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# 7. Light pollution

Artificial lighting at earth contributes via tropospheric scattering to the night sky brightness over a large area around the source of light. Both a continuous component as well as distinct emission lines are present in the light pollution spectrum. A recent review of sky pollution is given in McNally (1994).

## 7.1. Observations of sky pollution

Figure 31: Variation with city population of the distance at which the lights of a city produce an artificial increase of the night sky brightness at 45 deg altitude toward the city by 0.20 mag. This increase refers to an assumed natural sky brightness of V = 21.9 mag/''. Observations by Walker (1977) are indicated by dots. Two models by Garstang (1986) are shown as solid lines. K is a measure for the relative importance of aerosols for scattering light. The uppermost dot refers to Los Angeles County, the cross below it to Los Angeles City. From Garstang (1986)

Figure 32: Variation with distance from the city of the sky brightness at 45 deg altitude in the direction of the city. The dots indicate observations in V band by Walker (1977) near the city of Salinas. The solid curves are according to models by Garstang (1986). The brightness ratio is defined as , where b = sky brightness. Zenith distance +45 is towards and away from the city. The solid curves are according to models by Garstang (1986). Curve 1: L0 = 986 lumens per head, K =0.43, . Curve 2: L0 = 1000 lumens per head, K =0.5, . L0 is the artificial lighting in lumens produced per head of the population. K is a measure for the relative importance of aerosols for scattering light. F: a fraction F of the light produced by the city is radiated directly into the sky at angles above the horizontal plane, and the remainder (1-F) is radiated toward the ground. The dashed line is the relation . From Garstang (1986)

Figure 33: Zenith distance dependence of sky pollution light according to the model calculations of Garstang (1986). Results are for sky pollution due to Denver as seen from a distance of 40 km in the vertical plane containing the observer and the center of Denver. Curve 1: sky background; Curve 2: Denver only; Curve 3: Denver and sky background. Negative zenith distances are away from Denver. From Garstang (1986)

## 7.2. Modelling of sky pollution

Treanor (1973) and Bertiau et al. (1973) have used an empirical formula, based on a simplified model of the tropospheric scattering, to fit the sky pollution observations near cities. Garstang (1986, 1989a,b, 1991) has used radiative transfer models including 1st and 2nd order Rayleigh and aerosol scattering, effects of ground albedo and curvature of the earth's surface, and the areal distribution of the light source to calculate the sky pollution light intensity. He has compared and scaled his model results against the above mentioned observational results. Garstang's fitted models are shown in Figs. 31 (click here) and 32 (click here). superimposed on the observational points of Walker (1977). Garstang (1986, 1989a,b) gives also the calculated zenith distance dependence of the sky pollution light intensity both towards and away from the source of light. These results are reproduced in Fig. 33 (click here).

## 7.3. Spectrum of the sky pollution light

The emission line spectra of the different types of street lamps are visible in the night sky light even at good observatory sites, such as Kitt Peak in Arizona. While the most commonly used street lamps until the 1970's were filled with Hg there has been since then a general change over to sodium lamps, both of the high pressure (HPS) and low pressure sodium (LPS) types. The most important sky pollution lines are given in Table 14 (click here) according to Osterbrock et al. (1976), Osterbrock & Martel (1992) and Massey et al. (1990). At good sites (e.g. Kitt Peak), the strongest pollution lines are about a factor of two weaker than the strongest airglow lines. The opposite is true for strongly contaminated sites (e.g. Mt Hamilton). Whereas the pollution lines are normally restricted to a relatively narrow wavelenth range the Na D line wings produced by the HPS lamps are extremely broad, extending over . Thus the LPS lamps are highly preferable over the HPS ones from the astronomer's point of view.

Other studies of the night sky spectrum, including the artificial pollution lines, have been presented by Broadfoot & Kendall (1968) for Kitt Peak, Turnrose (1974) for Mt. Palomar and Mt. Wilson, and Louistisserand et al. (1987) for Pic du Midi.

 Line Sources Hg I 3650 Hg lighting Hg I 3663 Hg lighting Hg I 4047 Hg lighting Hg I 4078 Hg lighting Hg I 4358 Hg lighting Na I 4665, 4669 HPS Na I 4748, 4752 HPS Na I 4978, 4983 HPS Na I 5149, 5153 HPS Hg I 5461 Hg lighting Na I 5683, 5688 HPS, LPS Hg I 5770 Hg lighting Hg I 5791 Hg lighting Na I 5890, 5896 HPS, LPS, airglow Na I 5700 - 6100 HPS (wings) Na I 6154, 6161 HPS, LPS K I 7665 HPS, LPS K I 7699 HPS, LPS Na I 8183 HPS, LPS Na I 8195 HPS, LPS
Table 14: The strongest artficial emission lines in the night sky spectrum between . The most intense features are shown in boldface

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