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Atmospheric Chemistry

Atmospheric Chemistry Instruments

Airborne Oxygen |  AO2

Atmospheric OxygenMonitored by :: Britt Stephens & Jonathan Bent
Chemical Species Measured :: O2 & CO2
There is great value in measuring O2 concentrations in carbon cycle studies as relationships between CO2 and O2 provide clues to what is affecting CO2 concentrations. For example, photosynthesis produces O2 and consumes CO2, while fossil-fuel burning does the opposite with a slightly different ratio. Because the atmospheric concentration of O2 is so high (~ 21%), measuring the small changes that trees and power plants make is very difficult.

The NCAR Airborne Oxygen Instrument measures O2 concentration using a vacuum-ultraviolet absorption technique. AO2 is based on earlier ship-board and laboratory instruments using the same technique, but has been designed specifically for airborne use to minimize motion and thermal sensitivity and with a pressure and flow controlled inlet system. AO2 flew on the Wyoming King-Air during the ACME-2007 campaign and on the NSF/NCAR G-V during the START-08 and HIPPO-1 campaigns. To achieve the high-levels of precision needed, AO2 switches between sample gas and air from a high-pressure reference cylinder every 2.5 seconds. Atmospheric O2 concentrations are typically reported in units of one part in 106 relative deviations in the O2/N2 ratio, which are referred to as “per meg.”  AO2 has a precision of +/- 2 per meg on a 5 second measurement. For comparison, this is equivalent to detecting the removal of one O2 molecule from 2.5 million molecules of air. At typical HIPPO operational speeds of 300 kts or climb/descent rates of 1500 fpm, 5-seconds correspond to a horizontal resolution of 750 m and a vertical resolution of 40 m. The AO2 system consists of a pump module, a cylinder module, an instrument module, and a dewar.

Carbon Monoxide | CO

CO InstrumentMonitored by :: Teresa Campos
Chemical Species Measured :: CO
Carbon monoxide (CO) enters the atmosphere as a by-product of fossil fuel combustion, biomass burning, and through natural emissions from plants, forest fires and volcanos. Natural and human-related emissions are about 50%-50%. Historically, aircraft- and ground-based instruments have provided a patchwork of measurements in space and time, however the HIPPO measurements of carbon monoxide will play a key role in providing a clear and complete picture of global distribution.

Carbon monoxide is a weaker greenhouse gas, yet still plays a very important role in climate change. CO has an indirect effect by increasing the concentrations of methane (CH4) and tropospheric ozone (O3) through chemical reactions.

CO_HIPPO_I
CO_HIPPO_II

Carbon Dioxide | CO2

CO2 InstrumentMonitored by: Bruce Daube, Eric Kort & Jasna Pittman
Chemical Species Measured :: CO2
The high-altitude fast response CO2 instrument used on the NSF/NCAR GV 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 30 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. This instrument has been flown successfully on over 200 flights since 1996 on various platforms, including high altitude balloons (to 32 km), the NASA WB-57F,  the UND Citation II, the University of Wyoming King Air, and the NSF/NCAR GV.

CO2 Instrument VideoWatch a video on how the CO2 instrument
operates on the HIPPO campaign
.


CO2_HIPPO_I
CO2_HIPPO_II

Multiple Enclosure Device for Unfractionated Sampling of Air | MEDUSA

MEDUSAMonitored by :: Britt Stephens, Ralph Keeling & Jonathan Bent
Chemical Species Measured :: O2, CO2, Ar/N2, 13CO2, C18O16O
The Multiple Enclosure Device for Unfractionated Sampling of Air, “MEDUSA”, with its 64 lines of black flexible tubing, somewhat resembles its mythical namesake. MEDUSA serves several roles in the HIPPO global campaign. It acts as a discretely-sampled comparison for onboard (“in-situ”) real-time O2/N2 ratio measurements from the AO2 instrument, as a redundant measurement of CO2, and as the only measurement of argon and 13C, 14C, and 18O isotopes. The complementary measurements (CO2, O2/N2) allow ground-truthing of onboard instrument measurements in a laboratory setting, where analysis conditions can often be more stringently proscribed, and carefully monitored. Isotope and argon measurements can provide additional information about land and ocean controls over the carbon cycle, about the age and source of the air sampled, and about the convective activity of the troposphere.

MEDUSA has an onboard computer, two pressure controllers, two pumps, three multi-position selector valves, and a host of other hardware that control and direct the air samples. All air is dehydrated by passing it through traps immersed in a -80° Celsius dry ice bath, adjusted to match atmospheric pressure at sea level, and then automatically sealed in place by a valve. Flasks are later analyzed on a sector-magnet mass spectrometer and a LiCor non-dispersive infrared CO2 analyzer by the Atmospheric Oxygen Research Group at Scripps Institution of Oceanography.

An earlier version of MEDUSA flew on the University of North Dakota Citation II in the COBRA-2000 and COBRA-2003 campaigns, and on the NSF/NCAR C-130 during the ACME-2004 campaign. The MEDUSA system has since been repackaged and expanded, and has flown on the NSF/NCAR GV during the START-08 and HIPPO I campaigns.

