Geoscience Reference
In-Depth Information
18.2
Dust Measurement Techniques
18.2.1
Coulter Counter
Most dust concentration and size data reported for snow and ice have been deter-
mined using Coulter Counter (CC) instruments. Ice samples are melted and mixed
with filtered, particle-free electrolyte solution (usually NaCl). The sample is drawn
through a small aperture, across which the electrical conductivity is continuously
monitored. As each particle crosses the aperture, a change in conductivity across
the aperture is recorded, with the magnitude of the conductivity change being
proportional to the volume of displaced electrolyte, that is, the volume of the
dust particle. The coulter counter is calibrated using latex beads of specified size
and concentration, allowing the determination of both particle concentration and
size. Although CCs are reliable and sensitive instruments, they are limited to
discrete measurements with a minimum sample size of approximately 10 mL. The
technique is slow (approximately 5 min per measurement) and CCs are sensitive to
electromagnetic noise. The particle size range that can be determined is dependent
upon the aperture size; the smallest aperture allows particle measurements between
0.6 and 10 m diameters. As with most particle detectors, there is the difficulty
of coincidence - the phenomenon of simultaneously counting two small particles,
which are then counted as one particle, however as particle numbers in ice core
samples are usually low, the coincidence effect is often negligible.
18.2.2
Laser Particle Detector
Laser particle detectors (LPD) have been in use since the 1980s and compensate for
some limitations of CCs, the primary one being that LPDs sample continuously. The
LPD detector consists of small optical cell, through which the melted ice sample is
passed. A high-powered laser illuminates the optical cell, opposite a very sensitive
light detector. The detector records scattering of light due to the presence of particles
in the optical cell, thus allowing the determination of particle concentrations and
size ranges. The sensitivity and particle size range depends on the instrument
wavelength range and power; a 680 nm laser allows the determination of 0.9-15 m
particle diameters, appropriate for ice core measurements. Similar to CC, LPDs are
calibrated using manufactured latex bead standards. There are concerns regarding
such materials, as to whether they accurately represent the response from real dust
particles. An alternative is to use standards produced from filtered desert dust,
although they are expensive and less reproducible. CC and LPD techniques have
been compared by parallel measurements of EPICA Dome C (EDC) samples (Ruth
et al. 2008 ). Very good agreement was found for insoluble particle concentrations,
although it appears that the LPD did not reproduce the full variance of the CC,
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