Visibility & Light Scattering Effects From Air Pollution

According to the U.S. Environmental Protection Agency, air pollution develops when tiny particles made up of sulfates, soot, soil dust and organic carbon settle within the air. As sunlight makes its way through the atmosphere, tiny particles can scatter incoming light rays, which results in reduced visibility. The size and composition of particles determine the light scattering effects that occur.

Visibility

• The term "haze" or "smog" refers to the effects of air pollution on visibility in the atmosphere. According to the Encyclopedia of Earth, haze develops when large concentrations of hydrocarbons and organic compounds combine with sunlight. It is most prominent over cities, though wind currents can transport smog to rural areas as well. The particles contained inside haze appear in different sizes, with some as small or smaller than the lightwaves themselves. Light scattering effects occur when lightwaves encounter particles of smaller, or comparable, sizes.

  According to Windows to the Universe, air pollution particles can also affect visibility by altering rainfall patterns within the atmosphere. Smaller particles, such as organic carbons, attract water vapor and hold onto vapor molecules, while other particles, such as dust, eventually release these molecules in the form of rain. This can lead to reduced rainfall amounts, which leaves existing particle materials in the atmosphere. In effect, more particles in the atmosphere reduce visibility levels.

Rayleigh Effect

• According to the Encyclopedia of Earth, the effects of air pollution on light movement can alter horizontal visibility and create what's known as the Rayleigh effect. This occurs as the light particles that make up lightwaves come into contact with smaller particles. As sunlight contains different wavelength sizes and colors, the size of a particular wavelength determines how it interacts with different particle sizes. When sunlight encounters smaller-sized particles, visibility can be limited to 80 to 160 miles, depending on wavelength and particle sizes. The Rayleigh effect results in a reduction in the amount of light an object emits, which in turn reduces its visibility.

Mie Scatter Effect

• The particle materials found in air pollution can absorb oncoming lightwaves as well as scatter them. According to the Encyclopedia of Earth, the Mie scatter effect happens when similar-sized particles (similar in size to light particles) absorb and deflect oncoming lightwaves. The resulting effect causes light to scatter in all directions and some rays may even scatter back toward the direction they came from.

  According to the Georgia Institute of Technology, the Mie scatter effect can be further determined using the Mie theory based on a particle's size and the directions in which light scatters when it makes contact with the particle. This theory measures the ability of a particle to absorb and deflect lightwaves. As the Mie scatter effect occurs only with particles of similar size, the Mie theory applies only to similar-sized particles.