NASA - National Aeronautics and Space Administration

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.

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.

The Unmanned Aircraft Systems (UAS) Chromatograph for Atmospheric Trace Species (UCATS) was designed and built for autonomous operation on remotely piloted aircraft, but has also been used on manned aircraft. It uses chromatography to separate atmospheric trace gases along narrow heated columns, followed by precise and accurate detection with electron capture detectors. There are currently three chromatography channels on UCATS, which measure nitrous oxide (N2O) and sulfur hexafluoride (SF6); CFC-11, CFC-12, CFC-113, and halon 1211; and chloroform (CHCl3) and carbon tetrachloride. On an earlier version of UCATS, with only two channels, we also measured methane, hydrogen, and carbon monoxide, along with N2O and SF6. 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 outside the aircraft, measured, and vented. UCATS has flown on the Altair UAS, the GV during HIPPO, the NASA Global Hawk UAS during the Global Hawk Pacific (GloPac) and ATTREX missions, where a record was set for the longest duration research flight (more than 28 hours), the DC-8 for ATom, and the ER-2 for DCOTSS. 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 or Global Hawk. 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. 3-channel (formerly 2-channel, up until 2020) gas chromatograph (GC)
2. Dual-beam ozone photometer (OZ)
3. Tunable diode laser (TDL) spectrometer for water vapor (WV)

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.

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.

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.

The TAMMS is composed of several subsystems including: (1) distributed pressure ports coupled with absolute and differential pressure transducers and temperature sensors, (2) aircraft inertial and satellite navigation systems, (3) a central data acquisition/processing system, and (4) water vapor instruments and potentially other trace gas or aerosol sensors.