Laser Hygrometer Research Articles

Comparisons of in situ measurements of cirrus cloud ice water content

Aircraft‐based in situ measurements of cirrus cloud ice water content (IWC) are important for cloud microphysical/radiative modeling and satellite validation studies, yet the measurements have proven challenging due, in part, to the large dynamic range of IWC values present in cirrus clouds. To date, three instruments designed for the measurement of IWC have been flown aboard the NASA WB‐57F research aircraft: the University of Colorado closed‐path laser hygrometer, the Droplet Measurement Technologies Cloud Spectrometer and Impactor, and the Harvard University Lyman‐α total water photofragment‐fluorescence hygrometer. This paper compares IWC measurements from these three instruments taken during the Midlatitude Cirrus Experiment (MidCiX) in 2004. At larger values of IWC (IWC > 10 mg m−3), the three instruments agree, on average, to within 20%, which is of the order of their estimated instrumental uncertainties. At smaller IWC values (<10 mg m−3), the agreement is worse, in part due to increasing instrument uncertainties. These results have implications for measurements in thin and subvisual cirrus. Particle sampling and evaporation, instrument background levels, and hysteresis are not found to be significant contributors to discrepancies among the measurements. For remote sensing validation studies where IWC data are vertically integrated to obtain ice water path, the agreement between the instruments is better than 20% for thick cirrus (τ > 1), implying that IWC measurements in thicker clouds are of sufficient accuracy for validation studies.

Measurement of Total Water with a Tunable Diode Laser Hygrometer: Inlet Analysis, Calibration Procedure, and Ice Water Content Determination

AbstractThe University of Colorado closed-path tunable diode laser hygrometer (CLH), a new instrument for the in situ measurement of enhanced total water (eTW, the sum of water vapor and condensed water enhanced by a subisokinetic inlet), has recently been flown aboard the NASA DC-8 and WB-57F aircrafts. The CLH has the sensitivity necessary to quantify the ice water content (IWC) of extremely thin subvisual cirrus clouds (∼0.1 mg m−3), while still providing measurements over a large range of conditions typical of upper-tropospheric cirrus (up to 1 g m−3). A key feature of the CLH is its subisokinetic inlet system, which is described in detail in this paper. The enhancement and evaporation of ice particles that results from the heated subisokinetic inlet is described both analytically and based on computational fluid dynamical simulations of the flow around the aircraft. Laboratory mixtures of water vapor with an accuracy of 2%–10% (2σ) were used to calibrate the CLH over a wide range of water vapor mixing ratios (∼50–50 000 ppm) and pressures (∼100–1000 mb). The water vapor retrieval algorithm, which is based on the CLH instrument properties as well as on the spectroscopic properties of the water absorption line, accurately fits the calibration data to within the uncertainty of the calibration mixtures and instrument signal-to-noise ratio. A method for calculating cirrus IWC from the CLH enhanced total water measurement is presented. In this method, the particle enhancement factor is determined from an independent particle size distribution measurement and the size-dependent CLH inlet efficiency. It is shown that despite the potentially large uncertainty in particle size measurements, the error introduced by this method adds ∼5% error to the IWC calculation. IWC accuracy ranges from 20% at the largest IWC to 50% at small IWC (<5 mg m−3).

Compact and Low-Power Diode Laser Hygrometer for Weather Balloons

Abstract A vertical cavity laser hygrometer has been developed for balloon-borne measurements. This hygrometer meets many of the specifications that are required for common weather balloon instrumentation, including weight, size, time response, and sensitivity. In addition, the system cost has been dramatically lowered relative to prior diode laser instruments. Sensitivity of 2 and 0.9 μbar at low- and high-altitude conditions, respectively, has been obtained in a 1-s measurement period, with the strong possibility of improved performance in the near future. The engineering advances that are demonstrated by this system make balloon-borne diode laser hygrometry a practical technique for more frequent use.

Laser Hygrometer Using a Vertical-Cavity Surface-Emitting Laser (VCSEL) With an Emission Wavelength of 1.84<tex>$muhboxm$</tex>

This paper presents a hygrometer setup for measuring the concentration of water vapor in air by means of infrared absorption spectroscopy. Recently developed vertical-cavity surface-emitting lasers (VCSELs) emitting near 1.84 /spl mu/m are used as source of the infrared light. The laser hygrometer is able to calculate the density of water molecules from the measurement of one single light-current characteristic, without any additional reference measurement. The absolute and relative humidity are calculated from the water molecule density and the temperature of the ambient air.

