Some aerosols, such as salt and certain dust particles, have an affinity to water. For this reason, the relative humidity does not have to reach 100% before water vapor condenses onto them. At a relative humidity of about 70%, these aerosols start to attract water vapor and dissolve, forming a tiny solution droplet much smaller than a cloud droplet. (Refer to Droplet Growth Processes in this session for a table of relative nuclei and droplet sizes.) High concentrations of these solution particles in the air form what we call haze. Though haze does not qualify physically as either a cloud or fog, haze particles are large enough to scatter all wavelengths of light efficiently (Mie scattering) which greatly reduces visibility in the air both near the ground and aloft.
The decreasing visual air quality as a result of haze is an indicator of the increasing concentrations of pollutants in our air and the need to control the various emission sources which produce them. The illustration below is borrowed from a publication of the EPA's Office of Air and Radiation. The document, which can be found on-line, is titled Air Quality Trends - 1994. The three maps show the amount of haze present during the summer months of 1970, 1980, and 1990. The dark blue color represents the best visibility and red represents the worst visibility. These maps show that visibility in the eastern US. decreased greatly between 1970 and 1980, and increased slightly in 1990. These changes in visibility correlate with the sulfur emissions data during that same 20 year period.
Below, we have provided several photographs provided by the National Park Service Air Quality Office. These images illustrate varying degrees of visibility impairment caused by haze within our national parks. The National Park Service in cooperation with the EPA has established a visibility monitoring program that serves as a means of keeping track of the pollution problem that exists in our country. In addition to the images below, you can visit our Gallery of Visibility Images provided by the National Park Service Air Quality Office (all images in our gallery are in JPEG format). You may also visit the National Park Service Visibility Program web site for more information regarding this program and the visibility problem that exists in our national parks.
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