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.

Instrument: Reactive Nitrogen

Principal Investigator: David W. Fahey

Organization:
NOAA Aeronomy Laboratory
325 Broadway,R/E/AL6
Boulder, CO 80303

Principle of Operation:
The instrument is designed to measure nitric oxide (NO) and the sum of reactive nitrogen oxides (NOy). Species included in NOy are NO, NO2, HNO3, N2O5 and ClONO2. NO is measured by detecting light from the chemiluminescent reaction between reagent ozone and NO in the ambient sample. NOy is reduced to NO by catalytic reduction on a gold surface with carbon monoxide (CO) acting as a reducing agent. The catalyst is located outside the aircraft fuselage in order to avoid inlet line losses. Two reaction vessels are incorporated in the instrument to allow for simultaneous measurement of NO and NOy. Ca1ibration with NO or NO2 is made by standard addition several times during a flight. The baseline of each measurement is determined in part by the addition of synthetic air that contains no reactive nitrogen. The difference between the sample flow velocity in the inlet opening and the aircraft velocity cause aerosol particles in the atmosphere to be oversampled. For sizes below 5 micrometers in diameter, this feature assists in the identification of aerosol particles that contain NOy.

Accuracy: < 20% plus precision
Detection limit: < 0.1 ppbv NOy, ~0.02 ppbv NO
Response time: 1 sec
Location on the ER2: Lower Q-bay rack

Reference: D.W. Fahey et al., In situ aerosol measurements of total reactive nitrogen, total water, and aerosol in a polar stratospheric cloud in the Antarctic, J. Geophys. Res. 94 11-99-11315, 1959.

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 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 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 Charged-coupled device Actinic Flux Spectroradiometers (CAFS) instruments measure in situ down- and up-welling radiation and combine to provide 4 pi steradian actinic flux density spectra from 280 to 650 nm. The sampling resolution is ~0.8 nm with a full width at half maximum (FWHM) of 1.7 nm at 297 nm. From the measured flux, photolysis frequencies are calculated for ~40 important atmospheric trace gases including O3, NO2, HCHO, HONO and NO3 using a modified version of the NCAR Tropospheric Ultraviolet and Visible (TUV) radiative transfer model. The absolute spectral sensitivity of the instruments is determined in the laboratory with 1000 W NIST-traceable tungsten-halogen lamps with a wavelength dependent uncertainty of 3–5%. During deployments, spectral sensitivity is assessed with secondary calibration lamps while wavelength assignment is tracked with Hg line sources and comparisons to spectral features in the extraterrestrial flux. The optical collectors are characterized for angular and azimuthal response and the effective planar receptor distance. CAFS have an excellent legacy of performance on the NASA DC-8 and WB-57 platforms during atmospheric chemistry and satellite validation mission. These include AVE Houston 2004 and 2005, PAVE, CR-AVE, TC4, ARCTAS, DC3, SEAC4RS, KORUS-AQ, ATom and FIREX-AQ. For FIREX-AQ, upgraded electronics and cooling reduced noise and allowed for a decrease to 1 Hz acquisition.