PhD defence: Isotope measurements of carbonyl sulfide

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 uptake by plants, but measuring this uptake is difficult because plants also ‘breathe out’ CO₂. Another uncertainty in current climate models is the cooling effect of small sulfur particles in the stratosphere.

Carbonyl sulfide (COS) is the most abundant sulfur-containing trace gas in the atmosphere, 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 CO₂ uptake. Furthermore, COS stays in the atmosphere for a relatively long time, and can therefore be transported to the higher atmosphere. Therefore, it could be the main contributor to sulfur particles in the stratosphere.

In this PhD research, we used measurements of COS isotopes to answer questions on the COS budget, including uptake by plants and transport to the stratosphere. For this, we developed a new measurement system. We measured COS isotopes in air samples from Utrecht, conducted plant chamber experiments, and sampled air in the stratosphere using a large balloon launched from northern Sweden.