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

Abstract

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.