John V. Molenar

Examining the relationship among atmospheric aerosols and light scattering and extinction in the Grand Canyon area

During the winter and summer months of 1990 a special study called Project MOHAVE (measurement of haze and visual effects) was carried out with the principle objective of attributing aerosol species to extinction and scattering and the aerosol species to sources and/or source regions. The study area included much of southern California and Nevada, Arizona, and Utah; however, the intensive monitoring sites and primary focus of the study was on the Colorado Plateau of northern Arizona, southern Nevada, and Utah. This paper reports on the apportionment of various aerosol species to measured fine and coarse mass concentrations and these species to scattering and extinction. The study is unique in that a number of “ambient” integrating nephelometers were operated to measure the ambient scattering coefficient, while transmissometers were used to measure atmospheric extinction. Comparison of measured scattering, extinction, and aerosol species concentration, both statistically and theoretically, allows for an estimate of scattering and absorption efficiencies. Analysis suggests that using elemental carbon, derived from thermal optical techniques, to estimate absorption may significantly underestimate absorption. Using elemental carbon, absorption is estimated to be 5% of extinction, while direct measurements of absorption suggest that it is about 30% of measured extinction. Furthermore, because light absorption by soil is usually not accounted for, soil extinction is underestimated by about 30%.

Human perception of visual air quality (uniform haze)

Abstract The National Park Service and the U.S. Environmental Protection Agency are cooperatively conducting ongoing studies of human perception of visual air quality. Major objectives of this program include: (1) determination of the relationship between judgments of visual air quality of actual three dimensional scenes and a surrogate slide representation of that scene, (2) examination of the effect of sun angle and meteorological conditions on perceived visual air quality, (3) examination of the effect of demographic background on observers judgments of visual air quality, (4) establishment of a functional relationship between human perception of visual air quality and various electro-optical parameters for several different scenic vistas and (5) development of a model capable of predicting the sensitivity of a park to visual air pollution impact. Preliminary results of a previous study involving one vista revealed a linear relationship between human perception and apparent vista contrast for constant vista illumination and ground cover. A more general formalism for averaging vista color contrast appeared to account for effects that snow cover and varying illumination have on the sensitivity of perceived visual air quality to air pollution. These functional relationships are re-examined using a number of southwestern vistas. A first order model capable of predicting perceived visual air quality as a function of change in air pollution is developed. In addition, the relationship between perceived visual air quality of actual three dimensional scenes and pictoral surrogates is examined.

Human perception of visual air quality

The characterization of visibility should involve a selection of physical variables that (1) correlate with human perception, and (2) can be directly measured with appropriate instrumentation. To address these questions, the National Park Service in cooperation with US Environmental Protection Agency formulated a field program that involved a visitor survey. Visitors to Canyonlands National Park were asked to rate slides, on a scale of 1 to 10, that represented various levels of visual air quality of a pre-selected vista. In addition visitors rated on the same scale the visual air quality on the day that they visited the park. Objectives of the study included (1) evaluation of existing visibility indicators, such as visual range, contrast, etc. in their ability to characterize visual air quality, (2) formulate new physical variables that are better suited for characterization of visibility, and (3) establish the functional relationship between new and old physical variables and human perception.

Visual air quality simulation techniques

Abstract Visual air quality is primarily a human perceptual phenomenon beginning with the transfer of image-forming information through an illuminated, scattering and absorbing atmosphere. Visibility, especially the visual appearance of industrial emissions or the degradation of a scenic view, is the principal atmospheric characteristic through which humans perceive air pollution, and is more sensitive to changing pollution levels than any other air pollution effect. Every attempt to quantify economic costs and benefits of air pollution has indicated that good visibility is a highly valued and desired environmental condition. Measurement programs can at best approximate the state of the ambient atmosphere at a few points in a scenic vista viewed by an observer. To fully understand the visual effect of various changes in the concentration and distribution of optically important atmospheric pollutants requires the use of aerosol and radiative transfer models. Communication of the output of these models to scientists, decision makers and the public is best done by applying modern image-processing systems to generate synthetic images representing the modeled air quality conditions. This combination of modeling techniques has been under development for the past 15 yr. Initially, visual air quality simulations were limited by a lack of computational power to simplified models depicting Gaussian plumes or uniform haze conditions. Recent explosive growth in low cost, high powered computer technology has allowed the development of sophisticated aerosol and radiative transfer models that incorporate realistic terrain, multiple scattering, non-uniform illumination, varying spatial distribution, concentration and optical properties of atmospheric constituents, and relative humidity effects on aerosol scattering properties. This paper discusses these improved models and image-processing techniques in detail. Results addressing uniform and non-uniform layered haze conditions in both urban and remote pristine areas will be presented.

On the potential of regional-scale emissions zoning as an air quality management tool for the grand canyon

Abstract Air arriving at the Grand Canyon of the Colorado River during 1988–1989 is attributed to one of four geographic quadrants—NE, SE, SW, NW—on the basis of routinely calculated back-trajectories. Most of the haze observed at the Canyon is attributed to the SW quadrant, which contains the populous and industrialized areas of southern California. Air from either northern quadrant tends to be significantly clearer than air from either southern quadrant. Clear northern air is most common during the winter, and is rarely observed during the summer tourist season, when steady flow from the southwest is the norm. Various possible interpretations of these empirical results are discussed, with varying implications for emissions management policy.

