Polar stratospheric clouds (PSCs)
Polar stratospheric clouds (PSCs), also known as nacreous clouds, are unique atmospheric phenomena that form in the lower stratosphere at altitudes between 15 and 25 kilometers, particularly in high-latitude regions during winter. These clouds are characterized by their composition of ice crystals and particles, which are uniform in size, measuring about 10 micrometers in diameter. One of the most visually striking features of PSCs is their iridescent colors, which can manifest as bright red and blue zones, especially visible during twilight hours in locations such as Oslo, Norway, and other northern regions like Scotland and Antarctica.
Scientifically, PSCs are categorized into two main types based on chemical and thermal properties. Type I PSCs form at around -78° Celsius and include subcategories containing various acidic compounds, while Type II PSCs consist of water ice crystals at even lower temperatures of approximately -85° Celsius. These clouds are significant in the context of climate change as they play a crucial role in the depletion of the ozone layer, particularly over Antarctica. The particles within PSCs provide surfaces that facilitate chemical reactions, particularly involving chlorine compounds, which contribute to ozone destruction. Additionally, PSCs are involved in processes that lead to denoxification, converting nitrogen oxides into nitric acid, which further influences ozone dynamics in the stratosphere. Understanding PSCs is essential for comprehending their environmental impact and the broader implications for climate change.
Polar stratospheric clouds (PSCs)
Definition
Polar stratospheric clouds (referred to as “PSCs” by scientists and called nacreous clouds by the general public) form in the lower stratosphere between heights of 15 kilometers and 25 kilometers in the lee of high-latitude mountain ranges in winter. Ice crystals and particles composing the visible cloud are of uniform size, about 10 micrometers in diameter. After sunset, light from the Sun below the horizon continues to illuminate particles within PSCs, producing iridescent bright red and blue coloration zones in the clouds. Colorful PSC displays are most frequently photographed before dawn and after sunset when they appear in Oslo, Norway, but photographs and videotapes have also been taken in Scotland, England, Scandinavia, Iceland, Alaska, and Antarctica.
Atmospheric scientists have divided PSCs into two “Types” as defined by chemical and thermal differences. Type I PSCs have temperatures of about -78° Celsius. Type I(a) PSCs contain crystalline compounds of water and nitric acid; Type I(b) PSCs contain droplets of nitric (HNO3) and sulfuric (H2SO4) acids; Type I(c) PSCs contain nonspherical particles of metastable nitric acid in water; and Type II PSCs are composed of water ice crystals at temperatures of -85° Celsius.
Significance for Climate Change
Discovery of the formation of the in the above the Antarctic continent each spring focused scientific attention on chemical reactions depleting ozone. Scientists deduced that ultraviolet light from the rising Sun promoted chemical reactions leading to ozone destruction, and investigations linked chemical reactions going on within the PSCs to regional ozone depletion above the Antarctic.
Particles within Antarctic PSCs act as catalytic surfaces for reactions involving the destruction of relatively stable atmospheric chlorine compounds(including artificial chlorofluorocarbons sometimes called Freons). Through a series of reactions, these chlorine compounds are transformed into highly reactive molecular chlorine gas (C12) and hypochlorous acid (HCl) molecules, which rapidly form chlorine radicals, which in turn destroy ozone molecules. During winter and early spring, there is a strong easterly flow of air in the Antarctic stratosphere, the Antarctic Polar Vortex. Little air from lower latitudes mixes with the ozone-depleted air within the vortex, allowing greater depletion of ozone to occur, creating the ozone hole.
PSCs also catalyze other important chemical reactions, including denoxification, the conversion of oxides of nitrogen to nitric acid, which is found in all three Type I PSCs. Because decreasing the level of nitric oxide (NO2) allows high levels of ozone-removing hypochlorite (ClO) to remain in the stratosphere, this denoxification promotes development of the ozone hole. Denoxification leaves the waste product nitric acid behind in the cloud particles, which eventually exits the stratosphere, a process called denitrification.
"Polar Stratospheric Clouds." Australian Antarctic Program, 5 Aug. 2024, www.antarctica.gov.au/about-antarctica/ice-and-atmosphere/atmosphere/clouds-and-radiation/polar-stratospheric-clouds/. Accessed 21 Dec. 2024.
"Polar Stratospheric Clouds." UCAR Center for Science Education, scied.ucar.edu/image/polar-stratospheric-clouds. Accessed 21 Dec. 2024.