ER-2 - AFRC

The Cloud Droplet Probe (CDP), manufactured by Droplet Measurement Technologies, measures the concentration and size distribution of cloud droplets in the size range from 2-50 µm. The instrument counts and sizes individual droplets by detecting pulses of light scattered from a laser beam in the near-forward direction, using a sample area of 0.24 mm2 or a sample rate of 48 cm3 at a flight speed of 200 m/s. The probe is mounted in an underwing canister and is designed to operate at up to 200 m/s; the G-V often exceeds this flight speed, but usually not in penetrations of clouds containing cloud droplets. Droplet sizes are accumulated in 30 bins with variable sizes, as specied in the header of the netCDF data files. Measurements are usually provided at a rate of 1 Hz in the standard data files but can be made available at 10 Hz in special high-rate processing. The instrument is similar to, and might be considered a high-speed replacement for, the Forward Scattering Spectrometer Probe. At high droplet concentration (> 500 cm-3), coincidence losses have been observed with this probe, and these are especially serious at G-V flight speeds. The probe is designed for cloud droplets, and its response to ice crystals is not intended to be quantitative; measurements in ice clouds should not be used except as qualitative indications of cloud.

Total water is measured in situ as vapor with a Lyman-Alpha hygrometer. High ambient sample flows through a closed cell minimize the effect of trapped water. Lyman-a light (121.6 nm) photodissociates water to produce an excited OH radical. The fluorescence from this radical at 309 nm is detected with a phototube and counting system. At aircraft pressures the fluorescence signal is quenched by air which gives a signal that is proportional to mixing ratio. The Lyman-Alpha radiation produced with a DC-discharge lamp is monitored with an iodine ionization cell that is sensitive from 115 nm to 135 nm. Calibration occurs in flight by injecting water vapor directly into the ambient sample flow.

The NOy instrument has three independent chemiluminescence detectors for simultaneous measurements of NOy, NO2, and NO. Each detector utilizes the reaction between NO in the sample with reagent O3. The NO/O3 reaction produces excited state NO2 which emits light of near 1µ m wavelength. Emitted photons are detected with a cooled photomultiplier tube.

Because NOy species other than NO do not respond in the chemiluminescence detector, NOy component species are reduced to NO by catalytic reduction on a gold surface with carbon monoxide (CO) acting as a reducing agent. Conversion efficiencies are > 90% at surface temperatures of 300°C. An NO signal representing NOy is then detected by chemiluminescence in the detector module. The catalyst is located outside the aircraft fuselage in order to avoid inlet line losses. NO2 is photolytically converted to NO in a glass cell in the presence of intense UV light between 300 and 400 nm. The conversion fraction is > 50% for a residence time of 1 s. The chemiluminescence detector detects NO as well as the additional NO from NO2. The third channel measures NO directly by passing the ambient sample through the detector module.

The response of each detector is checked several times in flight by standard addition of NO or NO2 calibration gas. The baseline of each measurement is determined in part by the addition of synthetic air that contains no reactive nitrogen. A continuous flow of water vapor is added directly to the sample flow in order to reduce the background signal in the detectors.

The sampling inlet for NOy is located outside the fuselage of the aircraft in a separate football-shaped housing. The shape of the housing allows for the inertial separation of large aerosols (> 5 µm diameter) from the NOy inlet at the downstream end of the housing.

The RC-30 is an airborne film camera system, using color infrared, natural color and black and white film, to obtain high resolution earth imagery.

The Itek Iris II Panoramic Camera has been employed to acquire high resolution land use and land cover data. The Forest Service has used this camera extensively for assessing timber resources and monitoring gypsy moth defoliation in the Appalachian hardwood forests. The Iris II provides a 4.5 x 34.7 inch (11.4 x 88.1 cm) image covering 2.0 x 21.4 miles (3.2 x 34.2 km) on the ground. The high resolution twenty-four inch lens provides a scale at nadir on the panoramic image of half-mile to the inch. With its 10,000 foot film capacity the Iris II allows extended flight duration allowing photography acquisition over very large areas.

Hycon HR-732 cameras are used to acquire high resolution photography in a 9 x 18 inch format. These cameras can be flown in pairs or one camera may be paired with an RC-10 mapping camera. The HR-732s acquire high resolution photography with twenty-four inch focal length lenses providing an image scale of half-mile to the inch. The large scale high resolution photography provided by these cameras is used by agencies such as the Forest Service for timber resource management and by the Fish and Wildlife Service for wetlands inventories and wildlife habitat mapping.

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.