Synonyms: 
Water Vapor
Water

Vertical Cavity Surface Emitting Laser Hygrometer

The VCSEL hygrometer is an open-path, laser-based hygrometer that measures absolute concentration of water vapor (molecules per cm-3) at a rate of 25 Hz. The instrument is designed for operation throughout the troposphere and lower stratosphere. Two water vapor absorption lines are used: a “weak” line at 1853.37 nm for lower tropospheric mixing ratios and a “strong” line at 1854.03 nm for middle and upper tropospheric concentrations. VCSELs have a wide current tuning capability and can probe each line by changing the laser injection current with only slight adjustments to the laser temperature. Switching between the absorption lines generally occurs near a fractional absorption of 10-3, though a hysteresis is built in to prevent rapid switching near this transition (generally a factor of four in each direction). The thresholds for line switching changes slightly with temperature and pressure, but it generally is in the -15 to -25 C frost point range.

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Gulfstream V - NSF
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Harvard Harriot Hygrometer

The Harvard Herriott Hygrometer (HHH) is a multipass Herriott cell that measures water vapor via direct detection. Predicted accuracy and precision are ± 3–5% and ± 0.05 ppmv H2O, in the lower stratosphere, for a 10-s integration time, respectively. The theory and application of HHH as a water vapor instrument are laid out in the context of making accurate measurements traceable to laboratory standards. In conjunction with the Harvard Water Vapor (HWV) instrument, HHH will establish ultimate credibility via three, independent detection methods in-flight and five for laboratory and in-field calibration. A multi-detection, calibration system of this nature is beyond the scope of any in existence today. Because HHH promises such high reliability and slight margins of error, the data acquired by this instrument should minimize the uncertainty associated with natural and anthropogenic climate forcing. HHH may serve as a prototype instrument for the use of miniaturized, TDL systems as in situ quantifiers of atmospheric gases via the straightforward method of direct detection, thus extending the scientific payback of this new system.

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UAS Chromatograph for Atmospheric Trace Species

The Unmanned Aircraft Systems (UAS) Chromatograph for Atmospheric Trace Species (UCATS) was designed and built for autonomous operation on pilotless aircraft. It uses chromatography to separate atmospheric trace gases along a narrow heated column, followed by precise and accurate detection with electron capture detectors. There are two chromatographs on UCATS, one of which measures nitrous oxide and sulfur hexafluoride, the other of which measures methane, hydrogen, and carbon monoxide. In addition, there is a small ozone instrument and a tunable diode laser instrument for water vapor. Gas is pumped into the instruments from an inlet below the GV, measured, and vented. UCATS has flown on the Altair UAS, the GV during HIPPO I and II, and most recently on the NASA/NOAA Global Hawk UAS during the Global Hawk Pacific (GloPac) mission, where a record was set for the longest duration research flight (more than 28 hours). UCATS is relatively lightweight and compact, making it ideal for smaller platforms, but it is easily adaptable to a mid-size platform like the GV for HIPPO. The data are used to measure sources and sinks of trace gases involved in climate and air quality, as well as transport through the atmosphere.

UCATS is three different instruments in one enclosure:

1. 2-channel gas chromatograph (GC)
2. Dual-beam ozone photometer (OZ)
3. Tunable diode laser (TDL) spectrometer for water vapor (WV)

Measurements: 
N2O, SF6, CH4, CO, O3, H2, H2O
Aircraft: 
Altair, Global Hawk - AFRC, DC-8 - AFRC, Gulfstream V - NSF, WB-57 - JSC, ER-2 - AFRC
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Lidar Atmosphere Sensing Experiment

The Differential Absorption Lidar uses the backscatter of two simultaneous laser wavelengths through zenith and nadir windows to measure the vertical profiles of H2O and aerosols/clouds.

NASA's Lidar Atmospheric Sensing Experiment (LASE) system is an airborne DIAL (Differential Absorption Lidar) system used to measure water vapor, aerosols, and clouds throughout the troposphere. LASE probes the atmosphere using lasers to transmit light in the 815-nm absorption band of water vapor. Pulses of laser light are fired vertically below the aircraft. A small fraction of the transmitted laser light is reflected from the atmosphere back to the aircraft and collected with a telescope receiver. The received light indicates the amount of water vapor along the path of the laser beam.

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Syed Ismail (Prev PI)

Balloonsondes (NOAA)

NOAA Balloonsonde payloads include a NOAA Frost Point Hygrometer (FPH), an Electrochemical Concentration Cell (ECC) ozonesonde, and a radiosonde to telemeter data to the ground and provide in situ measurements of temperature, pressure, relative humidity (surface to upper troposphere), and GPS coordinates. Sounding data typically reach an altitude of 28 km.

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Balloon
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Balloon Borne Frost Point Hygrometer

The NOAA Balloon-borne Frost Point Hygrometer is based on the chilled mirror principle. The FPH measures the temperature of a small mirror controlled to maintain a constant, thin layer of frost. Under stable conditions the mirror temperature equals the frost point temperature of the air passing over the mirror. The frost coverage on the mirror is detected by a photodiode that senses the light of a light-emitting diode (LED) reflected off the mirror surface. Both optical components are rigorously temperature controlled, minimizing drift in the LED's intensity and the photodiode's sensitivity. The reflectance signal is used to control the temperature of the mirror using P-I-D logic. The mirror temperature is measured by a well-calibrated bead thermistor. The mirror temperature is telemetered to the ground station (along with a large array of other data) by a radiosonde that also provides in situ measurements of ambient temperature, pressure, relative humidity (only in the lower and middle troposphere), and GPS coordinates.

