NASA - National Aeronautics and Space Administration

Langley Aerosol Research Group Experiment (LARGE).  The "classic" suite of instrumenation measures in-situ aerosol micrphysical and optical properties. The package can be tailored for specific science objectives and to operate on a variety of aircraft. Depending on the aircraft, measurments are made from either a shrouded single-diffuser "Clarke" inlet, from a BMI (Brechtel Manufacturing Inc.) isokinetic inlet, or from a HIML inlet. Primary measurements include:

1.) total and non-volatile particle concentrations (3nm and 10nm nominal size cuts),
2.) dry size distributions from 3nm to 5µm diameter using a combination of mobilty-optical-aerodynamic sizing techniques,
3.) dry and humidified scattering coefficients (at 450, 550, and 700nm wavelength), and
4.) dry absorption coefficients (470, 532, and 670nm wavelength). 

LARGE derived products include particle size statistics (integrated number, surface area, and volume concentrations for ultrafine, accumulation, and coarse modes), dry and ambient aerosol extinction coefficients, single scattering albedo, angstrom exponent coefficients, and scattering hygroscopicity parameter f(RH).

A solar tracking Direct beam Irradiance Airborne Spectrometer (DIAS) is used for calculation of line of sight ozone and wavelength dependent aerosol optical depths.

GAMS/LAABS is a combination of the Gas and Aerosol Measurement System (GAMS) and the Langley Airborne A-Band Spectrometer (LAABS). The instruments are optically co-aligned and use a common pointing system to track the Sun through an aircraft view port. In the field the instrument provides line-of-sight (LOS) O3, NO2, O4, and water vapor measurements using both a SAGE III-like multiple linear regression algorithm and a full spectrum algorithm. Aerosol may also be derived for ‘enhanced’ conditions including polar stratospheric clouds and optically thin cirrus. Using profile data (1-D/2-D) transformed to GAMS/LAABS LOS geometry, quick-look validation/comparison products for SAGE III, AROTAL, AATS-14, SCIAMACHY, and other instruments will be obtained. The data from GAMS/LAABS will make possible crucial evaluations of SAGE III data processing possible following deployment. These activities include SAGE III etaloning/mirror correction validation, O2 spectroscopy and forward model verification, ozone spectroscopy near the O2 A band and 940-nm water vapor features, evaluation of the relative strength of spectroscopic features (e.g., water vapor features at 600 nm and 940 nm) and altitude registration validation using oxygen measurements.

Instrument Details:

Gas and Aerosol Measurement Sensor (GAMS)

o Solar spectrometer with 1024 channels from ~ 430 to 1030 nm
o Provides measurements of LOS transmission spectra and
differential O3, H2O, O2, O4, and aerosol
o Solar imager to monitor scene homogeneity
o Focus on UV-Vis-Near-IR solar occultation only
o Designed to extend the technique into the troposphere
o Built space flight-like spectrometer, telescope, photon-to-bits
boards, & MCM (detector controller).
o COTS imager to establish imager performance requirements

Langley Airborne A-Band Spectrometer (LAABS)

• High spectral resolution (~0.035 nm) grating spectrometer with > 800
channels from ~759 to 771 nm
• Provides measurements of LOS transmission spectra for evaluation of
SAGE III O2 A-band forward model
• Originally developed to support CALIPSO (formerly PICASSO-CENA)
spaceborne A-band spectrometer
• Designed and fabricated by BATC to provide high spectral resolution
(~0.035 nm) radiance measurements in O2 A-band spectral region
(~765 nm)

• Technique: Chemluminescent reaction of ozone with nitric oxide
• Dynamic Range: 0.6 - 1600 ppb
• Accuracy: 5% or 2 ppb
• Precision: 2% or 0.6 ppb
• Response: 2-3 Hz; recorded at 6 Hz, reported at 1 Hz, faster data on request
• Spatial Resolution: <10 m vertical (aircraft spiral), 200 m horizontal (at 400 kts)

PANTHER uses Gas Chromatography with Electron Capture Detection and (GC-ECD) and Gas Chromatography with Mass Selective Detection (GC-MSD) to measure numerous trace gases, including methyl halides, HCFCs, peroxyacetyl nitrate, nitrous oxide, SF6, CFC-12, CFC-11, Halon-1211, methyl chloroform, carbon tetrachloride.

3 ECDs with packed columns (OV-101, Porapak-Q, molecular sieve).

1 ECD with a TE (thermal electric) cooled RTX-200 capillary column.

2-channel MSD (mass selective detector). The MSD analyzes two independent samples air concentrated onto TE cooled Haysep traps, which are then heated to desorb the analytes and separate using through two temperature programmed RTX-624 capillary columns.

