NASA - National Aeronautics and Space Administration

Solar viewing Fourier Transform Interferometer (FTIR). This is a ground based instrument stationed at the NOAA Mauna Loa Observatory (MLO). It operates daily in an autonomous mode taking middle infrared solar spectra of the terrestrial atmosphere. It began operation in 1995 and has run continuously since. The data are used for long term studies of many trace species in the atmosphere. Its operated as part of the Network for the Detection for Atmospheric Composition Change (NDACC www.ndacc.org). See https://www2.acom.ucar.edu/irwg for information on the network and https://www2.acom.ucar.edu/irwg for info on PI J. Hannigan. Data are publicly available at www.ndacc.org. Data products consist of retrievals from the remote sensing spectra of vertical profiles of CO, CH4, ClONO2, HCOOH, C2H6, HCN, HCl, HF, HNO3, H2O, HDO, OCS, N2O, O3, H2CO. Other species are available.

The CIMS instrument consists of a low pressure ion molecule reactor (IMR) coupled to a quadrupole mass filter by an actively pumped collisional dissociation chamber (CDC) and an octopole ion guide. The vacuum system is a 100 mm outer diameter stainless steel chamber evacuated with two small turbo pumps (70 l s^-1). The mass filter is a set of 9.5 mm diameter quadrupole rods housed in the main vacuum chamber. The CDC is a short 80 mm diameter chamber that houses an octopole ion guide and is evacuated with a hybrid molecular drag pump. The IMR is evacuated with a scroll pump (300 l min^-1) that also serves as the backing pump for the mass spectrometer.

Click here for the Collaborative Ground and Airborne Observations description page.

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 ASUR (Airborne SUbmillimeter Radiometer) is an airborne radiometer measuring the thermal emission of trace gases in the stratosphere (in an altitude range between 15 and 50 km). The instrument detects the radiation in a frequency range between 604.3 and 662.3 GHz. This corresponds to wavelengths of about 0.45-0.5 mm. In this frequency range a major part of the radiation is absorbed by atmospheric water vapor. As most of the water vapor is found in the troposphere (in the Arctic up to 8 km, in the tropics up to 16 km altitude) the instrument is operated on board of an aircraft flying at an altitude of 10-12 km, such that a major part of the water vapor absorption is avoided. Using appropriate inversion techniques vertical profiles from 15 to over 50 km altitude can be retrieved with a vertical resolution of typically 6 km and 12 km in the lower and upper stratosphere, respectively.

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 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 NOAA chemical ionization mass spectrometer (CIMS) instrument was developed for high-precision measurements of gaseous nitric acid (HNO3) specifically under high- and variable-humidity conditions in the boundary layer. The instrument’s background signals (i.e., signals detected when HNO3-free air is measured), which depend on the humidity and HNO3 concentration of the sample air, are the most important factor affecting the limit of detection (LOD). A new system to provide HNO3-free air without changing both the humidity and the pressure of the sampled air was developed to measure the background level accurately. The detection limit was about 23 parts per trillion by volume (pptv) for 50-s averages. Field tests, including an intercomparison with the diffusion scrubber technique, were carried out at a surface site in Tokyo, Japan, in October 2003 and June 2004. A comparison between the measured concentrations of HNO3 and particulate nitrate indicated that the interference from particulate nitrate was not detectable (i.e., less than about 1%). The intercomparison indicated that the two independent measurements of HNO3 agreed to within the combined uncertainties of these measurements.

ALIAS (Aircraft Laser Infrared Absorption Spectrometer) measures total water, total water isotopes, carbon monoxide, and carbon dioxide isotope ratios. No other instrument provides real-time measurements of carbon dioxide isotope ratios which are clear identifiers of atmospheric transport (18O/17O/16O for stratospheric intrusion, 13C/12C for anthropogenic signals). ALIAS easily adapts to changing mission priorities and can be configured to measure HCl, CH4, SO2, and N2O by simply replacing a semiconductor laser. These measurements contribute to Atmospheric Composition Focus Area research by providing key data on how convective processes affect stratospheric composition, the development of cirrus particles and their affect on Earth's radiative balance, and health of the ozone layer through measurement of chlorine partitioning.

The Airborne Laser Infrared Absorption Spectrometer (ALIAS-II) is a very high resolution scanning tunable diode laser spectrometer which makes direct, simultaneous measurements of selectable combinations of HCl, NO2, CO, CO2, CH4, and N2O at sub-part-per-billion levels over a 3-30 second integration time. The measurement technique is based upon using tunable lead-salt and/or quantum cascade lasers operating from 3.4 to 8 microns wavelength scanning over absorption lines at 10 Hz recording second harmonic spectra. The instrument features an open-cradle multipass Herriott absorption cell with 15.24-cm diameter spherical zerodur mirrors coated with gold on chrome. The separation between the mirrors is adjustable allowing for a relatively small cell (0.75-m to 1.5-m) to contain an optical path length up to 120-m, depending on the spacing of the mirrors. Lasers and detectors are contained in a lightweight aluminum liquid nitrogen Dewar which can achieve a 28-hour hold time with only a 2 liter charge of liquid nitrogen. The instrument features custom laser current drives, signal chains, InSb detectors and preamps, 16-bit signal averager, analog signal conditioner, and digital I/O which are controlled by an onboard Pentium processor. Data is written to a ruggedized 2-Gb hard disk every 30 seconds and simultaneously transmitted via telemetry to ground station computers which provide backup storage of the data. The instrument weighs 36 kg and requires <56 watts for operation. Additional power up to 250 watts is available for structural heaters and current draw varies with atmospheric conditions.