Whole Air Sampler | WAS

Monitored by :: Elliot Atlas, Fred Moore, Ben Miller & Steve Montzka
Chemical Species Monitored :: CFCs, HCFCs, HFCs, Solvents, Methyl Halides, Organic Nitrates, Non-Methane Hydrocarbons, Perfluorocarbons; See the complete list of chemical species
The Whole Air Sampler provides a means of semi-automated filling of flasks with whole air from the sample port (called a HIMIL (HIAPER MODULAR INLET)) on the belly of the NSF/NCAR GV aircraft. These flasks are later analyzed in the laboratory by a variety of instruments, yielding data for more than 80 trace gases found in the global atmosphere at mole fractions that range from parts-per-million (10^-6 mole fraction), e.g., carbon dioxide, down to less than 1 part-per-trillion (10^-12 mole fraction), e.g., HFC365mfc. The sampler is designed to fill two different types of flasks:  evacuated stainless steel flasks and glass flasks. For glass flask filling, a water trap removes excess water vapor to a dew point of about +2 C from the sampled air. Flasks are filled to about 2 bar without contamination. For stainless steel flasks, condensed water is removed from the inlet line with an all-metal water trap, and flasks are typically filled to 2.5 bar.

Ozone | O3

Monitored by :: David Fahey, Ru-Shan Gao & Ryan Spackman
Chemical Species Measured :: O3
Ozone is measured in situ by a photometer using a mercury lamp, two sample chambers and two detectors that measure the 254-nm difference in the light intensity in the sample chamber vs the ozone scrubbed chamber (Proffitt et al. [1983]). The ozone number density is calculated using the ozone absorption cross-section at 254 nm and the Beer-Lambert Law. Since the two absorption chambers are identical, virtually continuous measurements of ozone are made by alternating the ambient air sample and ozone-scrubbed sample between the two chambers. A flow of ambient air through the chambers is maintained with dynamic pressure at the inlet located outside the fuselage. At a one-second data collection rate, the minimum detectable concentration of ozone (one standard deviation) is 1.5 x 1010 molecules/cm3 (0.6ppbv at STP). This instrument has a long and successful history of operation on the NASA ER-2 and WB-57 high-altitude research aircraft. Over 300 flights have been logged (~1800 flight hours) during stratospheric missions dating back to 1985.

O3_HIPPO_I O3_HIPPO_II O3_HIPPO_III

PAN and other Trace Hydrohalocarbon ExpeRiment | PANTHER

nullMonitored by :: Fred Moore and James Elkins

200 lbs, 6-channel GC (gas chromatograph)

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

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

2-channel MSD (mass selective detector). The MSD analyzes two independent samples concentrated onto TE cooled Hayesep 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 are 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.

Quantum Cascade Laser System | QCLS

QCLSMonitored by :: Bruce Daube. Eric Kort , Jasna Pittman, & Greg Santoni

Chemical Species Measured :: CO2, CO, CH4, N2O
The Harvard QCLS (DUAL and CO2) instrument package contains 2 separate optical assemblies and calibration systems, and a common data system and power supply. The two systems are mounted in a single standard HIAPER rack, and are described separately below:

The Harvard QCL DUAL instrument simultaneously measures CO, CH4, and N2O concentrations in situ using two thermoelectrically cooled pulsed-quantum cascade lasers (QCL) light sources, a multiple pass absorption cell, and two liquid nitrogen-cooled solid-state detectors. These components are mounted on a temperature-stabilized, vibrationally isolated optical bench with heated cover. The sample air is preconditioned using a Nafion drier (to remove water vapor), and is reduced in pressure to 60 mbar using a Teflon diaphragm pump. The trace gas mixing ratios of air flowing through the multiple pass absorption cell are determined by measuring absorption from their infrared transition lines at 4.59 microns for CO and 7.87 microns for CH4 and N2O using molecular line parameters from the HITRAN data base. 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. A prototype of this instrument was flown on the NOAA P3 in the summer of 2004.

The Harvard QCL CO2 instrument measures CO2 concentrations in situ using a thermoelectrically cooled pulsed-quantum cascade laser (QCL) light source, gas cells, and liquid nitrogen cooled solid-state detectors. 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 reduced in pressure to 60 mbar using 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 from a single infrared transition line at 4.32 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, and with a low-span and a high-span gas every 20 minutes.

CO2_HIPPO_I
CO2_HIPPO_II
CO_HIPPO_I
CO_HIPPO_II
CH4_HIPPO_I CH4_HIPPO_II
N2O_HIPPO_I N2O_HIPPO_II

Unmanned Aircraft Systems (UAS) Chromatograph for Atmospheric Trace Species | UCATS

UCATSMonitored by :: Eric Hintsa, Fred Moore, and James Elkins
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 (“channels”) 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)

    The UCATS enclosure measures 41 x 46 x 25 cm (W x L x H) and weighs 28 kg (62 lbs).

    External to the UCATS enclosure are:

    • A Teflon diaphragm pump (KNF) for sampling air through an external inlet
    • 2 high-pressure aluminum cylinders: Nitrogen and calibrated whole air
    • Keyboard, monitor, and mouse
    About HIPPO

    HIPPO is a landmark study for many reasons, not the least of which is it is the first time scientists have systematically mapped global distribution of carbon dioxide and other greenhouse gases in the atmosphere, covering the full troposphere in all seasons and multiple years.

    HIPPO Dates
    • HIPPO I :: 8 January-30 January 2009
    • HIPPO II :: 31 October-22 November 2009
    • HIPPO III :: 24 March-16 April 2010
    • HIPPO IV :: 14 June-11 July 2011
    • HIPPO V :: 9 August-9 September 2011