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A Simplified Model of All-Sky Artificial Sky Glow Derived from VIIRS Day

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A Simplified Model of All-Sky Artificial Sky Glow Derived from VIIRS Day Journal of Quantitative Spectroscopy & Radiative Transfer 214 (2018) 133–145 Contents lists available at ScienceDirect Journal of Quantitative Spectroscopy & Radiative Transfer journal homepage: www.elsevier.com/locate/jqsrt A simplified model of all-sky artificial sky glow derived from VIIRS Day/Night band data ∗ Dan M. Duriscoe a, Sharolyn J. Anderson b, , Christian B. Luginbuhl c, Kimberly E. Baugh d a U.S. National Park Service Night Skies Program (retired), Big Pine, CA, USA b National Park Service, Natural Sounds and Night Skies, Fort Collins, CO, USA c Dark Sky Partners LLC, Tucson, AZ, USA d University of Colorado- CIRES, Boulder, CO, USA a r t i c l e i n f o a b s t r a c t Article history: We present a simplified method using geographic analysis tools to predict the average artificial luminance Received 15 November 2017 over the hemisphere of the night sky, expressed as a ratio to the natural condition. The VIIRS Day/Night Revised 25 April 2018 Band upward radiance data from the Suomi NPP orbiting satellite was used for input to the model. The Accepted 25 April 2018 method is based upon a relation between sky glow brightness and the distance from the observer to the Available online 26 April 2018 source of upward radiance. This relationship was developed using a Garstang radiative transfer model Keywords: with Day/Night Band data as input, then refined and calibrated with ground-based all-sky V-band photo- Night sky brightness metric data taken under cloudless and low atmospheric aerosol conditions. An excellent correlation was Light pollution found between observed sky quality and the predicted values from the remotely sensed data. Thematic Landscape scale assessments maps of large regions of the earth showing predicted artificial V-band sky brightness may be quickly gen- erated with modest computing resources. We have found a fast and accurate method based on previous work to model all-sky quality. We provide limitations to this method. The proposed model meets require- ments needed by decision makers and land managers of an easy to interpret and understand metric of sky quality. Published by Elsevier Ltd. This is an open access article under the CC BY license. ( http://creativecommons.org/licenses/by/4.0/ ) 1. Introduction and previous work velopment of this tool to advance conservation of scenic resources and the quality of visitor experience. Sky glow from escaped anthropogenic light degrades visual night sky quality. Sky glow results from reflection and scattering of stray light by molecules and aerosols in the atmosphere. Mea- 1.1. Measurements of artificial sky glow surements and models of this phenomenon are useful in evaluat- ing the effect of sky glow on the visual nightscape. This paper de- Astronomical observations of background night sky brightness velops a method to predict sky glow luminance using Geographic from large observatory telescopes provide one measure of night Information System (GIS) analysis applied to readily available re- sky quality. These observations are nearly always of an area near motely sensed upward radiance measures. The result, distance- the zenith or the darkest part of the night sky, e.g. [1,2] . Zenith weighted neighborhood sums of upward radiance around each ob- sky brightness has also been measured by citizen science projects servation point, have been calibrated with contemporaneous sky and researchers using low cost devices such as the Unihedron SQM brightness measurements of artificial sky glow. The model works and SQM-L. [3–5] . Zenith sky brightness, or zenith luminance (ZL), best in protected areas distant from major cities and other artificial is important to astronomical observations and is an informative in- light sources. It provides an accessible tool to evaluate the amount dicator of sky quality in moderate to severely light polluted areas. of sky glow and the quality of the night sky on large landscape The U.S. National Park Service (NPS) and natural resource managers scales. The U. S. National Park Service has participated in the de- characterize the condition of the entire night sky using average sky luminance (or ASL) [7] . This is a more sensitive measure of sky glow because light domes are more intense near the horizon than near the zenith. It is a more relevant measure of sky quality be- ∗ Corresponding author. cause wildlife and park visitors are more likely to have the horizon E-mail address: [email protected] (S.J. Anderson). in view than the zenith. https://doi.org/10.1016/j.jqsrt.2018.04.028 0022-4073/Published by Elsevier Ltd. This is an open access article under the CC BY