Question

Some time ago formation of polar stratospheric clouds was reported over Antarctica. Why were these formed? What happens when such clouds break up by warmth of sunlight?

Answer 1

To know about the formation of the clouds and the effect of sunlight on them we need to know what these clouds are. Polar stratospheric clouds are clouds in the winter polar stratosphere. The reaction on these clouds is mainly due to chlorine compounds.

Complete step by step answer:
Scientists working in Antarctica reported depletion of the ozone layer which is also known as ozone hole over the South Pole in Antarctica region. It was found that a unique set of conditions are responsible for the ozone hole.

In the summer season, nitrogen dioxide and methane gas react with chlorine monoxide and chlorine atoms which form chlorine sinks thus preventing much ozone depletion. While in winter, special type of clouds called polar stratospheric clouds are formed over Antarctica.
Polar stratospheric clouds abbreviated as PSCs are clouds in the winter polar stratosphere at altitudes in range 15,000 to 25,000 m. These clouds are best observed during civil twilight when the sun is between 1 and 6 degrees below the horizon as well as in winter and in more northerly latitudes. Due to the high altitude of clouds and the curvature of the surface of the Earth, they receive sunlight from below the horizon and reflect it to the ground shining brightly well before dawn or after dusk.

One main type of PSC is made up mostly of supercooled droplets of water and nitric acid and is implicated in the formation of ozone holes. The other main type consists only of ice crystals which are not harmful. This type of PSC is also denoted as nacreous.
Now, knowing about how these clouds are formed. These clouds provide a surface on which chlorine nitrate formed gets hydrolysed to form hypochlorous acid. It also reacts with hydrogen chlorine produced to give molecular chlorine.
$ClO(g) + N{O_2}(g) \to ClON{O_2}(g)$
$Cl(g) + C{H_4}(g) \to C{H_3}(g) + HCl(g)$
$ClON{O_2}(g) + {H_2}O(g) \to HOCl(g) + HN{O_3}(g)$
$ClON{O_2}(g) + HCl(g) \to C{l_2}(g) + HN{O_3}(g)$
Also, when sunlight returns to Antarctica in the spring the sun’s warmth breaks up the clouds and $HOCl$ and $C{l_2}$ are photolyzed by sunlight which is shown in the reactions given below.
$HOCl(g) \to OH(g) + Cl(g)$
$C{l_2}(g) \to 2Cl(g)$
Here, the chlorine radicals formed initiate the chain reaction for ozone depletion.

Note:
PSCs provide a surface on which chlorine nitrate is formed which gets hydrolysed to form hypochlorous acid and it reacts with hydrogen chlorine to give molecular chlorine. When sunlight breaks the clouds chlorine radicals are formed which initiates the chain reaction for ozone depletion.