Arctic Ozone

Research and Observation: The State of the Arctic Ozone Layer

OBSERVATION AND MONITORING

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Canadian scientists have been active in stratospheric ozone research since the late 1920s, but continuous monitoring of ozone levels over Canada did not begin until the late 1950s. The present Canadian observation network consists of a dozen stations, of which three are located in the Arctic (Figure 10). One of these, at Resolute Bay, has observations dating back to 1957. Observations at the other two, Alert and Eureka, began in 1987 and 1992 respectively.

Ozone at all Canadian stations is measured with ground-based instruments. The Dobson ozone spectrophotometer, devised in the 1920s by the pioneer British ozone researcher G.M.B. Dobson, was used for these measurements until 1988. It was then replaced by the Canadian-developed Brewer ozone spectrophotometer, an automated instrument that is now the standard apparatus for ground-based ozone measurements worldwide. In addition, some 300 ozonesondes, which are instrument packages carried aloft by balloons, are launched throughout the year from half a dozen Canadian locations, including all three Arctic sites, to provide direct measurements of ozone concentrations at different altitudes. Ozonesondes have been launched from Resolute Bay since 1966 and from Alert and Eureka since 1987 and 1992 respectively.

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The downward trend in the ozone content of the lower stratosphere can be seen to begin in the late 1970s, and by the 1990s severe depletion episodes were being observed in some years (Figure 5). The longest record, from Resolute Bay, provides a particularly striking illustration of seasonal and annual trends in the Arctic ozone layer over the past 40 years. Figure 11 shows the seasonal pattern, with ozone amounts peaking in February and March and reaching a minimum in August and September. The general downward movement of ozone levels in the 1990s during all seasons is also abundantly clear, with springtime values below 300 DU occurring more frequently after 1993. Before 1993, springtime values below 300 DU were occasionally recorded at Resolute Bay, but they were relatively infrequent and lasted for only a few days.

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During March 1996, ozone values over the high Arctic were as much as 30% below normal, while in March 1997 they dropped to as much as 45% below the normal values. During the 1997 episode, ozone values remained below 300 DU for much of March and remained significantly below normal minimum values until the middle of April (Figure 12). Although this depletion was very large by Arctic standards, ozone amounts were still well above those in the Antarctic, where values of 100 DU or less have commonly been recorded during ozone hole episodes.

ARCTIC OZONE RESEARCH

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Arctic ozone research received a boost in 1992 with the opening of the Eureka Stratospheric Ozone Observatory on Ellesmere Island (Figure 13). The observatory is a primary component of the Network for the Detection of Stratospheric Change, an international group of high quality, ground-based research stations for investigating the physical and chemical processes of the stratosphere. The Eureka observatory is used by university and government researchers from Canada, Japan, and the United States.

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Total ozone amounts over Eureka are measured year-round by a Brewer spectrophotometer, which uses the absorption of ultraviolet radiation in sunlight and moonlight to determine how much ozone is in the atmosphere. In addition, ozonesondes are launched weekly to provide vertical profiles of ozone and temperature. From early December to March an instrument known as a lidar is used to obtain vertical profiles of ozone and temperature. The lidar, which measures the reflection of laser pulses much as a radar measures reflected radio waves, can also be used to determine atmospheric concentrations of fine sulphate particles associated with volcanic eruptions, polar stratospheric clouds, and Arctic haze (a kind of smog that is transported into the Arctic during the winter from industrial regions to the south). Because the lidar works best in total darkness, it is shut down as Arctic summer approaches and the nights get shorter. Overlapping measurements from these instruments not only provide an accuracy check on total ozone amounts but can also be used to detect the atmospheric temperatures and light-scattering patterns that indicate the presence of polar stratospheric clouds (Figure 14).

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Another instrument, known as a Fourier Transform Infrared Spectrometer (FTIR), can be used to measure the chemical content of the stratosphere. One of the most important chemicals that it tracks is chlorine monoxide, since a rise in chlorine monoxide concentrations in the stratosphere is a key indicator that the catalytic destruction of ozone is taking place (Figure 15). The results of these measurements are also used to determine the relationship between chlorine monoxide concentrations and ozone depletion.

Computer models are essential tools for improving our understanding of atmospheric processes and how they are affected by both natural and human-induced changes. The Canadian Middle Atmosphere Model, a joint effort involving university and government scientists, has recently been developed to include a more detailed representation of chemical and physical processes in the stratosphere. It will be used to study ozone depletion issues in the near future.

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As a party to the Vienna Convention and the Montreal Protocol, the major international agreements for protecting the ozone layer, Canada coordinates its research and monitoring activities closely with those of other nations. Since 1991, for example, it has participated in the Match ozonesonde program, which coordinates the launches of ozonesondes in Europe and Canada to probe the same air mass at different points as it travels around the Arctic vortex. By analyzing the differences between measurements taken at different times and places within this air mass, it is possible to determine the amount of ozone loss that is due to chemical processes alone and to avoid any distortions caused by ozone brought in by other air masses. Among other things, data from the Match program, clearly reveal the crucial role of sunlight in ozone destruction (Figure 16).

Canada has also collaborated closely with the U.S. National Aeronautics and Space Administration (NASA) in Project Polaris, a campaign to investigate ozone chemistry in the Arctic. Dr. Tom McElroy of Environment Canada, for example, works with a team using a high-flying ER-2 aircraft to measure the chemical composition of the upper troposphere and lower stratosphere. After analyzing data collected by these flights, McElroy recently discovered evidence that bromine monoxide, a compound associated with ozonedestruction, exists in the higher levels of the Arctic troposphere. The presence of this compound in the upper troposphere had not been expected. Because it is associated with the destruction of ozone by bromine, its detection suggests that further ozone depletion is occurring, at least occasionally, in the upper troposphere. These findings emphasize as well that our understanding of the chemistry of ozone depletion in the troposphere is incomplete.

In addition to these activities, Environment Canada runs the World Ozone and Ultraviolet Radiation Data Centre on behalf of the World Meteorological Organization and the world scientific community. The Centre's web site ( www.tor.ec.gc.ca/woudc/woudc.htm ) contains data from more than 150 stations around the world.

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