High 210Po Activity Concentration in the Surface Water of Malaysian Seas Driven by the Dry Season of the Southwest Monsoon (June–August 2009)

Abstract

Surface seawater was collected using a high-volume water pump in Malaysian waters during the first Prime Scientific Sailing Expedition in 2009 to obtain activity levels of lead-210 (210Pb) and polonium-210 (210Po). The 210Pb activity in surface water obtained from this sampling ranged from 0.22 ± 0.06 to 0.96 ± 0.31 mBq/L, averaging 0.58 ± 0.28 mBq/L, whereas the 210Po activity was between 1.52 ± 0.11 and 8.98 ± 0.96 mBq/L, averaging 4.19 ± 0.58 mBq/L. The 210Po/210Pb activity ratios in the surface water at the studied sites were between 3.48 ± 1.06 and 13.08 ± 3.78, with an average of 7.27 ± 3.73, which is distinctly higher than the range of 0.1 to 2.8 in the whole southern South China Sea and western Pacific regions. The relatively high activities of dissolved 210Po in Malaysian waters can be attributed to high depositional fluxes of the radionuclides from enhanced dry precipitations caused by haze events. The higher 210Po fractions found in this study compared with those found in the major oceans were due to terrestrial fires with estimated 210Po residence times between 0.15 and 0.51 years, slightly less than the 0.2- to 0.8-year residence times found in oceans with higher productivities. Higher residence times have also been observed more often offshore than near the shoreline. This phenomenon results in a slower rate of removal of 210Po from offshore surface water.