Q9.pdf

Q9 Why has an “ozone hole” appeared over Antarctica when ozone-depleting substances are present throughout the...

Q9 Why has an “ozone hole” appeared over Antarctica when ozone-depleting substances are present throughout the stratosphere? Ozone-depleting substances are present throughout the stratospheric ozone layer because they are transported great distances by atmospheric air motions. The severe depletion of the Antarctic ozone layer known as the “ozone hole” occurs because of the special meteorological and chemical conditions that exist there and nowhere else on the globe. The very low winter temperatures in the Antarctic stratosphere cause polar stratospheric clouds (PSCs) to form. Special reactions that occur on PSCs, combined with the isolation of polar stratospheric air in the polar vortex, allow chlorine and bromine reactions to produce the ozone hole in Antarctic springtime. The severe depletion of stratospheric ozone in late winter and gases (see Q7 and Q8). In addition to a large abundance of early spring in the Antarctic is known as the “ozone hole” (see these reactive gases, the formation of the Antarctic ozone hole Q10). The ozone hole appears over Antarctica because meteo- requires temperatures low enough to form polar stratospheric rological and chemical conditions unique to this region increase clouds (PSCs), isolation from air in other stratospheric regions, the effectiveness of ozone destruction by reactive halogen and sunlight (see Q8). Figure Q9-1. Arctic and Antarctic tem- peratures. Air temperatures in both polar Minimum Air Temperatures in the Polar Stratosphere regions reach minimum values in the lower stratosphere in the winter season. Average Arctic winter daily minimum values over Antarctica are Nov Dec Jan Feb March April as low as −92°C in July and August in a –50 – 60 Average daily values 50º to 90º Latitude typical year. Over the Arctic, average mini- –55 Arctic 1979/80 to 2017/18 mum values are near −80°C in late Decem- Antarctic 1979 to 2018 –70 ber and January. Polar stratospheric clouds – 60 Temperature (degrees Fahrenheit) (PSCs) are formed in the ozone layer when Ranges of daily values – 80 Temperature (degrees Celsius) winter minimum temperatures fall below PSC formation temperature their formation threshold of about −78°C. – 65 This occurs on average for 1 to 2 months – 90 over the Arctic and about 5 months over –70 Antarctica each year (see heavy red and –100 blue lines). Reactions on liquid and solid –75 Arctic PSC particles cause the highly reactive –110 chlorine gas ClO to be formed, which – 80 catalytically destroys ozone (see Q8). The range of winter minimum temperatures –85 –120 found in the Arctic is much greater than that in the Antarctic. In some years, PSC –90 –130 formation temperatures are not reached in the Arctic, and significant ozone depletion Antarctic –95 –140 does not occur. In contrast, PSC formation temperatures are always present for many May June July August Sept Oct months somewhere in the Antarctic, and Antarctic winter severe ozone depletion occurs each winter season (see Q10). 32 Section II: The ozone depletion process  2018 Update | Twenty Questions | Q9 Distribution of halogen gases. Halogen source gases that are Polar stratospheric clouds (PSCs). Reactions on the surfaces emitted at Earth’s surface and have lifetimes longer than about of liquid and solid PSCs can substantially increase the relative 1 year (see Table Q6-1) are present in comparable abundances abundances of the most reactive chlorine gases. These reactions throughout the stratosphere in both hemispheres, even though convert the reservoir forms of reactive chlorine gases, chlorine most of the emissions occur in the Northern Hemisphere. The nitrate (ClONO2) and hydrogen chloride (HCl), to the most reac- abundances are comparable because most long-lived source tive form, ClO (see Figure Q7-3). The abundance of ClO increases gases have no significant natural removal processes in the lower from a small fraction of available reactive chlorine to comprise atmosphere, and because winds and convection redistribute nearly all chlorine that is available. With increased ClO, the cata- and mix air efficiently throughout the troposphere on the times- lytic cycles involving ClO and BrO become active in the chemical cale of weeks to months. Halogen gases (in the form of source destruction of ozone whenever sunlight is available (see Q8). gases and some reactive products) enter the stratosphere pri- Different types of liquid and solid PSC particles form when marily from the tropical upper troposphere. Stratospheric air stratospheric temperatures fall below about −78°C (−108°F) in motions then transport these gases upward and toward the polar regions (see Figure Q9-1). As a result, PSCs are often found pole in both hemispheres. over large areas of the winter polar regions and over significant Low polar temperatures. The severe ozone destruction that leads to the