A Comparison of Aircraft SO2 Measurements in Remote and Polluted Marine Environments

Sulfur dioxide (SO₂) plays a pivotal role in the chemistry of the troposphere, ultimately affecting the Earth’s radiation balance and climate. Within the atmosphere, SO₂ is oxidised by gas- and aqueous-phase chemistry to sulfate and is therefore a major precursor to atmospheric aerosols, particularly in the remote marine atmosphere. Both the direct radiative forcing from aerosols and the indirect forcing from aerosol-cloud interactions are poorly understood and produce large uncertainties in climate models. Therefore, it is of interest to precisely quantify the concentration of atmospheric SO₂ if we are to predict the effects of changing emission rates on both climate and air quality.

Anthropogenic SO₂ emissions have fallen dramatically in recent decades, resulting in significant reductions in atmospheric concentrations. Current commercial SO₂ detection techniques, for example pulsed fluorescence, are no longer sensitive enough to detect trace levels of SO₂ such as those found in remote marine environments. We report the development of a laser-induced fluorescence instrument for in situ SO₂ measurements using a custom-built, tunable fiber-amplified laser system. Based on the system initially developed by Rollins et al. (2016), the University of York LIF-SO₂ system has a detection limit of 50 ppt for 30 seconds and its relatively small size, weight and power requirements makes this instrument suitable for a variety of field campaigns.

Here we present aircraft measurements of SO₂ made by the York LIF-SO₂ instrument on board the UK FAAM research aircraft in both the remote and polluted marine boundary layer. These are then compared to simultaneous SO₂ measurements made by the University of Manchester I^- chemical ionisation mass spectrometer (I^- CIMS) instrument and the FAAM pulsed fluorescence commercial SO₂ detector.