Studies in atmospheric ozone

• Main Author: Griggs, M.
• Published: University of Oxford 1961
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.734635

It has been known for many years that the variations, annual and day-to-day, in total ozone are caused by air movements in the stratosphere. To follow these air movements in detail, it is essential to measure the vertical distribution of ozone in the atmosphere. To date, only one instrument has been capable of measuring the fine structure of the ozone distribution. This is the transmogrifier developed by Brewer and Milford (1960). Unfortunately it is complicated in construction and is difficult to use. A new detector, the bubbler, is described. It is similar in principle to the transmogrifier, but is much simpler in both design and use* The detector is a galvanic cell with a platinum cathode and silver anode in buffered potassium iodide solution. The air containing ozone is bubbled through the solution, and the ozone is removed: 2KI + O₃ + H₂O → 2KOH + I₂ + O₂ The iodine reaches the platinum cathode by the mixing action of the bubbling, and the iodide ion is formed: I₂ + 2e → 2I^- This iodide ion is then removed from the solution at the silver anode: 2I^- - 2e + 2Ag → 2AgI. Thus, each ozone molecule entering the solution causes the passage or two electrons through the cell and external circuit. The detector is coulometric and gives a continuous measurement of the ozone with an exponential response time of about 20 seconds. For atmospheric concentrations of ozone, the bubbler output current can be up to 7μa, which is easily measured. The design of the bubbler, which is moulded in polystyrene, is perhaps unexpected, in that the air is taken through the pump and blown into the solution. In fact, little or no ozone is destroyed in the pump. Experiments were conducted to choose the best plastic materials, for the construction of the bubbler and pump, and the best lubricating oil for the pump. By certain treatments of the bubbler and pump, it is possible to keep the ozone losses below 8%. This is a small disadvantage compared with the advantages of a blowing system which are listed. The bubbler is incorporated into an ozonesonde, using much the same auxiliary equipment as used with the transmogrifier, although modification of the transductor which converts the d.c. ozone current into an audio signal for transmission by the Kew sonde, has given improved performance. Investigation of the temperature effects on the ozonesonde shows that a correction for a change in sensitivity of the transductor must be made. This was not realised with the transmogrifier, and the results obtained with it may be up to 10% low near the top of the ascents. The possible errors in the results are examined, and the maximum error in the absolute values is about ±20% or 1x10^-3cm/km, whichever is larger. Changes in ozone readings during a flight are significant if they are greater than ±15% or 1x10^-3cm/km. Analysis of the flights suggest that, in fact, the absolute accuracy of the results is better than indicated. More t > an 60 flights, with very few failures, have been made at Liverpool, Nairobi and Malta. Most flights were made at Liverpool once a week for over a year. The flights at Nairobi are the first to give detailed ozone structure near the equatorial tropopause, and they show only small ozone concentrations just above it. Two approaches are made to extract as much information as possible out of the Liverpool series of ascents. The first is to consider the variations of ozone and temperature in stratospheric layers in conjunction with the changes in total ozone. The second method la to consider the ozone and vorticity changes at three pressure levels in the lower stratosphere. Consideration of the ozone and temperature changes in layers enables a tentative discussion to be made about the mechanism of the day-to-day changes in the ozone distributions. The discussion is necessarily tentative due to the lack of a statistically significant number of ascents. The discussion suggests that in the period August to January, before the breakdown of the polar vortex, vertical movement is the main cause of ozone changes between the tropopause and 50mb. Between 50mb. and 25 mb, the lower limit of the photochemical equilibrium layer, advection is the main cause. Advection from the north (south) is generally associated with ascent (subsidence) in the lower layers. In the period January to August, it is suggested that advection is the main cause of ozone changes right up to 25mb., although vertical movement is playing a large part in the lowest levels of the stratosphere. The results for the stratosphere below 100mb. are confirmed in another tentative discussion based on the ozone-vorticity-temperature variations at 100mb, 150mb and 200mb. A brief discussion of the equatorial tropopause is made on the basis of the Nairobi ascents, and it is concluded that the air is rising through the tropopause. Finally, a brief discussion of the long-period use of a glass bubbler for surface measurements is given, along with some interesting results obtained in aircraft.