132 (2) operating the detector at 280° C rather than at room temperature, (3) using a pulsed powersupply, (4) using a larger volume (2 mi) for the sample loop, (5) using pure heliumas the carrier gas, and (6) using alumina as the column material. Additional work to insure the absolute accuracy and repeatability of the system and to refine analysis procedures will continue. BACKGROUND SFg MEASUREMENTS On February 27, 1969, six samples of air were col- lected at the Argonne site and analyzed for back- Four of the NAPCA continuous sample collectors were placed along Lake Shore Drive in Chicago, from 31st Street to Lincoln Park, on May 28. Samples were colleeted between 1135 and 1400 CST; none showed measurable SF, levels. In addition, a sampler was placed on the roof of the DAPC building, 320 North Clark Street. Four one-hour samples were collected between 1100 and 1500 CST; none was positive. It is clear that many more background samples must be collected and analyzed when the improved SF, detector system is completed and operational. SFg RELEASE EXPERIMENTS Three SF tracer gas experiments were made at the Argonne site in order to gain experience with the total ground levels of SFy. No gas was released during the collection period. The weather conditions at Argonne during the collection periods (about 1030 to 1130 CST) were: east winds at 10 mph, clear skies, temperature 23° F, very unstable air. In other words, the winds brought in polluted air from the City of Chicago, and very rapid dilution was occurring. Argonne were straight east (90°). Argonne buildings, and two were placed downwind of two Argonne buildings known to contain large quantities of SF,: the Tandem Van de Graaff accelerator building and the Zero Gradient Synchrotron. In 1968, 15,000 Ibs of SF, leaked from the Van de Graaff ma- were: wind ENE (70°) at 18 mph, elear skies, temperature 34° F, very unstable surface layer. The six samplers were placed along an cast-west road 45, 135, 300, 550, 750, and 950 meters downwind of the release point. Just as the releases began, the winds backed to second about 300 meters further downwind. Five of the six samplers showed SF, concentrations below 0.20 ppb, the maximum sensitivity of the de- the next one 5.65 ppb. The remaining four were below Two of the six samplers were placed upwind ofall chine. One sampler was placed near each building, the tector system at that time; the sample taken just outside the Van de Graaff building contained 0.78 ppb. Since projected changes in the analysis system will re- sult in sensitivities of one part in 101°, this background level and leak rate are disturbing. Sixteen more background samples were collected at Argonne during the morning of April 23, 1969; two of these were found to have measurable levels of SFe (>0.05 ppb). These two were located downwind of the Van de Graaff building. Additional background air samples were collected system: release, collection, and analysis. The gas release rate was about 3.5 g/see. The first of these releases was made between 1030 and 1100 CST on February 28, 1969. At the time the SF, tank and the samplers were located, the winds at The actual conditions during the release period ENE; as a result, only the two closest samplers were in the plume, The nearest sampler measured 342 ppb, 0.2 ppb. Another release was made at Argonne between 1518 and 1550 CST on March 3, 1969. The weather during this release period was: wind NNE (30°) 14 mph, sky clear, temperature 40° F, very unstable surface layer, (a sigma meter showed a standard deviation of the horizontal wind direction of 11°; for the vertical component, 5.5°). Seven samplers were placed in an are about 450 meters downwind of the source. One failed to collect any gas due to a poor connection between the pump and the sample bag; the other six showed concentrations between 5.6 and 23.0 ppb. The results of this release are not shownin figure form since the in Chicago on May 27 and 28, 1969, during the third fuel switch test. Strong WSWto SWwinds carried air absolute accuracy of the system was not known. A the air samples were collected along Lake Shore Drive stability from Argonne into the city. For this reason, most of and in the Loop; any large SFg sourees in the city would have been detected. Twelve instantaneous air samples were collected on May 27, eighteen more the next day. None showed SF, concentrations greater than the limit of detect- ability (again 0.05 ppb). Two samples yielded readings in the vicinity of the limit of detectability; it is believed that these represent baseline excursions rather than the positive results. simple diffusion calculation (Turner) for class “D” vields a center-line concentration of 18.2 ppb, somewhat lowerthan the 23.0 observed. The final release was made between 1145 and 1220 CST on April 17, 1969, with the tracer gas being emitted from the 34-meter Argonne experimental stack. Seven samplers were placed in an are 1500 meters downwind. Weather conditions during the release were: wind SE (140°) at 11 mph, cumulus overcast at 1500 meters, temperature, 19°C. A light shower dropped 0.01 inches of rain during the release.