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PACDEX Airborne Instrumentation

Facility aerosol instruments

Condensation nuclei (CN) will be measured with a new water-based instrument that has been acquired by EOL/Research Aviation Facility (RAF) as part of the HIAPER instrument infrastructure. This instrument is being modified by the manufacturer (Aerosol Dynamics, Inc.) to improve its response across the wide range of pressures flown by the G-V (~1000 to 100 mb). It detects particles larger than ~3 nm (Hering et al., 2005). Two instruments can be flown, with one operating at this size threshold and the second one at ~20 nm.  Nucleation-mode particles can then be identified by differencing.  These will be available by early 2006.

A new Ultra-High Sensitivity Aerosol Spectrometer (UHSAS, manufactured by ParticleMetrics, Inc.) is being acquired by RAF for use on the G-V.  This is an optical scattering, single-particle instrument that spans the size range 55 to 1000 nm.  It is an under-wing pod-mounted instrument, and will be available for research by Fall 2006.

Measurements of aerosol light scattering will be made using two integrating nephelometers, Radiance Research model M903, currently available at RAF. The units are identical, and an in-line humidifier can be used in front of either one to assess the response of the aerosol to humidity (adjustable from ambient to 90%+).

Optical and bulk aerosol instrumentation

Elemental carbon will be characterized by the DMT Single particle soot photometer (SP-2).  The SP-2 measures absorbing (assumed to be carbon) mass by laser induced incandescence and particle size by light scattering in the range of 0.08 to 5.0 μm.
The Arizona State University analyses will be conducted on samples from the CVI and the giant aerosol collector and from two small samplers which are used to collect particles for individual particle analysis, a Programmable Streaker sampler (PIXE International) set up as a filter sampler for SEM analysis and a 3-stage micro-impactor (California Microsystems) for impaction onto 3 mm grids for TEM analysis.  The Streaker has a rotary stage and can acquire up to 40 separate samples (each 2 x 8 mm) on a 90 mm polycarbonate filter; the streaker stage advance is controlled by a signal from a laptop.  For the micro-impactor, only one set of sample stages can be mounted at a time and sample change is accomplished by hand, requiring the presence of an operator.

Automated SEM analysis of filter samples from the streaker measures the size, shape, and elemental composition of particles 0.1 microns in diameter and larger on a population of approximately 1000 particles per sample analyzed.  Because it takes 24 hours to analyze this many particles, a subset of the collected samples are selected for analysis.  The Arizona State University also will perform manual high-resolution SEM imaging of the filter samples to investigate mixing states (e.g. aggregation of dust and black carbon) and aerosol aging.

Manual TEM analysis investigates the structure and physical properties of individual aerosol particles.  Details of the mixing state of black carbon, sulfate, and high-molecular-weight organics are of particular interest, as well as the variety of forms of black carbon.

Cloud Condensation Nuclei Measurements

The measurements described here directly address how the physico-chemical evolution of the Asian plume affect the ability of aerosols to serve as CCN and the potential subsequent indirect effects of aerosols. On the G-V aircraft, fast response CCN measurements are with a multi-column Continuous-Flow Streamwise Thermal Gradient CCN Chamber (MCSTGCC), being developed at Scripps Institution of Oceanography. The continuous-flow thermal gradient diffusion chamber [Roberts and Nenes, 2005] was developed for autonomous operation in airborne studies employing a novel technique of generating a supersaturation along the streamwise axis of the instrument. Roberts and Nenes are transforming the single-column streamwise CCN instrument into a compact, automated multi-column device to retrieve CCN activation spectra over a range of supersaturations appropriate for aerosol/cloud interactions. CCN spectra are especially important for the proposed airborne measurements as supersaturations can change spatially and temporally in different parts of a cloud system. The multi-column CCN instrument will provide 1 Hz measurements at four supersaturations between 0.1% and 1% supersaturation. The measured CCN spectra during PACDEX will yield valuable insight on how the aging of the urban pollution and dust affects cloud microphysics and subsequent changes to cloud radiative properties.

Giant (>1 µm) and ultra-giant (>10 µm) CCN will be measured by a giant nuclei collection system that is based on a proven technique of impacting aerosol on microscope media directly exposed to the airstream. This technique, at typical G-V airspeed, collects particles larger than a few microns. Many of these larger particles require a large sample volume for proper statistics, which is provided by the giant nuclei sampling system. Two techniques will be used to analyze the aerosol. Some of the sampling will be done on electron microscope grids, which will be analyzed at the University of Arizona for size, composition and morphology, as will be done for the Hawaii TAS samples. The reset of the sampling will be done on optical microscope slides. Digitized optical analysis of the size distribution will be made under dry and humidified conditions. This allows the identification of giant and ultra-giant cloud condensation nuclei, which are critical to the production of rain by collision and coalescence. In addition, it allows the determination of the size segregated soluble mass of particles.

Air sample inlets on HIAPER

Inlets for bringing air to instruments inside the aircraft cabin have been developed to support HIAPER.  These HIAPER Modular Inlets (HIMIL) can be configured for sampling trace gases and aerosol particles.  HIMIL consists of a aerodynamic strut and the cylindrical tube it supports.  Metal or Teflon piping can feed through the strut and terminate in the tube. Tip and tail ends of the tube can be modified as needed; for example, Figure 14 shows a flow-alignment shroud. Other options may include a diffuser tip, converging tail, or open tube with small piping inserted to draw samples from the centerline. Leading edges have temperature-controlled heating to prevent ice accumulation in supercooled water clouds. First flights with HIMIL are underway (November 2005). Flow modeling work is being done to characterize the performance. For more information,  see HIMIL inlets.

