Climate Dictionary

PSCs

Stratospheric Ozone

Tropospheric Ozone

Images

POLAR STRATOSPHERIC CLOUDS & OZONE DEPLETION

We explain the role played by polar stratospheric clouds (PSCs) in the destruction of the ozone layer. The article also examines the links between global warming of the troposphere and cooling of the stratosphere, where the ozone layer is located.

When Gabriell Walker, features editor of the New Scientist, saw PSCs over Kiruna in Swedish Lapland, she described them as "petrol blue and green, rimmed with vibrant pink—lurid colours that have no business in a sunset. Against the monochrome backdrop of snow and forest, they are shocking." In Scandinavia, PSCs are called mother of pearl clouds because of their resemblance to the shimmering inside of shells.

Despite their beauty, PSCs are not a welcome sight because they destroy ozone (a molecule made of three oxygen atoms). For a layperson, it can seem odd that clouds are able to destroy anything. Aren't clouds simply full of water or ice? Unfortunately, it's more complicated than that.

PSCs form in the cold, sunless months of the Antarctic and Arctic winter when stratospheric temperatures (10 to 25 km above the earth's surface) dip below minus 80 deg. C (minus 110 deg. F). The clouds consist of nitric acid trihydrate (a mixture of nitric acid and water). As the temperature gets colder, clouds made of larger droplets of water-ice with nitric acid dissolved in them can form. 

For ozone loss to occur, chlorine compounds, derived from chlorofluorcarbons (CFCs), and bromine are needed. Nearly all of the chlorine, and half of the bromine, in the stratosphere comes from human industrial activities. The main long-lived inorganic carriers (reservoirs) of chlorine are hydrochloric acid (HCl) and chlorine nitrate (ClONO₂). These form from the breakdown products of CFCs. 

The resevoir species of chlorine and their bromine equivalents are inactive, ie they do not themselves destroy ozone, but are converted into active, molecular, forms on the surface of PSCs.

Sunlight, which returns to Antarctica and the Arctic in spring,  breaks apart molecular forms of chlorine and bromine into atomic chlorine and bromine . Thus begins an extremely rapid light-driven catalytic cycle destroying ozone. A chlorine atom can react with ozone and break it down to chlorine oxide and O2. Chlorine oxide will break down as well, releasing the chlorine to go on destroying ozone. In fact, one Cl can destroy up to 10,000 ozone molecules.

To summarise, in order for PSCs to destroy ozone: 

1)  The sunless polar winter leads to the formation of the polar vortex wind, which isolates the air within it;

2)  Cold temperatures, below minus 80 deg. C,  inside the vortex enable the formation of PSCs;

3)  Once the PSCs form, reactions take place on the surface of the clouds and convert inactive chlorine and bromine reservoirs to more active forms of chlorine and bromine;

4)  No ozone loss occurs until sunlight returns to the air inside the polar vortex and allows the production of active chlorine and initiates very rapid ozone destruction.

PSCs and Global Warming

It was once thought that global warming (ie warming of the earth's surface as a result of emission of greenhouse gases) and cooling of the stratosphere, and thereby ozone destruction via PSCs, were unrelated processes. But it is now understood that the two are linked. In the words of NASA researcher Azadeh Tabazadeh: "As the earth's surface gets warmer, due to heat trapped by so-called greenhouse gases, the stratosphere gets colder, making it an even better place to create the ozone-depleting clouds."

This is perhaps a greater concern over populated Arctic and sub-Arctic regions than over the unpopulated Antarctic. In the words of NASA scientist Paul Newman: "Because ozone screens harmful ultraviolet or sun-burning radiation, large losses in spring, when plants are blooming and people are starting to venture outdoors again, can result in biological damage."

(Thanks to the Centre for Atmospheric Science at the University of Cambridge for vital information on the chemistry of ozone destruction on the surface of PSCs.)

Polar stratospheric clouds over Lapland