Heterogeneous ice nucleation on dust particles sourced from nine deserts worldwide – Part 1: Immersion freezing
Desert dust is one of the most abundant ice nucleating particle types in the atmosphere. Traditionally, clay minerals were assumed to determine the ice nucleation ability of desert dust and constituted the focus of ice nucleation studies over several decades. Recently some feldspar species were i...
| Main Authors: | , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-12-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/15075/2016/acp-16-15075-2016.pdf |
| Summary: | Desert dust is one of the most abundant ice nucleating particle types in the
atmosphere. Traditionally, clay minerals were assumed to determine the ice
nucleation ability of desert dust and constituted the focus of ice nucleation
studies over several decades. Recently some feldspar species were identified
to be ice active at much higher temperatures than clay minerals, redirecting
studies to investigate the contribution of feldspar to ice nucleation on
desert dust. However, so far no study has shown the atmospheric relevance of
this mineral phase.<br><br>For this study four dust samples were collected after
airborne transport in the troposphere from the Sahara to different locations
(Crete, the Peloponnese, Canary Islands, and the Sinai Peninsula).
Additionally, 11 dust samples were collected from the surface from nine
of the biggest deserts worldwide. The samples were used to study the ice
nucleation behavior specific to different desert dusts. Furthermore, we
investigated how representative surface-collected dust is for the atmosphere
by comparing to the ice nucleation activity of the airborne samples. We used
the IMCA-ZINC setup to form droplets on single aerosol particles which were
subsequently exposed to temperatures between 233 and 250 K. Dust particles were
collected in parallel on filters for offline cold-stage ice nucleation
experiments at 253–263 K. To help the interpretation of the ice nucleation
experiments the mineralogical composition of the dusts was investigated. We
find that a higher ice nucleation activity in a given sample at 253 K can be
attributed to the K-feldspar content present in this sample, whereas at
temperatures between 238 and 245 K it is attributed to the sum of feldspar and
quartz content present. A high clay content, in contrast, is associated
with lower ice nucleation activity. This confirms the importance of feldspar
above 250 K and the role of quartz and feldspars determining the ice
nucleation activities at lower temperatures as found by earlier studies for
monomineral dusts. The airborne samples show on average a lower ice
nucleation activity than the surface-collected ones. Furthermore, we find
that under certain conditions milling can lead to a decrease in the ice
nucleation ability of polymineral samples due to the different hardness and
cleavage of individual mineral phases causing an increase of minerals with
low ice nucleation ability in the atmospherically relevant size fraction.
Comparison of our data set to an existing desert dust parameterization
confirms its applicability for climate models. Our results suggest that for
an improved prediction of the ice nucleation ability of desert dust in the
atmosphere, the modeling of emission and atmospheric transport of the
feldspar and quartz mineral phases would be key, while other minerals are only
of minor importance. |
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| ISSN: | 1680-7316 1680-7324 |