NASA - National Aeronautics and Space Administration

The NOy instrument has three independent chemiluminescence detectors for simultaneous measurements of NOy, NO2, and NO. Each detector utilizes the reaction between NO in the sample with reagent O3. The NO/O3 reaction produces excited state NO2 which emits light of near 1µ m wavelength. Emitted photons are detected with a cooled photomultiplier tube.

Because NOy species other than NO do not respond in the chemiluminescence detector, NOy component species are reduced to NO by catalytic reduction on a gold surface with carbon monoxide (CO) acting as a reducing agent. Conversion efficiencies are > 90% at surface temperatures of 300°C. An NO signal representing NOy is then detected by chemiluminescence in the detector module. The catalyst is located outside the aircraft fuselage in order to avoid inlet line losses. NO2 is photolytically converted to NO in a glass cell in the presence of intense UV light between 300 and 400 nm. The conversion fraction is > 50% for a residence time of 1 s. The chemiluminescence detector detects NO as well as the additional NO from NO2. The third channel measures NO directly by passing the ambient sample through the detector module.

The response of each detector is checked several times in flight by standard addition of NO or NO2 calibration gas. The baseline of each measurement is determined in part by the addition of synthetic air that contains no reactive nitrogen. A continuous flow of water vapor is added directly to the sample flow in order to reduce the background signal in the detectors.

The sampling inlet for NOy is located outside the fuselage of the aircraft in a separate football-shaped housing. The shape of the housing allows for the inertial separation of large aerosols (> 5 µm diameter) from the NOy inlet at the downstream end of the housing.

The NO2-ClO-ClONO2-BrO instrument is composed of two separate instruments: A laser-induced fluorescence instrument for the detection of NO2 and a thermal dissociation/resonance fluorescence instrument for the detection of ClO, ClONO2 and BrO.

The NO2 detection system uses laser-induced resonance fluorescence (LIF) for the direct detection of NO2. Ambient air passes through a detection axis where the output of a narrow bandwidth (0.06 cm-1), tunable dye laser operating near 585 nm is used to excite a rovibronic transition in NO2. The excited NO2 molecules are either quenched by collision with air or fluorescence. The NO2 fluorescence is strongly red-shifted, with emission occurring over a broad range of wavelengths from 585 nm to the mid-infrared. The specificity of the technique is accomplished by tuning the laser frequency on and off resonance with a narrow spectral feature (0.04 cm-1) in the NO2 absorption spectrum. The difference between the fluorescence signal on and off resonance is related to the mixing ratio of NO2 through laboratory and in-flight calibrations. The observations are determined with an accuracy (1 sigma) of ±10% ±50 pptv, precision (1 sigma) of ±40 pptv, and a reporting interval of 10 seconds. Higher resolution (0.25 sec) data available on request.

The halogen detection system uses gas-phase thermal dissociation of ambient ClONO2 to produce ClO and NO2 radicals. The pyrolysis is accomplished by passing the air sampled in a 5-cm-square duct through a grid of resistively heated silicon strips at 10 to 20 m/sec, rapidly heating the air to 520 K. The ClO fragment from ClONO2 is converted to Cl atoms by reaction with added NO, and Cl atoms are detected using ultra-violet resonance fluorescence at 118.9 nm. A similar detection axis upstream of the heater provides simultaneous detection of ambient ClO. An identical twin sampling duct provides the capability for diagnostic checks. The flight instrument is calibrated in a laboratory setting with known addition of ClONO2 as a function of pressure, heater temperature and flow velocity. The concentration of ClONO2 is measured with an accuracy and detection limit of ±20% and 10 pptv, respectively, in 35 seconds (all error estimates are 1 sigma). The concentration of ClO is measured with an accuracy and detection limit of ±17% and 3 pptv, respectively, in 35 seconds.