ozone hole requires low temperatures to be present over a range of stratospheric altitudes, over large geographical Arctic Polar Stratospheric Clouds (PSCs) regions, and for extended time periods. Low temperatures are important because they allow liquid and solid PSCs to form. Reactions on the surfaces of these PSCs initiate a remarkable increase in the most reactive chlorine gas, chlorine monoxide (ClO) (see below as well as Q7 and Q8). Stratospheric tempera- tures are lowest in the polar regions in winter. In the Antarctic winter, minimum daily temperatures are generally much lower and less variable than those in the Arctic winter (see Figure ­Q9-1). Antarctic temperatures also remain below PSC formation temperatures for much longer periods during winter. These and other meteorological differences occur because of variations be- tween the hemispheres in the distributions of land, ocean, and mountains at middle and high latitudes. As a consequence, win- ter temperatures are low enough for PSCs to form somewhere in the Antarctic for nearly the entire winter (about 5 months), and only for limited periods (10–60 days) in the Arctic for most winters. Isolated conditions. Stratospheric air in the polar regions is rel- atively isolated for long periods in the winter months. The isola- tion is provided by strong winds that encircle the poles during winter, forming a polar vortex, which prevents substantial trans- port and mixing of air into or out of the polar stratosphere. This circulation strengthens in winter as stratospheric temperatures decrease. The Southern Hemisphere polar vortex circulation tends to be stronger than that in the Northern Hemisphere be- Figure Q9-2. Polar stratospheric clouds. This photograph cause northern polar latitudes have more land and mountain- of an Arctic polar stratospheric cloud (PSC) was taken in ous regions than southern polar latitudes. This situation leads to Kiruna, Sweden (67°N), on 27 January 2000. PSCs form in more meteorological disturbances in the Northern Hemisphere, the ozone layer during winters in the Arctic and Antarctic, which increase the mixing in of air from lower latitudes that wherever low temperatures occur (see Figure Q9-1). The warms the Arctic stratosphere. Since winter temperatures are particles grow from the condensation of water, nitric acid lower in the Southern than in the Northern Hemisphere polar (HNO3), and sulfuric acid (H2SO4). The clouds often can be stratosphere, the isolation of air in the polar vortex is much more seen with the human eye when the Sun is near the horizon. effective in the Antarctic than in the Arctic. Once temperatures Reactions on PSCs cause the formation of the highly reac- drop low enough, PSCs form within the polar vortex and induce tive gas chlorine monoxide (ClO), which is very effective in chemical changes such as an increase in the abundance of ClO the chemical destruction of ozone (see Q7 and Q8). (see Q8) that are preserved for many weeks to months due to the isolation of polar air. Section II: The ozone depletion process 33  Q9 | Twenty Questions | 2018 Update altitude ranges, with significantly larger regions and for longer the reservoir gas ClONO2. As a result, ClO remains chemically time periods in the Antarctic than in the Arctic. The most com- active for a longer period, thereby increasing chemical ozone mon type of PSC forms from nitric acid (HNO3) and water con- destruction. Significant denitrification occurs each winter in the densing on pre-existing liquid sulfuric acid-containing particles. Antarctic and only for occasional winters in the Arctic, because Some of these particles freeze to form solid particles. At even PSC formation temperatures must be sustained over an exten- lower temperatures (−85°C or −121°F), water condenses to form sive altitude region and time period to lead to denitrification ice particles. PSC particles grow large enough and are numer- (see Figure Q9-1). ous enough that cloud-like features can be observed from the Ice particles form at temperatures that are a few degrees lower ground under certain conditions, particularly when the Sun is than those required for PSC formation from HNO3. If ice parti- near the horizon (see Figure Q9-2). PSCs are often found near cles grow large enough, they can fall several kilometers due to mountain ranges in polar regions because the motion of air over gravity. As a result, a significant fraction of water vapor can be the mountains can cause localized cooling in the stratosphere, removed from regions of the ozone layer over the course of a which increases condensation of water and HNO3. winter. This process is called dehydration of the stratosphere. When average temperatures begin increasing in late winter, Because of the very low temperatures required to form ice, de- PSCs form less frequently, which slows down the production hydration is common in the Antarctic and rare in the Arctic. The of ClO by conversion reactions