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| Titel |
Cloud droplet activity changes of soot aerosol upon smog chamber ageing |
| VerfasserIn |
C. Wittbom, A. C. Eriksson, J. Rissler, J. E. Carlsson, P. Roldin, E. Z. Nordin, P. T. Nilsson, E. Swietlicki, J. H. Pagels, B. Svenningsson |
| 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 ; 14, no. 18 ; Nr. 14, no. 18 (2014-09-17), S.9831-9854 |
| Datensatznummer |
250119043
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| Publikation (Nr.) |
 copernicus.org/acp-14-9831-2014.pdf |
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| Zusammenfassung |
Particles containing soot, or black carbon, are generally considered to
contribute to global warming. However, large uncertainties remain in the net
climate forcing resulting from anthropogenic emissions of black carbon (BC),
to a large extent due to the fact that BC is co-emitted with gases and
primary particles, both organic and inorganic, and subject to atmospheric
ageing processes. In this study, diesel exhaust particles and particles from
a flame soot generator spiked with light aromatic secondary organic aerosol
(SOA) precursors were processed by UV radiation in a 6 m3 Teflon
chamber in the presence of NOx. The time-dependent changes of the soot
nanoparticle properties were characterised using a Cloud Condensation Nuclei
Counter, an Aerosol Particle Mass Analyzer and a Soot Particle Aerosol Mass
Spectrometer. The results show that freshly emitted soot particles do not
activate into cloud droplets at supersaturations ≤2%, i.e. the BC core coated with primary organic aerosol (POA) from the exhaust is
limited in hygroscopicity. Before the onset of UV radiation it is unlikely
that any substantial SOA formation is taking place. An immediate change in
cloud-activation properties occurs at the onset of UV exposure. This change
in hygroscopicity is likely attributed to SOA formed from intermediate
volatility organic compounds (IVOCs) in the diesel engine exhaust. The change
of cloud condensation nuclei (CCN) properties at the onset of UV radiation
implies that the lifetime of soot particles in the atmosphere is affected by
the access to sunlight, which differs between latitudes. The ageing of soot
particles progressively enhances their ability to act as cloud condensation
nuclei, due to changes in: (I) organic fraction of the particle, (II)
chemical properties of this fraction (e.g. primary or secondary organic
aerosol), (III) particle size, and (IV) particle morphology. Applying
κ-Köhler theory, using a κSOA value of 0.13
(derived from independent input parameters describing the organic material),
showed good agreement with cloud droplet activation measurements for
particles with a SOA mass fraction ≥0.12 (slightly aged particles). The
activation properties are enhanced with only a slight increase in organic
material coating the soot particles (SOA mass fraction < 0.12),
however not as much as predicted by Köhler theory. The discrepancy
between theory and experiments during the early stages of ageing might be
due to solubility limitations, unevenly distributed organic material or
hindering particle morphology.
The change in properties of soot nanoparticles upon photochemical processing
clearly increases their hygroscopicity, which affects their behaviour both
in the atmosphere and in the human respiratory system. |
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