UNH Mercury

The UNHMERC instrument provides detailed information on atmospheric mercury. Measurements of total gaseous mercury (TGM) and gaseous elemental mercury (Hg°) are performed simultaneously with one minute time resolution using a custom four-channel atomic fluorescence spectrometer. The relative amount of reactive gaseous mercury (RGM = HgCl2 + HgBr2+ HgOBr + …) will be assessed through careful examination of the difference between TGM and Hg°. TGM is defined as the sum Hg° + RGM. Targeted aerosol sampling will also be conducted for particulate-phase mercury (HgP).

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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, Gulfstream V - NSF
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Proton-Transfer-Reaction Mass Spectrometer

PTR-MS is a state-of-the-art chemical ionization mass spectrometry technique that allows for fast (seconds) and highly sensitive (detection limits in the tens of pptv range) of volatile organic compounds (VOCs). A detailed description of the measurement principle and the instrument can be found at: http://en.wikipedia.org/wiki/Proton-transfer-reaction_mass_spectrometry

The technique was developed in the mid-1990s at the University of Innsbruck in Austria and has since then become a routine and integral part of most tropospheric chemistry field campaigns including INDOEX 1999, SOS 1999, TEXAQS 2000, AOE 2001, BEWA 2002, ECHO 2003, ARCTAS-2008, DISCOVER-AQ 2011 – just to name a few of the campaigns in which the University of Innsbruck measurement team has been involved. An interactive presentation of the airborne PTR-MS instrument can be found at: http://discover-aq.larc.nasa.gov/instruments.php (click on interactive viewer and on PTR-MS)

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Difference Frequency Generation Absorption Spectrometer

The DFGAS instrument utilizes a room temperature infrared (IR) laser source based upon non-linear difference frequency generation (DFG) in the measurement of CH2O.

Mid-IR laser light is generated in the DFG system by mixing the output of two near-IR room temperature laser sources (one at 1562-nm and the other at 1083-nm) in a periodically poled lithium niobate (PPLN) non-linear wavelength conversion crystal. The mid-IR difference frequency at 2831.6 cm-1 (3.53-μm) is generated at the PPLN output and directed through a multipass astigmatic Herriott cell (100-m pathlength using ~ 4-liter sampling volume) and ultimately onto IR detectors 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].

Ambient air is continuously drawn through a heated rear-facing inlet at flow rates around 9 standard liters per minute (slm), through a pressure controller, and through the multipass Herriott cell maintained at a constant pressure around 50-Torr. Ambient measurements are acquired in 1-second increments for time periods as long as 60 to 120-seconds (to be determined during the campaign), and this will be followed by 15-seconds of background zero air acquisition, using an onboard CH2O scrubbing unit. The zero air is added back to the inlet a few centimeters from the tip at flow rates ~ 2 to 3 slm higher than the cell flow. This frequent zeroing procedure very effectively captures and removes optical noise as well as residual outgassing from inlet line and cell contaminants. Retrieved CH2O mixing ratios are determined for each 1-second ambient spectrum by fitting to a reference spectrum, obtained by introducing high concentration calibration standards (~ 3 to 7-ppbv) from an onboard permeation calibration system into the inlet approximately every hour. The calibration outputs for the two permeation tubes employed are determined before and after the field campaign using multiple means, including direct absorption employing the Beer-Lambert Law relationship. The 1-second ambient CH2O results can be further averaged into longer time intervals for improved precision. However, in all cases the 1-second results are retained. This flexibility allows one to further study pollution plumes with high temporal resolution, and at the same time study more temporally constant background CH2O levels in the upper troposphere using longer integration times.

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Point(s) of Contact: 
Alan Fried (Co-I)

Atmospheric Vertical Observations of CO2 in the Earth's Troposphere

The NASA Langley CO2 sampling system (AVOCET) has an extensive measurement heritage in tropospheric field campaigns, delivering high reliability over 3400 flight hours (452 science flights) and is recognized within the CO2 community as a benchmark for evaluating newly evolving remote CO2. This instrument was adapted by the investigators for airborne sampling and has been successfully deployed aboard NASA research aircraft beginning with the PEM-West A mission in 1992, and more recently during the 2016 KORUS-AQ, 2017 ACSENDS/ABoVE, and 2019 FIREX-AQ missions. The newest iteration of the technique as of 2017 has at its core a modified LI-COR model 7000 non-dispersive infrared spectrometer (NDIR). The basic instrument is small (13 x 25 x 37 cm) and composed of dual 11.9 cm^3 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 (~405 ppm), minute fluctuations in atmospheric concentration can be quantified with high precision. Calbrations are peformed frequently during flight using WMO-traceable standards from NOAA ESRL. Precisions of ≤ 0.1 ppm (±1σ) for 1 Hz sampling rates are typical for our present airborne CO2 system when operated at 600 torr sample pressure.

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CU Aircraft High-Resolution Time-of-Flight Aerosol Mass Spectrometer

Principle: The CU aircraft version of the Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) detects non-refractory submicron aerosol composition by impaction on a vaporizer at 600°C, followed by electron ionization and time-of-flight mass spectral analysis. Size-resolved composition can be quantified by measuring the arrival times of the aerosol at the vaporizer.