With the exception of PAN, all channels of chromatography are normalized to a stable in-flight calibration gas references to NOAA scales. The PAN data are normalized to an in-flight PAN source of ≈ 100 ppt with ±5 % reproducibility. This source is generated by efficient photolytic conversion of NO in the presence of acetone. Detector non-linearity is taken out by lab calibrations for all molecules.

The Microwave Temperature Profiler (MTP) is a passive microwave radiometer, which measures the natural thermal emission from oxygen molecules in the earth’s atmosphere for a selection of elevation angles between zenith and nadir. The current observing frequencies are 55.51, 56.65 and 58.80 GHz. The measured "brightness temperatures" versus elevation angle are converted to air temperature versus altitude using a quasi-Bayesian statistical retrieval procedure. The MTP has no ITAR restrictions, has export compliance classification number EAR99/NLR. An MTP generally consists of two assemblies: a sensor unit (SU), which receives and detects the signal, and a data unit (DU), which controls the SU and records the data. In addition, on some platforms there may be a third element, a real-time analysis computer (RAC), which analyzes the data to produce temperature profiles and other data products in real time. The SU is connected to the DU with power, control, and data cables. In addition the DU has interfaces to the aircraft navigation data bus and the RAC, if one is present. Navigation data is needed so that information such as altitude, pitch and roll are available. Aircraft altitude is needed to perform retrievals (which are altitude dependent), while pitch and roll are needed for controlling the position of a stepper motor which must drive a scanning mirror to predetermined elevation angles. Generally, the feed horn is nearly normal to the flight direction and the scanning mirror is oriented at 45-degrees with respect to receiving feed horn to allow viewing from near nadir to near zenith. At each viewing position a local oscillator (LO) is sequenced through two or more frequencies. Since a double sideband receiver is used, the LO is generally located near the "valley" between two spectral lines, so that the upper and lower sidebands are located near the spectral line peaks to ensure the maximum absorption. This is especially important at high altitudes where "transparency" corrections become important if the lines are too "thin." Because each frequency has a different effective viewing distance, the MTP is able to "see" to different distances by changing frequency. In addition, because the viewing direction is also varied and because the atmospheric opacity is temperature and pressure dependent, different effective viewing distances are also achieved through scanning in elevation . If the scanning is done so that the applicable altitudes (that is, the effective viewing distance times the sine of the elevation angle) at different frequencies and elevation angles are the same, then inter-frequency calibration can also be done, which improves the quality of the retrieved profiles. For a two-frequency radiometer with 10 elevation angles, each 15-second observing cycle produces a set of 20 brightness temperatures, which are converted by a linear retrieval algorithm to a profile of air temperature versus altitude, T(z). Finally, radiometric calibration is performed using the outside air temperature (OAT) and a heated reference target to determine the instrument gain. However, complete calibration of the system to include "window corrections" and other effects, requires tedious analysis and comparison with radiosondes near the aircraft flight path. This is probably the most important single factor contributing to reliable calibration. For stable MTPs, like that on the DC8, such calibrations appear to be reliable for many years. Such analysis is always performed before MTP data are placed on mission archive computers.

The nucleation-mode aerosol size spectrometer (NMASS) measures the concentration of particles as a function of diameter from approximately 4 to 60 nm. A sample flow is continuously extracted from the free stream using a decelerating inlet and is transported to the NMASS. Within the instrument, the sample flow is carried to 5 parallel condensation nucleus counters (CNCs) as shown in Fig. 1. Each CNC is tuned to measure the cumulative concentration of particles larger than certain diameter. The minimum detectable diameters for the 5 CNCs are 4.0, 7.5, 15, 30 and 55 nm, respectively. An inversion algorithm is applied to recover a continuous size distribution in the 4 to 60 nm diameter range.

The NMASS has been proven particularly useful in measurements of nucleation-mode size distribution in environments where concentrations are relatively high and fast instrumental response is required. The instrument has made valuable measurements vicinity of cirrus clouds in the upper troposphere and lower stratosphere (WAM), in the near-field exhaust of flying aircraft (SULFUR 6), in newly created rocket plumes (ACCENT), and in the plumes of coal-fired power plants (SOS ’99). The instrument has flown on 3 different aircraft and operated effectively at altitudes from 50 m to 19 km and ambient temperatures from 35 to -80ºC.

