1 Light pollution and the concentration of anthropogenic photons in the terrestrial atmosphere

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Light pollution and the concentration of anthropogenic

photons in the terrestrial atmosphere

Salvador Bará1,*, Carmen Bao-Varela2, and Fabio Falchi2,3

1 A. Astronómica 'Ío', 15005 A Coruña, Galicia (Spain)

2 Photonics4Life Research Group, Applied Physics Department, Universidade de

Santiago de Compostela, Campus Vida, E-15782 Santiago de Compostela, Spain

3 Istituto di Scienza e Tecnologia dell’Inquinamento Luminoso (ISTIL), 36016 Thiene,

Italy

(*) Corresponding author. email: salva.bara@usc.gal

ORCID SB: https://orcid.org/0000-0003-1274-8043

ORCID CB-V: https://orcid.org/0000-0002-0602-800X

ORCID FF: https://orcid.org/0000-0002-3706-5639

Abstract

Light pollution can be rigorously described in terms of the volume concentration of

anthropogenic photons (light quanta) in the terrestrial atmosphere. This formulation,

consistent with the basic physics of the emission, scattering and absorption of light,

allows one to express light pollution levels in terms of particle volume concentrations,

in a completely analogous way as it is currently done with other classical pollutants, like

particulate matter or molecular contaminants. In this work we provide the explicit

conversion equations between the photon volume concentration and the traditional

light photometry quantities. This equivalent description of the light pollution levels

provides some relevant insights that help to identify artificial light at night as a standard

pollutant. It also enables a complementary way of expressing artificial light exposures

for environmental and public health research and regulatory purposes.

Keywords

Light pollution ; nocturnal environment ; radiometry ; photometry ; air pollution

CC-BY-NC-ND This is an author-formatted version of the accepted manuscript whose version of

record has been published in Atmospheric Pollution Research, 2022, 13(9):101541,

https://doi.org/10.1016/j.apr.2022.101541

1. Introduction

Artificial light at night is a key factor for the improvement of the living conditions of

humankind. Its production and control have provided us an essential level of freedom,

enabling many activities at nighttime that otherwise would be limited by the low

performance of the human visual system under typical natural night light levels. Artificial

light at night is also a longtime recognized air pollutant. Already in 1979, the UN

Convention on Long-range Transboundary Air Pollution, in its art. 1.a), established that

“"Air Pollution" means the introduction by man, directly or indirectly, of substances

or energy into the air resulting in deleterious effects of such a nature as to endanger

human health, harm living resources and ecosystems and material property and

impair or interfere with amenities and other legitimate uses of the environment,

and "air pollutants" shall be construed accordingly" (United Nations, 1996)

with an explicit and consistent interpretation by the UN International Law Commission

in the sense that, in the context of the protection of the atmosphere,

““Energy” is understood to include heat, light, noise and radioactivity introduced

and released into the atmosphere through human activities” (United Nations,

2018).

Artificial light at night has been shown to produce undesired disruptive effects on

wildlife (Longcore and Rich, 2004; Rich and Longcore, 2006; Hölker et al., 2010; Davies

et al., 2013; Svechkina et al., 2020; Gaston et al., 2021) as well as in relevant aspects of

human health (Davis et al., 2001; Haim and Portnov, 2013; Stevens et al., 2014;

Smolensky et al., 2015; García-Sáenz et al., 2018; Russart and Nelson, 2018; Boyce,

2022). These unwanted effects add to the progressive loss of the starry night sky

(Cinzano et al., 2001; Falchi et al., 2016; Bará, 2016), whose negative consequences for

the sustainability of the scientific activity of ground-based astronomical observatories

(Walker, 1970; Garstang, 1989; Green et al., 2022), and the preservation of humankind's

intangible cultural heritage (Marin and Jafari, 2008) have been noticed since longtime

ago. Light pollution levels increase at a steady rate worldwide (Kyba et al., 2017), and

their overall impact on the nocturnal environment is an issue of concern, as expressed

in a growing body of recommendations (see, e.g., International Astronomical Union,

1976; Americal Medical Association, 2012; European Union, 2018; IARC, 2019;

Convention on Migratory Species, 2020; International Union for Conservation of Nature,

2021; Brown et al., 2022).

Being inextricably linked to human visual experience, light has been traditionally

measured and described using specific visual photometric quantities based on the

candela (), one of the seven basic units of the international system, SI (BIPM, 2018).

Visual photometric quantities are related to their physical radiometric counterparts

through an internationally agreed set of well-defined conversions, mostly based on the

CIE  function, the photopic spectral sensitivity of the human visual system (CIE, 1926).

By applying these conversions, the spectral radiance of the electromagnetic field

(usually given in energy units    ) can be straightforwardly

transformed to visual luminance ().

The quantum nature of light is an essential feature of our universe, and the

emission, scattering and absorption processes of light are often more adequately

described in terms of discrete light quanta (photons) than in terms of continuous

distributions of energy (Einstein, 1905). The discrete formulation is particularly well

suited for environmental studies where light plays an essential role as the basic visual

input (Nilsson and Smolka, 2021; Nilsson et al, 2022), since the photochemical

interactions in the eye retina take place on a photon by photon basis, and also in those

cases where very low illumination levels reveal the granular nature of light. Accordingly,

the spectral radiance is also commonly specified using discrete photon numbers

(   ) based on the photon energy  

, where   is the exact value of the Planck

constant after the last SI reform (BIPM, 2018), and  is the frequency of light ().

The description of the environmental light exposure in terms of surface fluxes of

energy or particles is of course entirely correct and, when given with enough spectral

resolution and complementary information about the polarization state of the light, it

provides a complete description of the light polluting field as pertinent for

environmental and health sciences studies. However, expressing light pollution levels in

terms of fluxes through surfaces has helped to frame light pollution as a somehow

exceptional type of pollution, not clearly assimilable to the canonical air pollution forms

(e.g. particulate matter of different sizes, ground-level ozone, carbon monoxide, sulfur

dioxide, nitrogen dioxide, or lead), whose levels are normally expressed in terms of their

volume concentrations in the environment.

As a matter of fact, light pollution levels can be equivalenty expressed in terms of

the volume concentration of anthropogenic photons (in ). This

formulation gives the same quantitative results as the traditional

radiometric/photometric descriptions in terms of surface flux densities and helps to

highlight the fundamental similarity between light pollution and other pollution types.

The quantitative relations linking the surface flux and the volume concentration

formulations are explicited in this paper. Section 2 contains the main transformation

equations and a brief analysis of their most relevant features. In section 3 we provide

quantitative results of these transformations applied to two spectral bands of

environmental interest: the human photopic () and the Johnson-Cousins V (hereafter

). Discussion and conclusions are summarized in sections 4 and 5, respectively.

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1LightpollutionandtheconcentrationofanthropogenicphotonsintheterrestrialatmosphereSalvadorBará1,*,CarmenBao-Varela2,andFabioFalchi2,31A.Astronómica'Ío',15005ACoruña,Galicia(Spain)2Photonics4LifeResearchGroup,AppliedPhysicsDepartment,UniversidadedeSantiagodeCompostela,CampusVida,E-15782SantiagodeComp...

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