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| Titel |
State transformations and ice nucleation in amorphous (semi-)solid organic aerosol |
| VerfasserIn |
K. J. Baustian, M. E. Wise, E. J. Jensen, G. P. Schill, M. A. Freedman, M. A. Tolbert |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 13, no. 11 ; Nr. 13, no. 11 (2013-06-07), S.5615-5628 |
| Datensatznummer |
250018692
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| Publikation (Nr.) |
 copernicus.org/acp-13-5615-2013.pdf |
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| Zusammenfassung |
Amorphous (semi-)solid organic aerosol particles have the potential to serve
as surfaces for heterogeneous ice nucleation in cirrus clouds. Raman
spectroscopy and optical microscopy have been used in conjunction with a
cold stage to examine water uptake and ice nucleation on individual
amorphous (semi-)solid particles at atmospherically relevant temperatures
(200–273 K). Three organic compounds considered proxies for atmospheric
secondary organic aerosol (SOA) were used in this investigation: sucrose,
citric acid and glucose. Internally mixed particles consisting of each
organic and ammonium sulfate were also investigated.
Results from water uptake experiments followed the shape of a
humidity-induced glass transition (Tg(RH)) curve and were used to
construct state diagrams for each organic and corresponding mixture.
Experimentally derived Tg(RH) curves are in good agreement with
theoretical predictions of Tg(RH) following the approach of
Koop et al. (2011). A unique humidity-induced glass transition point
on each state diagram, Tg'(RH), was used to quantify and
compare results from this study to previous works. Values of Tg'(RH)
determined for sucrose, glucose and citric acid glasses were 236,
230 and 220 K, respectively. Values of Tg'(RH) for internally
mixed organic/sulfate particles were always significantly lower; 210, 207
and 215 K for sucrose/sulfate, glucose/sulfate and citric acid/sulfate,
respectively.
All investigated SOA proxies were observed to act as heterogeneous ice
nuclei at tropospheric temperatures. Heterogeneous ice nucleation on pure
organic particles occurred at Sice = 1.1–1.4 for temperatures below
235 K. Particles consisting of 1:1 organic-sulfate mixtures took up water
over a greater range of conditions but were in some cases also observed to
heterogeneously nucleate ice at temperatures below 202 K (Sice= 1.25–1.38).
Polynomial curves were fitted to experimental water uptake data and then incorporated into the Community Aerosol Radiation Model for Atmospheres (CARMA) along
with the predicted range of humidity-induced glass transition temperatures
for atmospheric SOA from Koop et al. (2011). Model results suggest
that organic and organic/sulfate aerosol could be glassy more than 60%
of the time in the midlatitude upper troposphere and more than 40% of
the time in the tropical tropopause region (TTL). At conditions favorable
for ice formation (Sice > 1), particles in the TTL are
expected to be glassy more than 50% of the time for temperatures below
200 K. Results from this study suggests that amorphous (semi-)solid organic
particles are often present in the upper troposphere and that heterogeneous
ice formation on this type of particle may play an important role in cirrus
cloud formation. |
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