Better measurements reveal seasonal changes in sulfur

Researchers from the University of Maryland (UMD) and the National Institute of Standards and Technology (NIST) have developed a new and improved technique for the simultaneous measurement of sulfur isotopic ratios and concentrations of atmospheric sulfate using snow samples from Greenland and Kyrgyzstan.

Sulfur plays an important role in the Earth’s climate. Sulfate particles in the atmosphere scatter and absorb sunlight, provide “seeds” for cloud formation, and affect the reflectivity and radiance of clouds, and thus the temperatures at the Earth’s surface. Atmospheric sulfate comes from natural sources, including oceans and volcanoes, but a large fraction comes from the burning of fossil fuels. Researchers can distinguish between various natural and anthropogenic sources in snow by measuring sulfur isotopes–forms of the element with different numbers of neutrons.

To study how these particulates have changed over time, scientists dig holes in snow that provide an archive of atmospheric particles deposited on the Earth’s surface. The standard analysis technique, gas-source isotope ratio mass spectroscopy (GIRMS), requires relatively large samples–up to four kilograms (about 9 pounds) of snow and ice, but the cycling of sulfur in the atmosphere is dynamic and variable, so samples this large blur seasonal changes.

To solve this problem, the UMD/NIST team developed a new analytical tool based on thermal ionization mass spectrometry (TIMS), which requires much smaller samples. The researchers used an advanced calibration technique known as double isotopic spiking to correct measurement drift and obtain isotope ratio measurements comparable to or better than GIRMS. The smaller snow samples required for TIMS make it possible to distinguish seasonal changes in sulfur particulate composition. The technique also can be used for making highly precise and accurate measurements of sulfur in low-sulfur fossil fuels, and similar applications.