Measuring the missing molecules: a history of overseeing ozone

Around 90% of ozone is located in the lower stratosphere (beginning at a height of between 10-16 km above the ground). Flickr/NASA Earth Observatory

SAVING THE OZONE: Part eight in our series exploring the Montreal Protocol on Substances that Deplete the Ozone Layer – dubbed “the world’s most successful environmental agreement” – looks at how we measure ozone’s ups and downs.

In the 1980s, scientists realised Earth’s ozone layer was diminishing, endangering the health of humans and other species. But they would never have known this if not for an 80-year-old instrument still in use today.

The Dobson spectrophotometer (or simply “the Dobson”) was developed by Gordon Dobson at Oxford University in the 1920s. It allowed for the first time a quick but very accurate determination of Total Column Ozone at a given location.

Essentially, the Dobson works by comparing the intensity of light at two pairs of wavelengths. For each pair, one wavelength is much more strongly absorbed by ozone in the atmosphere than the other. By comparing the ratios of the intensities, the total amount of ozone overhead can be calculated. The unit of measurement for Column Ozone is named the “Dobson Unit” (DU), in honour of Gordon Dobson.

It is this measurement which shows the diminishing levels of ozone in the stratosphere at specific locations.

The Dobson in Australia

In 1936, three Australian organisations placed a combined order for three of the new instruments at a cost of £400 each – the Bureau of Meteorology, Mt Stromlo Observatory and the Council for Scientific and Industrial Research (CSIR) Radio Research Board. Although the Dobsons were manufactured and tested, none began operational use in Australia until after the Second World War.

It wasn’t until The International Geophysical Year of 1957 that a concerted global effort was made to establish a worldwide Dobson network. Australia established ongoing monitoring programs at Brisbane, Melbourne and the sub-Antarctic Macquarie Island. These programs continue to the present day.

On the January 1 1983, the Bureau of Meteorology assumed full responsibility for the Australian ozone network (now including Darwin as well as the other three locations). The Bureau’s network Dobsons are all still fully manual in operation.

Despite their age, Dobsons remain in wide use across the world and continue to form the backbone of the surface-based component of the global ozone monitoring network. Although modern and more capable instruments have been developed in the 80 years since its creation, the Dobson has in its favour, as well as continuity with long standing measurement programs, great stability and operational reliability.

As with any long-term monitoring program, in order to detect small changes over long periods of time, questions of calibration and consistency of operation become crucial. Unfortunately in the case of Dobsons, standardisation was only really put on a solid footing in 1977 when the World Meteorological Organisation designated a World Standard Dobson and a number of Regional Standards. From this time on each Dobson became traceable to the Regional Standard and thus to the World Standard. The Bureau operates the Standard Dobson for the South-West Pacific Region.

Bureau observer Janelle Inkster makes a Dobson observation at Macquarie Island. Ashleigh Wilson

Ozone over Australia

At all of the Australian locations, ozone follows a distinct cycle over the course of each year, increasing in winter and spring and then decreasing over summer through to autumn. The size of the increase however is much greater with increasing distance from the tropics - the typical annual range expressed as monthly mean values over the last decade have been around 240 to 260 Dobson units at Darwin, 260 to 310 DU at Brisbane, 270 to 340 DU at Melbourne and 290 to 400 DU at Macquarie. For comparison, in Antarctica during springtime total ozone can drop below 100 DU.

Superimposed upon the annual cycle there is considerable day-to-day variability (again more so in the mid-latitudes than the tropics or subtropics), primarily due to the effect of passing weather systems. Total ozone will often increase or decrease by 50 DU within the space of a day or two. Individual readings on a given day can therefore be much higher or lower than the monthly mean values.

Figure 1 – Monthly mean Total Column Ozone values for Melbourne 1978-2012, smoothed by a 132 month running mean to remove the effects of the annual cycle and long-term periodic influences. (The first and last five years are not shown due to the smoothing).

Figure 1 shows that the ozone trend is broadly consistent (in reverse) with the trend in ozone depleting substances, decreasing steadily through the 1980s and early 1990s, followed by a steadying and finally a small increase in recent years.

Two natural phenomena

For year to year variations in ozone, two large-scale natural phenomena are particularly important over mid-latitudes: the 11-year Solar Cycle and the so-called Quasi-Biennial Oscillation (QBO).

The Solar Cycle refers to the approximately cyclical variation in various processes taking place in the sun, first discovered by astronomers noting patterns in the number of sunspots. The amount of solar UV flux received by Earth is modulated by this cycle, and this directly affects the amount of ozone produced in the atmosphere. This may also indirectly influence ozone by modifying the circulation. The effect of the solar cycle over ozone in summer over Melbourne, for example, is about 5 DU.

The Quasi-Biennial Oscillation (QBO) refers to the reversal in direction of the mean east-west wind in the tropical stratosphere. It is observed to take place about every 27-29 months (hence the name “quasi-biennial”). Although not all the mechanisms are fully resolved, the QBO appears to have a very significant influence on the stratosphere at latitudes outside the tropics as well. The phase of the QBO in a given year affects springtime ozone values by up to 20 DU.

Other factors which have been shown to influence ozone levels include debris from volcanic eruptions injected into the stratosphere, and various dynamic influences including the El Niño–Southern Oscillation (ENSO).

Read more on the Montreal Protocol’s 25th anniversary.