license. ( http://creativecommons.org/licenses/by/4.0/ ) 134 D.M. Duriscoe et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 214 (2018) 133–145 Accurate measurements of artificial sky glow in areas distant Berry [11] , and translated into a computer model by Pike [12] in from the sources, may only be obtained when the natural light the 1970 s. These models utilized human population as a surrogate from the night sky is removed from luminance observations, since for the amount of escaped outdoor light produced by a city. They the observed sky brightness is the combination of artificial and also included the distance from the observer to the city and a mea- natural sources. For this reason the NPS developed a method of sure of the aerosol content of the air as input parameters. In the pixel-by-pixel subtraction of a natural sky model from photomet- late 1970 s, Walker [13] developed a brightness/distance relation- rically calibrated all sky image mosaics [6] . A similar method has ship from a source of fixed escaped light from observations of sky been used to improve zenith brightness measures with the SQM-L brightness at progressively greater distances from the same city. A [7] . Values of 171 μcd m −2 and 250 μcd m −2 have been proposed simple function where the distance is raised to power −2.5 was as the reference conditions for ZL and ASL, respectively, in the ab- found to fit the data, the relationship is known as "Walker’s Law." sence of artificial sky glow [8] . These values provide a baseline This relationship was expanded by Albers and Duriscoe [14] who level for expressing the scale of sky glow. For many audiences, the derived a constant to convert from population to predicted artifi- condition of the night sky is more expressively rendered in terms cial sky brightness for cities. Netzel and Netzel [15] used high res- of a unitless ratio to natural conditions. The ratio indicators uti- olution daytime satellite imagery classified as "percent of built up lized in this paper are termed all-sky average light pollution ratio area" as a surrogate for escaped artificial light and Berry’s empiri- (ALR) and zenith light pollution ratio (ZLR). Numeric indicators of cal model to produce an estimate of sky brightness at zenith over artificial sky brightness were discussed in detail by Duriscoe [8] , the country of Poland at 100 m resolution with modest computer including their derivation from all sky photometric measurements resources using GIS tools. The model was calibrated with observa- and application to night sky quality assessments. tional sky brightness data from 65 sites. These empirical models To summarize the important sky brightness measures used in paved the way to constructing landscape scale models of the sky this paper, for the hemisphere of sky with azimuth angle ϕ, zenith brightness at zenith. −2 angle z , and punctual luminance B ϕ,z in μcd m : zenith lumi- nance: 1.2.2. Physical models A complex physical model of atmospheric scattering of light ZL = B ϕ, (1) 0 from ground based sources was described by Garstang [16–18] . The anthropogenic zenith luminance: model predicts the punctual sky brightness as seen by an observer on earth at any given point in the sky from any number of sources. Z L = Z L − Z L (2) a n Garstang’s radiative transfer model was adapted to produce the where ZL n = natural sky background brightness at zenith, reference first World Atlas of Artificial Night Sky Brightness in 2001 by Cin- condition set to 171 μcd m −2 zenith light pollution ratio: zano et al. [19] . Covering most of the land area of earth and using upward radiance data derived from DMSP-OLS satellite imagery as ZLR = Z L / 171 (3) a input [20] , the atlas is the pioneering landscape-scale assessment average sky luminance: of night sky quality based upon a physical model. The New World π/ π Atlas of Artificial Night Sky Brightness [7] utilized VIIRS Day/Night 2 2 1 ASL = B ϕ,z sin ( z )d ϕ d z (4) band upward radiance data [21] from 2014. The atlas used an en- π 2 0 0 hanced version of the Garstang model [22] was calibrated to thou- anthropogenic average sky luminance: sands of ground based observations over a several year period. π/ π Both atlases expressed artificial sky brightness as a ratio to a natu- 2 2 1 2 2 AS L a = ( B − B n )ϕ , sin ( z )d ϕ d z (5) ral background condition set at 251 μcd/m and 174 μcd/m in the π z 2 0 0 "first" and "new" editions, respectively. Both required substantial where B n = natural sky background brightness at φ, z computational resources and the assumption of a constant concen- With the reference condition for the all-sky average set to 250 tration of aerosols over the entire earth. μcd m −2 , all-sky light pollution ratio: Physical models have been used to refine the Walker’s Law neg- ative exponential zenith brightness/distance relationship.
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