Measurement of low-ppm mixing ratios of water vapor in the upper troposphere and lower stratosphere using chemical ionization mass spectrometry

[摘要] A chemical ionization mass spectrometer (CIMS) instrument has been developedfor the fast, precise, and accurate measurement of water vapor (H₂O) atlow mixing ratios in the upper troposphere and lower stratosphere (UT/LS). Alow-pressure flow of sample air passes through an ionization volumecontaining an α-particle radiation source, resulting in a cascade ofion-molecule reactions that produce hydronium ions (H₃O⁺) fromambient H₂O. The production of H₃O⁺ ions from ambientH₂O depends on pressure and flow through the ion source, which weretightly controlled in order to maintain the measurement sensitivityindependent of changes in the airborne sampling environment. The instrumentwas calibrated every 45 min in flight by introducing a series ofH₂O mixing ratios between 0.5 and 153 parts per million (ppm,10⁻⁶ mol mol⁻¹) generated by Pt-catalyzed oxidation of H₂ standards whileoverflowing the inlet with dry synthetic air. The CIMS H₂O instrumentwas deployed in an unpressurized payload area aboard the NASA WB-57F high-altituderesearch aircraft during the Mid-latitude Airborne CirrusProperties Experiment (MACPEX) mission in March and April 2011. Theinstrument performed successfully during seven flights, measuring H₂Omixing ratios below 5 ppm in the lower stratosphere at altitudes up to17.7 km, and as low as 3.5 ppm near the tropopause. Data were acquired at 10 Hzand reported as 1 s averages. In-flight calibrations demonstrated a typicalsensitivity of 2000 Hz ppm⁻¹ at 3 ppm with a signal to noise ratio(2 σ, 1 s) greater than 32. The total measurement uncertainty was 9to 11%, derived from the uncertainty in the in situ calibrations.

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