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 Real Time Camera

Causes Of Poor Visibility

When we visit a national park or look at the skyline of a city, often we do not enjoy a clear vista -- a white or brown haze hangs in the air and affects the view. This haze is not natural. It is caused by man-made air pollution, often carried by the wind hundreds of miles from where it originated.

Typical visual range in the eastern U.S. is 15 to 30 miles, or about one-third of what it would be without manmade air pollution. In the West, the typical visual range is 60 to 90 miles, or about one-half of the visual range under natural conditions. Haze diminishes the natural visual range.

Haze is caused by fine particles that scatter and absorb light before it reaches the observer. As the number of fine particles increases, more light is absorbed and scattered, resulting in less clarity, color, and visual range.

Five types of fine particles contribute to haze: sulfates, nitrates, organic carbon, elemental carbon, and crustal material. The importance of each type of particle varies across the U.S. and from season to season. The typical importance of each particle type in the eastern and western U.S. is shown in the figure to the right. Details on each particle type are provided below. (Note in the Class I areas in the upper Midwest, the contributions from nitrates and organics are somewhat greater and from sulfates somewhat less than indicated in the figure for the eastern U.S.).

Contribution of Various Particulates to Haze
Eastern U.S.
Eastern US
Western U.S.
Western US
  • Sulfate particles form in the air from sulfur dioxide gas. Most of this gas is released from coal-burning power plants and other industrial sources, such as smelters, industrial boilers, and oil refineries. Sulfates are the largest contributor to haze in the eastern U.S., due to the region's large number of coal-fired power plants. In humid environments, sulfate particles grow rapidly to a size that are very efficient at scattering light, thereby exacerbating the problem in the East.
  • Organic carbon particles are emitted directly into the air and also form there as a reaction of various gaseous hydrocarbons. Sources of direct and indirect organic carbon particles include vehicle exhaust, vehicle refueling, solvent evaporation (e.g., paints), food cooking, and various commercial and industrial sources. Gaseous hydrocarbons are also emitted naturally from trees and from fires.
  • Nitrate particles form in the air from nitrogen oxide gas. This gas is released from virtually all combustion activities, especially those involving cars, trucks, off-road engines (e.g., construction equipment, lawn mowers, and boats), power plants, and other industrial sources. Like sulfates, nitrates scatter more light in humid environments. Nitrate contributes relatively more to particle light extinction in winter than in summer because of its chemical properties (i.e., ammonium nitrate is more stable and less likely to dissociate at lower temperatures than at higher temperatures).
  • Elemental carbon particles are very similar to soot. They are smaller than most other particles and tend to absorb rather than scatter light. The "brown clouds" often seen in winter over urban areas and in mountain valleys can be largely attributed to elemental carbon. These particles are emitted directly into the air from virtually all combustion activities, but are especially prevalent in diesel exhaust and smoke from the burning of wood and wastes.
  • Crustal material is very similar to dust. It enters the air from dirt roads, fields, and other open spaces as a result of wind, traffic, and other surface activities. Whereas other types of particles form from the condensation and growth of microscopic particles and gasses, crustal material results from the crushing and grinding of larger, earth-born material. Because it is difficult to reduce this material to microscopic sizes, crustal material tends to be larger than other particles and tends to fall from the air sooner, contributing less to the overall effect of haze.

 Haze generally appears either as uniform haze, layered haze, or plumes.

  • A uniform haze degrades visibility evenly across the horizon and from the ground to a height well above the highest features of the landscape. Uniform haze often travels long distances and covers large geographic areas, in which case it is called a regional haze.
  • In a layered haze, you can see the top edge of the pollution layer. This is often the case when pollution is trapped near the ground beneath a temperature inversion.
  • Plumes result from local sources. Plumes and plume-like layers of elevated pollution take their shape under certain meteorological condition where the air is stable or constrained.

Some of the pollutants that form haze have been linked to serious health effects and environmental damage. Exposure to fine particles in the air have been linked with increased respiratory illness, decreased lung function, and premature death. In addition, sulfate and nitrate particles contribute to acid rain, which can damage forests, reduce fish populations, and erode buildings, historical monuments, and even car paint.

 Improving Visibility

To reduce haze we must reduce emissions of haze-forming pollutants across broad areas of the country. Cars, trucks, and industries are much cleaner than they were in the past, and several programs are in place to maintain this progress over the next several years. Nonetheless, these programs by themselves are unlikely to restore visibility to its natural conditions in may protected areas.

In April 1999 the U.S. Environmental Protection Agency (EPA) issued regulations to further reduce haze and protect visibility across the country. Given the effect of regional pollutant transport in contributing to haze in Class I areas, USEPA encourages states to work together in regional partnerships to develop and implement multi-state strategies to reduce emissions of visibility-impairing fine particle pollution. Five Regional Planning Organizations (RPOs) were formed to implement the regional haze regulations. The Midwest RPO is led by the States of Illinois, Indiana, Michigan, Ohio, and Wisconsin, and the tribes located in these states. Federal Land Managers, USEPA, and stakeholders also participate in the regional planning process.

The regional planning process will consist of three phases:

  1. Organization and Coordination
    During the first year or so of regional planning, the Midwest RPO has worked to "get organized" and "get smart". This initial phase included some preliminary work to build the technical foundation for regional planning.

  2. Technical Assessment Phase
    This phase, which is expected to take place over the first five years or so, will involve collecting and reviewing monitoring data, developing an emissions inventory, and analyzing future year emissions reductions scenarios using models or other appropriate methods. These activities are designed to meet several objectives: (a) develop an understanding of current pollution levels, (b) identify the principal contributing sources, (c) determine which states or areas contribute to another state’s visibility problem, and (d) estimate the impact of future strategies on air quality, costs, and other factors.

  3. Regulatory Phase
    This phase, which will take place after the technical assessment phase is completed, will include the development and submittal of the following SIP documents:

Initial SIP (due 1 year after PM2.5 designations)

  • show that emissions from your state contribute to visibility impairment in Class I area in another state (or emissions from another state contribute to visibility impairment in Class I area in your state)
  • describe regional planning process
  • list of BART-eligible sources
  • commit to submit control strategy SIP by appropriate date

Control Strategy SIP (due 3 years after PM2.5 designations)

  • establish reasonable progress goals to ensure improvement for most impaired days and no degradation for least impaired days
  • determine baseline and natural visibility conditions for most and least impaired days
  • long-term control strategy
  • monitoring strategy

Progress Reports (every 5 years)

  • evaluate progress toward reasonable progress goal; reinitiate regional planning, if not "on track"

Comprehensive SIP Revision (every 10 years)

  • evaluate and reassess control strategy SIP

We are challenged to do our part to help reduce air pollution. To learn more about what you can do to reduce air pollution, click on