One of the most basic symptoms of air pollution – haze – degrades the visibility in many of America's cities and scenic areas. Haze obscures the color, clarity, texture, and form of what we see. It affects how far and how well we can see landscape features. In many parts of the country, visibility is significantly reduced by air pollutants that are carried by the wind hundreds or even thousands of miles. The same pollutants (fine particles) that form haze also contribute to significant health and environmental problems across the United States.
What Causes Haze?
Visibility is reduced when light is absorbed, scattered, or interfered with. Large particles are efficient at absorbing light, thus darkening a distant image. Small particles can absorb light and scatter it (obscuring the image) and they can cause interfering light to be introduced to an image (adding a whitish appearance). Gases can cause light to scatter, adding or subtracting colors to a view of an image. Source: Malm, 2000
Our national parks and wilderness areas are places that possess many stunning vistas and scenery. Unfortunately, these scenes are diminished by haze, causing discoloration and loss of texture and visual range. Recognizing the importance of visual air quality, Congress included legislation in the 1977 Clean Air Act to prevent future degradation and remedy existing visibility impairment in Class I areas. Class I areas are defined as areas of special national or regional value from a natural, scenic, recreational or historic perspective
In 1999 the US Environmental Protection Agency announced a major effort to improve air quality in national parks and wilderness areas. The Regional Haze Rule was established, which calls for state and federal agencies to work together to improve visibility in 156 national parks and wilderness areas such as the Grand Canyon, Yosemite, the Great Smokies and Shenandoah. In New Hampshire, this includes the Great Gulf Wilderness and the Presidential – Dry River Wilderness, both of which surround Mt. Washington. The rule requires the states, in coordination with EPA, the National Park Service, US Fish and Wildlife Service, the US Forest Service, and other interested parties, to develop and implement air quality protection plans to reduce the pollution that causes visibility impairment. The first State plans for regional haze call for visibility improvements to occur around the year 2018 timeframe. Five multi-state regional planning organizations are working together to develop the technical basis for these plans. New Hampshire is a member of the Mid-Atlantic Northeast Visibility Union (MANE-VU).
Regional Planning Organizations (RPOs)
The pollutants primarily responsible for fine particle formation, and thus contributing to regional haze, include sulfur dioxide (SO2), oxides of nitrogen (NOX), volatile organic compounds (VOCs), ammonia (NH3), particulate matter less than 10 microns (PM10), and particulate matter less than 2.5 microns (PM2.5). Due to atmospheric chemistry, SO2 emissions convert to sulfate, which is the most important single constituent of haze-forming fine particle pollution and the principle cause of visibility impairment across the Northeast region. Sulfate alone accounts for anywhere from ½ to ⅔ of total fine particle mass on the 20 percent haziest days in New Hampshire. This translates to about ⅔ to ¾ of visibility impairment on those days. Organic carbon was shown to be a distant second largest contributor to haze. As a result of the dominant role of sulfate in the formation of regional haze in the Northeast and Mid-Atlantic Regions, MANE-VU concluded that an effective emissions management approach would rely heavily on broad-based regional SO2 control measures in the eastern United States.
Relative Importance of Key Pollutants in Haze Production on Worst Days
Visibility extinction is a measure of the ability of particles to scatter and absorb light. Extinction is expressed in units of inverse mega-meters (Mm-1). The figure above shows the dominance of sulfate in visibility extinction calculated from 2000-2004 baseline data for seven Northeast Class I Areas.
Visibility Trends at Great Gulf Wilderness
Over the past ten years, there has been a small improvement in visibility in New Hampshire. This improvement is due to several ongoing air pollution reduction programs, especially the Acid Raid program. In the chart above, the solid blue line represents the current visibility on the haziest days and the solid red line represents current conditions on the best visibility days. The dotted lines represent visibility targets for the year 2018. Visibility is reported in the units of deciviews, a logarithmic measure of light scattering and extinction.