Synonyms: 
HCHO
H2CO
Formaldehyde
Methanal

Compact Formaldehyde FluorescencE Experiment

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In Situ Airborne Formaldehyde

The NASA GSFC In Situ Airborne Formaldehyde (ISAF) instrument measures formaldehyde on high altitude NASA aircraft. The instrument uses laser induced fluorescence (LIF) to obtain the high detection sensitivity needed to detect formaldehyde in the upper troposphere and lower stratosphere where abundances are 10 parts per trillion. LIF also enables a fast time response needed to measure the abundance of formaldehyde in the finely structured outflow of convective storms. These measurements of formaldehyde will be used elucidate mechanisms of convective transport and quantify the effects of boundary layer pollutants on the ozone photochemistry and cloud microphysics of the upper atmosphere.

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Compact Atmospheric Multispecies Spectrometer

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].

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NSF G-V, DC-8 - AFRC
Instrument Team: 
Alan Fried (Co-I)

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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Alan Fried (Co-I)

URI Peroxides and Formaldehyde Instrument

POPS measures CH2O, H2O2, and CH3OOH.

CH2O is measured by aqueous collection followed by enzyme fluorescence detection.

H2O2 and CH3OOH is measured by aqueous collection followed by HPLC separation and enzyme fluorescence detection.

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Georgia Tech Laser-Induced Fluorescence

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.

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Dual Channel Airborne tunable diode Laser Spectrometer

The instrument uses two-tone frequency modulation (TTFM) with signal detection at approximately equals 12 MHz. Multiplexing is achieved using a dichroic optical element and a mechanical chopper which blocks each beam alternately. A control program running on a dedicated digital signal processor (DSP) allows the registration of the full absorption line shape each millisecond and simultaneous zero overhead on-line data reduction using a multiple linear regression algorithm. Gas exchange through the compact multireflection cell (2.71 volume, total path 53 m.) takes place in approximately equals 200 ms and thus determines the instrument response time.

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Alan Fried (Co-I)

Airborne Compact Atmospheric Mapper

Two spectrographs + HD video camera

Air Quality (AQ) 304:520 nm 0.8 nm resolution (NO2, O3, UV absorbing aerosols, SO2, HCHO)

Ocean Color (OC) 460:900 nm 1.5 nm resolution

Video camera (2592x1936 pixels) –3 pixel FWHM

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Airborne Scanning Microwave Limb Sounder

The National Research Council decadal survey for earth science identified the need for a Global Atmospheric Composition Mission (GACM) to address crucial issues on how changes in atmospheric composition affect the quality and well-being of life on earth. The baseline GACM instrument suite comprises UV/Vis and IR/SWIR spectrometers and an advanced microwave limb sounder working together to retrieve atmospheric composition worldwide with high spatial resolution. The Scanning Microwave Limb Sounder (SMLS) is designed to meet the measurement requirements of GACM by providing complete orbit-to-orbit retrieval of O3, N2O, temperature, water vapor, CO, HNO3, ClO, and volcanic SO2 in the upper troposphere and lower stratosphere. Unlike previous MLS instruments that only scanned the limb vertically leaving large orbit to orbit gaps, SMLS will simultaneously scan both in azimuth and elevation providing complete global coverage with 6 or more repeat measurements per day. SMLS will employ extremely sensitive, broadband, sideband-separating, SIS receivers centered at 230 and 640 GHz that provide the same precision as those on Aura MLS with a 100 fold reduction in integration time. SMLS will use a novel antenna design that provides high vertical resolution and enables rapid horizontal scanning of the field of view.

Since the late summer 2008, the development of the SMLS instrument technology has been underway within NASA Earth Science Technology Office’s Instrument Incubator Program. The objective of this development is to advance the core signal path technologies required for a microwave limb sounder with the capability to map the composition of the upper troposphere and stratosphere with 50x50x1 km spatial sampling and six times daily mid-latitude repeat coverage. The specific goals of this effort include:

* the mitigation of the optics and calibration risks of the SMLS flight sensor design by constructing and testing an airborne prototype of the SMLS sensor and calibration system - A-SMLS - using prototype sideband-separating mixers, line sources, and advanced spectrometers and calibration targets;

* the mitigation of the development risks of the cryogenics system by developing a flight-like cryostat and demonstrating an end-to-end prototype of the SMLS signal path from the antenna interface through the back-end electronics, and quantifying its stability, calibration accuracy, linearity, and sensitivity; and

* the demonstration of the potential science measurement capability of SMLS through the A-SMLS science flights.

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