Abstract: This work describes the development and testing of a new instrument for in situ measurements of sulfur dioxide (SO2) on airborne platforms in the upper troposphere and lower stratosphere (UTLS). The instrument is based on the laser-induced fluorescence technique and uses the fifth harmonic of a tunable fiber-amplified semiconductor diode laser system at 1084.5 nm to excite SO2 at 216.9 nm. Sensitivity and background checks are achieved in flight by additions of SO2 calibration gas and zero ai…