Isotope measurements of carbonyl sulfide
Authors:   , Baartman Sophia Louise 
Published: August 2023

The Earth’s climate is currently changing due to excessive human greenhouse gas emissions. Carbon dioxide (CO2) is the most important greenhouse gas, so it is essential to accurately understand the carbon cycle and its response to climate change. The largest sink of CO2 is photosynthetic uptake by the terrestrial biosphere. However, directly measuring gross primary productivity (GPP) is difficult because the biosphere also emits CO2 through respiration.

Another uncertainty in climate models is the effect of stratospheric sulfur aerosols (SSA), which reflect incoming solar radiation, thereby cooling the planet. The main contributor to the formation of the SSA layer under volcanic quiescent conditions is, however, still under debate.
Carbonyl sulfide (COS) is the most abundant sulfur-containing trace gas, with a tropospheric mole fraction of around 500 parts per trillion (ppt). COS is taken up by plants through a similar pathway as CO2, and has therefore been proposed as a proxy for estimating GPP. COS has a relatively long lifetime of approximately 2 years, allowing it to be transported to the stratosphere. Therefore, it could be a likely candidate for the main precursor of SSA in volcanically quiescent times.

The largest source of COS is the ocean, through direct and indirect emissions. Other sources are mainly anthropogenic. The terrestrial sinks of COS are the large plant uptake flux and a small soil uptake flux. Atmospheric sinks include destruction reactions in the troposphere and stratosphere. The budget of COS is still not fully understood, with a large missing source in current budget estimations.

Isotope measurements can provide information regarding the budget of COS. During this PhD project, a measurement system was developed for determining the isotopic composition of COS in air, with the general goal of increasing the knowledge on the global COS budget. The system was optimized to obtain a precision that is sufficient for observing the small isotope signals expected in atmospheric COS, from relatively small air samples.

We present a series of ambient air measurements in Utrecht and found sulfur isotopic compositions of +1.0 ± 3.4 ‰ and +15.5 ± 0.8 ‰ for δ33S and δ34S, respectively. We observed a seasonal variation in COS mole fraction, but no significant change in isotope values.

To investigate whether COS could be the main precursor of SSA, we measured the isotopic composition of air samples taken in the upper troposphere lower stratosphere region, collected during two campaigns in Nepal and Sweden.

We found decreasing mole fractions with altitude, and small fractionation factors for 34ε, which indicate that COS is likely the main precursor of SSA. We did, however, find a much smaller than expected value for 13ε. Based on TM5 model results, we deduced that atmospheric mixing and transport processes likely dilute the isotope fractionation signal.

We also conducted flow-through plant chamber experiments, using both a C3 and a C4 species. While the plants were exposed to varying amounts of light, we measured CO2 and COS mole fractions online and took samples for analysis of both CO2 and COS isotopic composition.