Trace Organic Gas Analyzer

TOGA measures volatile organic compounds (VOCs). Specific data will be obtained for radical precursors, tracers of anthropogenic and biogenic activities, tracers of urban and biomass combustion emissions, products of oxidative processing, precursors to aerosol formation, and compounds important for aerosol modification and transformation. TOGA measures a wide range of VOCs with high sensitivity (ppt or lower), frequency (2.0 min.), accuracy (15% or better), and precision (<3%). Over 50 species are routinely measured (see PDF for full table) throughout the full DC-8 altitude range. TOGA is contained in a standard DC-8 rack, weighs less than 200 kg and consumes ~1 kW of power. The major components of the instrument are the inlet, cryogenic preconcentrator, gas chromatograph, mass spectrometer detector, zero air/calibration system, and the control/data acquisition system. All processes and data acquisition are computer controlled.

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DC-8 - AFRC, NSF G-V
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UAS Chromatograph for Atmospheric Trace Species

The Unmanned Aircraft Systems (UAS) Chromatograph for Atmospheric Trace Species (UCATS) was designed and built for autonomous operation on pilotless aircraft. It uses chromatography to separate atmospheric trace gases along a narrow heated column, followed by precise and accurate detection with electron capture detectors. There are two chromatographs on UCATS, one of which measures nitrous oxide and sulfur hexafluoride, the other of which measures methane, hydrogen, and carbon monoxide. In addition, there is a small ozone instrument and a tunable diode laser instrument for water vapor. Gas is pumped into the instruments from an inlet below the GV, measured, and vented. UCATS has flown on the Altair UAS, the GV during HIPPO I and II, and most recently on the NASA/NOAA Global Hawk UAS during the Global Hawk Pacific (GloPac) mission, where a record was set for the longest duration research flight (more than 28 hours). UCATS is relatively lightweight and compact, making it ideal for smaller platforms, but it is easily adaptable to a mid-size platform like the GV for HIPPO. The data are used to measure sources and sinks of trace gases involved in climate and air quality, as well as transport through the atmosphere.

UCATS is three different instruments in one enclosure:

1. 2-channel gas chromatograph (GC)
2. Dual-beam ozone photometer (OZ)
3. Tunable diode laser (TDL) spectrometer for water vapor (WV)

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PAN and Trace Hydrohalocarbon ExpeRiment

PANTHER uses Electron Capture Detection and Gas Chromatography (ECD-GC) and Mass Selective Detection and Gas Chromatography (MSD-GC) to measure numerous trace gases, including Methyl halides, HCFCs, PAN, N20, SF6, CFC-12, CFC-11, Halon-1211, methyl chloroform, carbon tetrachloride.

3 ECD (electron capture detectors), packed columns (OV-101, Porpak-Q, molecular sieve).

1 ECD with a TE (thermal electric) cooled RTX-200 capillary column.

2-channel MSD (mass selective detector). The MSD analyses two independent samples concentrated onto TE cooled Haysep traps, then passed through two temperature programmed RTX-624 capillary columns.

With the exception of PAN, all channels of chromatography are normalized to a stable in-flight calibration gas references to NOAA scales. The PAN data is normalized to an in-flight PAN source of ≈ 100 ppt with ±5 % reproducibility. This source is generated by efficient photolytic conversion of NO in the presence of acetone. Detector non-linearity is taken out by lab calibrations for all molecules.

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PeroxyAcetylNitrate, Aldehydes and Ketones

The Ames PANAK instrument is a computerized 3- channel Capillary Gas Chromatographic system designed for the collection and analysis of low ppt (10-12 v/v) levels of peroxyacyl nitrates (PANs), alkyl nitrates, and tertrachloroethene in Channels 1 and 2; and C2-C3 aldehydes, C1-C2 alcohols, C3-C4 ketones, and C1-C2 nitriles in channel 3. Channels 1 and 2 use ECD detectors and have a sampling frequency of 2.5 minutes. Channel 3 uses a Photo Ionization detector placed in series with a Reduction Gas detector and has a sampling frequency of 5 minutes. The main manifold draws 5 SL/min of ambient air through a heated Teflon lined probe from which each of the three instrument channels draws a 200 ml aliquot of sample air. This aliquot is dried by passing it through a –35 °C cold trap, cooled to -140 °C for constituent pre concentration, and then heat desorbed into the gas chromatographic columns. All calibrations are performed in-flight by using an installed dilution system and in a manner that mimics ambient air sampling. Primary standards are generally referred to a series of permeation tubes. In addition high concentration standards are also carried on board. Sensitivities under typical conditions are: 1-3 ppt PANs, 1-5 ppt alkyl nitrates, 5-20 ppt OVOC, and 20-30 ppt nitriles.

