NASA - National Aeronautics and Space Administration

NO is measured using a chemiluminescence detector. One of the four NO detectors is used for the NO measurements. NOy is measured simultaneously by catalytically converting it to NO on the surface of gold tubes heated to ±° C, with carbon monoxide (CO) acting as a reducing agent. The converter system is contained in a pod mounted outside the cabin to minimize the length of the inlet tubes. Gas phase-NOy measurements are made by sampling air through the rearward facing inlet which discriminates against particles of diameter larger than 1 mm. The mixing ratios of total NOy (gas phase-NOy + amplified particulate-NOy) are measured by sampling air through the forward facing inlet which is heated to 100° C. The mixing ratios of gas phase and total NOy are measured independently. A humidifier maintains the H2O mixing ratio in sample flows at a few % in order to stabilize the instrument background against humidity variations in the ambient air. The absolute sensitivities of the NO and NOy channels are measured every 80 minutes by adding NO or NO2 standard gases. The pressure in the gold catalytic converter for gas-phase NOy is maintained at a constant value of about 50 hPa, independent of the ambient pressure. The pressure is held constant by controlling the sample flow using a servo-controlled Teflon valve mounted upstream of the converter tube. All parts of the inlet system upstream of the gold catalyst are made of Perfluoroalkoxy (PFA) Teflon which is temperature controlled at 40˚C. The NO2 conversion efficiency is 99.0611%. The HCN conversion efficiency is lower than 5% for dry air with O3 mixing ratios lower than 100 ppbv. It decreases to 2% for humid air with an H2O mixing ratio of 0.1% and O3 mixing ratios lower than 100 ppbv. This instrument is also equipped with an NO2 photolytic converter combined with an NO detector in our first attempt to access the accuracy of the NO2 measurements.

The Georgia Tech Laser-Induced Fluorescence instrument measures nitric oxide (NO), formaldehyde (HCHO), and nitrogen dioxide (NO2). Each species is measured by laser-induced fluorescence at reduced pressure. Ambient air is drawn in through a pinhole orifice into a pair of multipass White cells. The pressure in the White cells is maintained at 5-10 mbar to extend the fluorescence lifetime, and the multiple passes (typically 32-40) effectively extends the probe interaction volume. The ambient air is probed at 90o from the flow and the fluorescence collected at 90o to the flow and probe.

NO is probed at the 226 absorption line and monitored at the 247 nm fluorescence. The laser pulse and scattering will be time-gated out using microchannel plate detectors. The expected 2-sigma limit of detection is 5 pptv/min. Formaldehyde is probed at 353 nm and the fluorescence monitored in a range from 400 to 450 nm. The expected performance is 10 pptv/min. NO2 will be probed near 435 nm and the fluorescence around 780 nm collected. Its expected performance is 15 pptv/min. In each case, the probe wavelength will be alternately switched from the absorption feature to a nearby “off-line” position to determine the background. The actual frequency will be monitored in reference cells. Calbration is done by standard addition to the airflow. The light sources used are custom-built cavities pumped by a diode-pumped Nd:YAG laser operating at ~10 kHz.

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.

Two spectrographs + HD video camera
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Air Quality (AQ) 304:520 nm 0.8 nm resolution (NO2, O3, UV absorbing aerosols, SO2, HCHO)
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Ocean Color (OC) 460:900 nm 1.5 nm resolution
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Video camera (2592x1936 pixels) –3 pixel FWHM

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.

AATS-14 measures direct solar beam transmission at 14 wavelengths between 354 and 2139 nm in narrow channels with bandwidths between 2 and 5.6 nm for the wavelengths less than 1640 nm and 17.3 nm for the 2139 nm channel. The transmission measurements at all channels except 940 nm are used to retrieve spectra of aerosol optical depth (AOD). In addition, the transmission at 940 nm and surrounding channels is used to derive columnar water vapor (CWV) [Livingston et al., 2008]. Methods for AATS-14 data reduction, calibration, and error analysis have been described extensively, for example, by Russell et al. [2007] and Shinozuka et al. [2011]. AATS-14 measurements of spectral AOD and CWV obtained during aircraft vertical profiles can be differentiated to determine corresponding vertical profiles of spectral aerosol extinction and water vapor density. Such measurements have been used extensively in the characterization of the horizontal and vertical distribution of aerosol optical properties and in the validation of satellite aerosol sensors. For example, in the Aerosol Characterization Experiment-Asia (ACE-Asia), AATS measurements were used for closure (consistency) studies with in situ aerosol samplers aboard the NCAR C-130 and the CIRPAS Twin-Otter aircraft, and with ground-based lidar systems. In ACE-Asia, CLAMS (Chesapeake Lighthouse & Aircraft Measurements for Satellites, 2001), the Extended-MODIS-λ Validation Experiment (EVE), INTEX-A, INTEX-B, and ARCTAS, AATS results have been used in the validation of satellite sensors aboard various EOS platforms, providing important aerosol information used in the improvement of retrieval algorithms for the MISR and MODIS sensors among others.

Aerosols (particulate matter) have a dramatic effect on radiative forcing of the climate, in some cases cooling and in other cases warming. The Fourth Assessment Report of the IPCC estimates that direct radiative forcing due to all aerosols is a cooling of -0.50 W m-2 with absorbing aerosol (black carbon) responsible for a warming of +0.22 W m-2, but the uncertainties associated with these numbers are very large. Better measurements of the optical properties of aerosols, especially absorption coefficient and asymmetry parameter, and their spatial and temporal distribution are required to reduce these uncertainties and improve the ability of models to predict climate change. Aero3X was designed to provide such measurements. It is a light weight (11 kg), compact (0.25 x 0.30 x 0.6 m), and fast (1 Hz sample rate) instrument intended for use on an Unmanned Aerial System (UAS) but suitable for flight on other aircraft and for surface measurements. Aero3X uses an off-axis cavity ring-down technique to measure extinction coefficient and a reciprocal nephelometry technique for measurement of total-, forward- and back-scatter coefficients at wavelengths of 405 nm and 675 nm. Its outstanding precision (0.1 Mm-1) and sensitivity (0.2 Mm- 1) allow the determination of absorption coefficient, single-scattering albedo, estimates of backscatter to extinction ratio and asymmetry parameter at both wavelengths, and Angstrom exponent. Together with its humidification system for measurement of the dependence of aerosol optical properties on relative humidity, these represent a complete set of the aerosol optical properties important to climate and air quality. Aero3X was designed to operate in pollution plumes where NO2 may cause interference with the measurement, therefore, a measurement of NO2 mixing ratio is also made.