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Stratospheric aerosols, influence on Earth's climate

Stratospheric aerosols are constituted by thin particles.

In addition to the simple components (e.g. molecular oxygen, ozone, nitrogen) that constitute the Earth’s atmosphere, the atmosphere also contains microscopic particles, of which the composition, form and size are far less known.

Their origine can be:

Thin droplets with an effect on climate

These small droplets play an important role in the physico chemistry of the stratosphere. They can:

  • change its optical characteristics (absorption and diffusion of light)
  • induce a warming of the stratosphere due to light absorption
  • induce a cooling of the troposphere because of the decreasing amount of light in this region of the atmospher
  • trigger important chemical reactions at their surface, in particular the destruction of ozone

Aerosols also contribute to the changing colours of the twilight.

The structure of aerosols and their impact on the stratosphere depend on several parameters, including the local temperature. When temperatures are not too low, these particles appear as fine droplets (aerosols). When temperatures fall, these stratospheric particles can turn into microscopic ice crystals better known as polar stratospheric clouds.

Aerosols influence the climate by modifying the circulation of stratospheric air masses. Higher up, at an altitude of about 90 km, the general circulation is dominated by gravity waves that reflect climatological disturbances undergone by the lower atmospheric layers (one of the many aspects of global changes).

Earth’s climate is certainly influenced by major volcanic eruptions, which are the main responsible for the massive injection of aerosols in the stratosphere. During these eruptions, sulphur compounds are injected into the stratosphere, giving rise to thin droplets of sulfuric acid and water.

Stratospheric aerosols reflect and absorb a part of the solar radiation.

In the period following a volcanic eruption, the size and number of particles varies, and the most part of aerosol falls back into the lower part of the atmosphere. Due to this fact, the influence of aerosols on the light transmission through the atmosphere can vary considerably, depending on the importance of the volcanic activity at the considered period.

The intensity decay per length unit of a sunbeam is quantified by the extinction coefficient. The extinction value depends on the altitude and of course, on the aerosol mass loading of the atmosphere.

May 18th, 1980: Explosion of the Mount St Helens (Oregon, USA)