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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

  Systematic broad-band observations of the sky pollution light near cities have been carried out by Bertiau et al. (1973) in Italy, Berry (1976) in Canada & Walker (1970, 1977) in California. Berry showed that there is a relationship between the population of a city and the zenith sky brightness as observed in or near to the city. Walker interpreted his extensive observations by deriving the following relationships: (1) between the population and luminosity of a city; (2) the sky brightness as a function of distance from the city; and (3) between the population and the distance from a city for a given sky pollution light contribution. The last two relationships are shown in Figs. 31 (click here) and 32 (click here). These figures can be utilized to derive an estimate for the sky pollution at 45 deg altitude caused by a city with 2000 - 4 million population and with a similar street lighting power per head as California. Starting with the city population Fig. 31 (click here) gives the distance at which the artificial lighting contribution increases the natural sky brightness by 20% (0.2 mag/tex2html_wrap_inline11293''). With this distance one can enter Fig. 32 (click here) and obtain a scaling for the (arbitrary) intensity axis of this figure. Thus the artificially caused sky brightness at 45 deg altitude at tex2html_wrap_inline12621 distance from the city can be estimated from this figure.

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/tex2html_wrap_inline11293''. 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 tex2html_wrap_inline12633, where b = sky brightness. Zenith distance +45tex2html_wrap_inline11647 is towards and tex2html_wrap_inline12639 away from the city. The solid curves are according to models by Garstang (1986). Curve 1: L0 = 986 lumens per head, K =0.43, tex2html_wrap_inline12645. Curve 2: L0 = 1000 lumens per head, K =0.5, tex2html_wrap_inline12651. 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 tex2html_wrap_inline12663. 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 tex2html_wrap_inline12671. 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
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 tex2html_wrap_inline12611. The most intense features are shown in boldface

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