Aircraft Operation: (1 min cycles, can be adjusted to meet mission goals):
46 s total concentration measurements (1 s resolution, can be increased to up to 10 Hz upon request)
5 s speciated size distribution measurements (with improved S/N detection due to ePToF acquisition)
9 s Background + Overhead
Higher accuracy due to flight day calibrations using built-in system
Custom pressure controlled inlet with confirmed performance up to 41 kft

Real Time Data Products: 
PM1 Aerosol Mass Concentrations:
Organic aerosol (OA) , SO4, NO3, NH4, Chloride 
OA Chemical Markers: f44 (Secondary OA), f57 (hydrocarbon-like OA), f60 (biomass burning OA), f82 (isoprene epoxide-SOA), other fx upon request

More Advanced Products:
- PM1 Seasalt, ClO4, total I, total Br, MSA concentrations
- O/C, H/C, OA/OC, OSc
- Particle organic nitrates (pRONO2)
- Ammonium Balance, estimated pH
- OA components by positive matrix factorization (PMF)
- Particle eddy covariance fluxes of all species
- Speciated Aerosol size distributions

Detection Limits (1s, ng sm-3), (1 min, ng sm-3) from start of the flight (due to custom cyropump):
Sulfate: 40, 15
Nitrate: 15, 6
Ammonium: 3, 1
Chloride: 30, 12
OA: 200, 80
For detailed OA analysis, longer averaging (3-30 s, depending on OA concentration) is needed. A 1 min product is hence provided as well.

 

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DC-8 - AFRC, C-130 - NSF
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Cloud Aerosol and Precipitation Spectrometer

Measures concentration and records images of cloud particles from approximately 50-1600 microns in diameter with a resolution of 25 microns per pixel. Measures cloud droplet and aerosol concentrations within the size range of 0.5-50 microns.

The three DMT instruments included in the CAPS are the Cloud Imaging Probe (CIP), the Cloud and Aerosol Spectrometer (CAS), and the Hotwire Liquid Water Content Sensor (Hotwire LWC).

The CIP, which measures larger particles, operates as follows. Shadow images of particles passing through a collimated laser beam are projected onto a linear array of 64 photodetectors. The presence of a particle is registered by a change in the light level on each diode. The registered changes in the photodetectors are stored at a rate consistent with probe velocity and the instrument’s size resolution. Particle images are reconstructed from individual “slices,” where a slice is the state of the 64-element linear array at a given moment in time. A slice must be stored each time interval that the particle advances through the beam a distance equal to the resolution of the probe. Optional grayscale imaging gives three levels of shadow recording on each photodetector, allowing more detailed information on the particles.

The CAS, which measures smaller particles, relies on light-scattering rather than imaging techniques. Particles scatter light from an incident laser, and collecting optics guide the light scattered in the 4° to 12° range into a forward-sizing photodetector. This light is measured and used to infer particle size. Backscatter optics also measure light in the 168° to 176° range, which allows determination of the real component of a particle’s refractive index for spherical particles.

The Hotwire LWC instrument estimates liquid water content using a heated sensing coil. The system maintains the coil at a constant temperature, usually 125 °C, and measures the power necessary to maintain this temperature. More power is needed to maintain the temperature as droplets evaporate on the coil surface and cool the surface and surrounding air. Hence, this power reading can be used to estimate LWC. Both the LWC design and the optional PADS software contain features to ensure the LWC reading is not affected by conductive heat loss.

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Ultra High Sensitivity Aerosol Spectrometer

The UHSAS is an optical-scattering, laser-based aerosol particle spectrometer for sizing particles in the 0.06 – 1 μm range. The instrument counts particles in up to 100 user-specified sizing bins, with a resolution as fine as 1 nm/bin. This high sensitivity makes the UHSAS ideal for aerosol research and filter testing.

A laser illuminates particles, which scatter light that is then collected by two pairs of Mangin optics. One pair of optics images onto a highly sensitive avalanche photodiode (APD) for detecting the smallest particles. The other pair images onto a low-gain PIN photodiode for detecting particles in the larger size range of the instrument. Each detector is amplified in a current-to-voltage stage that feeds into the analog electronics system. The amplification allows the system to detect particles as small as 60 nm.

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Global Hawk - AFRC, Gulfstream V - NSF
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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)

Measurements: 
N2O, SF6, CH4, CO, O3, H2, H2O
Aircraft: 
Altair, Global Hawk - AFRC, DC-8 - AFRC, Gulfstream V - NSF, WB-57 - JSC, ER-2 - AFRC
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High Volume Precipitation Spectrometer

SPEC previously built the Version 1 and Version 2 HVPS probes that have now been discontinued due to obsolete parts and significant advances in technology. The HVPS-3 uses the same 128-photodiode array and electronics that are used in the 2D-S and 2D-128 probes. The optics are configured for 150 micron pixel resolution, resulting in a maximum field of view of 1.92 cm (i.e., particles up to 1.92 cm are completely imaged, although even larger particles can be sized in the direction of flight).

Sample volume of the HVPS-3 is 400 L s-1 at 100 m s-1. The 2D-S or 2D-128 and HVPS make an excellent pair of probes that completely image particles from 10 microns to 1.92 cm.

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