A new laser induced fluorescence instrument for in-situ measurements of SO2 throughout the troposphere and lower stratosphere

Abstract:

Sulfur dioxide (SO₂) is a ubiquitous naturally and anthropogenically emitted species, impacting air quality and climate in many ways. Measurements of SO₂ are challenging in the stratosphere and remote troposphere due to very low mixing ratios (~10 ppt), and in polluted regions due to interfering species. New scientific understanding will be achievable with SO₂ instrumentation capable of making accurate measurements in the low ppt range in the upper troposphere and lower stratosphere.

We have developed a new laser-induced fluorescence based instrument for aircraft measurements of SO₂ from the surface up to the lower stratosphere. The instrument uses a custom-built tunable fiber laser that produces > 1mW at 216.9 nm. Photons in the range 250-400 nm fluorescing from laser-excited SO₂ are counted, yielding a 1-second detection limit (S:N=2) of < 20 ppt. The instrument has been compared on the ground to a commercial pulsed fluorescence instrument (TECO) and has been flown on the NASA WB-57F in the lower stratosphere. Here we will present details of the instrument, and results from the initial field deployment.