Disclaimer: This material is being kept online for historical purposes. Though accurate at the time of publication, it is no longer being updated. The page may contain broken links or outdated information, and parts may not function in current web browsers. Visit espo.nasa.gov for information about our current projects.

 

Synonyms: 
STEP87
STEP 1987
Associated content: 

Water vapor and cloud water measurements over Darwin during STEP 1987 tropical cloud mission

Condensation Nucleus Counters (CNCs) and Electrical Aerosol Sampler (EAS)

Instrument: Condensation Nucleus Counters (CNCs) and

Electrical Aerosol Sampler (EAS)

Principal Investigator: James C. Wilson

Organization:
University of Denver
Department of Engineering
2390 S. York Street
Denver, CO 80208

Instrument Description:
Two condensation nucleus counters (CNCs) have been developed for use on the NASA ER-2 high altitude research aircraft. One CNC measures the number concentration of aerosol particles having diameters in the 0.01 to about 1.0 micron range, while the second uses a heated (150 oC) inlet to vaporize volatile components and then measures the number concentration of residue particles. Used together, the CNCs discriminate between particles composed of volatile materials (i.e., sulfuric acid), and those containing components that are non-volatile at temperatures of 150 oC. For SPADE, the CNCs should be able to distinguish between particles containing carbon soot (an aircraft exhaust product) and background sulfate particles.

Two aerosol collectors have been developed for the SPADE mission. The first uses electrical precipitation to collect particles with diameters greater than 0.01 microns on electron microscope grids. Similar samples, but of particles with diameters greater than about 0.1 microns, are taken concurrently with an impactor. The sealed samples are then returned to the laboratory for analysis by analytical electron microscopy. Up to 25 samples may be collected by each method during a single flight.

Instrument Function: The CNCs function by saturating an aerosol sample with warm alcohol vapor and then cooling the sample so that the alcohol vapor condenses on the particles. The particles grow by vapor deposition to a size such that the individual particles are easily detected by a simple optical particle counter.

The aerosol collectors function by two different principles. In the electrical collector, a needle is forced into a corona discharge by high voltage. The sample is carried past the corona point, and unipolar ions produced by the corona attatch to the particles, causing them to become charged. The charged particles are then collected on a grounded electron microscope grid. In the impactor, the air sample is accelerated through a nozzle and forced around a sharp bend. Particles larger than about 0.1 micron diameter cannot follow the streamlines and instead impact onto an electron microscope grid located at the bend.

Accuracy: The accuracy and precision of the CNCs is highly dependent on the aerosol size distribution. For aerosols whose number distribution is dominated by particles larger than 0.01 microns in diameter, the submicron number concentration is usually measured with an uncertainty of less than 20% and a precision smaller than 10%.

Reference: Wilson, James Charles, Edmund D. Blackshear and Jong Ho Hyun. "The Function and Response of an Improved Stratospheric Condensation Nucleus Counter." J. Geophys. Res. 88 (1983): 6781-6785.

Aircraft: 
Point(s) of Contact: 

FSSP-300 Aerosol Spectrometer

Instrument: FSSP-300 Aerosol Spectrometer

 

Principal Investigator: Guy V. Ferry

 

Organization:

NASA-Ames Research Center

M.S. 245-5

Moffett Field, CA 94035-1000

 

Principal Investigators: James E. Dye (303) 497-8944 Darrel Baumgardner (303) 497-1054 FAX (303) 497-8181 Organization: National Center for Atmospheric Research 1850 Table Mesa Drive Boulder, Co 80307 Principle of Operation: The Forward Scattering Spectrometer Probe (FSSP) Model 300 sizes particles by measuring the amount of laser light scattered from angles of 4 to 12&degree; by aerosol particles in situ as they pass through a focused laser beam. Comparison of voltage outputs from the signal detector and a masked slit detector is used to electro-optically define the sample area. Fig. 1 shows the configuration of the instrument. The instrument system is composed of two parts: (l) a Particle Measuring Systems model FSSP-300 aerosol spectrometer, and (2) a data acquisition and recording system. The FSSP-300 aerosol spectrometer is located on the front of the starboard spear pod of the ER-2. The data acquisition and recording system is part of the package that houses the FPCAS aerosol spectrometer located in the bottom, rear portion of the starboard spear pod of the ER-2. The FSSP-300 aerosol spectrometer sizes particles in the 0.4 to 20 micron diameter size range (depending on the refractive index of the aerosol particles measured) in the free air stream outside the ER-2. The measured particles are divided into 31 size intervals with more resolution at smaller sizes.

 

Detection Limit: 0.4 to 20 micrometers diameter Sampling Rate: 0.1 Hertz Location on ER-2: Nose of right pod. Reference: Baumgardner, D., et al. ~Interpretation of Measurements made by the FSSP-300 during the Airborne Arctic Stratosphenc Expedition." J. Geophys. Res. In press. 1992.

 
Aircraft: 

NOAA Lyman-Alpha Total Water Hygrometer

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.

Measurements: 
Point(s) of Contact: 

NOAA NOy Instrument

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.

Instrument Type: 
Measurements: 
Aircraft: 
ER-2 - AFRC, Balloon
Point(s) of Contact: 

Dual-Beam UV-Absorption Ozone Photometer

The NOAA-O3 instrument consists of a mercury lamp, two sample chambers that can be periodically scrubbed of ozone, and two detectors that measure the 254-nm radiation transmitted through the chamber. The ozone absorption cross-section at this wavelength is accurately known; hence, the ozone number density can be easily calculated. 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. At a one-second data collection rate, the minimum detectable concentration of ozone (one standard deviation) is 1.5 x 10 10 molecules/cm 3 (0.6ppbv at STP).

