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 (UT–LS). 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 air, respectively. Aircraft demonstration was performed during the NASA Volcano-Plume Investigation Readiness and Gas-Phase and Aerosol Sulfur (VIRGAS) experiment, which was a series of flights using the NASA WB-57F during October 2015 based at Ellington Field and Harlingen, Texas. During these flights, the instrument successfully measured SO₂ in the UT–LS at background (non-volcanic) conditions with a precision of 2 ppt at 10 s and an overall uncertainty determined primarily by instrument drifts of ±(16 % + 0.9 ppt).