phased it out by 1996
• Montreal Protocol amended at subsequent meetings
• 2016 Kigali amendment to phase down production and consumption of HFCs -> used as alternatives to ozone depleting substances such as HCFCs and CFCs as they can contribute to climate change.

Question 8

What might have been

Answer 8

In 2014, UN Environment Programme published a report on the progress of the ozone layer and Montreal Protocol:

• without Montreal Protocol, ozone depleting substances would have increased 10x
• protocol may have saved 2 million skin cancer cases
• ozone declined over 80s and 90s but has remained unchanged in the 2000s