NASA - National Aeronautics and Space Administration

The NASA GSFC In Situ Airborne Formaldehyde (ISAF) instrument measures formaldehyde (CH2O) on both pressurized and unpressurized (high-altitude) aircraft. Using laser induced fluorescence (LIF), ISAF possesses the high sensitivity, fast time response, and dynamic range needed to observe CH2O throughout the troposphere and lower stratosphere, where concentrations can range from 10 pptv to hundreds of ppbv.

Formaldehyde is produced via the oxidation of hydrocarbons, notably methane (a ubiquitous greenhouse gas) and isoprene (the primary hydrocarbon emitted by vegetation). Observations of CH2O can thus provide information on many atmospheric processes, including:
 - Convective transport of air from the surface to the upper troposphere
 - Emissions of reactive hydrocarbons from cities, forests, and fires
 - Atmospheric oxidizing capacity, which relates to formation of ozone and destruction of methane
In situ observations of CH2O are also crucial for validating retrievals from satellite instruments, such as OMI, TROPOMI, and TEMPO.

The HIAPER Airborne Radiation Package (HARP) instrumentation is a comprehensive atmospheric radiation suite to measure spectrally resolved actinic flux and horizontally stabilized irradiance. HARP was developed in a collaborative effort between NCAR, the University of Colorado, the Leibniz-Institute for Tropospheric Research, Metcon, Inc and Enviscope GmbH. The package is part of the HIAPER Aircraft Instrumentation Solicitation (HAIS), funded by NSF.

The CAMS instrument’s core design and operation is similar to the DFGAS (Difference Frequency Generation Absorption Spectrometer) instrument, which has been successfully deployed for fast, accurate, and sensitive airborne measurements of the important trace gas formaldehyde (CH2O). CAMS like DFGAS is based on tunable mid-IR (3.53-μm) absorption spectroscopy utilizing advanced fiber optically pumped difference-frequency generation (DFG) laser sources. Mid-Infrared light at 2831.6-cm^-1 (3.53 μm) is generated by mixing two near-IR room temperature lasers (one at 1562 nm and the other at 1083 nm) in a non-linear crystal (periodically poled lithium niobate). The DFG laser output is directed through a multipass Herriott absorption cell (90-m pathlength in ~ 1.7 liter volume) where the laser light is selectively absorbed by a moderately strong and isolated vibrational-rotational absorption feature of CH2O. The transmitted light from the cell is directed onto an IR detector employing a number of optical elements. A portion of the IR beam is split off by a special beam splitter (BS) before the multipass cell and focused onto an Amplitude Modulation Detector (AMD) to capture and remove optical noise from various components in the difference frequency generation process. A third detection channel from light emanating out the back of the beam splitter is directed through a low pressure CH2O reference cell and onto a reference detector (RD) for locking the center of the wavelength scan to the absorption line center. The mid-IR DFG output is simultaneously scanned and modulated over the CH2O absorption feature, and the second harmonic signals at twice the modulation frequency from the 3 detectors are processed using a computer lock-in amplifier [Weibring et al., 2006].

The measurement of gas phase peroxide species, H2O2 and CH3OOH, contribute to our scientific understanding of the photochemistry of trace gases and particles prior to and after their transport and processing through deep convective clouds. The PCIMS instrument used to make these measurements in the DC3/SEAC4RS mission is new and this will be its first use in an airborne science campaign.

The PCIMS instrument is a slightly modified CIMS instrument manufactured by THS Instruments LLC. Mechanically it consists of a differentially pumped quadrupole mass spectrometer. The instrument operates in negative ion mode and currently I^- and O2^- reagent ions are used to measure hydrogen peroxide and methylhydroperoxide, respectively, by the formation of cluster ions at masses 80 and 161. The reagent ions are produced by flowing a N2/CH3I/O2 mixture past a ²¹⁰Po foil.

On the G-V, the PCIMS inlet system starts with a PFA Teflon lined heated G-V HIMIL inlet. From the HIMIL the inlet line is comprised of PFA Teflon and is also heated (Hot-Tube, Clayborn Lab). Analytical blanks are performed by diverting the ambient sample flow through a trap filled with Carulite 200 catalyst. Gas phase calibrations are performed through standard additions to ambient air. H2O2 is added from a urea hydrogen peroxide solid decomposition source or by the evaporation of a nano-fluidic flow of a dilute aqueous solution. CH3OOH is added by the evaporation of a nano-fluidic flow of a dilute aqueous solution. The ambient, calibration and reagent gases are vented overboard through the G-V common exhaust.

The NCAR/NSF G-V vacuum UV resonance fluorescence instrument is a commercial version of the instrument published by Gerbig, et al. (Journal of Geophysical Research, Vol. 104, No. D1, 1699-1704, 1999). The source is a flowing RF discharge gas lamp emitting in the VUV. An optical filter provides a narrow band of source radiation centered at 151 nm with a 10 nm bandpass. CO fluorescence is detected using photon counting. The internal data system can accommodate sampling rates from 1-18 samples/second. For SEAC4RS, the instrument was integrated into the HAIS ozone instrument rack and shared a pressure-controlled inlet.

The operating principle of the O3 instrument is the measurement of chemiluminescence from the reaction of nitric oxide (NO) with ambient O3 using a dry-ice cooled, red-sensitive photomultiplier employing photon counting electronics. The reagent NO (grade > 99%) is supplied from a commercially purchased lecture bottle filled to a maximum pressure of 500 psig. Since NO is a toxic gas, the small high pressure cylinder, its regulator, and several safety features are contained inside a specially designed pressure safe vessel that is vented overboard the aircraft. Ambient air is sampled through a standard HIMIL inlet protruding outside the aircraft boundary layer. Ambient air sample flow is controlled to 500 sccm, while the NO reagent is introduced to the reaction vessel in near-excess flow of ~ 4 sccm. Gas flows as well as the reaction vessel temperature (35 ± 0.1°C) and pressure (10 ± 0.05 torr) are all controlled at constant conditions resulting in maximum stability of the detected signal and instrument sensitivity. The instrument sensitivity (~2000 cps/ppbv) is determined from calibrations performed on the ground before and after each flight or set of back-to-back flights using a UV absorption based calibrator (TECO model 49PS) operated with high-quality ultra-pure air. A near-linear calibration curve is generated in 100 ppb intervals from 0 to 1 ppm. This calibration range is sufficient to measure O3 mixing ratios over the altitude range of the aircraft.

This is a 2-channel instrument based on the chemiluminescence detection of NO via reaction with O3 to form excited NO2, which is detected via photon counting. One sample channel is used to measure nitric oxide, NO, and the second measures nitrogen dioxide, NO2, by flowing ambient air through a glass cell illuminated by light-emitting diodes at 395 nm, for the conversion of NO2 to NO via photolysis. The instrument is similar to instruments previously built at NCAR [Ridley and Grahek, 1990; Ridley et al., 2004]. In the UTLS region, NOx (= NO + NO2) is mostly in the form of NO and is formed in situ by lightning, is emitted by aircraft, and may be transported to the UTLS from the boundary layer by convection.