Atmosphere Watch Section

Monitoring Stratospheric Ozone

NASA Ozone Archive | Other Map Archives

Stratospheric ozone, why monitor it?

The initial monitoring of ozone was driven by curiosity about the circulation in the upper levels of the atmosphere. Because measurements of total ozone were observed to be related to the passage of weather systems, it was used for many years as an aid to weather forecasting. Now, of course, the focus is very much on the depletion of the ozone layer due to anthropogenic pollutants and the ensuing negative biological impacts. The Bureau monitors ozone so that it can have data for the intitialisation and verification of global modelling and analysis products, so that we can detect long term trends and in order to resolve questions about the dynamics of the stratosphere and the ozone layer.

The Bureau’s network is a part of the WMO’s Global Atmosphere Watch. It is in a relatively data sparse part of the globe and the global modelling effort is greatly enhanced by the availability of the Australian data, especially the profile data which are becoming increasingly important as reductions in mid-latitude lower stratospheric ozone are observed. It is clear that the Bureau’s surface based network is a vital ingredient in this ‘public good’ research. The fact that atmospheric ozone was monitored for many years before it became an issue highlights the benefit of long term monitoring. Without the measurements that Dobson began in the 1920s, we would be far less capable of reaching conclusions about the status of, and trends in, the ozone layer today.

Ozone is also an important issue in the Climate Change debate. Ozone is a greenhouse gas and stratospheric ozone is a radiatively important constituent responsible for heating of the upper troposphere. Most crucial for climate simulations is the distribution of ozone near the tropopause, as the minimum temperatures here contrast most with the Earth’s surface and allows for the maximum forcing of the earth-troposphere system.

Stratospheric ozone observations- a brief history

Schönbein discovered ozone in 1839 and in 1850 it was determined that it was a naturally occurring atmospheric constituent. By 1860 surface ozone was being measured at hundreds of locations in Europe. The measurements at Paris from 1873 show levels there were half of what they are today. In 1879 Cornu suggested that the rather sharp limitation of the end of the solar spectrum as received at the ground was due to absorption in the atmosphere and, in 1880, Hartley postulated the existence of a layer above the troposphere, the stratosphere, where ozone was responsible for the absorption of solar ultra-violet radiation at wavelengths between 200 and 300nm. In 1921 Dobson and Lindeman, both working on meteors at the Clarendon Laboratory at Oxford University discovered that the temperature in the stratosphere increased with height in contrast to the troposphere. They concluded that radiative processes must dominate in the stratosphere and, as had been predicted by Hartley, that the source of the energy must be from the absorption of solar UV radiation by ozone.

The first measurements of total ozone, namely the total amount of atmospheric ozone between the surface of the Earth and the outermost reaches of the atmosphere, were driven by curiosity about the upper atmosphere. Since it was known that most of the atmospheric ozone should reside in the stratosphere, it was apparent that aspects of the stratosphere could be studied by measuring total ozone. In 1919 and 1920 Fabry and Buisson designed a special spectrograph and began making measurements at Marseilles. They found, quite rightly, that the amount of ozone in an atmospheric column was about 3mm (STP). In 1924 the first Fry spectrograph was built at Oxford. Seven of these instruments were eventually deployed in a network that included the Southern Hemisphere (Montezuma, Chile). In 1927 the Chilean instrument was placed in New Zealand. The Fry spectrographs were very successful, provided data which allowed the general relationship between ozone and the pressure distribution to be found and gave the first picture of the seasonal variation of ozone over much of the globe. Their chief drawbacks were that they required direct sunlight, when the sun was not too low and the measurements took a very long time to obtain because of the photographic developing required. The first photoelectric spectrophotometer that could reliably measure total ozone was produced by Dobson in 1926 (Dobson #1) and about 14 were made before the Second World War. During the 1930s-40s, further research was undertaken on how total ozone amounts correlated with weather patterns, and increased interest was directed on monitoring ozone in support of weather forecasting. This was of particular interest during the War when ground-based total ozone measurements could supplement the loss of weather information caused by the sinking of weather monitoring ships in the Atlantic. Also, during the War, stratospheric humidity was of importance for long range, high flying reconnaissance planes for which a vapour trail would be a dead give away.

Internationally, concerted measurements of ozone commenced in the International Geophysical Year (1957) and following this, during the 1960s, it became evident that the measurement of ozone could provide information concerning large-scale planetary atmospheric circulation. This is possible since in the lower middle atmosphere ozone is a tracer whose lifetime is large compared to the time involved in atmospheric circulation processes. Models of this circulation were initially derived based on the information provided by ozone measurements.

During the 1960s concerns were raised about the effect of atmospheric nuclear tests on the ozone layer but the first concrete indication that human activities could damage the ozone layer came in 1971 when Johnston pointed out that the large fleet of supersonic aircraft proposed by the US would feed considerable amounts of nitric oxide into or just below the ozone layer. Research had shown that oxides of nitrogen were very efficient destroyers of ozone. In 1974 a landmark paper by Molina and raised the possibility of ozone loss in the stratosphere due to presence of halogen-containing man-made chemicals, including the CFCs; first produced commercially in the early 1930s by DuPont, in the atmosphere. Since ozone absorbs almost all solar radiation below about 300nm and prevents this harmful radiation from arriving at the Earth’s surface, intense international debates followed. Also as a result was the upgrading and expansion of ozone measurements worldwide and the development of more sophisticated satellite-borne instruments for measuring ozone and ozone-related trace gases. The first comprehensive satellite observations were started in 1978 with the Nimbus-7 satellite which carried a Total Ozone Mapping Spectrometer (TOMS) instrument. International debate led to international treaty (Vienna Convention and its Montreal Protocol on Substances that Deplete the Ozone Layer). However it was not until concern following the discovery of the Antarctic ozone hole in 1985, followed by the air-borne measurements over Antarctica by NASA in the Spring of 1987 that a causal link between ozone loss and man-made chemicals was established, that the international treaties became strongly influential.

