Air pollution mitigation can reduce the brightness of the night sky

Light pollution is a novel environmental problem whose extent and severity are rapidly increasing. Among other concerns, it threatens global biodiversity, nocturnal animal migration, and the integrity of the ground-based astronomy research enterprise. The most familiar manifestation of light pollution is skyglow, the result of the interplay of outdoor artificial light at night (ALAN) and atmospheric scattering that obscures views of naturally dark night skies. Interventions to reduce night sky brightness (NSB) involving the adoption of modern lighting technologies are expected to yield the greatest positive environmental consequences, but other aspects of the problem have not been fully explored as bases for public policies aimed at reducing light pollution.

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Fig: Angular distribution of scattered light intensity for light of wavelength (λ) 530 nm computed for a mixture of urban-dominated aerosol components (details are discussed in ). The number of scattered photons proportional to the differential scattering cross section is shown on y-axis relative to the number concentration of urban aerosols. (A) Black crosses in the left plot indicate results for the baseline conditions. The plot demonstrates that an increase of the particle size by 20% (solid blue triangles), 50% (solid green circles), and 100% (hollow red squares) over baseline while holding the number concentration constant enhances the scattered light intensity in all directions. (B) The range of backscattering features due to the varying complex refractive index (m=n+iκm=n+iκ). The baseline conditions are for a mean refractive index mm = 1.5 + 0.05i (solid black line with cross symbols). The scattered intensity drops when lowering nn to 1.4 (blue stars) and increases when nn is raised to 1.6 (green squares). Highly absorbing elements in the aerosol mixture flatten the scattering phase function at large angles (long dashed black line for κκ = 0.2), which is contrasted by the results for non-absorbing particles (short dashed black line forκκ = 0.0).

Here we show that reducing air pollution, specifically aerosols, decreases NSB by tens of percent at relatively small distances from light sources. Cleaner city air lowers aerosol optical depth and darkens night skies, particularly in directions toward light sources, due to relatively short path lengths traversed by photons from source to observer. A field experiment demonstrating the expected changes when transitioning from conditions of elevated turbidity to cleaner air validated our hypothesis. Our results suggest new policy actions to augment and enhance existing light pollution reduction techniques targeting lighting technology and design.

Kocifaj, M., Barentine, J.C. Air pollution mitigation can reduce the brightness of the night sky in and near cities. Sci Rep 11, 14622 (2021). https://doi.org/10.1038/s41598-021-94241-1

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