MEASUREMENTS OF RESIDUAL TRACES OF CS-137 IN THE ATMOSPHERE IN NEW-ZEALAND

High-volume air samplers were installed in New Zealand during 1985 but atmospheric weapons-tests Cs-137 levels were already below the 0.5 mu Bq/m(3) 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 d efection limits of 0.07 or 0.02 mu Bq/m(3), respectively. Caesium-137 was detected in quarterly aggregates from 1986, 1987 and 1988 at level s of 0.08-0.36 mu Bq/m(3), with annual trends indicative of stratosphe ric origin. It was detected in all yearly filter aggregates with annua l average concentrations decreasing from 0.25 mu Bg/m(3) in 1986 to 0. 02 mu Bq/m(3) during 1991-1993. A comparison of this trend in New Zeal and with that observed prior to 1986 in Australia indicated that the C s-137 was of Chinese weapons-tests origin, with no contribution from t he Chernobyl nuclear reactor accident.