Tropospheric Ozone and Tracers from Commercial Aircraft Platforms

Ozone is measured in a dual-beam ultraviolet (254 nm) absorption analyzer. Ambient air flows through one absorption cell while air scrubbed of ozone flows through an adjacent one. This allows continuous measurement of both background and absorption signals. Flows are switched between cells by a pair of solenoid valves, which permits monitoring of optical changes. Water vapor is detected with a tunable diode laser spectrometer designed and built by Randy May. This sensor employs a room-temperature near-infrared laser (single mode at about 1.37 microns) and second harmonic detection, rather than direct absorption. Unlike the JPL water instrument, this sensor has an internal absorption path, optimized for the mid-troposphere. Carbon dioxide is measured by its absorption in the infrared (4.25 microns) using a LiCor NDIR instrument. This is also a dual-cell device, in which the absorption caused by the ambient air sample is compared to that from a reference gas of known composition. Halocarbons are monitored with a custom-built gas chromatograph, using short, packed columns and small ovens, and HP micro-electron capture detectors. Ambient sample and standard will be run simultaneously on paired columns to reduce errors associated with drift in ECD response.

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Composition and Photo-Dissociative Flux Measurement

The instrument used for the CPFM is a spectroradiometer based on a concave, holographic diffraction grating and a 1024-element diode array detector. It measures the intensities of the two linear polarization components of radiation propagating upward at the aircraft location from a range of elevation angles near the horizon. In addition, a measurement of the intensity of the direct solar beam is made by viewing a horizontal diffusing surface mounted under a quartz dome on board the aircraft. These measurements are used to verify atmospheric light-scattering calculations, which are essential for the accurate modeling of the chemistry of the stratosphere where POLARIS makes its measurements.

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CO2 Weather Balloon Spectrometer

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Charged-coupled device Actinic Flux Spectroradiometers

The Charged-coupled device Actinic Flux Spectroradiometers (CAFS) instruments measure in situ down- and up-welling radiation and combine to provide 4 pi steradian actinic flux density spectra from 280 to 650 nm. The sampling resolution is ~0.8 nm with a full width at half maximum (FWHM) of 1.7 nm at 297 nm. From the measured flux, photolysis frequencies are calculated for ~40 important atmospheric trace gases including O3, NO2, HCHO, HONO and NO3 using a modified version of the NCAR Tropospheric Ultraviolet and Visible (TUV) radiative transfer model. The absolute spectral sensitivity of the instruments is determined in the laboratory with 1000 W NIST-traceable tungsten-halogen lamps with a wavelength dependent uncertainty of 3–5%. During deployments, spectral sensitivity is assessed with secondary calibration lamps while wavelength assignment is tracked with Hg line sources and comparisons to spectral features in the extraterrestrial flux. The optical collectors are characterized for angular and azimuthal response and the effective planar receptor distance. CAFS have an excellent legacy of performance on the NASA DC-8 and WB-57 platforms during atmospheric chemistry and satellite validation mission. These include AVE Houston 2004 and 2005, PAVE, CR-AVE, TC4, ARCTAS, DC3, SEAC4RS, KORUS-AQ, ATom and FIREX-AQ. For FIREX-AQ, upgraded electronics and cooling reduced noise and allowed for a decrease to 1 Hz acquisition.

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Cloud, Aerosol, and Precipitation Spectrometer

This multipurpose particle spectrometer includes three Droplet Measurement Technologies instruments plus temperature and relative humidity sensors that are packaged into a single, integrated measurement system. The CAPS provides the following data:

- Aerosol particle and cloud hydrometeor size distributions from 0.51 to 50 µm

- Precipitation size distributions from 25 µm to 1550 µm, or 15-930 um with optional 15-micron resolution

- Particle optical properties (refractive index)

- Particle shape assessments (discrimination between water and ice for probes with depolarization feature)

- Liquid water content from 0.01 to 3 g/m3

- Aircraft velocity

- Atmospheric temperature and pressure

This instrument replaces the older PMS/PMI FSSP-100, FSSP-300, 2D-C, 2D-P and KLWC and can be used in many applications, including weather modification, aircraft icing, hurricane and storm research, and agricultural and industrial spray characterization.

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Airborne Tunable Laser Absorption Spectrometer

ATLAS uses a tunable laser to detect an infrared-active target gas such as N2O, methane, carbon monoxide, or ozone. The laser source is tuned to an individual roto-vibrational line in an infrared absorption band of the target gas, and is frequency modulated at 2 kHz. The instrument detects the infrared target gas by measuring the fractional absorption of the infrared beam from the tunable diode laser as it traverses a multipass White cell containing an atmospheric sample at ambient pressure.

