Therefore, all visibility monitoring programs photographically document the appearance of the scene under various levels of visibility. Because it is difficult to extract quantitative information from photographs, visibility monitoring also includes instruments to record optical characteristics of the atmosphere and the composition of visibility reducing aerosols. Most often optical instruments measure either the scattering or extinction coefficient. Aerosol monitoring determines the composition of visibility reducing aerosols to help identify the source type and source strength of particles and gaseous precursors to secondary particles. This network began with 20 long-term monitoring sites in 1987 and now includes over 40 sites in parks and wilderness areas across the nation.
Some light extinction occurs naturally.
Scattering by the primary atmospheric gasses such as nitrogen and oxygen is called Rayleigh scattering. It is this natural scattering from atmospheric gases that causes the sky to appear blue. Naturally occurring particles also affect light extinction. Particle concentrations have only recently been routinelymeasured, so it is difficult to estimate natural occurring particulate levels. Visibility monitoring programs and airport observations are used to estimate the current visibility levels across the United States and to identify the relative contribution of various particle types to reduced visibility.