Characterization of Upper-Troposphere Water Vapor Measurements during AFWEX Using LASE

AbstractWater vapor mass mixing ratio profiles from NASA's Lidar Atmospheric Sensing Experiment (LASE) system acquired during the Atmospheric Radiation Measurement (ARM)–First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) Water Vapor Experiment (AFWEX) are used as a reference to characterize upper-troposphere water vapor (UTWV) measured by ground-based Raman lidars, radiosondes, and in situ aircraft sensors over the Department of Energy (DOE) ARM Southern Great Plains (SGP) site in northern Oklahoma. LASE was deployed from the NASA DC-8 aircraft and measured water vapor over the ARM SGP Central Facility (CF) site during seven flights between 27 November and 10 December 2000. Initially, the DOE ARM SGP Cloud and Radiation Testbed (CART) Raman lidar (CARL) UTWV profiles were about 5%–7% wetter than LASE in the upper troposphere, and the Vaisala RS80-H radiosonde profiles were about 10% drier than LASE between 8 and 12 km. Scaling the Vaisala water vapor profiles to match the precipitable water vapor (PWV) measured by the ARM SGP microwave radiometer (MWR) did not change these results significantly. By accounting for an overlap correction of the CARL water vapor profiles and by employing schemes designed to correct the Vaisala RS80-H calibration method and account for the time response of the Vaisala RS80-H water vapor sensor, the average differences between the CARL and Vaisala radiosonde upper-troposphere water vapor profiles are reduced to about 5%, which is within the ARM goal of mean differences of less than 10%. The LASE and DC-8 in situ diode laser hygrometer (DLH) UTWV measurements generally agreed to within about 3%–4%. The DC-8 in situ frost point cryogenic hygrometer and Snow White chilled-mirror measurements were drier than the LASE, Raman lidars, and corrected Vaisala RS80H measurements by about 10%–25% and 10%–15%, respectively. Sippican (formerly VIZ Manufacturing) carbon hygristor radiosondes exhibited large variabilities and poor agreement with the other measurements. PWV derived from the LASE profiles agreed to within about 3% on average with PWV derived from the ARM SGP microwave radiometer. The agreement between the LASE and MWR PWV and the LASE and CARL UTWV measurements supports the hypotheses that MWR measurements of the 22-GHz water vapor line can accurately constrain the total water vapor amount and that the CART Raman lidar, when calibrated using the MWR PWV, can provide an accurate, stable reference for characterizing upper-troposphere water vapor.

Measurements of ice water content in tropopause region Arctic cirrus during the SAGE III Ozone Loss and Validation Experiment (SOLVE)

A new instrument, the closed‐path laser hygrometer (CLH), was flown on the NASA DC‐8 aircraft during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) campaign in 1999–2000 to measure condensed‐phase water. The ice water content (IWC) of Arctic cirrus was determined from in situ measurements of condensed‐ and gas‐phase water. The IWC values obtained from the CLH observations are compared to those determined by integrating particle size distributions measured by a Forward Scattering Spectrometer Probe (FSSP) also flown on the DC‐8. The considerably greater IWC seen by the CLH implies the presence of particles with diameters greater than the FSSP's upper limit of 20 μm. The evidence for and implications of the presence of large ice crystals in Arctic cirrus is discussed.

Atomic layer deposition and characterization of hafnium oxide grown on silicon from tetrakis(diethylamino)hafnium and water vapor

In this work thin films of hafnium oxide are deposited on Si(100) substrates by means of atomic layer deposition (ALD) using tetrakis(diethylamino)hafnium and water vapor at substrate temperatures of 250–350ºC. Our system capabilities include fast transient delivery of reactive fluids, real-time vapor phase detection (in situ tunable diode laser hygrometer), precursor thermochemical capabilities, and ppt level elemental analysis by inductive coupling plasma mass spectrometry. The composition, purity, and other properties of the films and resulting interfaces are determined using x-ray and Fourier transform infrared spectroscopies, Z-contrast imaging and electron energy loss spectroscopy in a scanning transmission electron microscope with Å scale resolution, and spectroscopic ellipsometry. The observed ALD rate is ∼1.4Å per cycle. The nonuniformity across the film is less than 4%. Negligible carbon contamination is found in the resulting stoichiometric films under all conditions studied. The pulse sequence was optimized to prevent disastrous particulate problems while still minimizing purge times. The film deposition is investigated as a function of substrate temperature and reagent pulsing characteristics. A mild inverse temperature dependence of the ALD rate is observed. The initial stage of the HfO2 growth is investigated in detail.

Mars laser hygrometer.

