Dobson Ozone Spectrophotometer

Scientists at the British Antarctic Survey (BAS) research base at Halley Bay on the Brunt Ice shelf had always measured the amount of UVA and UVB arriving at the station from which they could plot the behaviour of the ozone layer.  Like most instruments of the period, readings were taken by hand and plotted on charts.  And for decades the graph was a more or less straight line.  However, in the late 70s it started to droop and eventually fell off the bottom of the charts.  This oddity was assumed to be caused by a fault in the spectrophotometer, so the instrument was sent back to the BAS facility in Cambridge for re-calibrating.  This was duly done but it was found that there was no fault with the instrument so in 1985 they published a paper showing that there was a hole developing in the Ozone layer over the Antarctic.This caused quite a stir and most researchers refused to believe it.  In particular, NASA said that the hole didn’t exist as there was no sign of it on any of their measurements from satellites.  So, BAS became convinced they had been wrong and went through the calibration procedure more meticulously and still found no fault.  This time they convinced NASA it must mean NASA was wrong.  NASA took it seriously and eventually discovered a fault in the software.  It had been programmed to ignore ‘anomalous’ data and so was ignoring the increase in the UV levels. On rewriting the code, they agreed with BAS and found the hole was worse than had been suggested and was growing at an increasing rate. This lead eventually to the 1987 Montreal Treaty where every world government signed up to a total ban on the manufacture and use of CFCs.

The instrument is a Dobson UV spectrophotometer designed by Gordon Dobson in the early 20s. With Dobson’s guidance, in 1924, it was taken on as a product by R & J Beck (of London). And, over the decades about 120 units were made. BAS and NASA insisted they were all made to the same drawings and only by Beck. This meant they would all calibrate identically, eliminating the possibility of anomalies being introduced by differences in the instruments.

There are two Dobsons with NASA which have been calibrated to be the world’s Primary and Secondary standards. And it is estimated that 100 or so survive. The last one was made in the 1980s.

The picture at the top of this section shows BAS scientists Joe Farman, Brian Gardiner and Jon Shanklin who announced their findings in Nature in 1985 along with their Beck Spectrophotometer.