Measurements of residual traces of 137Cs in the atmosphere in New Zealand

High-volume air samplers were installed in New Zealand during 1985 but atmospheric weapons-tests 137Cs levels were already below the 0.5 μBq/m3 detection limit achieved with weekly sampling periods. During 1993 all air filters collected from the Kaitaia monitoring site since January 1986 were reanalysed as quarterly or yearly aggregates with detection limits of 0.07 or 0.02 μBq/m3, respectively. Caesium-137 was detected in quarterly aggregates from 1986, 1987 and 1988 at levels of 0.08–0.36 μBq/m3, with annual trends indicative of stratospheric origin. It was detected in all yearly filter aggregates with annual average concentrations decreasing from 0.25 μBq/ m3 in 1986 to 0.02 μBq/m3 during 1991–1993. A comparison of this trend in New Zealand with that observed prior to 1986 in Australia indicated that the 137Cs was of Chinese weapons-tests origin, with no contribution from the Chernobyl nuclear reactor accident.