Artificial light at night (ALAN), commonly known as light pollution, is the excessive or misdirected use of outdoor lighting that brightens the night environment. While the impacts on wildlife behavior and astronomical observation are widely recognized, this light also fundamentally alters the Earth’s atmosphere. Artificial light interacts with gaseous molecules and particulate matter suspended in the air, creating effects that range from physical scattering to chemical disruption. These interactions modify the atmosphere’s natural nighttime state, influencing meteorology and complicating scientific efforts to monitor natural phenomena.
The Physics of Sky Glow
The most visible atmospheric effect of light pollution is sky glow, the diffuse dome of light seen over populated areas. This phenomenon results from artificial light scattering off atmospheric components and redirecting back toward the ground. The scattering process involves two primary physical mechanisms, dictated by the size of the particles the light encounters.
Rayleigh scattering occurs when light interacts with particles much smaller than its wavelength, primarily nitrogen and oxygen gas molecules. This scattering is highly dependent on the wavelength of light, favoring the shorter, bluer end of the spectrum. Consequently, blue light from artificial sources is scattered more efficiently, responsible for the bluish or whitish tint often seen in light domes over cities.
The second major mechanism is Mie scattering, which involves light interacting with larger particles, such as aerosols, dust, and water droplets. Since these particles are comparable to or larger than the wavelength of visible light, Mie scattering affects all colors of light more uniformly. In hazy or polluted urban atmospheres, this process dominates the overall brightness of the sky glow. Light emitted upward or reflected from the ground is intercepted by these particles and molecules, redirecting the photons and creating the characteristic light dome visible dozens of miles away from the source.
Alterations to Atmospheric Chemistry
Artificial light at night introduces an unnatural source of energy that disrupts the balance of nocturnal chemical reactions in the lower atmosphere (troposphere). This interference is noticeable in the nocturnal nitrogen cycle, a major pathway for cleaning the air of pollutants. During dark hours, nitrogen dioxide (\(\text{NO}_2\)) converts into the nitrate radical (\(\text{NO}_3\)), a highly reactive molecule.
The \(\text{NO}_3\) radical is a powerful nighttime oxidant that helps remove volatile organic compounds and other pollutants. Natural darkness is necessary for the buildup of these \(\text{NO}_3\) radicals, allowing them to perform their cleaning function. However, artificial light can photolyze, or chemically break down, the \(\text{NO}_3\) radical back into its constituent parts. This light-induced destruction significantly reduces the concentration of this nighttime cleaning agent.
By prematurely destroying the nitrate radical, artificial light halts a key nocturnal self-cleaning process of the air. This disruption can prolong the lifespan of certain atmospheric pollutants, including components that contribute to the formation of smog and ozone the following day. Research indicates that the degree of \(\text{NO}_3\) loss due to city light can be comparable to its natural loss mechanisms, demonstrating a measurable chemical alteration in the illuminated urban atmosphere.
Localized Thermal and Meteorological Changes
The energy emitted by artificial light sources contributes to localized atmospheric heating, creating distinct microclimates separate from the broader urban heat island effect. Outdoor lighting systems, particularly high-intensity lamps, radiate heat directly into the lower atmosphere. This energy input alters the surface energy balance, locally suppressing the natural radiative cooling that occurs at night.
In rural areas, uninterrupted long-wave radiation from the ground into space causes the air near the surface to cool significantly, leading to the formation of a deep, stable nocturnal boundary layer (NBL). This stability facilitates the formation of ground-level phenomena like radiation fog and dew. The heat emitted by artificial lighting counteracts this cooling process, resulting in a shallower or less stable NBL in illuminated areas.
The reduced atmospheric stability and warmer surface temperatures in areas with intense lighting directly influence local meteorology. The prevention of strong surface temperature inversions can inhibit the conditions necessary for the formation of dense radiation fog or heavy dew. Consequently, microclimates in brightly lit areas exhibit higher minimum nighttime temperatures and lower occurrences of fog and dew compared to nearby unlit environments.
Interference with Atmospheric Monitoring
The omnipresent background brightness caused by sky glow introduces a significant challenge for scientists monitoring natural atmospheric phenomena. Observatories and monitoring stations rely on a truly dark sky to accurately measure faint, natural light sources and processes. The artificial light scattered by the atmosphere acts as noise, overwhelming the subtle signals scientists need to detect.
One specific area of interference is the measurement of natural airglow, a faint, diffuse emission of light produced by chemical reactions in the upper atmosphere. Airglow provides data on upper atmospheric dynamics and composition, which are important for climate modeling. Sky glow from cities can be many times brighter than natural airglow, making it nearly impossible to characterize the natural emission from ground-based instruments.
The bright, fluctuating artificial background also complicates the study of faint natural light sources, such as distant atmospheric electrical events like sprites or blue jets. Furthermore, the variability of the artificial light background, which changes with weather and atmospheric aerosol load, makes it difficult for researchers to accurately differentiate between natural aerosol concentrations and the artificial light being scattered by those aerosols.