Very high 14C activity in Abisko, Sweden during summer, 1965

During the summer of 1965 there was remarkably high 14C activity in the atmosphere at Abisko in the northern part of Sweden. The 14C activity over Spitsbergen was much lower and the increase started much later. The excess at Abisko was more than 30 000 per mille of the standard and was decreasing very rapidly. Samples collected at the end of April show the authors' measured maximum. Ten days earlier the intensity was about 760 per mille and ten days later about 7000 per mille above the standard. Twenty days after the large peak the excess was about 2000 per mille. In the beginning of September the excess was still a little more than 1000 per mille above the standard. The Abisko values indicate a second peak in July. The second peak is, however, only about 1300 per mille over the standard. The values for the winters before and after the summer of 1965 were about 760 and 700 per mille, respectively. In May 1966 the excess in Abisko was still below 750 per mille. The international 14C standard, 95% of the activity in 1950 of oxalic acid, from the National Bureau of Standards (NBS) in Washington, D.C., was used as the reference sample. The samples were collected by absorption in NaOH solution in a tray with an open surface of about 900 cm2. Each collection period was three or four days. The samplers were mounted in the open air a few metres above ground level. From the beginning of April to the end of November 1965 about 15 samples were measured from Abisko but only five from Spitsbergen. A possible explanation of the activity peak is contamination due to special 14C experiments in Abisko. (author)