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.

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.

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.

The NASA Langley Airborne Differential Absorption Lidar (DIAL) system uses four lasers to make DIAL O3 profile measurements in the ultraviolet (UV) simultaneously with aerosol profile measurements in the visible and IR. Recent changes incorporate an additional laser and modifications to the receiver system that will provide aerosol backscatter, extinction, and depolarization profile measurements at three wavelengths (UV, visible, and NIR). For SEAC4RS, the DIAL instrument will include for the first time aerosol and cloud measurements implementing the High Spectral Resolution Lidar (HSRL) technique [Hair, 2008]. The modifications include integrating an additional 3-wavelength (355 nm, 532 nm, 1064 nm) narrowband laser and the receiver to make the following measurements; depolarization at all three wavelengths, aerosol/cloud backscatter and extinction at 532 nm via the HSRL technique, and aerosol/cloud backscatter at the 355 and 1064 nm via the standard backscatter lidar technique. Integration of the aerosol extinction profile at 532nm above and below the aircraft also provides aerosol optical depth (AOD) along the aircraft flight track.

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 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.

The Whole Air Sampler (WAS) collects samples from airborne platforms for detailed analysis of a wide range of trace gases. The compounds that are typically measured from the WAS includes trace gases with sources from industrial midlatitude emissions, from biomass burning, and from the marine boundary layer, with certain compounds (e.g. organic nitrates) that have a unique source in the equatorial surface ocean. The use of a broad suite of tracers with different sources and lifetimes provides powerful diagnostic information on air mass history and chemical processing that currently is only available from measurements from whole air samples. Previous deployments of the whole air sampler have shown that the sampling and analytical procedures employed by our group are capable of accessing the wide range of mixing ratios at sufficient precision to be used for tracer studies. Thus, routine measurement of species, such as methyl iodide, at <= 0.1 x 10-12 mole fraction, or NMHC at levels of a few x 10-12 mole fraction are possible. In addition to the tracer aspects of the whole air sampler measurements, we measure a full suite of halocarbon species that provide information on the role of short-lived halocarbons in the tropical UT/LS region, on halogen budgets in the UT/LS region, and on continuing increasing temporal trends of HFCs (such as 134a), HCFCs (such as HCFC 141b), PFCs (such as C2F6), as well as declining levels of some of the major CFCs and halogenated solvents. The measurements of those species that are changing rapidly in the troposphere also give direct indications of the age and origin of air entering the stratosphere.

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.