We have designed and built a miniature near-IR tunable diode laser (TDL) spectrometer for measuring in situ the water vapor mixing ratio either in the Martian atmosphere or thermally evolved from Martian soil or ice samples. The laser hygrometer uses a thermoelectrically cooled single-mode distributed-feedback TDL at 1.87 microm to scan over a selected vibration-rotation line of both H2O and CO2 near 5327.3 cm(-1). A working prototype that weighs only 230 g has been built and used to generate spectra whose analysis demonstrates precision sensitivities as fine as 1 part in 10(6) by volume in 1 s or 0.1 part in 10(6) in 10 s at Martian pressures and temperatures. Absolute uncertainties of approximately 5% are calculated.

Calibration and data retrieval algorithms for the NASA Langley/Ames Diode Laser Hygrometer for the NASA Transport and Chemical Evolution Over the Pacific (TRACE‐P) mission

This paper describes the procedures and algorithms for the laboratory calibration and field data retrieval of the NASA Langley/Ames Diode Laser Hygrometer as implemented during the NASA Transport and Chemical Evolution Over the Pacific (TRACE‐P) mission during February–April 2000. The calibration is based on a National Institute of Standards and Technology traceable dew point hygrometer using relatively high humidity and short optical path length. Two near‐infrared water vapor absorption lines of widely different strengths are used to increase the dynamic range of the instrument in the course of a flight. The laboratory results are incorporated into a numerical model of the second harmonic spectrum for each of the two spectral window regions using spectroscopic parameters from the HITRAN database and other sources, allowing water vapor retrieval at upper tropospheric and lower stratospheric temperatures and humidity levels. The data retrieval algorithm is simple, numerically stable, and accurate. A comparison with other water vapor instruments on board the NASA DC‐8 and ER‐2 aircraft is presented.

Comparison of ER‐2 aircraft and POAM III, MLS, and SAGE II satellite measurements during SOLVE using traditional correlative analysis and trajectory hunting technique

We compared the version 5 Microwave Limb Sounder (MLS), version 3 Polar Ozone and Aerosol Measurement III (POAM III), version 6.0 Stratospheric Aerosol and Gas Experiment II (SAGE II), and NASA ER‐2 aircraft measurements made in the Northern Hemisphere in January–February 2000 during the SAGE III Ozone Loss and Validation Experiment (SOLVE). This study addresses one of the key scientific objectives of the SOLVE campaign, namely, to validate multiplatform satellite measurements made in the polar stratosphere during winter. This intercomparison was performed by using a traditional correlative analysis (TCA) and a trajectory hunting technique (THT). TCA compares profiles colocated within a chosen spatial‐temporal vicinity. Launching backward and forward trajectories from the points of measurement, the THT identifies air parcels sampled at least twice within a prescribed match criterion during the course of 5 days. We found that the ozone measurements made by these four instruments agree most of the time within ±10% in the stratosphere up to 1400 K (∼35 km). The water vapor measurements from POAM III and the ER‐2 Harvard Lyman α hygrometer and Jet Propulsion Laboratory laser hygrometer agree to within ±0.5 ppmv (or about ±10%) in the lower stratosphere above 380 K. The MLS and ER‐2 ClO measurements agree within their error bars for the TCA. The MLS and ER‐2 nitric acid measurements near 17‐ to 20‐km altitude agree within their uncertainties most of the time with a hint of a positive offset by MLS according to the TCA. We also applied the Atmospheric and Environmental Research, Inc. box model constrained by the ER‐2 measurements for analysis of the ClO and HNO3 measurements using the THT. We found that: (1) the model values of ClO are smaller by about 0.3–0.4 (0.2) ppbv below (above) 400 K than those by MLS and (2) the HNO3 comparison shows a positive offset of MLS values by ∼1 and 1–2 ppbv below 400 K and near 450 K, respectively. Our study shows that, with some limitations (like HNO3 comparison under polar stratospheric cloud conditions), the THT is a more powerful tool for validation studies than the TCA, making conclusions of the comparison statistically more robust.

Wavelength-modulation laser hygrometer for ultrasensitive detection of water vapor in semiconductor gases

Water vapor is measured by use of a near-infrared diode laser and wavelength-modulation absorption spectroscopy. Humidity levels as low as 5 nmol/mol [1 nmol/mol = 1 ppb (1 ppb equals 1 part in 10(9))] of water vapor in air are measured with a sensitivity of better than 0.2 nmol/mol (3varsigma). The sensitivity, linearity, and stability of the technique are determined in experiments conducted at the National Institute of Standards and Technology, Gaithersburg, Maryland, by use of the low frost-point humidity generator over the range from 5 nmol/mol to 2.5 mumol/mol of water vapor in air. The pressure-broadening coefficients for water broadened by helium [0.0199(6) cm(-1) atm(-1) HWHM] and by hydrogen chloride [0.268(6) cm(-1) atm(-1) HWHM] are reported for the water line at 1392.5 nm.

