6.2 Stratospheric Ozone

Facts about Ozone

absorbs most UV (all UV-C)

allow life to proliferate

can be degraded by CFCs

3 O2 → 6 O → 2 O3 (ozone)

sunlight catalyzes this reaction

O3 ←→ O2

happens naturally and in equilibrium w/o pollution + human interference

UV Absorption by Ozone

UV-A → all passes

UV-B → some passes

• sunblock

• cause sunburn, cancer, reduce PS/crop production/productivity

UV-C → none passes

• lethal

• disinfection purposes

Measuring Ozone

Dobson Units (DU)

• the depth ozone molecules would occupy at STP

• 1 DU → 0.01 mm of ozone

• normal = 300-500 DU (3-5 mm thick)

• ozone thinning = O3 breaks apart → molecules scattered

SUN INCREASES PRODUCTION OF OZONE

• more made at tropics

Ozone Depleting Substances (ODS)

ODS = Cl, Br, F

• CFCs + HCFCs = (hydro)chlorofluorocarbons used to replace CFCs

• used as refrigerants

discovered in 1930s

many types

• stable, long life

dissipate into air, not soluble (gases, can’t be removed)

MEDCs phased out first → LEDCs

CFCs migrate up to the stratosphere

UV breaks the Cl off the CFC ‘cule

Cl reacts with O3, breaking it apart

1 Cl can destroy ~300,000 O3

Cl can form HCl (hydrochloric acid) → drop to troposphere → precipitates down

Other ODS’s

Methyl Bromide

• soil fumigator to rid pests + soil pathogens

• naturally formed in the ocean + burning biomass

• 2 yr lifespan

• Br IS 50X STRONGER THAN Cl IN DESTROYING O3

Halons

• fire suppressant

Hydrobromofluorocarbons

• solvents, cleaning agents, fire suppressants

Carbon tetrachloride

• solvent, dry cleaning agent, refrigerant, propellant

Methyl chloroform

• industrial solvent, degreasing agent, correction fluid

Volcanic emissions (natty occuring)

Changes in Stratospheric Ozone Layers

Ozone varies over time + szn

measurement didn’t begin until the 70s

1980s → 5-15% reduction measured (NOT seasonal variation)

Ozone Hole

Seasonal Loss at Poles

@ antarctic + arctic

ozone = lowest spring to summer at the poles

• no new O3 being made in winter, takes months for sunlight to help re-establish the layer

Polar Stratospheric Clouds

• winter → CFCs build up in clouds

  • Cl isn’t breaking off yet (sun catalyzes Cl break off)

• when sun returns in spring, the reaction breaking off Cl happens → ozone decreases

• sun breaks the polar vortex which allows air movement + O3 can move in (helps reestablish layer)

  • ozone depleted air pushes toward the tropics

• ozone made in summer and fall in polar regions (strongest sunlight)

International Agreements to Manage the Ozone Layer

UN legislation

• Vienna Convention

  • protects ozone

  • support research + exchange info

• Montreal Protocol (MOST SUCCESSFUL INTL AGREEMENT TO PROTECT ENVIRONMENT)

  • Targets to reduce emissions of ODS’s

  • targets on ODS are legally binding

  • assistance to LEDCs

More Agreements

Montreal Protocol

• timetable reduce CFCs by 50%, freeze halon production

• ALLOW each country to set own law + compliance

London Amendment

• target to phase out main ODS

• deadline 2000, LEDCS = 2010

Copenhagen

• moved deadline up

Beijing

• more stringent (strict) controls on HCFCs

• added more ODSs to be phased out by 2004

Montreal Protocol

amendments hv been made due to new knowledge of ozone + ODSs

ratified by all 197 nations in 2015

is MOST SUCCESSFUL INTL AGREEMENT that has used SCIENCE to direct CHANGE + POLICIES

• an exemplar of intl cooperation

Challenges

CFC lifespan (+ other ODSs)

• long lived, even when eliminated, they’re still up in the atm

ODSs in discarded equipment

• they can release into the atm

lack of alternatives to replace the ODSs that aren’t ODSs (some do hv shorter lifespan tho)

many alternatives are also GHGs

HCFCs are to be phased out by 2030

Illegal trade of ODSs

Lack of policing + enforcement

Progress on Ozone Protection

Production of the main ODSs is banned (except for some med + research applications)

Overall emission reduced

Atm lvls of ODSs are leveling/declining (will take a bit of time due to lifespan)

Long life of CFCs will make ozone recovery a long road