Factors controlling temporal and spatial variations of 7Be and 210Pb deposition in northern Taiwan
Abstract
Fluxes of 7Be and 210Pb monitored at two contrasting sites in northern Taiwan were studied along with meteorological data to elucidate factors controlling their temporal and spatial variations. The nine-year time series of both nuclides at Nankang in the Taipei Basin show dependence on wet precipitation and follow an annual cycle regulated by typhoons, monsoons and mei-yu. Superimposed on the annual cycle are interannual variation caused by ENSO and intra-seasonal oscillations due to the passage/invasion of fronts, cold surges, dust storms and the Pacific high pressure system. The observed 7Be/210Pb ratios are indicative of the source regions of the fallout nuclides, with higher ratios from high-altitude rain and lower ratios from dry fallout and low-altitude rain. The two-year time series at Yangminshan shows that 7Be and 210Pb fluxes at the mountainous site are in phase with those at Nankang. However, nuclide fluxes at Yangminshan are 4-5 times of those at Nankang. Combining results from this and other ancillary studies at Yangminshan we suggest that: (1) in applying fallout nuclides to study the Earth's surface processes, the assumption of constant flux should be made judicially according to the timescales involved; and that (2) the application of fallout nuclides to study soil erosion in hilly areas must consider not only wet and dry precipitation but also direct contact and interaction of clouds with soils.