The Harvard water vapor instrument

The Harvard Lyman-a instrument has been previously described in detail [Weinstock et al., 1994]. A simple detection axis diagram is shown in Figure 1. Briefly, 121.6 nm (Lyman_a) radiation from an RF discharge lamp photodissociates water vapor in a 2-inch duct. A fraction of the resulting OH fragments are formed in their first excited electronic state (A²S⁺), and the OH fluorescence at ~315 nm is collected at right angles to the Lyman_a beam through a filter and detected with a photomultiplier tube (PMT). Because the fluorescence is strongly quenched by collisions with O₂ and N₂ at a rate proportional to the air density, at altitudes of the upper troposphere and lower stratosphere the observed detector signal is proportional to the water vapor volume mixing ratio. Solar scatter in the WB57 instrument is eliminated by a light trap in the duct. Lamp scatter near 315 nm is measured by using a quartz window to periodically block the L_a beam. Changes in lamp intensity monitored with a vacuum photodiode opposite the lamp are used to normalize the fluorescence signal. A rear-surface MgF₂ mirror surrounding the diode (changed form a less robust front- surface mirror in June 1996) reflects some of the radiation back across the duct to a second diode, allowing water measurements by direct (Beer's Law) absorption at sufficiently high water vapor (mid to upper troposphere). The instrument is functionally identical to the ER-2 version, differing only in duct dimensions (2" rather than 3" square cross section) and in the details of inlet design.

The detection axis is mounted in the spear pod of the WB-57, as is schematically illustrated in Figure 2. The 5 cm id secondary duct picks up the central laminar core of flow from the 10 cm id primary duct that protrudes from the spearpod. The duct flow velocities can be controlled using the primary and secondary duct throttle valves. In previous missions, flow velocities in the duct have typically been varied from 30 to 70 m/s for fast time response and to avoid (and directly test for) contamination from walls. Because of the increasing scientific focus on cloud properties in upcoming campaigns, the decoupling of the water vapor measurement from the OH/HO₂ measurement, and the concern for particle evaporation in the ram-heated duct, the duct flow velocity in the secondary duct will be significantly increased, potentially approaching the WB-57 true air speed, with a resulting concurrent decrease in ram heating. Pitot tubes and thermistors respectively monitor the velocity and temperature in each of the ducts.

The instrument is calibrated in the laboratory both before and after each field deployment by flowing known amounts of water vapor in the air through the detection axis [Weinstock et al., 1994]. Saturated air-water mixtures are prepared by passing air through a two-stage bubbler apparatus. These are diluted with additional dry air before being sent to the detection axis, which is integrated into the laboratory flow system. The instrument response is linear up to concentrations of ~10¹⁶ H₂O molecules/cm³ (typical of the mid troposphere), and the calibration is determined as a function of density to directly measure the effect of quenching on the OH fluorescence. The accuracy of the laboratory water vapor calibration system is 3%, based on uncertainties in temperature, pressure, and gas flow, and has been verified by direct absorption both along and across the detection axis.

A critical part of the calibration strategy is that we test the calibration in flight by comparing L_a photofragment fluorescence with direct absorption measurements of water vapor on ascent and descent—a calibration method first used by Kley et al. [1979]. This approach uses the atmosphere to provide a wide range of H₂O concentrations. Data in the very dry overworld are used for a reference signal (I₀), since at cruise altitudes, stratospheric concentrations are too low (~10¹³) to measure H₂O by direct absorption across our detection axis (l = 5.9 cm). At lower altitudes, concentrations are high enough (10¹⁴-10¹⁶/cm³) for measurements of I, and hence the water vapor concentrations by direct (Beer's Law) absorption:

where s is the absorption cross section of H₂O at 121.6 nm and l is the path length. The first diode measures the lamp intensity at the mirror surface (to account for changes in lamp flux), and the second diode detects the intensity of the reflected beam after it has passed through the absorption path, so the ratio of diode 1 signal to diode 2 signal is used for I and I₀. As a diagnostic check, diode 1 or diode 2 signal can also be used separately to calculate absorption, with the assumption that the lamp output is constant. L_a radiation is also absorbed by O₂, and we account for this by using the formula

We use a cross section of 1.57 · 10^-17 cm² for water vapor from Lewis et al. [1983] and an O₂ cross section from Kley [1984]. Lamp spectra have been checked for radiation other than at L_a.

Water Vapor Instrument

ER-2 Performance

WB-57 Performance

References