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SPADE H2O Measurements and the Seasonal Cycle of Stratospheric Water Vapor

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.

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Multiple Axis Resonance Fluorescence Chemical Conversion Detector for ClO and BrO

Vacuum ultraviolet radiation produced in a low pressure plasma discharge lamp is used to induce resonance scattering in Cl and Br atoms within a flowing sample. ClO and BrO are converted to Cl and Br by the addition of NO such that the rapid bimolecular reaction ClO + NO → Cl + NO2 (BrO + NO → Br + NO2) yields one halogen atom for each halogen oxide radical present in the flowing sample. Three detection axes are used to diagnose the spatial (and thus temporal) dependence of the ClO (BrO) to Cl (Br) conversion and to detect any removal of Cl (Br) following its formation. A double duct system is used both to maintain laminar flow through the detection region and to step the flow velocity in the detection region down from free stream (200 m/sec) to 20 m/sec in order to optimize the kinetic diagnosis.

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Lyman Alpha-Hygrometer

 

Instrument: Lyman Alpha-Hygrometer

Principal Investigator: Ken Kelly

Organization:
NOAA/ERL/Aeronomy Laboratory
325 Broadway MS R/E/AL6
Boulder, CO 80303

Principle of Operation: A 121.6 nm light source dissociates a fraction of the water and forms excited hydroxyl radicals. These radicals will either fluoresce at 309 nm or be quenched by air molecules. A PMT measures the 309 nm light, which is proportional to the water vapor mixing ratio. A photodiode monitors the 121.6 nm intensity at the same distance as the sample chamber center. An in-flight calibration is obtained from the measured absorption of 121.6 nm light by injected water vapor, the known absorption cross section and the chamber pressure. The hygrometer will measure total water.

Accuracy: 6%
Detection Limit: 0.1 ppmv
Response Time: 1 Second
Location on ER-2: Q-Bay

Reference: Kley, D., A. Schmeltekopf, K. Kelly, R. Winkler, T. Thompson, M. McFarland. "The U-2 Lyman-Alpha results from the 1980 Panama Experiment." The 1980 Stratospheric-Tropospheric Exchange Experiment. Ed: A.P.Margozzi. NASA Technical Memo 84297 (1983): 85-125.

 

 

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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.

 
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Reactive Nitrogen

Instrument: Reactive Nitrogen

Principal Investigator: David W. Fahey

Organization:
NOAA Aeronomy Laboratory
325 Broadway,R/E/AL6
Boulder, CO 80303

Principle of Operation:
The instrument is designed to measure nitric oxide (NO) and the sum of reactive nitrogen oxides (NOy). Species included in NOy are NO, NO2, HNO3, N2O5 and ClONO2. NO is measured by detecting light from the chemiluminescent reaction between reagent ozone and NO in the ambient sample. NOy is reduced to NO by catalytic reduction on a gold surface with carbon monoxide (CO) acting as a reducing agent. The catalyst is located outside the aircraft fuselage in order to avoid inlet line losses. Two reaction vessels are incorporated in the instrument to allow for simultaneous measurement of NO and NOy. Ca1ibration with NO or NO2 is made by standard addition several times during a flight. The baseline of each measurement is determined in part by the addition of synthetic air that contains no reactive nitrogen. The difference between the sample flow velocity in the inlet opening and the aircraft velocity cause aerosol particles in the atmosphere to be oversampled. For sizes below 5 micrometers in diameter, this feature assists in the identification of aerosol particles that contain NOy.

 

 

Accuracy: < 20% plus precision
Detection limit: < 0.1 ppbv NOy, ~0.02 ppbv NO
Response time: 1 sec
Location on the ER2: Lower Q-bay rack

Reference: D.W. Fahey et al., In situ aerosol measurements of total reactive nitrogen, total water, and aerosol in a polar stratospheric cloud in the Antarctic, J. Geophys. Res. 94 11-99-11315, 1959.

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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.

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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.

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ER-2 - AFRC, Balloon
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Forward Scattering Spectrometer Probe

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.

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