The Whole Air Sampler (WAS) collects samples from airborne platforms for detailed analysis of a wide range of trace gases. The compounds that are typically measured from the WAS includes trace gases with sources from industrial midlatitude emissions, from biomass burning, and from the marine boundary layer, with certain compounds (e.g. organic nitrates) that have a unique source in the equatorial surface ocean. The use of a broad suite of tracers with different sources and lifetimes provides powerful diagnostic information on air mass history and chemical processing that currently is only available from measurements from whole air samples. Previous deployments of the whole air sampler have shown that the sampling and analytical procedures employed by our group are capable of accessing the wide range of mixing ratios at sufficient precision to be used for tracer studies. Thus, routine measurement of species, such as methyl iodide, at <= 0.1 x 10-12 mole fraction, or NMHC at levels of a few x 10-12 mole fraction are possible. In addition to the tracer aspects of the whole air sampler measurements, we measure a full suite of halocarbon species that provide information on the role of short-lived halocarbons in the tropical UT/LS region, on halogen budgets in the UT/LS region, and on continuing increasing temporal trends of HFCs (such as 134a), HCFCs (such as HCFC 141b), PFCs (such as C2F6), as well as declining levels of some of the major CFCs and halogenated solvents. The measurements of those species that are changing rapidly in the troposphere also give direct indications of the age and origin of air entering the stratosphere.

The Submillimeterwave Limb Sounder (SLS) is a heterodyne radiometer measuring thermal emission spectra near 640 GHz (for detection of ClO, HCl, and O3) and 604 GHz. (for detection of HNO3 and N2O) designed for use on high altitude balloons and aircraft. The instrument consists of five subsystems:

-optics which define the instrument field of view (FOV)
-radiometer front-ends which down converts incoming radiance signals
-intermediate frequency (IF) stage which selects and frequency shifts signal bands
-spectrometers which frequency resolve and detect the incoming power spectrum
-command and data handling which controls the instrument and transmits data to the ground

Limb scanning is accomplished by a flat mirror (~20 cm diameter) connected to a stepper motor (0.2 steps) and 14 bit position encoder. This mirror is also used for gain and zero calibration by viewing an absorber target located below the mirror and upward at 47° elevation angle to view the cold sky. A set of three off-axis parabolic reflectors form the instrument field of view (0.35 full width at half maximum) and couple limb radiance to the mixer input waveguide. These reflectors are oversized (~30 dB edge taper) to minimize side lobes in the FOV. Pointing and beam shape were verified by scanning the instrument FOV across the emission from a 600 GHz transmitter (multiplied output of a Gunn oscillator) located in the receiver optical far-field.

The radiometer front-end is an uncooled second harmonic mixer using a waveguide mounted Schottky diode. The radiometer is operated double side band (DSB), i.e., spectral features occurring symmetrically above and below the effective local oscillator frequency (637.050 GHz) appear together in the IF output spectrum. The diode is pumped at a 318.525 GHz. This source is generated by a tripled 106.175 GHz phase-locked InP Gunn oscillator and wave guide coupled to the mixer block. The mixer produces an IF output spectrum of 10.5 to 13 GHz, which corresponds to signals at the mixer input at 647.5 GHz to 650.0 GHz (in the radiometer upper side band) and 626.5 GHz to 624.1 GHz ( in the lower side band). The design of the 604 GHz radiometer system is similar to 637 GHz system but operates at a lower IF frequency of 2 to 3 GHz.

Diagram of the SLS frequency down-conversion scheme. RF signals enter the signal flow path through mixer feeds at the left of the diagram. At the right side, the signal flow enters a set of UARS MLS-type filterbank spectrometers where bands are further spectrally resolved, power detected, and digitized.

The NOAA-O3 instrument consists of a mercury lamp, two sample chambers that can be periodically scrubbed of ozone, and two detectors that measure the 254-nm radiation transmitted through the chamber. The ozone absorption cross-section at this wavelength is accurately known; hence, the ozone number density can be easily calculated. Since the two absorption chambers are identical, virtually continuous measurements of ozone are made by alternating the ambient air sample and ozone scrubbed sample between the two chambers. At a one-second data collection rate, the minimum detectable concentration of ozone (one standard deviation) is 1.5 x 10 10 molecules/cm 3 (0.6ppbv at STP).

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 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.