Synchronous detection of the resultant amplitude modulation at 2kHz and 4kHz yields the first and second harmonics of the generally weak absorption feature with high sensitivity (DI/I < 1E-5). Part of the main beam is split off through a short cell containing a known amount of the target gas to a reference detector. The reference first harmonic signal is used to lock the laser frequency to the absorption line center, while the second harmonic signal is used to derive the calibration factor needed to convert the measurement beam second harmonic amplitude into absolute gas concentration. A zero beam is included to correct for background gas absorption occurring outside the multipass cell. The response time of the instrument is set by the gas flow rate through the White cell, which is normally adjusted to give a new sample every second. Periodic standard additions of the target gas are injected into the sample stream as a second method to calibrate the measurement technique and as an overall instrument diagnostic.

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Argus Tunable Diode Laser Instrument

Argus is a two channel, tunable diode laser instrument set up for the simultaneous, in situ measurement of CO (carbon monoxide), N2O (nitrous oxide) and CH4 (methane) in the troposphere and lower stratosphere. The instrument measures 40 x 30 x 30 cm and weighs 21 kg. An auxiliary, in-flight calibration system has dimensions 42 x 26 x 34 cm and weighs 17 kg.

The instrument is an absorption spectrometer operating in rapid scan, secondharmonic mode using frequency-modulated tunable lead-salt diode lasers emitting in the mid-infrared. Spectra are co-added for two seconds and are stored on a solid state disk for later analysis. The diode laser infrared beam is shaped by two anti-refection coated lenses into an f/40 beam focused at the entrance aperture of a multi-pass Herriott cell. The Herriott cell is common to both optical channels and is a modified astigmatic cell (New Focus Inc., Santa Clara, California).

The aspherical mirrors are coated with protected silver for optimal infrared reflectivity. The cell is set up for a 182-pass state for a total path of 36m. The pass number can be confirmed by visual spot pattern verification on the mirrors observed through the glass cell body when the cell is illuminated with a visible laser beam. However, instrument calibration is always carried out using calibrated gas standards with the Argus instrument operating at its infrared design wavelengths, 3.3 and 4.7 micrometers respectively for CH4 and CO detection. The electronic processing of the second harmonic spectra is done by standard phase sensitive amplifier techniques with demodulation occurring at twice the laser modulation frequency of 40 kHz. To optimize the secondharmonic signal amplitude in a changing ambient pressure environment the laser modulation amplitude is updated every 2 seconds to its optimal theoretical value based upon the measured pressure in the Herriott cell.

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Cloud, Aerosol, and Precipitation Spectrometer

The CAPS is a combination probe designed around the newest technologies and the experience gained with over 20 years of using similar probes. It meets the goals of measuring a large range of particle sizes--0.5μm to 1.55mm--with one probe, thus minimizing space, cable connections, and data systems necessary for measurement of this range. Today's technology also provides the CAPS the processing power necessary to perform at speeds up to 200m/s. An intuitive graphical user interface, the Particle Analysis and Collection System (PACS), at the host computer, provides simple but powerful control of measurement parameters, while simultaneously displaying on-the-fly size distributions and derived parameters. All data interfaces are done via line drivers meeting the RS-422 electrical specification, allowing cable lengths of up to 100 meters--an improvement over RS-232 lines capable of only 15-meter cable lengths.

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Airborne Laser Isotope Spectrometer

Isotopic CO2 measurements have been identified as an important component of NASA's Earth Science Enterprise's Carbon Cycle Initiative as part of its program in global climate change. The isotopic composition of atmospheric CO2, and especially its 13CO2/ 12CO2 ratio, is an established tool for understanding the details of the global carbon cycle, since this ratio can distinguish between oceanic and terrestrial biospheric sinks of CO2.

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Aircraft Laser Infrared Absorption Spectrometer

ALIAS (Aircraft Laser Infrared Absorption Spectrometer) measures total water, total water isotopes, carbon monoxide, and carbon dioxide isotope ratios. No other instrument provides real-time measurements of carbon dioxide isotope ratios which are clear identifiers of atmospheric transport (18O/17O/16O for stratospheric intrusion, 13C/12C for anthropogenic signals). ALIAS easily adapts to changing mission priorities and can be configured to measure HCl, CH4, SO2, and N2O by simply replacing a semiconductor laser. These measurements contribute to Atmospheric Composition Focus Area research by providing key data on how convective processes affect stratospheric composition, the development of cirrus particles and their affect on Earth's radiative balance, and health of the ozone layer through measurement of chlorine partitioning.

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