oan doBante asicbabel Discussion There are at least five possible explanations for the anomalously high Pu-239/Sr-90 ratios observed in surface air since mid-1968. These are as follows: 1. Systematic analytical bias. The possibility of a systematic error in the radiochemical analyses seems very remote because through the period of anomalous ratios, both the stratospheric and surface air samples were analyzed by a single contractor laboratory (Trapelo/Wwest). The performance of the contractor during this period was monitoried by analysis of coded quality control samples and found to be excellent. Furthermore, the data are seen to be internally consistent, allowing for relatively smooth contouring. 2. Non-representative sampling. The representativeness of the Samples in both the Airstream and Surface Air programs have always been considered adequate. the debris, The filter media, the particle size of the face velocities and the general conditions of sampling were all considered in design of the equipment in these programs. In this light, erratic bias in the samplers seems highly unlikely. In addition the continuing balance in the Sr-90 budget in I cod - mame eco ae - oe - 18 - er . - FR Oy reerag pe a wen em . . . pt! 2 ‘ ey — - mr ew"