Visibility Measurements lo National Parks In the Western United States

The Nationl Park Service (NPS)in cooperation with the Environmental Protection Agency, Environmental Monitoring Systems Laboratory, has operated a regional (western US) visibility monitoring network since the fall of 1978. At each of the 27 monitoring sites NPS personnel record sky-target contrast three times a day using a multiwavelength teleradiometer. After each measurement, operators also photograph the vista to pictorially record the appearance of the target when the measurement was made. This paper discusses teleradiometer data reduction and editing techniques as well as temporal and spatial visibility trends in the western US. Throughout the West, winter months correspond to highest visibility, while summer months have the lowest overall air quality. Visibilities which were highest in 1978 decreased until 1981; 1982 showed a slight increase in visibility. Examination of visibilities throughout the West in summer of 1982 shows that visibilities are highest in northern Nevada, northern Utah and southern Idaho. The Colorado Plateau, the geographic area containing Grand Canyon, Bryce, Zion and Canyonlands National Parks as well as a number of national monuments, has the second highest visibility. Other areas of the west have intermediate visibilities with the coastal areas of California having the lowest visual air quality.

Long-path transmissometer for measuring ambient atmospheric extinction

With the passage of the 1977 Clean Air Act Amendments, the United States Congress emphasized visibility protection for the pristine areas of the country as a major national goal. Federal programs were implemented to estimate existing levels of atmospheric extinction. Until recently, this has been accomplished primarily by the use of various techniques to measure apparent target/horizon sky contrast. While the ability of these methods to accurately determine contrast is well proven, the estimation of extinction using the Koschmieder relationship is usually compromised by theoretical assumptions that are typically not met in practice. To address this problem, the United States National Park Service has supported the development, testing, and deployment of an instrument that accurately and reliably measures the transmission of an ambient atmospheric optical path. The average atmospheric extinction coefficient of the path can be calculated directly from this measurement. This transmissometer consists of a constant output light source transmitter and a computer-controlled photometer receiver. The irradiance at visible wavelengths from the transmitter can be measured to a high degree of accuracy both day and night over path lengths of 0.5 to 30km. A complete description of the transmissometer will be presented. Atmospheric optical considerations will be combined with design parameters and experimental data to confirm the ability of this instrument to make an accurate, precise, and reliable extinction measurement.

Statistical comparisons between teleradiometer-derived and slide-derived visibility parameters

An automated scanning densitometer system developed to estimate visibility-related parameters from photographic slides was tested in comparison to similar teleradiometric measurements. Parameters such as target-sky contrast, visual range, atmospheric light extinction coefficient, and plume and layered contrast can be estimated using this system; however, only target-sky radiance ratios and standard visual ranges were compared. More than 1600 concurrent densitometer and teleradiometer data pairs from eight National Park Service air quality and visibility monitoring locations in the western United States were analyzed using the techniques of correlation, linear regression, average bias and difference calculation, and cumulative frequency distribution generation. Correlation coefficients were good, especially with middle-range camera vistas around 50 km distant. Regression slopes approached unity with intercepts near zero. Average bias introduced into the determination of radiance ratios from slides ranged f...

Which visibility indicators best represent a population’s preference for a level of visual air quality?

ABSTRACT Several studies have been carried out over the past 20 or so years to assess the level of visual air quality that is judged to be acceptable in urban settings. Groups of individuals were shown slides or computer-projected scenes under a variety of haze conditions and asked to judge whether each image represented acceptable visual air quality. The goal was to assess the level of haziness found to be acceptable for purposes of setting an urban visibility regulatory standard. More recently, similar studies were carried out in Beijing, China, and the more pristine Grand Canyon National Park and Great Gulf Wilderness. The studies clearly showed that when preference ratings were compared to measures of atmospheric haze such as atmospheric extinction, visual range, or deciview (dv), there was not a single indicator that represented acceptable levels of visual air quality for the varied urban or more remote settings. For instance, using a Washington, D.C., setting, 50% of the observers rated the landscape feature as not having acceptable visual air quality at an extinction of 0.19 km−1 (21 km visual range, 29 dv), while the 50% acceptability point for a Denver, Colorado, setting was 0.075 km−1 (52 km visual range, 20 dv) and for the Grand Canyon it was 0.023 km−1 (170 km visual range, 7 dv). Over the past three or four decades, many scene-specific visibility indices have been put forth as potential indicators of visibility levels as perceived by human observers. They include, but are not limited to, color and achromatic contrast of single landscape features, average and equivalent contrast of the entire image, edge detection algorithms such as the Sobel index, and just-noticeable difference or change indexes. This paper explores various scene-specific visual air quality indices and examines their applicability for use in quantifying visibility preference levels and judgments of visual air quality. Implications: Visibility acceptability studies clearly show that visibility become more unacceptable as haze increases. However, there are large variations in the preference levels for different scenes when universal haze indicators, such as atmospheric extinction, are used. This variability is significantly reduced when the sky–landscape contrast of the more distant landscape features in the observed scene is used. Analysis suggest that about 50% of individuals would find the visibility unacceptable if at any time the more distant landscape features nearly disappear, that is, they are at the visual range. This common metric could form the basis for setting an urban visibility standard.