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High-Sensitivity Fast-Response CO2 Analyzer

The high-sensitivity fast response CO2 instrument measures CO2 concentrations in situ using the light source, gas cells, and solid-state detector from a modified nondispersive infrared CO2 analyzer (Li-Cor, Inc., Lincoln, NE). These components are stabilized along the detection axis, vibrationally isolated, and housed in a temperature-controlled pressure vessel. Sample air enters a rear-facing inlet, is preconditioned using a Nafion drier (to remove water vapor), then is compressed by a Teflon diaphragm pump. A second water trap, using dry ice, reduces the sample air dewpoint to less than 70C prior to detection. The CO2 mixing ratio of air flowing through the sample gas cell is determined by measuring absorption at 4.26 microns relative to a reference gas of known concentration. In-flight calibrations are performed by replacing the air sample with reference gas every 10 minutes, with a low-span and a high-span gas every 20 minutes, and with a long-term primary standard every 2 hours. The long-term standard is used sparingly and serves as a check of the flight-to-flight accuracy and precision of the measurements, augmented by ground-based calibrations before and after flights.

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Measurements of Atmospheric Carbon Dioxide Over Northwestern North America

A modified LI-COR model 6252 infrared gas analyzer forms the basis of a CO2 sampling system. The LI-COR is small (13 x 24 x 34 cm) and composed of dual 12 cm3 volume sample/reference cells; a feedback stabilized infrared source; 500 Hz chopper; thermoelectrically-cooled solid state PbSe detector; and a narrow band (150 nm) interference filter centered on the 4.26 μm CO2 absorption band. Using synchronous signal detection techniques, it operates by sensing the difference in light absorption between the continuously flowing sample and reference gases occupying each side of the dual absorption cell. Thus, by selecting a reference gas of approximately the same concentration as background air (~ 378 ppmv), very minute fluctuations in atmospheric concentration can be quantified with high precision (≤ 0.07 ppmv). The system is operated at constant pressure (250 torr) and has a 0.1 second electronic time response.

During ambient sampling, air is continuously drawn through a Rosemount inlet probe, a permeable membrane dryer to remove H2O(v), the LI-COR, and then exchanged through a diaphragm pump that vents overboard. In-flight calibrations are performed every 15 minutes using standards traceable to the primary standards maintained by the WMO Central CO2 Laboratory. By interpolating between these calibrations, slow drifts in instrument response are effectively suppressed, yielding high precision values. Temperature control of the instrument minimizes thermal drift thus maximizing ambient sampling time by decreasing calibration frequency. The CO2 measurement accuracy is closely tied to the accuracy of the standards obtained from NOAA/CMDL, Boulder, CO prior to the mission.

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Airborne Chromatograph For Atmospheric Trace Species

ACATS-IV is a 4-channel gas chromatograph with electron capture detection (ECD) that measures a variety of halocarbons and other long-lived trace gases in the stratosphere. The instrument is currently configured to measure CFC-11 (CCl3F), CFC-12 (CCl2F2), CFC-113 (CCl2FCClF2), methyl chloroform (CH3CCl3), carbon tetrachloride (CCl4), halon-1211 (CBrClF2), chloroform (CHCl3), methane (CH4), and hydrogen (H2) every 125 s, and nitrous oxide (N2O) and sulfur hexafluoride (SF6) every 250 s. Each channel is comprised of a sample loop (2-10 cm3 volume), gas sampling valve (GSV), chromatographic column pair, ECD, electrometer, and several flow, temperature, and pressure controllers. In-flight calibration is carried out every 625 s (1250 s for N2O and SF6) by injecting a dried, whole air standard containing approximately 80% of tropospheric mixing ratios.

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