Climate changeClimate change (global warming and global dimming) is impacting the entire planet on which we live. Many governments throughout the world have realized the size and complexity of the problem and are beginning to take decisive action to ameliorate the effects of global warming. A key component of taking action is to understand:
- What is happening?
- Why is it happening?
- How is it happening?
Global warming/global dimming have many interrelated factors that influence the type, rate and direction of changes that can affect the earth’s radiation balance.
The Earth is warmed principally by the sun’s radiation which enters the atmosphere. Not all the radiation penetrates the atmosphere as some is scattered back into space. The amount of back-scattering into space alters the amount of energy that is absorbed into the atmosphere, this is known as global dimming (see figure below). Measuring the amount of backscattering of this light provides valuable information on the change in the earth’s radiation balance and how different activities (artificial and anthropogenic) can affect this delicately balanced system.
Global dimming has, by all accounts, reduced the radiation reaching the earth's surface and thus cooled the planets heating. This is explained best in the folowing transcript at www.pbs.org. Measuring the effect global dimming throughout various parts of the world will help dramatically in predicting global temperature rises and the effect these will have on climate change.
Integrating nephelometers directly measure the light scattered by aerosols and gases in an enclosed sample volume. The sampling chamber and light source are confined to a small volume so that the instrument makes a “point” or localised measurement of scattering, continuously and in real-time. The total measurements are then combined with a backscatter measurement that will only sample between 90° and 170° to give a more in-depth analysis of particle scattering.
This information can be combined with data measured by other aerosol instruments and then inserted into mathematical models to derive the following additional parameters:
Aerosol asymmetry parameter (g) Defined as the cosine-weighted average of the phase function, where the phase function is the probability of radiation being scattered in a given direction. It can be derived, under certain assumptions, from the measured backscatter fraction (Ogren et al, 2006).
Angstom Exponent (Å) Can be used to describe the dependency between aerosol optical depth and wavelength. The Ångström exponent is inversely related to the average size of aerosol particles: the smaller the particles, the larger the exponent. Its determination using integrating nephelometers and satellite data provides valuable information into the optical depth of the atmosphere and the radiative forcing effect of aerosol.
Single Scattering Albedo The ratio of light scattering to light extinction by atmospheric particles. It is an important parameter when assessing the climatic effects of aerosols. It is understood that aerosols with a single-scattering albedo greater than 0.85 generally cool the planet, while those with less than 0.85 warm the planet (Hansen et al. 1981). This is also dependent on the surface albedo and the backscatter fraction.
The Aurora 3000 Three wavelength nephelometer with backscatter is an ideal instrument for assisting in determining the above parameters. An extension of this instrument is the Aurora 4000 polar nephelometer which can measure scatter in specific segments, giving a much more detailed understanding.