An intercomparison of aircraft sulfur dioxide measurements in clean and polluted marine environments

The University of York's laser-induced fluorescence (LIF) instrument for measuring sulfur dioxide (SO2) was compared to a commercial pulsed fluorescence (PF) analyser and iodide chemical ionisation mass spectrometer (I−CIMS) aboard the UK FAAM research aircraft in both remote and ship-polluted marine environments. In high SO2 concentration plumes, the LIF instrument and PF analyser compared well, but LIF was the only instrument capable of SO2 measurements in the remote marine boundary layer due to its campaign limit of detection (LoD, 3σ) of 0.07 ppb at 10 s compared with 0.4 ppb for the PF analyser. Quantification of SO2 using I−CIMS was challenging due to a significant interference, but good signal correlation with the other instruments was observed in polluted air masses. A comparison of response time was also made, for which the I−CIMS and LIF instrument proved much faster than the PF analyser with 3 e-folding times of 0.6, 2 and 17 s respectively. This work demonstrates the importance of sensitive instrumentation like the LIF system for quantifying low concentrations of SO2, such as over remote marine environments, at the time resolutions required for a fast moving platform. This is particularly relevant now as a result of more stringent sulfur emission regulations for shipping, and likely more so in the future as anthropogenic SO2 concentrations continue to decline.