Notes

Outline

Lecture 11: Ozone Hole Depletion  Ozone Production and Destruction  Catalytic Cycle  Polar Stratospheric Clouds  CFC and Ozone Depletion

Ozone Production and Destruction

Ozone Distribution

Other Ozone Destruction Processes Other atmospheric trace constituents, such as nitrous oxide (N2O), water vapor, and freons, can also be photolyzed. They produce highly reactive radicals that keep ozone abundances lower than they would otherwise be. These radicals include: nitric oxide (NO), atomic chlorine (Cl), bromine (Br) radicals, and hydroxyl (OH) radicals. These radicals can destroy stratospheric ozone through “catalytic cycles”.

The Chlorine Catalytic Cycle

The Chlorine Catalytic Cycle

The Nitrogen Catalytic Cycle

Chlorine Sources

Man-Made Sources for CFCs There are two kinds of CFCs: freon-11 (CCl3F) and freon-12 (CCl2F2). Freon-11 has been used:       (1) as a propellant in spray cans       (2) as a blowing agent for producing foams       (3)  to clean semiconductor chips. Freon-12 has been used as       (1) a refrigerant       (2) working fluid in most car air conditioners.

Polar Stratospheric Clouds (PSCs) In winter the polar stratosphere is so cold (-80°C or below) that certain trace atmospheric constituents can condense. These clouds are called “polar stratospheric clouds” (PSCs).

How PSCs Affect Ozone Hole The ice crystals in the polar stratospheric clouds provide surface for the ozone depletion surface to occur more easily. On these cloud surfaces, certain forms of chlorine that do not react with ozone are converted into forms that do. Polar stratospheric clouds set up the stage for massive destruction of ozone to happen when sunlight returns in the spring.

Two Factors for the Ozone Hole Very Cold temperature     To form  polar stratospheric clouds (PSCs) Return of the Sunlight      To start the ozone depletion reaction

Why No Ozone Hole in Artic?

The Polar Vortex The wintertime circulation over the South Pole is characterized by a gigantic whirlpool of cold and dense air, called the polar vortex. The cold and dense cold air in the middle of the vortex is subsiding. The sinking air carries cloud particles along with it. Remove odd nitrogen from the stratosphere. Very little ozone and odd nitrogen can be brought into the south pole.

The 1997 Ozone Hole