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A winning proposal for the Innovative Research Program, 2008: Measurement of low water-vapor mixing ratios using mass spectrometryInvestigators: Troy Thornberry1,2,3, Ru-Shan Gao2, David Fahey1,2 Objective: Design and construction of a prototype instrument using mass spectrometry capable of making fast time response (< 1s) measurements of water vapor at the low (1 to 10 parts per million) mixing ratios typical of the upper troposphere and lower stratosphere. Background: Water vapor is the most important greenhouse gas in the atmosphere and represents a major feedback to warming and other changes in the climate system [Trenberth et al., 2007]. Knowledge of the distribution of water vapor and how it is changing as climate changes is especially important in the upper troposphere and lower stratosphere (UT/LS) where water vapor plays a critical role in determining atmospheric radiative balance, cirrus cloud formation, and photochemistry. Dehydration processes reduce water vapor amounts to part per million (ppm) values in the UT/LS. The microphysics related to dehydration and cirrus cloud nucleation are poorly understood at present, limiting our ability to accurately model the dehydration process and, hence, our ability to estimate the influence of climate change on the UT/LS water vapor distribution. Part of this limitation derives from the large uncertainty in available water measurements in the 1 to 10 ppm range typical of this region of the atmosphere. For example, in situ instruments operated on airborne platforms in the UT/LS consistently show significant disagreement (~25 - 50%) at low water values. The consequence of these differences is that there is a wide range of supersaturation over ice observed in the UT/LS with the largest values being over 200% [Peter et al., 2006]. Such values are not consistent with our understanding of the fundamental microphysics of ice formation. A technique that combines an in-flight internal reference measurement with high sensitivity and fast time response, and that could be developed into a small payload instrument (< 50 kg) for deployment aboard a high altitude aircraft or unmanned aircraft system would do much to advance the state-of-the-science regarding water vapor in the UT/LS. This proposed instrument builds upon the extensive experience in ESRL/CSD with using mass spectrometry in atmospheric measurements. Research Plan: Mass spectrometry is a highly sensitive tool used for the measurement of many gases, including low levels of water vapor contamination in semiconductor fabrication processes. This monitoring is typically accomplished with a residual gas analyzer (RGA). These are compact quadrupole mass spectrometers, typically with limited mass range. The stated detection limit of a number of commercial RGA instruments of < 1 x 10-13 mbar with an upper sampling pressure of ~1 x 10-5 mbar yields a mixing ratio detection limit of 1 x 10-8, or 0.01 ppm. We propose to build a prototype instrument by adapting a commercially available residual gas analyzer and coupling it to a custom-designed inlet interface. A sketch of the proposed configuration is shown in Figure 1. The custom inlet will be designed to interface the RGA to the atmosphere through a pumped, pressure-controlled region. A common problem measuring low ambient H2O levels is desorption of water from instrument surfaces that results in a large, slowly decaying background signal in instruments launched from the surface to measure in the UT/LS. Adsorption and desorption can also lead to a measurement hysteresis artifacts when the sampled water vapor concentration changes sufficiently rapidly. The desorption artifact will be minimized or eliminated by using an inlet with very small surface area and porous walls (frit) through which a strong sample flow will be maintained by active pumping and the pressure kept constant using an active pressure control valve. The inlet minimization will be unique amongst existing water vapor instruments. Flow from the pressure-controlled inlet into the mass spectrometer will then pass through a pair of differentially pumped orifices. The absolute instrument calibration in the laboratory will be initially derived using a frost-point hygrometer instrument that has been constructed in ESRL. Later, links to a commercial frost-point hygrometer will be made. More importantly, the new instrument will reference the water vapor signal to that produced by ionization of ambient air (e.g., N2+ and O2+). The ionizer of the RGA will be operated at a reduced ionization energy of ~16 eV in order to yield molecular ions H2O+, N2+, and O2+ from H2O, N2, and O2, respectively, without producing significant quantities of fragment ions or doubly charged ions. Determination of the H2O+/O2+ or H2O+/N2+ ratios during laboratory calibration procedures due to simultaneous ionization of O2 and N2 will then be used as a continuous internal reference during ambient sampling to derive the time series of water vapor mixing ratios. Ambient pressure measurements, accurate to a few percent or better, will link the laboratory calibration to in-flight conditions. Other flow system components to simulate ambient flow and water vapor mixing ratios are available in our laboratory. 
Figure 1: Envisioned design for H2O mass spectrometer. Why is it innovative? There are a number of instruments that have been developed and deployed to measure the low levels of water vapor found in the UT/LS, but when operated together produce divergent results at the lowest mixing ratios. The instrument proposed here utilizes a fundamentally different analytical technique than the extant instruments and includes several potential advantages, including: 1) an internal reference measurement for determining water vapor mixing ratio, 2) insensitivity to ambient condition changes, and 3) low detection limit. This instrument has the potential to contribute significantly to the ongoing community effort to understand the processes controlling water vapor in the UT/LS. While confidence in the absolute mixing ratio and stability of calibration are the priority objectives with this new technique, the intrinsic fast response time of the instrument (< 1 s) will also provide an unprecedented view of the variability of water vapor in regions where microphysical processes such as ice formation and sedimentation are active. The fast response derives from the low inlet volume and the high sensitivity. Furthermore, the instrument will have high sensitivity for detecting water isotopes, which are an emerging diagnostic for the transport of water vapor into the UT/LS [Hanisco et al., 2007]. References |
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