Types of Monitoring

The Dobson Spectrophotometer

The Dobson Spectrophotometer is a double monochromator that measures the relative intensities of pair wavelengths in the Huggins ozone band (300-350nm) and the Hartley Chappuis band (440-1180nm). Long term ozone measurement precision for the instrument is estimated to be about 1% (at the 2 level). The instrument is primarily designed to estimate total column ozone amounts, though the vertical distribution of ozone can also be estimated using a sequence of special observations.

Producing reliable data from routine Dobson spectrophotometer data is an involved process and it is vital that it is carried out with the primary intention of gaining the highest quality data, rather than the most readily available. The speed with which it can be produced is a question of the resources that are brought to bear. The whole process, from the time measurements are taken until they are properly quality controlled can take 3 to 4 months.

Umkehrs

Umkehrs (the German word for reversal) are a sequence of special ozone measurements made from sunrise or to sunset using a spectrophotometer. These measurements require clear skies and provide a crude atmospheric ozone profile at nine levels (0-49km) [a recent development has been a technique that enables 15 levels to be extracted from the data]. Within each layer there is assumed to be no ozone variation. The systematic uncertainty or bias associated with these measurements is about 15% to 20%. In Australia, meteorological staff at Brisbane and Darwin perform umkehrs. Explained simply, a sequence of umkehr observations would comprise almost continuous zenith observations between a zenith angle of 60^o to when the sun is just below the horizon. The restriction of umkehr measurements to clear skies means that at stations such as Macquarie Island and Melbourne for many months of the year they cannot be obtained.

The automated Dobson in Perth can produce up to two umkehrs per day. The corrected data are sent to WOUDC for archiving. Although umkehrs date back to the 1930s the WOUDC archive is useful from the time of IGY (1957). Though the archive contains more than 35,000 profiles only thirteen stations globally have sufficient observations to be useful for trend analysis and only one of these is in the Southern Hemisphere (Aspendale/Melbourne).

The Ozone Sonde

Ozone sondes are balloon-borne instruments that continuously estimate ozone concentration as they ascend into the atmosphere. A profile of ozone is obtained up to the burst point of the balloon - typically in excess of 30 km. In roughly 10% of cases the instrument remains within range during descent and these values are also obtained. The Bureau uses electrochemical types of ozone sondes. These rely on the reaction of ozone with a solution of potassium iodide. Air containing ozone is passed through an accurately calibrated air pump and bubbled through the potassium iodide solution. Electrodes placed in the solution monitor small electrical currents that are proportional to the concentration of ozone. Attached to the ozone sensor is a radiosonde - a device that has been specially modified to estimate ozone as well as the normal atmospheric parameters of pressure, temperature and relative humidity. The transmitted information is received by a ground station then digitally recorded and processed. The utmost care in cleanliness, skill in calibration and procedures are required for a successful flight. Pollutants, especially sulfur dioxide and nitrous oxide can interfere with the measurement of ozone. Once an ozone profile is obtained, the residual ozone amounts above the burst point are obtained by extrapolation and the total amount of ozone compared to that of a nearby spectrophotometer measurement (which is used as a reference measurement). An ozone sonde flight is deemed to be successful if the spectrophotometer and ozone sonde agree within 10%. A flight that did not reach 18 hPa would generally not be successful. Good flights reach as high as 5 hPa. Ozone sonde measurements are the most accurate means of determining at what heights ozone variations are occurring. The detection limit of the instrument is less than 2 parts per billion. Measurement uncertainty is about 10% in the troposphere, 5% in the stratosphere up to 10 hPa and about 25% above that. This knowledge greatly assists researchers in pinpointing whether the variations are natural or anthropogenic in origin.

In addition to the ground based equipment the Bureau also conducts monitoring using satellite data. We routinely retrieve analyse and archive total ozone data from the Tiros Operational Vertical Scanner (TOVS). These data come from the NOAA polar orbiting satellites. These have provided a near continuous record of operational vertical sounder radiance data from 19 October, 1978 to the present. These data are received routinely via the World Meteorological Oganisation's Global Telecommunications System (GTS), they are analysed and converted into images. The ozone analyses are also used as input for the Bureau’s UV analysis and forecasting scheme. There are other more accurate satellite data available (Total Ozone mapping Spectrometer (TOMS) and Solar Backscatter UV (SBUV)) but these are not available in real time. Data from the Bureau’s surface network are used by NOAA and NASA to validate the satellite data (so-called overpass data). We use the data ourselves to quality control the surface based data. A significant drop in readings on an instrument can, for example, be checked by inspection of the regional situation.

Images from June 1996 are available to browse at NASA Ozone Archive