Photofragment laser-induced fluorescence instrument for the detection of HDO and H₂O

[Hoxotope Instrument Paper] [Isotopes GRL Draft]

The HOx isotope instrument combines a new water photolysis system with our pre-existing instrument for laser induced fluorescence detection of OH. Water is photolyzed with an excimer lamp at 172 nm, producing ground state OH and OD radicals. The radicals are detected with rotational state selective laser induced fluorescence at 287 nm. The instrument has the sensitivity to measure the relative abundance of H₂O and HDO in situ aboard the WB-57 throughout the upper troposphere and lower stratosphere. The fluorescence experiment has two unique characteristics. The first is the high sensitivity: At stratospheric mixing ratios of H₂O (4 ppm at 50 mbar), the relative abundance of H₂O and HDO can be measured with a S/N > 12 in a 16 s acquisition cycle. The second is fast time response: The fluorescence is collected a 1 cm³ region in the center of a well defined flow. This minimizes the detection of isotopes from the boundary layer of the duct where wall exchange, uptake, and desorption occur. This also enables the use of a small sampling volume (< 500 cm³), since long path lengths are not required.