throughout the polar region. removal of water vapor does not directly affect the catalytic re- Without continued production, the abundance of ClO decreases actions that destroy ozone. Dehydration indirectly affects ozone as other chemical reactions re-form the reservoir gases, ClONO2 destruction by suppressing PSC formation later in winter, which and HCl. When temperatures rise above PSC formation thresh- reduces the production of ClO by PSC reactions. olds, usually sometime between late January and early March in Discovering the role of PSCs. Ground-based observations of the Arctic and by mid-October in the Antarctic (see Figure Q9-1), PSCs were available many decades before the role of PSCs in the most intense period of ozone depletion ends. polar ozone destruction was recognized. The geographical and Nitric acid and water removal. Once formed, the largest PSC altitude extent of PSCs in both polar regions was not known particles fall to lower altitudes because of gravity. The largest fully until PSCs were observed by a satellite instrument in the particles can descend several kilometers or more in the strato- late 1970s. The role of PSC particles in converting reactive chlo- sphere within a few days during the low-temperature winter/ rine gases to ClO was not understood until after the discovery spring period. Because PSCs often contain a significant fraction of the Antarctic ozone hole in 1985. Our understanding of the of available HNO3, their descent removes HNO3 from regions of chemical role of PSC particles developed from laboratory stud- the ozone layer. This process is called denitrification of the strato- ies of their surface reactivity, computer modeling studies of sphere. Because HNO3 is a source for nitrogen oxides (NOx) in polar stratospheric chemistry, and measurements that directly the stratosphere, denitrification removes the NOx available sampled particles and reactive chlorine gases, such as ClO, in for converting the highly reactive chlorine gas ClO back into the polar stratosphere. 34 Section II: The ozone depletion process  2018 Update | Twenty Questions | Q9 The Discovery of the Antarctic Ozone Hole The first decreases in Antarctic total ozone were observed in the early 1980s over research stations located on the Antarctic continent. The measurements were made with ground-based Dobson spectrophotometers (see box in Q4) installed as part of the effort to increase observations of Earth’s atmosphere during the International Geophysical Year that began in 1957 (see Figure Q0-1).The observations showed unusually low total ozone during the late winter/early spring months of September, October, and November. Total ozone was lower in these months compared with previous observations made as early as 1957. The early published reports came from the Japan Meteorological Agency and the British Antarctic Sur- vey. The results became widely known to the world after three scientists from the British Antarctic Survey published their observations in the prestigious scientific journal Nature in 1985. They suggested that rising abundances of atmospheric CFCs were the cause of the steady decline in total ozone over the Halley Bay research station (76°S) observed during suc- cessive Octobers starting in the early 1970s. Soon after, satellite measurements confirmed the spring ozone depletion and further showed that for each late winter/early spring season starting in the early 1980s, the depletion of ozone extended over a large region centered near the South Pole. The term “ozone hole” came about as a description of the very low values of total ozone, apparent in satellite images, that encircle the Antarctic continent for many weeks each October (spring in the Southern Hemisphere) (see Q10). Currently, the formation and severity of the Antarctic ozone hole are documented each year by a combination of satellite, ground-based, and balloon observations of ozone. Very early Antarctic ozone measurements. The first total ozone measurements made in Antarctica with Dobson spec- trophotometers occurred in the 1950s following extensive measurements in the Northern Hemisphere and Arctic region. Total ozone values observed in the Antarctic spring were found to be around 300 Dobson units (DU), lower than those in the Arctic spring. The Antarctic values were surprising because the assumption at the time was that the two polar regions would have similar values. We now know that these 1950s Antarctic values were not anomalous; in fact, similar values were observed near the South Pole in the 1970s, before the ozone hole appeared (see Figure Q10-3). Antarctic total ozone values in early spring are systematically lower than those in the Arctic early spring because the Southern Hemisphere polar vortex is much stronger and colder and, therefore, much more effective in reducing the transport of ozone-rich air from midlati- tudes to the pole (compare Figures Q10-3 and Q11-2). Section II: The ozone depletion process 35