Unit IX. The Ozone Hole(s)A.Discovery: Antarctic Ozone Hole

Satellites have been monitoring the totalamount of ozone (mostly in the stratosphere)since the late 1970s.

Satellites accurately recorded the suddendisappearance of at least half of all the ozone,but the computer rejected the results for many

years.

B. What is Ozone? 1. Ozone: A gaseous molecule that contains

three oxygen atoms (O3), rather than the normal two atoms (O2).This is NASTY stuff: Extremely reactiveStrongly oxidizing

Formation and destruction of ozone is going on all the time in a natural cycle.

This natural cycle is also being impacted by human activity.

Unit IX. The Ozone Hole(s)A. Discovery: Antarctic Ozone HoleB. What is Ozone?

1. Ozone: A gaseous molecule (O3).2. Where does it come from?

O2 + UV photon O + O

O + O2 + M O3 + M

O3 + photon O2 + OO + O3 2 O2

slow

fast

fast

slow

O2 + UV photon O + O

O + O2 + M O3 + M

O3 + photon O2 + O

O+ O3 2 O2

slow

fast

fast

slow

Production

Destruction

O2 + UV photon O + O

O + O2 + M O3 + M

O3 + photon O2 + O

O+ O3 2 O2

slow

fast

fast

slow

Recycling

Destruction

O2 + UV photon O + O

O + O2 + M O3 + M

O3 + photon O2 + O

O+ O3 2 O2

slow

fast

fast

slow

Production

Destruction

Recycling

Photodissociation of O3 both heats the stratosphere, and consumes (blocks) most of the shorter UV energy from the Sun: hence, the “ozone shield”

Most O3 production is in the tropical stratosphere, because the solar flux is highest there (perpendicular to Sun’s energy). Stratospheric winds move O3 to higher latitudes.

3. Why is it mostly in the stratosphere?The small amount of very short wave energy(<200 nm) required to split O2 molecules is consumed in the stratosphere and is not foundbelow 20 km.

C. UV RadiationUV radiation is between 200 and 400 nmWe divide the UV portion into 3 segments:UVA: longer ans youUVB: middle Burns you; skin cancersBCC (most will get), melanoma, squamousUVC: shortest Destroys DNA

Tropopause

90% of O3 is in the stratosphere

We can repair minor (UVB) damage; not UVC

DNA

O3 absorbs very efficiently at UVC, and moderately well at UVB, but not at UVA.Consequently, at Earth surface, UVA and UVB

More ozone in the stratosphere means less UVB at Earth’s surface.

D. Measuring ozone concentrationsDobson Units (DU): 1 DU = layer of pureozone 0.001 cm thick at 1 atmosphere.

Typical concentration is 300 DU

Tropospheric ozone: 10% of all ozone is in the troposphere from both natural and industrial sources. Its reactive nature cleans the troposphere of pollutants, but at ground level it is a major eye and lung irritant and kills plants. LA and Denver photochemical smog….

Unit IX. The Ozone Hole(s)E. Other ozone depletion mechanisms.

1. Total amount of ozone is always 30% less than predicted if photodissociation is the only loss.

There are other trace gases in the stratosphere…. Can these interact with ozone?

2. Chlorine catalytic cycleChlorine as a catalyst. Catalyst: A substance

that increases the rate of a reaction, but is itself unchanged by the reaction.

Cl + O3 ClO +O2

ClO + O Cl + O2

fast

fast

Cl + O3 ClO +O2fast

As a catalyst, Chlorine is now free to repeat the cycle of ozone destruction

Unit IX. The Ozone Hole(s)E. Other ozone depletion mechanisms.

1. 30% less ozone than predicted2. Chlorine catalytic cycle

3. Other catalytic radicalsNitrogen (NO + O3 = NO2 +

O2)(NO2 + O = NO +

O2)N2O + O = 2 NO

Unit IX. The Ozone Hole(s)E. Other ozone depletion mechanisms.

1. 30% less ozone than predicted2. Chlorine catalytic cycle

3. Other catalytic radicalsNitrogen (NO + O3 = NO2 +

O2)(NO2 + O = NO +

O2)Sources of N2O in the stratosphere…• Natural from microbial activity in soils• Soil microbials enhanced by fertilizers• Also from jet exhaust (NO)……particularly

if they fly in the stratosphere (SST)

Unit IX. The Ozone Hole(s)E. Other ozone depletion mechanisms.

4. CFCs and the chlorine catalytic cycle

What are CFCs?Chloro-Fluoro-Carbon compounds: compounds

containing chlorine, fluorine and carbon.A simple CFC: CCl3FCarbon tetrachloride: CCl4

Unit IX. The Ozone Hole(s)E. Other ozone depletion mechanisms.

4. CFCs and the chlorine catalytic cyclea) sources: leaky fridges/AC/spraysb) fate of CFCs.

Because they are an inert gas, CFCs have a long residence time in the troposphere.

But how do they then get through the tropopauseinto the stratosphere? Through rare large convective storms in the tropics. Mixed slowly(1 to 2 yr) by stratospheric winds.

Unit IX. The Ozone Hole(s)4. CFCs and the chlorine catalytic cycle

a) sources: leaky fridges/AC/spraysb) fate of CFCs. c) solar radiation and CFCs.

Short-wave UV radiation breaks CFC bonds,releasing atomic Cl, which is then free to participate in the chlorine catalytic cycle, whereeach Cl atom destroys tens of thousands of O3 molecules. The chlorine cycle is eventually broken when Cl combines with CH4 or H2 toform HCl, which is scrubbed out by rain.

solar radiation

Destruction of CFCs in the stratosphere

UVC

d) Natural sources of Cl in the stratosphere:• HCl (from mild volcanic eruptions)• Methyl chloride (CH3Cl) from marine biota.

But CFCs supply a larger portion of stratospheric Cl.

Unit IX. The Ozone Hole(s)F. The Antarctic Ozone Hole

1. Only occurs in the Antarctic spring (September, October [strongest]). 2. Was not present in the early 1970s…. only after ~1976.

Global ozone distribution in October

Unit IX. The Ozone Hole(s)F. The Antarctic Ozone Hole

1. Only occurs in the Antarctic spring (September, October [strongest]). 2. Was not present in the early 1970s…. only after ~1976.3. In 2003, the Antarctic ozone hole was the second largest on record.

Unit IX. The Ozone Hole(s)F. The Antarctic Ozone Hole

1. Only occurs in the Antarctic spring (September, October [strongest]). 2. Was not present in the early 1970s…. only after ~1976.3. In 2003, the Antarctic ozone hole was the second largest on record.4. Is ozone depletion restricted to the Antarctic?

Unit IX. The Ozone Hole(s)F. The Antarctic Ozone Hole

1. Only occurs in the Antarctic spring (September, October [strongest]). 2. Was not present in the early 1970s…. only after ~1976.3. In 2003, the Antarctic ozone hole was the second largest on record.4. Is ozone depletion restricted to the Antarctic? No: also small global decreases5. Is there an Arctic Ozone Hole? Yes, but much less severe and more variable.

Unit IX. The Ozone Hole(s)F. The Antarctic Ozone HoleG. Mitigation

Unit IX. The Ozone Hole(s)F. The Antarctic Ozone HoleG. Mitigation:

Montreal Protocols and ammendments have nearly eliminated CFC and related production.

Long residence times of CFCs means it will take abot 50 years for the ozone to fully recover. But it will recover.