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(3,5,6) Given the ubiquity in nature, ideal half-life relevant to atmospheric and hydrological processes with a time scale of <1 year, and multiphase chemistry (gas, particle, aqueous solution), 35S is a unique and useful tracer to track and date a wide range of physical, chemical, and potentially biological processes on Earth’s surface. After dry/wet depositions of atmospheric radiosulfur on Earth’s surface, 35S embeds in the sulfur cycle in the regolith, cryosphere, hydrosphere, and potentially biosphere. (4) Due to the longer atmospheric residence time of sulfate than SO 2, most 35S in the atmosphere occurs as 35SO 4 2–. (3) Most gaseous 35SO 2 is subsequently oxidized to gaseous radiosulfuric acid or particulate radiosulfate ( 35SO 4 2–), and the rest is removed from the atmosphere via dry and wet depositions. (1) 35S is oxidized to radiosulfur dioxide ( 35SO 2) immediately after production (∼1 s) (2) and participates in the active atmospheric sulfur cycle. Specifically, the production rate of 35S at Earth’s surface is 1–2 orders of magnitude less than the maximum production rate in the stratosphere. As a result of cosmic ray cascades in the atmosphere, 35S production peaks at high altitudes (the stratosphere and upper troposphere). Radiosulfur ( 35S half-life: 87.4 days) is a naturally formed radionuclide by the high-energy cosmic ray bombardment of 40Ar in the Earth’s atmosphere. Based on these results, we conclude that our protocol allows the continuing utility of 35S measurements using a new-type LSC for a deeper understanding of the atmospheric sulfur cycle and its influences on the environment, climate, and public health. The obtained values are within the range of previously reported data from various mid-latitude sampling sites. As an application example, we measured concentrations of 35S in sulfate aerosols collected in Guangzhou, a megacity in subtropical South China. We therefore developed a new data processing protocol to determine 35S activities accurately and precisely in the range between ∼1 and ∼13 disintegrations per minute, and its validity was tested by working standards with known 35S activities. The counting efficiency of the new-type LSC for low- 35S activity samples (e.g., natural samples) is low and highly variable because a portion of true signals from 35S decay events was undesirably removed by GCT. To solve this problem, we extended the methods to a new-type LSC equipped with new mathematics-based active shielding techniques (Guard Compensation Technology GCT). These LSC methods heavily rely on instruments conventionally equipped with 650 kg lead blocks that passively shield cosmic and environmental background radiation, but this type of instrument is not commercially available anymore, hindering further applications of 35S.
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High-sensitivity measurements of radiosulfur (cosmogenic 35S half-life: 87.4 days) at natural abundance using ultra-low-level liquid scintillation counter (LSC) methods have been developed and optimized in the last decade, providing new details in space, atmospheric, and hydrological sciences.