Sulfur and carbon isotope measurements of carbonyl sulfide (COS) from small air samples; an overview and recent findings

Carbonyl sulfide (COS) is the most abundant sulfur-containing trace gas in the atmosphere, with an average mixing ratio of 500 parts per trillion (ppt). It has a relatively long lifetime of about 2 years, which permits it to travel into the stratosphere. There, it likely plays an important role in the formation of stratospheric sulfur aerosols (SSA), which have a cooling effect on the Earth’s climate. Furthermore, during photosynthetic uptake by plants, COS follows essentially the same pathway as CO₂, and therefore COS could be used to estimate gross primary production (GPP). Unfortunately, significant uncertainties still exist in the sources, sinks and global cycling of COS, which need to be overcome. Isotopic measurements of COS could be a promising tool for constraining the COS budget, as well as for investigating its role in the formation of stratospheric sulfur aerosols.

Within the framework of the COS-OCS project, we developed a GC-IRMS based measurement system at Utrecht University that can measure δ³³S, δ³⁴S and δ¹³C from S⁺ and CO⁺ fragment ions of COS from small air samples of 2 to 5 L. With this system, we have measured various types of air samples, including outside air, firn air from Greenland, and air from the upper troposphere – lower stratosphere region. We conducted photosynthesis experiments using a plant gas exchange chamber and we are also planning to measure firn air from Antarctica. Here, we will present an overview of the COS isotope measurements conducted within the COS-OCS project, and we will highlight the most interesting findings.