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Analytical Chemistry Summer Studentship 2022:

Carbonyl sulfide and its isotopic analogues – sentinels of the carbon cycle and driver of stratospheric climate

Development opportunities for the summer student

The project will expose the student to an analytical challenge involving environmental topics (stratospheric chemistry, photosynthesis, isotopic fractionation). It provides an opportunity for collaborative work in a scientific team, making contributions to potential publications. The student will gain skills in lab experiments and analytical data collection under the guidance of Prof. Kaiser and Dr Marca in the School of Environmental Sciences at the University of East Anglia. A hands-on approach will be needed to work with gases, cryogenics, vacuum systems, GC and IRMS. The measurements are interpreted quantitatively, to enhance the student's deductive and analytical skills.

Specific skills and qualities required:

Background

Carbonyl sulfide (COS) is a sulfur analogue of carbon dioxide (CO2). At a mole fraction of about 500 pmol mol-1, it is the most abundant reduced atmospheric sulfur compound and is emitted in large, but not very well known, quantities by oceans, wildfires and industry. The global lifetime is 2 to 4 years, governed primarily by plant and soil uptake as well as atmospheric loss reactions. Due to its relatively long lifetime, some COS reaches the stratosphere where it is photo-oxidised to form the stratospheric sulfate aerosol layer (see photo below), which is of major importance for ozone chemistry, the radiation balance and Earth's climate.

COS research in environmental sciences is on the rise because COS is similar enough to CO2 that its plant uptake rate can be used as a proxy for terrestrial gross photosynthesis (Asaf et al. 2013), a process of key ecological importance that is difficult to quantify otherwise. When taken up by the photosynthetic apparatus of plants, COS binds to the enzyme carbonic anhydrase. However, unlike for CO2, this is a one-way process for COS: It is hydrolysed irreversibly, and there is no corresponding release during respiration.

Atmospheric COS concentration measurements alone cannot distinguish between plant and soil uptake, so that the latter either has to be neglected or assumed to be equal to nighttime COS uptake rates. These assumptions are difficult to test with concentration measurements alone. Analyses of the mono- and polysubstituted isotopic analogues of COS (in terms of 13C/12C, 33S/32S, 34S/32S, 36S/32S, 17O/16O, 18O/16O and 34S13CO/32S12CO ratios) may provide additional constraints on sources and sinks of the gas and help quantify any confounding reactions, provided the associated isotopic fractionations are known.

Stratospheric aerosol layer (as seen by astronauts on the Space Shuttle)

Objectives

This project will investigate the potential of COS isotopologues to help address unresolved questions of the global COS cycle. The student will develop a gas chromatographic purification system coupled to an isotope ratio mass spectrometer (IRMS) with a bespoke COS detector array (see photo below). They will the use this system to measure rates and isotopic fractionation of selected COS removal processes.

Thermo MAT 253 isotope ratio mass spectrometer at UEA

Methodology

Our analytical setup for online GC-IRMS analysis uses a PoraBond Q column, with a pre-column and column backflush to avoid interference of late eluting peaks with subsequent samples. The GC will be optimised for separation of COS, CO2, CS2 and H2S (Kamezaki et al. 2019). We expect that adjusting column temperature, flow and valve timings will be sufficient to accomplish this. The photolysis experiments will use an antimony lamp (see photo below) with an emission spectrum close to stratospheric actinic fluxes, as previously used for measurements of CFC isotopologue fractionation (Zuiderweg et al. 2021). The temperature of the double-walled reactor will be controlled with a refrigerated thermostat to between –50 and +25 °C (the atmospherically relevant range). In addition, bandpass interference filters (centred at 200, 214 and 220 nm) will be used to modulate the lamp spectrum around the COS absorption peak at 223 nm (Hattori et al. 2011). Lastly, COS will be reacted on humidified neutral and alkaline surfaces to measure the isotopic fractionation associated with hydrolysis.

UV photolysis setup (back: turbopump and pressure gauges; front: lamp, reaction gas mixture, gas handling system)

References