Figure 1. HIAPER Modular Inlet (HIMIL)

Compressional heating of the air is a significant issue for airborne sampling of particles. At typical HIAPER airspeeds, ~225 m s-1, it amounts to ~25°C.  Small particles that have volatile components (e.g., sulfuric acid) are likely to evaporate partially or completely.  This is not a significant factor for mineral dust or soot, but it can be important for acids or organic species.  RAF is collaborating with aerosol researchers and exploring options to compensate for this effect and to avoid or reduce the impact of this issue.

Facility cloud particle instruments

An assortment of facility instruments for measuring cloud particles will be available in time for PACDEX.  For cloud droplets, RAF plans to purchase and install a CDP (Cloud Droplet Probe, made by Droplet Measurement Technologies). The CDP is an optical, single particle forward-scattering instrument, designed for airborne use and covering the size range 2-60 µm diameter at speeds up to 250 m s-1.

The Small Ice Detector, version 2, (SID-2) is being acquired with NSF funding through the Major Research Equipment and Facilities Construction program. SID-2 is a single particle optical scattering instrument. It has multiple detectors to measure scattering asymmetry, from which small ice particles and small water drops can be discriminated (Hirst et al., 2001; Field et al., 2004).  SID-2 is an under-wing pod-mounted instrument and should be available for research early in 2007.

Engineering staff at RAF are modifying a standard 2D-C (Two-dimensional Cloud) probe so that it will function at the faster speeds of HIAPER (up to 240 m s-1) and so that it can image a larger range of particle sizes (up to 1600 µm).  Standard versions of this probe with older electro-optics lose sensitivity above ~150  m s-1. This is an under-wing pod-mounted probe and will be available for use on HIAPER in Fall 2006.

The CSU Continuous Flow Diffusion Chamber (CFDC)

The continuous-flow (ice-thermal) diffusion chamber, is a device for processing populations of aerosol particles in order to promote ice formation by those particles capable of acting as ice nuclei (Rogers 1988; Rogers et al. 2001a). Air flow is directed vertically between two concentric ice-coated cylinders held at different temperatures, creating a supersaturated zone in the annular region. The sample air, ~ 10% (1 liter min-1) of the total flow, is sandwiched between two particle-free sheath flows. Particles in the sample flow are exposed to defined temperatures and ice (or water)  supersaturations and those particles active as IN are grown to ice crystal sizes larger than a few microns. These nucleated ice crystals are detected and counted by an optical particle counter (OPC) at the outlet of the instrument. Any activated cloud droplets are not counted due to the removal of the warm wall ice source in the lower third of the chamber, which reduces relative humidity toward ice saturation and causes the evaporation of liquid particles. By altering the ice wall temperatures and allowing a few minutes for stabilization, ice nuclei measurements may be made over a range of temperatures and supersaturations. The technique is most sensitive to deposition and condensation freezing nucleation due to limited (~10 s) residence times. Measurements are also limited at present to aerosol particles smaller than 1 micron in order to prevent false positive identification of large aerosol particles as nucleated ice crystals. This is accomplished by operating an impactor upstream of the CFDC. Developments are underway to replace the OPC detection system with a system capable of discriminating particle phase through spatial scattering properties, but it is not certain that this task will be complete prior to PACDEX. If available, the new detection system is not expected to require any selective removal of larger aerosols.

The CFDC instrument technique has a history of use for studies of mixed phase and ice phase clouds on a variety of aircraft (e.g., DeMott et al. 1998; Rogers et al. 2001b; DeMott et al. 2003a). Presently, two aircraft-capable versions and one laboratory version of this instrument exist. Cooling of the walls of the aircraft versions is accomplished by active refrigeration using compressors.  Air samples are typically obtained from an appropriate forward-facing aircraft inlet or alternately, cloud particle residuals can be sampled after a counterflow virtual impactor.  A web site exists that describes the instruments in more detail (http://lamar.colostate.edu/~pdemott/cfdc/cfd.html) and shows examples of installations.

Trace Gas Instruments

Although trace gas chemistry is not one of the major goals of PACDEX, we intend to measure trace gases to help identify anthropogenic pollution in the dust plumes from Asia.   In situ measurements of CO and ozone mixing ratios will be measured using HIAPER facility instruments, which are based on a modified UV photometer for ozone and a vacuum UV absorption method for CO.  The precision of the CO instrument is 3 ppbv for a 1 Hz sample rate and the ozone instrument is capable of 0.2 ppbv precision for a similar sampling rate.  These instruments can detect and measure both plume and ambient level of these gases.  We are also at present exploring ways to measure pollution levels of SO2, which is possible using available commercial instrumentation.  These trace gas measurements, when combined with the suite of aerosol measurements, will provide detailed documentation of the structure of the pollution and dust plumes in PACDEX.

Airborne Radiation measurements

Radiation measurements will be made using the HIAPER Airborne Radiation Package (HARP).   In includes the down and up welling spectral irradiances in the wavelength range from 300-2200 nm at various spectral resolutions using UV-Visible and Near Infrared spectrometers.    This instrument is suitable for determining layer properties, such as reflectance, transmittance and absorptance and for deriving broadband solar irradiances.   HARP is mounted on a horizontally stabilized leveling platform and views both zenith and nadir. 

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       Last update Tue 15 May 2007

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