Cleaner air could darken the night sky

For many people around the world, the darkness of the night sky is not all that dark . About 83% of the global population resides under light-polluted skies, according to a 2016 study , and nearly 80% of North Americans can’t see the arc of the Milky Way from where they live. Restoring the view will require addressing the source of the problem: outdoor lighting. Yet night-sky brightness is determined not only by the intensity of light from artificial sources but also by how that light propagates through the atmosphere. Astronomers Miroslav Kocifaj of Slovak Academy of Sciences and John Barentine of the International Dark-Sky Association have now quantified the effect of air quality on sky glow. Their model, which they validated with actual brightness measurements, illustrates how clearing pollutants would aid the escape of photons emitted by nighttime lighting.

In developing their model, Kocifaj and Barentine accounted for the interactions of photons with both air molecules and various aerosol particles—a typical urban mix of ammonium sulfates, organic matter, and black carbon. The researchers varied the aerosol optical depth (AOD), a property that corresponds to the total atmospheric aerosol concentration, and simulated the intensity of light at a range of locations about 1–15 km away from a single bright light source that emits 10% of its photons directly overhead. They started with AOD = 0.3, which is typical of the air over urban centers. The aerosols scattered photons back toward the surface and amplified the sky glow at locations close to the light source. Dropping the AOD to zero, however, removed that aerosol bottleneck. The night-sky brightness plummeted for observers near the light source—by 70% for someone located 1.3 km away. The effect was more nuanced for observers at greater distances. With fewer obstacles in their way, the photons on average traveled farther. So an observer 15 km from the light source would see their night-sky brightness increase by 40%, although the total perceived brightness would still be low.

Kocifaj and Barentine put their model to the test in April 2019, when a potent weather system abruptly displaced a stagnant, aerosol-laden air mass over Vienna, Austria, temporarily dropping the AOD from 0.4 to 0.06. Using a detector at the University of Vienna’s Institute for Astrophysics, which is located about 5 km from the city center, the researchers measured a 43% drop in zenith night-sky brightness. They inputted the distance to the primary light sources that create the sky glow observed at the institute and found the results matched up well with their model.

The researchers emphasize that dark-sky advocates should keep their focus on outdoor lighting, and they acknowledge that darkening the night sky isn’t the primary motivation for reducing air pollution. But along with enjoying the environmental and health benefits of cleaner air, future urban dwellers could also enjoy a better view of the stars. (M. Kocifaj, J. C. Barentine, Sci. Rep. 11, 14622, 2021 .)