Eighty amazing years of Dobson No 17
It looks like it could have come out of the Tardis of the first Doctor Who – an attempt at appearing technologically modern that long ago became an anachronism.
But the Dobson spectrophotometer goes much further back than early 1960s British television. And the heavy – it takes two people to move it – yet fragile block of cast iron remains an accurate and reliable way of measuring atmospheric ozone levels.
In the 1980s, Dobson instruments in the Antarctic registered dropping ozone values in the stratosphere, evidence that the springtime ozone hole had become a reality.
Those measurements propelled a concerned world to adopt the Montreal Protocol, to protect the ozone layer, in 1988. It took another decade before ozone-depleting chlorofluorocarbons peaked, but they have been declining since.
This is the story of Dobson No 17, now nearly 80 years old and still in fine working order at Arrival Heights near Scott Base, Antarctica.
A Dobson spectrophotometer measures the total amount of ozone in a column of air between the instrument on the ground and the top of the atmosphere.
It does this using quartz prisms to compare the absorption of ultraviolet of different intensities that are either strongly or weakly absorbed by ozone.
The amount of ozone is measured in Dobson Units, DUs, with 100DU equal to a 1mm-thick sliver of ozone at standard temperature and pressure.
The instrument was invented in the late 1920s by Oxford University physicist and meteorologist Dr Gordon Dobson.
The National Institute of Water and Atmospheric Research (Niwa) looks after
New Zealand’s two Dobsons. The original, No 17, is on the ice and its younger sibling, No 72, is at Lauder in Central Otago. There are about 100 Dobsons worldwide.
‘‘The Dobson is still highly regarded because they have been running for so long,’’ says Niwa atmospheric scientist Dr Sylvia Nicol. ‘‘It’s quite a simple instrument in some respects. Like a lot of meteorological equipment, it hasn’t changed much over the years.’’
Writing in Weather and Climate, Nicol says the
New Zealand Meteorological Service was invited to buy one of the first 20 units because Christchurch was one of five international sites already measuring ozone using a borrowed Fery quartz spectrograph.
Approval was given to buy a Dobson in 1937, but it was another 13 years before the instrument landed in New Zealand.
First, there were delays with calibration and then it was requisitioned for ‘‘urgent defence investigations’’ by the British Meteorological Office during World War II.
On No 17’s arrival in 1950, it was installed at Gracefield in Wellington, and then at Kelburn in July 1951. The first measurements were taken by Elizabeth Porter, who died in the Tangiwai train disaster on Christmas Eve 1953.
The work was then taken on by Edith Farkas, a scientist who had arrived from Hungary as a refugee a few years earlier. She then became leader of the Dobson No 17 programme from 1965 until her retirement in 1986.
Meanwhile, however, the unit was shifted to the met service’s Invercargill airport office in June 1970.
When No 72 was installed at Lauder in January 1987, the old instrument was sent to the Antarctic, where it made contributions to burgeoning ozone-hole studies.
That is where No 17 remains to this day, 31 years after moving south, an incongruous slab of metal still taking intricate daily ozone readings with its periscope through the roof of the observatory.
‘‘The Dobson is still highly regarded because they have been running for so long.’’ Dr Sylvia Nicol, Niwa