Accuracy. The instrument is calibrated using condensationally generated particles that are singly charged and classified by differential electrical mobility. Absolute counting efficiencies are determined by comparison with an electrometer. Monte carlo simulations of the propagation of uncertainties through the numerical inversion algorithm and comparison with established laboratory techniques are used to establish accuracies for particular size distributions, and may vary for different particle size distributions. A study of uncertainties in aircraft plume measurements demonstrated a combined uncertainty (accuracy and precision) of 38%, 36% and 38% for number, surface and volume, respectively.

Precision. The precision is controlled by particle counting statistics for each channel. If better precision is desired, it is necessary only to accumulate over longer time intervals.

Response Time: Data are recorded with 10 Hz resolution, and the instrument has demonstrated response times of this speed in airborne sampling. However the effective response time depends upon the precision required to detect the change in question. Small changes may require longer times to detect. Plume measurements with high concentrations of nucleation-mode particles may be processed at 10 Hz.

Specifications: Weight is approximately 96 lbs, including an external pump. External dimensions are approximately 15”x16”x32”. Power consumption is 350 W at 28 VDC, including the pump.

The MkIV interferometer operates in solar absorption mode, meaning that direct sunlight is spectrally analyzed and the amount of various gases at different heights in the Earth's atmosphere is derived from the shapes and depths of their absorption lines. The optical design of the MkIV interferometer is based largely on that of the ATMOS instrument, which has flown four times on the Space Shuttle. The first three mirrors in the optical path comprise the suntracker. Two of these mirrors are servo-controlled in order to compensate for any angular motion of the observation platform. The subsequent wedged KBr plates, flats, and cube-corner retro-reflectors comprise a double-passed Michelson interferometer, whose function is to impart a wavelength-dependent modulation to the solar beam. This is achieved by sliding one of the retro-reflectors at a uniform velocity so that the recombining beams interfere with each other. A paraboloid then focusses the solar beam onto infrared detectors, which measure the interferometrically modulated solar signal. Finally, Fourier transformation of the recorded detector outputs yields the solar spectrum. An important advantage of the MkIV Interferometer is that by employing a dichroic to feed two detectors in parallel, a HgCdTe photoconductor for the low frequencies (650-1850 cm-1) and a InSb photodiode for the high frequencies (1850-5650 cm-1), the entire mid-infrared region can be observed simultaneously with good linearity and signal-to-noise ratio. In this region over 30 different gases have identifiable spectral signatures including H2O, O3, N2O, CO, CH4, NO, NO2, HNO3, HNO4, N2O5, H2O2, ClNO3, HOCl, HCl, HF, COF2, CF4, SF6, CF2ClCFCl2, CHF2Cl, CF2Cl2, CFCl3, CCl4, CH3Cl, C2H2, C2H6, OCS, HCN, N2, O2, CO2 and many isotopic variants. The last three named gases, having well known atmospheric abundances, are important in establishing the observation geometry of each spectrum, which otherwise can be a major source of uncertainty. Similarly, from analysis of T-sensitive CO2 lines, the temperature profile can be accurately determined. The simultaneity of the observations of all these gases greatly simplifies the interpretation of the results, which are used for testing computer models of atmospheric transport and chemistry, validation of satellite data, and trend determination.

Although the MkIV can measure gas column abundances at any time during the day, the highest sensitivity to atmospheric trace gases is obtained by observing sunrise or sunset from a balloon. The very long (~ 400 km) atmospheric paths traversed by incoming rays in this observation geometry also make this so-called solar occultation technique insensitive to local contamination.

The DLH has been successfully flown during many previous field campaigns on several aircraft, most recently ACTIVATE (Falcon); FIREX-AQ, ATom, KORUS-AQ, and SEAC4RS (DC-8); POSIDON (WB-57); CARAFE (Sherpa); CAMP2Ex and DISCOVER-AQ (P-3); and ATTREX (Global Hawk). This sensor measures water vapor (H2O(v)) via absorption by one of three strong, isolated spectral lines near 1.4 μm and is comprised of a compact laser transceiver and a sheet of high grade retroflecting road sign material to form the optical path. Optical sampling geometry is aircraft-dependent, as each DLH instrument is custom-built to conform to aircraft geometric constraints. Using differential absorption detection techniques, H2O(v) is sensed along the external path negating any potential wall or inlet effects inherent in extractive sampling techniques. A laser power normalization scheme enables the sensor to accurately measure water vapor even when flying through clouds. An algorithm calculates H2O(v) concentration based on the differential absorption signal magnitude, ambient pressure, and temperature, and spectroscopic parameters found in the literature and/or measured in the laboratory. Preliminary water vapor mixing ratio and derived relative humidities are provided in real-time to investigators.