Recovering the city street lighting fraction from skyglow measurements in a large-scale municipal dimming experiment
Introduction
Light pollution is a global phenomenon caused by the prolific use of artificial light at night (ALAN) [1], [2]. ALAN offers clear benefits to human society by ensuring safe transit at night, enabling the nighttime economy, and enhancing public perception of outdoor spaces at night through placemaking, [3] but its use entails a number of known and suspected hazards to the natural nocturnal environment, e.g., [4], [5], [6], including potentially significant disruption of ecosystem services [7], [8], [9] and threats to biodiversity [10], [11]. On the other hand, actively preserving natural nighttime darkness not only appears to convey environmental benefits, but also can support sustainable rural economic development through ‘astrotourism.’ [12], [13], [14]
Given the negative environmental influence of ALAN, activists have called for addressing the problem through various lighting engineering and public policy means. Achieving these goals requires identifying best practices for reducing light emissions to levels strictly necessary to ensure public safety. The efficacy of these practices is in part determined by the participation rate of owners of light sources: the more sources whose output is reduced or eliminated presumably results in a greater reduction in the amount of ALAN in the nighttime environment. Public lighting is an attractive target for reduction efforts, especially in urban settings, given that many thousands of luminaires are often under the control of a single administrative entity.
Limited evidence exists to date suggesting that modifications to existing municipal lighting systems, in particular, can yield measurable environmental effects [15], [16]. However, current models are acutely deficient in the sense that the fraction of total light emissions comprised by municipal lighting in cities is not well constrained. While some cities have created inventories of publicly owned lighting, including light emission parameters of luminaires, a complete accounting of privately owned lighting is available only for the smallest municipalities. Currently available remote sensing data do not have sufficient spatial resolution to reliably determine the ownership status of lighting, although one approach to solving that problem is to make an initial guess of the function of outdoor lighting by matching the distribution of remotely sensed night lights to maps of land use patterns [17]. However, even high-resolution aerial imagery of cities is not presently subject to analysis techniques that can reliably discern between lighting types.
In order to better inform models, there is a need to determine the relative contributions of public and private sources to the total light emissions of cities. Reducing the influence of light pollution in cities may be well served by focusing on public lighting; however, if public lighting were a small fraction of total lighting, then the impact of such efforts would be proportionately less. Determining the fraction of total lighting attributable to publicly owned sources in cities is therefore important to guide decisions on how best to direct mitigation efforts.
Previous attempts to determine the relative contributions of public and private lighting in cities have found public lighting fractions ranging from % of total urban light emissions [16], [17], [18], [19], [20]. Several methods have been employed to determine these values, most of which rely on certain model assumptions. Perhaps the most robust approach to distinguish public lighting from private lighting is to change the output of only public light sources in some known way and measure by how much the total city light emission changes. This is relatively easy to arrange for smaller villages (e.g., [21]), but it is difficult for large cities. We arranged a test with the municipal government of Tucson, Arizona, U.S., that involved dimming the municipal street lighting system by certain amounts on a series of test nights. By controlling the output of street lights, we recovered the fraction of the total light emission of Tucson represented by the street lights through a combination of measurements of the brightness of the night sky and modeling skyglow with a radiative transfer code.
This paper is organized as follows. First, in Section 2, we outline the parameters of the street lighting dimming experiment and the intended goals of the project. Next, in Section 3, we describe the measurements made during each of the test nights. Then, in Section 4, we present the results of radiative transfer model runs used to predict skyglow changes over the city consequent to the dimming tests. We analyze the observations in the context of the model results in Section 5, and finally we summarize our work, point out its limitations, and offer suggestions to guide future experiments of this nature in Section 6.
Section snippets
Lighting system and test parameters
Tucson is a city of 535,000 inhabitants; the population of its metropolitan statistical area (MSA) is about one million. The Tucson municipality owns and manages approximately 20,000 roadway luminaires distributed across 587 square kilometers of its incorporated territory (Fig. 1); roadway luminaires operated by other municipalities in the MSA did not take part in the tests reported here. The inventory of luminaires consists of over 90% white LED products with a nominal correlated color
Skyglow measurements
We made a series of measurements of the brightness of the night sky over and adjacent to Tucson during the dimming tests in order to quantify the degree to which the sky brightness changed as a result of the different street light dimming configurations.
Skyglow simulator predictions
To model the influence of the municipal street lighting system in the different dimming configurations, we carried out model runs with SkyGlow Simulator (version v.5c)4 based on theory developed by Kocifaj [32]. Given the assumed linear relationship between the number of lumens emitted by the municipal lighting system and the brightness of the night sky at the zenith, and using the known experimental decrease in the zenith
Analysis and discussion
We hypothesized the following in terms of the expected results:
- 1.
The relative decrease in the zenith brightness on the 30% dimming nights should be considerably lower than the ~ 10% change observed previously as a consequence of the LED lighting conversion in 2016–17 [16]. The reason for this is that the fraction of lumens emitted by the LED luminaires after conversion is substantially lower than that emitted by the legacy (mostly) HPS lighting system. Reducing the output from a smaller starting
Summary and conclusions
We made photometric measurements of the brightness of the night sky over Tucson, Arizona, U.S., during a series of nights in March and April 2019 during which the municipal street lighting system was dimmed to a set of non-standard configurations. The experiment was designed to sense the dimming signal in the skyglow over the city, and the resulting measurements were compared against a radiative transfer model of skyglow to recover the fraction of total city light emissions specifically
CRediT authorship contribution statement
John C. Barentine: Project administration, Conceptualization, Methodology, Data curation, Formal analysis, Writing - original draft, Writing - review & editing. František Kundracik: Software, Methodology, Resources. Miroslav Kocifaj: Software, Resources. Jessie C. Sanders: Resources. Gilbert A. Esquerdo: Investigation. Adam M. Dalton: Investigation. Bettymaya Foott: Investigation. Albert Grauer: Investigation. Scott Tucker: Investigation. Christopher C.M. Kyba: Conceptualization, Methodology,
Declaration of Competing Interest
The authors declare that they do not have any financial or nonfinancial conflict of interests
Acknowledgments
The authors wish to thank the municipal government of the City of Tucson for participating in the experiment described here. They also recognize the valuable insights and comments of two anonymous reviewers whose suggestions helped sharpen the presentation.
Funding: FK and MK acknowledge support from the Slovak Research and Development Agency under contract number APVV-18-0014. CK acknowledges funding from the Helmholtz Association Initiative and Networking Fund under grant ERC-RA-0031, as well
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