Geoscience Reference
In-Depth Information
Also shown in Fig.
12.6
are results for two commercially available proxies for
natural mineral dust: Arizona Test Dust (ATD) and NX-illite. ATD has been the
subject of numerous studies and is made of material which has been milled to
produce particles with a specific range of sizes. It is sold on a commercial basis
for testing the efficiency of filters. It is attractive for ice nucleation experiments
because of its well-defined particle size, it is available in large quantities, and its
mineralogical composition has been reported (Broadley et al.
2012
). In Fig.
12.6
,
we summarize and compare ice-active site densities for a number of experiments
performed in the immersion mode with ATD. Its ice-nucleating ability has been
explored in the AIDA cloud chamber during experiments similar to those described
above for natural dusts (Connolly et al.
2009
; Niemand et al.
2012
). Niedermeier
et al. (
2010
) report
n
s
values for ATD determined with the LACIS (Leipzig Aerosol
Cloud Interaction Simulator) chamber which is a temperature-controlled laminar
flow tube, allowing the activated fraction of aerosol particles to be determined under
specific conditions. Using a CFDC, Hoyle et al. (
2011
) activated individual size-
selected ATD particles to droplets in order to measure the ice-nucleating fraction
from which Murray et al. (
2012
) determined
n
s
values. Inspection of the results
for ATD in Fig.
12.6
shows that
n
s
values from these diverse experiments are in
good agreement with one another. This suggests that ATD is a very useful material
for benchmarking instrumentation. However, it appears to be roughly an order of
magnitude more efficient at nucleating ice than the natural dusts based on the
information summarized in Fig.
12.6
. One explanation is that ATD has significantly
more feldspar in it than average natural dusts (see Figure 13 of Murray et al.
2012
)
but may also be related to the intense milling process used in the production of ATD.
On the basis of mineralogical makeup, it has been argued that NX-illite is a
better proxy for natural dusts (Broadley et al.
2012
; Murray et al.
2012
). The
n
s
values reported for NX-illite by Broadley et al. (
2012
), and shown in Fig.
12.6
,are
clearly much lower than any of the other values. However, it must be noted that there
are significant differences in experimental approach and in particular the measure
of surface area. All other measurements discussed above employed instruments
which yield equivalent spherical diameters based on aerodynamic or mobility size
measurements from which surface area was determined, whereas for NX-illite a
gas adsorption technique was used. The measured size distribution in the AIDA
experiments, for example, peaked at around 1 m diameter, but for clays the primary
grains are on the order of just 10s nanometers. Hence, a micron-scaled particle will
be made up of numerous smaller grains (see, e.g., Fig.
12.7
) and therefore has a
substantial internal surface area. Gas adsorption techniques report the total surface
area of all grains, whereas determining surface area from a spherical approximation
does not. Accounting for the internal surface area would bring the various results in
Fig.
12.6
into closer agreement.
When using
n
s
values to estimate the production of ice crystals in the atmo-
sphere, it is essential to consider how surface area was determined. If a spherical
approximation was made in the laboratory, then this assumption should also be
made for atmospheric aerosol if they are of a similar size. If a specific surface
area (gas adsorption) has been used, then a similar estimate should be made for
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