Twelve of the fourteen PRIMENet parks are or have been part of the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. Acadia, Big Bend, Canyonlands, Denali, Everglades, Glacier, Great Smoky Mountains, Rocky Mountain, and Shenandoah NPs have monitored visibility according to the IMPROVE protocol since the network’s inception in March 1988. Sequoia/Kings Canyon NP initiated visibility monitoring in September 1992, while visibility data collection at Hawaii Volcanoes NP occurred only between March 1988 and April 1993. Virgin Islands NP has measured fine mass with a simplified IMPROVE fine particle monitor since October 1990, and became a fully complemented site in 1997. Olympic and Theodore Roosevelt NPs are scheduled to begin visibility monitoring by year 2000. 

IMPROVE Particle Monitor Site Information. Data available at ftp://alta_vista.cira.colostate.edu.

        A fully complemented IMPROVE protocol visibility site employs three types of monitors—photographic, optical, and aerosol. Photographic monitoring documents the condition of a scene in a park several times a day using a 35mm camera. Optical monitoring directly measures the light extinction coefficient with transmissometers or the light scattering coefficient with nephelometers. The light extinction coefficient is a measure of the attenuation of light per unit distance caused by the scattering and absorption of gases and particles in the atmosphere. The scattering coefficient has a similar definition except absorption is not included. Aerosol monitoring is based on fine (PM-2.5) and coarse (PM-10) particle sampling and sample analysis. The aerosol sampler uses four independent modules to collect four simultaneous samples: three PM-2.5 samples on Teflon, nylon and quartz filters, and one PM-10 sample on a Teflon filter. The PM-2.5 filters are analyzed for mass, chemical elements, ions, organics and elemental carbon, and optical absorption. The PM-10 filter is analyzed for mass only. The concentrations of the various aerosol constituents are used to estimate their contributions to the light extinction coefficient. These "reconstructed" extinctions are briefly summarized below. IMPROVE aerosol and optical data sets are available at ftp://alta_vista.cira.colostate.edu.

        The 1991-1997 average reconstructed extinction at eleven PRIMENet sites is plotted in the following graph. (The mean extinction for Hawaii Volcanoes and Sequoia/Kings Canyon NPs is computed based on March 1988-February 1992, and January 1993-December 1997 data, respectively). Virgin Islands NP extinction is not plotted because only one of the four IMPROVE particle sampling modules was employed up until 1997, making an accurate estimate of extinction difficult. 

        The total reconstructed extinctions at these eleven sites vary from 20 to 120 inverse megameters (Mm^-1), a factor of six difference. These extinction values correspond to standard visual ranges of 186 km for Denali NP and 33 km for Shenandoah NP. Denali NP fine particle concentrations and total extinction are generally the lowest of all those observed in the IMPROVE network. Rocky Mountain and Canyonlands NPs have low total reconstructed extinctions of about 26 Mm^-1 (150 km visual range). The PRIMENet IMPROVE sites exhibit the well-documented, strong spatial gradient in visibility between eastern and western U.S. monitoring stations.

        The light extinction (and visibility reduction) at IMPROVE sites is typically explained by the following atmospheric constituents: fine particles of sulfates, nitrates, organic carbon, light absorbing carbon (soot), and soil, coarse particles, and atmospheric gas molecules like O₂ and N₂ which scatter light (Rayleigh scattering). Rayleigh scattering does vary somewhat with elevation but often it is assigned a constant contribution to extinction of 10 Mm^-1. Figure 2 illustrates that Rayleigh scattering accounts for a greater percentage of the total extinction at the cleaner sites like Denali and Canyonlands NPs than at lower visibility eastern sites like Acadia, Everglades, Great Smoky Mountains, and Shenandoah NPs.

        In the east, sulfates are usually the greatest contributor to extinction and visibility impairment. Sulfates contribute 57 to 69 percent of the total extinction in the four eastern sites displayed in Figure 2. At Denali, Rocky Mountain, and Canyonlands NPs sulfates account for less than one fourth of the total extinction. Sulfate extinction at Shenandoah NP (83.8 Mm^-1) is nearly fifteen times greater than that at Rocky Mountain NP (5.7 Mm^-1). Sulfate particles are typically formed in the atmosphere from the conversion of sulfur dioxide gas emitted from large anthropogenic sources such as fossil-fuel fired power plants. However, the monitor at Hawaii Volcanoes NP undoubtedly recorded the influence of the nearby large natural sources of volcanic sulfur emissions.

 

        The next largest chemical contributors to visibility impairment at most PRIMENet sites are organic carbon and light absorbing carbon (soot) which have their origins in vegetative burning and urban emissions. Nitrates present a smaller contribution to extinction at all PRIMENet sites with the exception of Sequoia/Kings Canyon NP where the sulfate, nitrate and Rayleigh extinctions are all each on the average about 10 Mm^-1. Nitrates are often formed in the atmosphere from precursor gaseous nitrogen oxides emitted from industrial and urban sources. Fine soil and larger coarse particles have a relatively large contribution to the extinction budget at Sequoia/Kings Canyon NP. This soil and coarse particle contribution is about two to six times larger than the corresponding ones measured at the other PRIMENet sites. Soil and coarse particles can result from unpaved roads, wind blown dust, and industrial sources.

        Sisler and Malm investigated trends in annual reconstructed visibility and fine particles at IMPROVE monitoring sites over the nine-year period 1988-1996. The 30 IMPROVE sites analyzed included eight of the following PRIMENet parks: Acadia, Big Bend, Canyonlands, Denali, Glacier, Great Smoky Mountains, Rocky Mountain, and Shenandoah NPs. For the worst visibility days, their analysis indicated a statistically significant (p<0.10) improving trend in reconstructed visibility at Canyonlands, Denali, and Glacier NPs and statistically insignificant changes at the other five PRIMENet IMPROVE sites. With respect to the average visibility days, Acadia, Canyonlands, Denali, Glacier, Rocky Mountain, and Shenandoah NPs showed statistically significant (p<0.05 or p<0.10) improving reconstructed visibility trends. No statistically significant trend was discernable at Big Bend and Great Smoky Mountains NPs. Finally, the trend analysis for the best visibility days indicated statistically significant (p<0.05) improvements in reconstructed visibility at Acadia, Canyonlands, Denali and Rocky Mountain NPs. Changes at the other four parks were judged to be statistically insignificant.

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