A sensitive laser-induced fluorescence system for the detection of trace level sulfur dioxide in the remote troposphere

Sulfur dioxide (SO2) impacts climate by influencing the cloud condensation nuclei budget, particularly in the remote marine troposphere, thereby contributing to uncertainties in climate model estimates of effective radiative forcing. It is thus crucial to accurately and precisely quantify trace levels of SO2 in these environments, however, typical commercial instruments lack the necessary sensitivity. This thesis describes the development and validation of the University of York’s laser-induced fluorescence (LIF) instrument, the second in the world, for highly sensitive in situ measurements of SO2 based on custom fibre-laser technology. Its performance is then demonstrated through airborne and ship-based measurements in the remote North Atlantic and Arctic respectively.

Aircraft SO2 measurements were made by the York LIF instrument in the remote and ship-polluted marine troposphere, providing a wide range of target concentrations. These were compared to simultaneous SO2 measurements from a commercial pulsed fluorescence (PF) analyser and iodide chemical ionisation mass spectrometer (I−CIMS). Given their limits of detection (LoD, 3 σ) of 0.07, 0.4 and 2 ppb at 10 seconds for the LIF, PF and I−CIMS respectively, the percentage of data below their LoDs across the three flights were 9, 91 and 98 % respectively. Therefore, a comparison of instruments could only be made in polluted environments where the LIF, PF and I−CIMS agreed well, once an interference affecting the sensitivity of the I−CIMS was accounted for.

The first ship-based LIF-SO2 measurements were performed to obtain a unique dataset of SO2 in the Arctic, and used to explore its potential sources. Peaks of enhanced SO2 were accounted for by local shipping activity while background measurements were attributed to oxidation of dimethyl sulfide, emitted from the open ocean and sea ice edge.