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 SO
2, 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.
Return
to the PACDEX page 