Tropospheric water vapor measurements over the North Atlantic during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX)

Fast‐response tunable diode laser measurements of water vapor were made over the North Atlantic during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) conducted in the fall of 1997. Resulting water vapor mixing ratios, in conjunction with air temperature and pressure measurements obtained by the DC‐8's Data Acquisition and Distribution System (DADS), were used to calculate the prevalence of ice‐saturated conditions (relative humidity with respect to ice >100% or frost saturation) in the upper troposphere over the SONEX sampling region. Additionally, Appleman theory was applied to the data to determine the subset of ice‐saturated regions which would support formation of contrails. Results suggest that studies to determine the potential climate or radiative impact of contrails and aviation‐induced cirrus clouds should not only consider the climatology of frost‐saturated regions, but should also address whether ambient conditions can support contrail formation within these regions. A separate discussion within the paper describes an in‐flight intercomparison between the SONEX diode laser hygrometer and the Pollution From Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT 2) cryogenic hygrometer and shows that the instruments perform to within their stated accuracies (∼10%) over a range of mixing ratios and altitudes.

On the accuracy of in situ water vapor measurements in the troposphere and lower stratosphere with the Harvard Lyman‐α hygrometer

In an effort to better constrain atmospheric water vapor mixing ratios and to understand the discrepancies between different measurements of water vapor in the stratosphere and troposphere, we have carefully examined data from the Harvard Lyman‐α photofragment fluorescence hygrometer, which has flown on the NASA ER‐2 aircraft from 1992 through 1998. The instrument is calibrated in the laboratory before and after each deployment, and the calibration is checked by direct absorption measurements in the troposphere. On certain flights, the ER‐2 flew level tracks during which water vapor varied by up to 80 ppmv, under nearly constant atmospheric conditions; These flights provide a stringent test of our calibration via direct absorption and indicate agreement to within 3%. During the 1997 Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) mission, our Lyman‐α instrument was compared with a new diode laser hygrometer from the Jet Propulsion Laboratory. Overall agreement was 5% during the June/July deployment and 1% for potential temperatures of 490 to 540 K. The accuracy of our instrument is shown to be ±5%, with an additional offset of at most 0.1 ppmv. Data from this instrument, combined with simultaneous measurements of CH4 and H2, are therefore ideal for studies of the hydrogen budget of the lower stratosphere.

In-situ sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer

molecules cm-3 Hz-1/2 (signal-to-noise ratio 3). This is equivalent to a mixing ratio of 0.3 ppmv at average midlatitude tropopause conditions or a mixing ratio of 0.6 ppmv under boundary-layer conditions. The corresponding minimum measurable absorbance is 10-5 Hz-1/2. The laser hygrometer was field-demonstrated aboard the NASA P3B research aircraft, during a series of flights spanning several weeks in the summer of 1997. During this demonstration, the laser hygrometer was intercompared with two optical chilled mirror hygrometers. In general, the laser hygrometer performed well; however, under some conditions, it reported water vapor number densities 20% greater than the cryogenic frost-point hygrometer. This difference is currently under study.

Environmental conditions required for contrail formation and persistence

The ambient temperatures and humidities required for contrail formation and persistence are determined from in situ measurements during the Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS) experiment. Ambient temperatures and water vapor concentrations were measured with the meteorological measurement system, a laser hygrometer, and a cryogenic hygrometer (all onboard the DC‐8). The threshold temperatures are compared with theoretical estimates based on simple models of plume evolution. Observed contrail onset temperatures for contrail formation are shown to be 0–2 K below the liquid‐saturation threshold temperature, implying that saturation with respect to liquid water must be reached at some point in the plume evolution. Visible contrails observed during SUCCESS persisted longer than a few minutes only when substantial ambient supersaturations with respect to ice existed over large regions. On some occasions, contrails formed at relatively high temperatures (≥−50°C) due to very high ambient supersaturations with respect to ice (of the order of 150%). These warm contrails usually formed in the presence of diffuse cirrus. Water vapor from sublimated ice crystals that entered the engine was probably necessary for contrail formation in some of these cases. At temperatures above about −50°C, contrails can only form if the ambient air is supersaturated with respect to ice, so these contrails should persist and grow.