SOURCES OF BIG BEND HAZE

Haze is caused by the scattering and absorption of light by fine liquid (>2.5 microns in diameter) and solid particles suspended in the air—these particles are known collectively as atmospheric aerosol. The sum of light scattering and absorption is known as light extinction and can be thought of as the fraction of light lost per unit of distance. The units of light extinction are inverse distance [e.g., 1/(million meters) or Mm -1)]. Levels of higher-light extinction correspond to hazier conditions. The contribution made to light extinction by the major components of atmospheric aerosol can be estimated by measuring the concentration of each component.

By contrast, light scattered by particle-free air, known as Rayleigh scattering, is assumed to be a constant value. Though Rayleigh scattering places limits on the ability to see distant scenic features clearly, many researchers do not consider Rayleigh scattering to be a component of haze, since it is a natural and fixed consequence of Earth’s atmosphere. Particulate haze, however, refers to the non-Rayleigh portion of light extinction and is the result of both manmade and naturally occurring particles in the atmosphere.

The following graph shows a five-year composite of light extinction based on measurements taken two to three times weekly at Big Bend with the following main observations.

• Generally, the two haziest periods of the year occur in spring, when particulate-sulfate compounds and carbonaceous particulate matter—organic compounds and light-absorbing carbon (LAC)—contribute similar amounts to haze, and in late summer and fall, when particulate-sulfate compounds are the largest contributors to park haze.

• Particulate-sulfate compounds generally contribute more to haze than any other individual component of the atmospheric aerosol.

• Carbonaceous particulate matter generally constituted the second largest contributor to park haze.

• Dust—represented by a combination of fine soil and coarse materials—contributed as much to the haze as particulate-sulfate compounds during the months of March and April.