When Pluto has an atmosphere, it is thought to be made up of methane, nitrogen and carbon monoxide in gaseous form. This atmosphere outgases from the ice on Pluto's surface when it is closest to the sun, which warms the planet and causes the ice to sublimate.
However, when Pluto moves away from the sun, the atmosphere freezes and falls to the surface.
Ironically, when the atmosphere is in place, Pluto's surface is colder than it is otherwise due to a kind of inverse greenhouse effect. Methane, which is a greenhouse gas, causes a temperature inversion in the atmosphere, with temperatures 6 miles above the planet's surface being warmer than on the surface.
The fact that Pluto has an atmosphere wasn't definitely proven until 1988, when it was discovered that a star slowly dimmed out when Pluto passed in front of it. Had the dwarf planet no atmosphere at all, the star would have simply vanished. Still, the Plutonian atmosphere is so thin that its atmospheric pressure is only 1/700,000 as strong as Earth's.
In 2002, scientists found more nitrogen than usual in Pluto's atmosphere. It was believed that this nitrogen had come from the south pole, which was receiving sunlight for the first time in over a century.