Instrument Type: 
Measurements: 
Aircraft: 
Point(s) of Contact: 

Radiometric Measurement System

Optics: We employed very simple optical arrangement for the radiometer. The diffuser-light trap arrangement provides a hemispherical field of view with incident radiation being collimated by the high reflectance walls of the exponential-logarithmic cavity. Enough collimation of the radiation is achieved with this design that narrow spectral bandpass interference filters can be used to select desired wavelength regions.

Electronics: The instrument electronics includes five major functional blocks. They are the detectors signal conditioning block, the data processing block, the system controller block, the shadow ring drive and control block, and the data storage block.

The signal detectors are silicon photodiodes operating in the photovoltaic mode and covering the spectral range from about 0.3 to 1.1µm. Their signals are converted into electrical voltages by low noise FET input operational amplifiers. Programmable gain amplifiers allow adjustments for dynamic range, and filter circuits condition the signals for analog to digital processing. Data processing units consist of an analog multiplexer circuit, a sample-and-hold circuit, and an analog to digital converter providing a 12-bit resolution output. The shadow ring is driven by a DC motor rotating at a constant speed. A motor controller is used to maintain motor speed. The system controller provides the timing necessary to perform all the system's tasks. It sets the shadow ring in motion and steps through the detector's outputs, maintaining the proper dynamic range for the analog to digital converter by selecting the proper amplifier gain. It also controls the analog to digital conversion and selectively stores data.

Instrument Type: 
Measurements: 
Point(s) of Contact: 

Microwave Temperature Profiler

The Microwave Temperature Profiler (MTP) is a passive microwave radiometer, which measures the natural thermal emission from oxygen molecules in the earth’s atmosphere for a selection of elevation angles between zenith and nadir. The current observing frequencies are 55.51, 56.65 and 58.80 GHz. The measured "brightness temperatures" versus elevation angle are converted to air temperature versus altitude using a quasi-Bayesian statistical retrieval procedure. The MTP has no ITAR restrictions, has export compliance classification number EAR99/NLR. An MTP generally consists of two assemblies: a sensor unit (SU), which receives and detects the signal, and a data unit (DU), which controls the SU and records the data. In addition, on some platforms there may be a third element, a real-time analysis computer (RAC), which analyzes the data to produce temperature profiles and other data products in real time. The SU is connected to the DU with power, control, and data cables. In addition the DU has interfaces to the aircraft navigation data bus and the RAC, if one is present. Navigation data is needed so that information such as altitude, pitch and roll are available. Aircraft altitude is needed to perform retrievals (which are altitude dependent), while pitch and roll are needed for controlling the position of a stepper motor which must drive a scanning mirror to predetermined elevation angles. Generally, the feed horn is nearly normal to the flight direction and the scanning mirror is oriented at 45-degrees with respect to receiving feed horn to allow viewing from near nadir to near zenith. At each viewing position a local oscillator (LO) is sequenced through two or more frequencies. Since a double sideband receiver is used, the LO is generally located near the "valley" between two spectral lines, so that the upper and lower sidebands are located near the spectral line peaks to ensure the maximum absorption. This is especially important at high altitudes where "transparency" corrections become important if the lines are too "thin." Because each frequency has a different effective viewing distance, the MTP is able to "see" to different distances by changing frequency. In addition, because the viewing direction is also varied and because the atmospheric opacity is temperature and pressure dependent, different effective viewing distances are also achieved through scanning in elevation . If the scanning is done so that the applicable altitudes (that is, the effective viewing distance times the sine of the elevation angle) at different frequencies and elevation angles are the same, then inter-frequency calibration can also be done, which improves the quality of the retrieved profiles. For a two-frequency radiometer with 10 elevation angles, each 15-second observing cycle produces a set of 20 brightness temperatures, which are converted by a linear retrieval algorithm to a profile of air temperature versus altitude, T(z). Finally, radiometric calibration is performed using the outside air temperature (OAT) and a heated reference target to determine the instrument gain. However, complete calibration of the system to include "window corrections" and other effects, requires tedious analysis and comparison with radiosondes near the aircraft flight path. This is probably the most important single factor contributing to reliable calibration. For stable MTPs, like that on the DC8, such calibrations appear to be reliable for many years. Such analysis is always performed before MTP data are placed on mission archive computers.

Instrument Type: 
Measurements: 
Aircraft: 
DC-8 - AFRC, ER-2 - AFRC, Global Hawk - AFRC, L-188C, M-55, Gulfstream V - NSF, WB-57 - JSC
Point(s) of Contact: 

Frost Point (NOAA)

The NOAA frost point instrument was designed to run unattended under the wing of NASA’s WB-57. An aircraft rated Stirling cooler provides cooling to 100 K. The cooler avoids consumables and provides a large temperature gradient that improves the response time. The vertical pylon houses the optics and provides aerodynamic pumping of the sample volume. At the bottom of the pylon there is a boundary layer plate and a vertical inlet that separates particles larger than 0.2 microns from the sampled air. There are two channels that use blue LEDs and scattered light to detect frost on the mirrors. Diamond mirrors are used for low thermal mass and high conductivity. The two channels are to be used to understand frost characteristics under flight conditions. High flow rates are used to decrease the shear boundary layer to facilitate diffusion through the boundary layer to the mirrors.

Measurements: 
Point(s) of Contact: 

Pages

Subscribe to RSS - STEP