Among the positive fallouts of the many nationwide lockdowns implemented following the outbreak of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) worldwide are the observed reductions in environmental pollution. Namely, reductions in nitrogen dioxide in the air, the release of pollutant aerosols, noise pollution and light pollution.
A novel study published on the preprints.org server in late November (2020) reports a favorable change in light pollution during the lockdown in Granada, Spain, from March 14 to May 31, 2020, and has traced it back to several different causes related to coronavirus disease 2019 (COVID-19) lockdown measures.
Many manmade sources of pollution related to economic activity have registered drastic declines in their pollutant output, providing ample opportunities for regional- and country-specific natural experiments. Satellite data, using the Suomi-North Polar Partnership/Visible Infrared Imaging Radiometer Suite-DayNightBand (SNPP/VIIRS-DNB), has been invaluable in many of these studies. However, light emissions at night are difficult to capture at a large scale in this manner because most of this data is recorded at 1:30 am (local time) when people are typically at home or asleep.
Data from other satellites lack such extensive coverage or require a lot of calibration – or their acquisition is not cost-effective. While the International Space Station (ISS) also uses DSLR cameras, these images are not systematically acquired to cover specific regions over time, making them unsuitable for this purpose, despite their high resolution and multi-spectral quality.
On the other hand, ground data on artificial lighting at night is limited in availability, but becomes very important as a source of information on this aspect of pollution during a lockdown. The current study combines both ground and satellite data to develop a reasonably accurate picture of the impact of the lockdown on light pollution.
The researchers found that the lockdown was associated with a reduction in light pollution, due both to fewer light emissions from the city and lower aerosol content in the air, causing less scattering of light. The aerosol content originates from human activity, and its reduction is thus directly related to the lockdown.
They found a clear link between the concentration of fine particulate matter (PM10) and the brightness of the sky, and were able to find a mathematical equation relating the two at three different wavelength bands in a linear fashion. The station where the PM10 concentration was measured was located about a kilometer away from the site of measurement of the sky's brightness, though both were at equal altitudes.
This finding corroborates earlier research, which states that "sky brightness seen close to the center of a city increases with the atmospheric aerosol content."
The sky's brightness is a factor of both aerosol content and the time from the beginning of the night. They found the night sky's brightness was darker after the lockdown began, indicating a reduction in the city's light output by about 20%. The reason for the reduction is probably reduced vehicular light and private lighting.
Image from the ASTMON FoV in the V filter on February 20th 2020 at 01:16 UT. The red circle highlights the region of the celestial sphere where sky brightness measurements were taken. The instrument is situated on the roof of the main building in the middle of the two domes.
However, when the blue-filter band is considered, they found that the light output of the city was about 45% less during the lockdown. They point out that private lighting is a major contributor to light pollution, and that private lighting is typically more blue-rich than municipal lighting, especially if cold LED lighting is not yet in wide use in that city. In other words, "the blue band night sky brightness appears to be a better human activity indicator than the sky brightness in the visible channel."
An important finding was that there was a reduction in human activities by over 90% but not in the city's light output, which was reduced by only half that amount. This indicates that most of the city lighting does not hold a clear function for its dwellers. In other words, much of the lighting is turned on at night, irrespective of whether the area lighted up is in use or not. The researchers call this "a clear waste of energy and resources."
The study also found that the sky glow from the city, observed at the observatory in Sierra Nevada, at an angle of 25 degrees, was also much dimmer. This, coupled with the reduced sky brightness (light pollution), seems to support the scientists' hypothesis. Namely, the reduced pollution is due to two reasons: first, the lower manmade aerosol production, and, second, the reduced net city light emission.
The lighting decrease may be due to less private lighting of buildings and private areas, as well as vehicle lights. At a much later time of night, the light emission is almost unchanged relative to before the lockdown, which explains the lack of significant difference with the late-night satellite images. This also agrees with the ground data, while supporting the unsuitability of VIIRS data acquired at late night to detect the lockdown's effects. For this purpose, another instrument with an earlier flyby time would be a boon.
Preprints.org publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.