A3.2.6 Continued…


Effects of Particulates:

Particulates can affect the climate in two different ways. The “direct effect” is caused by the fact that the particles scatter and absorb solar and infrared radiation in the atmosphere. As particles become increasingly absorbing, a point is reached where the overall effect of the particle layer changes from cooling to heating. The result of the scattering of sunlight caused by particles is an increase in the amount of light reflected back into space, which results in a decrease in the amount of solar radiation that reaches the surface.

The “indirect effect” of particles are more complex and more difficult to assess. Changes in the number concentration of aerosols in the atmosphere causes variations in the population and size of cloud droplets. There is a set amount of water available for clouds. The water can form large droplets within the clouds, which causes precipitation. The addition of PM into the atmosphere causes the water to condense on to the particles. This results in more, but smaller droplets in the clouds, which increases the cloud albedo.

[Albedo is a term to describe the extent to which an object diffusely reflects light from light sources such as the sun. It is a unitless measure. For example, fresh snow has an albedo of around 0.85. whilst bare soil has an albedo of around 0.17.]

Different types of clouds have different albedo values. Clouds keep Earth cool by reflecting sunlight, but they can also serve as blankets to trap warmth. In addition to increasing the albedo, this effect tends to decrease the chance of precipitation. If precipitation is suppressed, this results in excess water remaining in the atmosphere.

Other terms for particulates are as follows:


Technically, an aerosol is a suspension of fine solid particles (or liquid droplets) in a gas. Examples include smoke and smog:

  • Smoke is specifically the result of incomplete combustion. Smoke particles tend to be less than 2µm.
  • Smog is a generic term that refers to a fog containing pollutants (such as smoke, SOx or NOx).


Dust is a general term for small solid particles with an aerodynamic diameter of less than 100 µm. Sources of dust may be soil dust lifted up by wind, skin cells, textile fibres and minerals.


A fibre is a generic term for any solid particle with a length: width ratio of 3:1. This term is of more relevance to health, as a fibrous particle may be absorbed into the lungs but then get stuck, due to the shape. A commonly known fibrous material with associated health problems is asbestos.