The use of radiocarbon as an environmental tracer is widespread and yet the production rate in the atmosphere has not been studied experimentally in any great detail. The research we present here set out to make direct measurements of the production rate at the cruising altitude of commercial aircraft where the production rate is much higher than at ground level. Modelled estimates (eg Lingenfelter and Ramaty, 1970, O'Brien, 1979, Masarik and Reedy, 1995) put the estimated total production rate at between 1.8-2.4 14C atoms/(cm2 s). A previous experimental study (Mak et al 1999) measuring the production rate at the ground had found this to be lower than expected from the models by a factor of almost 2.
In this experiment 92 measurements were made on 14CO and 14CO2 generated in compressed air cylinders carried in a Lufthansa aircraft over a period of two years. Blank measurements were also made on identical cylinders stored at ground level. The correlation of radiocarbon production with flight time is good but the results were different from those expected in two respects. Firstly the 14CO/14CO2 ratio was lower than expected (5 rather than 20) which may be the result of the high pressure in the gas bottles, although there is no observed correlation with gas pressure over the range of samples we took. Even more significantly the production rate is considerably higher than expected from the models, even taking into account the calculated overall effects of the aircraft on 14C production.
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