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Balloon
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Fast In-situ Stratospheric Hygrometer

The Fast In situ Stratospheric Hygrometer (FISH), developed at the Forschungszentrum Jülich (Germany), is based on the Lyman-a photofragment fluorescence technique. Details of the instrument and the calibration procedure are described in Zöger et al. [1999]. FISH has been used in several campaigns both from balloon and aircraft and compared with a large number of other hygrometers [Kley et al., 2000].

FISH consists of a closed, vacuum-tight fluorescence cell, a Lyman-a radiation source, a PMT in photon-counting mode, detectors to monitor the VUV radiation output of the Lyman-a lamp, and a mirror drive that controls the measuring cycle (see diagram): determination of the fluorescence and background count rate and of the lamp intensity. With a measurement frequency of 1 Hz, the noise equivalent mixing ratio at 3 ppmv is 0.2-0.15 ppmv, and the detection limit is 0.18-0.13 ppmv.

FISH is calibrated between flights in the laboratory using a calibration bench under realistic conditions, that is varying the H2O mixing ratio of the test air from a few ppmv to several hundred ppmv and the pressure from 1000 to 10 hPa. A frost point hygrometer is used as a reference instrument. The overall accuracy of FISH measurements is 5-6 %.

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Cryogenic Hygrometer

Water vapor concentrations are measured using the cryogenically-cooled, chilled mirror hygrometer (Buck Research model CR-1). This instrument has a wide dynamic range (-90 to +30 C or approximately 1 to 30,000 ppmv H2O) and reasonably rapid response time (2 to 20 seconds, depending on the temporal and quantitative characteristics of the change in water vapor concentrations). The model CR-1 hygrometer utilizes a cryogenically chilled mirror and electro-optical technique to determine the dew/frost point of a gas. The primary difference between the CR-1 and other chilled mirror hygrometers is the mechanism used to cool the mirror surface. The mirror surface on which the dew/frost layer is preserved is coupled to a rod cooled by LN2 cryogen. The mirror surface is heated to the dew/frost point by means of a heater winding attached to the mirror rod. A control circuit controlled by optics monitors the reflectance from a LED off the mirror surface and maintains the condensate layer at a preset level. A thermistor embedded in the mirror measures the surface temperature and is output as a direct reading of the dew/frost point of the sample gas.

Air samples for the CR-1 hygrometer are provided by a separate window-mounted droplet-excluding inlet probe which has been used aboard the DC-8 platform in previous field missions. The in situ sampling probe consists of a stainless steel tubing inlet probe insert combined with a Rosemount type102 non-deiced temperature sensor housing. This type forward-facing probe provides inboard sampling of ambient air while maintaining efficient inertial separation of droplets and particles from the sampled air stream. The outer structural portion of the probe is manufactured by Rosemount Aerospace, Inc. and is flight-certified for use aboard both research and commercial jet aircraft. In normal subsonic flight, the inlet is self-pumping and develops enough pressure head to provide up to 15 liters/minute airflow through the approximately the 1 meter of ¼ “ stainless steel tubing which connects the inlet to the sensors. The tubing used to supply the sample air to the hygrometer is heated to approximately 50° C to avoid any chance of internal condensation in the sample line and reduce errors associated with wall effects.

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Fourier Transform Infrared Spectrometer

The absorption of infrared solar radiation along a slant path to the sun is recorded from 2 to 15 micrometers. Six spectral filters are used to cover the region from 2-15 microns. An interferogram is recorded in about 10 seconds. Interferograms are transformed to produce spectra. Column amounts are retrieved by fitting the observed spectra using the non-linear least squares fitting code SFIT2 that employs an Optimal Estimation retrieval algorithm.

The major chlorine reservoirs (HCl and ClONO2), the important nitrogen-containing gases in the stratosphere (N2O, NO, NO2, and HNO3), stratospheric and tropospheric tracers (HF, CH4, C2H6, H2O, CO2), a major source CFC (CF2Cl2) and ozone may be routinely retrieved.

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Unmanned Aerial System Laser Hygrometer

ULH measures water vapor at high accuracy in the upper troposphere and lower stratosphere to meet the following science objectives:

1. validation and scientific collaboration with NASA earth-monitoring satellite missions, principally the Aura satellite, http://aura.gsfc.nasa.gov/

2. observations of stratospheric trace gases in the upper troposphere and lower stratosphere from the mid-latitudes into the tropics,

3. sampling of polar stratospheric air and the break-up fragments of the air that move into the mid-latitudes, The ULH flights on Global Hawk will advance the state of the art technologically with remote command and control. ULH will provide real-time in-situ stratospheric water vapor measurements from Global Hawk. Additionally, ULH will make continuous measurements during long-duration flights up to 33 hours, which would be impossible with manned aircraft.

The advantages of ULH over other hygrometers are:

• Small and lightweight instrument package,
• No outgassing (achieved by mounting the open-path optical cell in the free air stream),
• Fast time response measurements in and out of clouds, without contamination,
• Accurate with a low detection limit <1 ppmv.

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