DC-8 - AFRC

The CIMS instrument consists of a low pressure ion molecule reactor (IMR) coupled to a quadrupole mass filter by an actively pumped collisional dissociation chamber (CDC) and an octopole ion guide. The vacuum system is a 100 mm outer diameter stainless steel chamber evacuated with two small turbo pumps (70 l s^-1). The mass filter is a set of 9.5 mm diameter quadrupole rods housed in the main vacuum chamber. The CDC is a short 80 mm diameter chamber that houses an octopole ion guide and is evacuated with a hybrid molecular drag pump. The IMR is evacuated with a scroll pump (300 l min^-1) that also serves as the backing pump for the mass spectrometer.

Click here for the Collaborative Ground and Airborne Observations description page.

GAMS/LAABS is a combination of the Gas and Aerosol Measurement System (GAMS) and the Langley Airborne A-Band Spectrometer (LAABS). The instruments are optically co-aligned and use a common pointing system to track the Sun through an aircraft view port. In the field the instrument provides line-of-sight (LOS) O3, NO2, O4, and water vapor measurements using both a SAGE III-like multiple linear regression algorithm and a full spectrum algorithm. Aerosol may also be derived for ‘enhanced’ conditions including polar stratospheric clouds and optically thin cirrus. Using profile data (1-D/2-D) transformed to GAMS/LAABS LOS geometry, quick-look validation/comparison products for SAGE III, AROTAL, AATS-14, SCIAMACHY, and other instruments will be obtained. The data from GAMS/LAABS will make possible crucial evaluations of SAGE III data processing possible following deployment. These activities include SAGE III etaloning/mirror correction validation, O2 spectroscopy and forward model verification, ozone spectroscopy near the O2 A band and 940-nm water vapor features, evaluation of the relative strength of spectroscopic features (e.g., water vapor features at 600 nm and 940 nm) and altitude registration validation using oxygen measurements.

Instrument Details:

Gas and Aerosol Measurement Sensor (GAMS)

o Solar spectrometer with 1024 channels from ~ 430 to 1030 nm
o Provides measurements of LOS transmission spectra and
differential O3, H2O, O2, O4, and aerosol
o Solar imager to monitor scene homogeneity
o Focus on UV-Vis-Near-IR solar occultation only
o Designed to extend the technique into the troposphere
o Built space flight-like spectrometer, telescope, photon-to-bits
boards, & MCM (detector controller).
o COTS imager to establish imager performance requirements

Langley Airborne A-Band Spectrometer (LAABS)

• High spectral resolution (~0.035 nm) grating spectrometer with > 800
channels from ~759 to 771 nm
• Provides measurements of LOS transmission spectra for evaluation of
SAGE III O2 A-band forward model
• Originally developed to support CALIPSO (formerly PICASSO-CENA)
spaceborne A-band spectrometer
• Designed and fabricated by BATC to provide high spectral resolution
(~0.035 nm) radiance measurements in O2 A-band spectral region
(~765 nm)

The absorption of infrared solar radiation along a slant path to the sun is recorded from 2 to 15 micrometers. Six spectral filters are used to cover the region from 2-15 microns. An interferogram is recorded in about 10 seconds. Interferograms are transformed to produce spectra. Column amounts are retrieved by fitting the observed spectra using the non-linear least squares fitting code SFIT2 that employs an Optimal Estimation retrieval algorithm.

The major chlorine reservoirs (HCl and ClONO2), the important nitrogen-containing gases in the stratosphere (N2O, NO, NO2, and HNO3), stratospheric and tropospheric tracers (HF, CH4, C2H6, H2O, CO2), a major source CFC (CF2Cl2) and ozone may be routinely retrieved.

• Technique: Chemluminescent reaction of ozone with nitric oxide
• Dynamic Range: 0.6 - 1600 ppb
• Accuracy: 5% or 2 ppb
• Precision: 2% or 0.6 ppb
• Response: 2-3 Hz; recorded at 6 Hz, reported at 1 Hz, faster data on request
• Spatial Resolution: <10 m vertical (aircraft spiral), 200 m horizontal (at 400 kts)

A Fabry-Perot interferometer is constructed of two very flat, partially reflecting mirrors held parallel to one another at a fixed distance. Interference occurs among the multiple reflections leading to the condition that wavelengths that exactly divide the spacing between the mirrors by an integer are transmitted very efficiently and all other wavelengths are reflected. Thus if the plates are held fixed at a separation of 10 μm, then radiation at 10, 5, 3.333, … μm will be transmitted. Note that these wavelengths are equally spaced in energy according to the relationship E=hc/l, where l is the wavelength of the light and h and c are Planck’s constant and the speed of light, respectively. This particular FPI technique makes use of these multiple passbands to increase the measurement signal and the resulting signal to noise ratio.

A Fabry-Perot can be tuned to transmit different wavelengths by changing the (optical) spacing between the mirrors. This is commonly done by employing piezo-electric transducers to translate the mirrors by very small distances, while maintaining the very precise parallelism between them. Fixed gap Fabry-Perots can be tuned by tilting, which changes the effective path length between the plates; by using the thermal expansion and contraction of the spacers between the mirrors; and by changing the composition or pressure of the gas that fills the space between the plates, which alters the index of refraction thereby changing the optical separation. Finally, Fabry-Perots can be constructed using a solid substrate of fused silica or optical quality glass onto which reflective coatings are deposited. These devices can be angle tuned or temperature tuned.

The FSSP is of that general class of instruments called optical particle counters (OPCs) that detect single particles and size them by measuring the intensity of light that the particle scatters when passing through a light beam. A Helium Neon laser beam is focused to a diameter of 0.2 mm at the center of an inlet that faces into the oncoming airstream. This laser beam is blocked on the opposite side of the inlet with an optical stop, a "dump spot" to prevent the beam from entering the collection optics. Particles that encounter this beam scatter light in all directions and some of that scattered in the forward direction is directed by a right angle prism though a condensing lens and onto a beam splitter. The "dump spot" on the prism and aperture of the condensing lens define a collection angle from about 4º - 12º.

The beam splitter divides the scattered light into two components, each of which impinge on a photodetector. One of these detectors, however, is optically masked to receive only scattered light when the particles pass through the laser beam displaced greater than approximately 1.5 mm either side of the center of focus. Particles that fall in that region are rejected when the signal from the masked detector exceeds that from the unmasked detector. This defines the sample volume needed to calculate particle concentrations.

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.

DC-8 dropsondes measure the vertical profile of atmospheric temperature, pressure, relative humidity, and wind speed and direction as the sonde falls from altitude to ocean surface.

A solar tracking Direct beam Irradiance Airborne Spectrometer (DIAS) is used for calculation of line of sight ozone and wavelength dependent aerosol optical depths.