Effects of flow rates and composition of the filter, and decay/ingrowth correction factors involved with the determination of in situ particulate Po-210 and Pb-210 in seawater

Abstract

Accurate measurements of particulate Po-210 (Po-210(p)) and Pb-210 (Pb-210(p)) are required in the investigation of (i) partitioning of Po and Pb between particulate and dissolved phases and (ii) export estimates of carbon and other key trace metals from the euphotic zone via sinking particulate matter. Based on the intercomparison of different composition of the filter material (QMA, Supor, Millipore, and Pall GN6) and flow rates (2 to 8 L min), we show how these factors affect the measured concentrations of Po-210(p) and Pb-210(p) and their activity ratios (AR). As such, we recommend using Supor 0.4 mu m filter and a flow rate of up to 8 L/min for the measurements of Po-210(p) and Pb-210(p). Furthermore, we inter-compared Po-210(p) and Pb-210(p) obtained by small-volume McLane and large-volume MULVFS pumps. The activities of Po-210(p) in MULVFS filter samples are somewhat higher than that of McLane filter samples, whereas the Po-210/Pb-210 AR collected by McLane pump is distinctly lower, suggesting some fractionation in the collection process by the pumping systems. Likewise comparison of vertical profiles of Po-210(p) and Pb-210(p) obtained using McLane pumps by two independent research groups at the two intercalibration stations in the Pacific Ocean show quite similar values except in the mesopelagic waters, suggesting possible uneven loading and presence of larger gelatinous plankton in the filter. Finally, we append a detailed analysis of various correction factors for the accurate calculation of in situ Po-210 and Pb-210. Presented results are relevant to the worldwide community that uses Po-210-Pb-210 as tracers in aquatic systems.