Visibility data collected at national parks and wilderness areas by the NPS and other IMPROVE program participants have helped define spatial and temporal variations in visibility throughout the United States.
The map below illustrates the distribution of visibility conditions across the country. Large differences exist in visibility between the eastern and western United States, with western visibility generally being substantially better than eastern conditions. Climatic factors such as higher relative humidities and the greater density, quantity, and mix of emissions in the East are some of the reasons for this difference. The best visibility in the contiguous U.S. occurs in an area centered around Great Basin National Park, Nevada. Within the National Park System, the lowest visibilities are measured in eastern national parks such as Mammoth Cave (KY), Great Smoky Mountains (TN/NC), Shenandoah (VA), and Acadia (ME). Parks in southern California, such as Sequoia National Park, also record lower visibilities.
2005-2007 Average Visual Range in Kilometers
Long-term visibility monitoring also shows that strong seasonal differences exist in visibility at national parks. For most areas of the country, visibility tends to be best during the winter months and worst during the summer.
Unfortunately, summer also coincides with the period of highest visitation in most national parks, and the hazy summertime conditions likely diminish visitor enjoyment of the parks' spectacular vistas
The bar chart at right illustrates the conditions at Mammoth Cave National Park. The higher concentrations of fine particle mass (as represented by the height of the bars) in the summer months equate to lower summertime park visibilities.
The chemical composition of the particulate matter samples are used to determine the amount that certain chemical species and source types (for example, power plants or smelters) contribute to visibility impairment. Knowing the chemical constituents responsible for visibility impairment allows scientists to infer the probable causes of impairment and the reductions in emissions that must occur to remedy it. Visibility monitoring and research by NPS and others have found fine particles less than 2.5 millionths of a meter in diameter (PM2.5) in the form of sulfates, nitrates, organics, elemental carbon, and soil particles are primarily responsible for visibility impairment.
Sulfate particles formed from sulfur dioxide emissions associated with fossil fuel combustion (mostly for electricity generation) account for 55 to 85 percent of the visibility impairment observed in eastern parks.
The pie chart at left presents the Great Smoky Mountains National Park summertime contributions that measured chemical species make towards visibility impairment (light extinction).
In contrast, sulfates account for 20 to 40 percent of the impairment in the western U.S. The contributions of the other chemical constituents is typically less than that of sulfates. Organics and elemental light absorbing carbon (LAC) play a much greater role in visibility impairment in certain regions of the West and Pacific Northwest. This is probably the result of a greater contribution of emissions from agricultural sources and forest fires to visibility impairment. Soil particles can be important contributors to visibility impairment in the western U.S. primarily due to the greater occurrence of wind-blown dust.
Additional Data Summaries
IMPROVE data summaries and raw data are available on the IMPROVE and VIEW web pages below: