Cloud droplet activity changes of soot aerosol upon smog chamber ageing

Research output: Contribution to journalArticle

Abstract

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 6m3 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 kappa-K hler theory, using a kappa SOA 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 K hler 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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Authors
Organisations
External organisations
  • University of Helsinki
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Subatomic Physics
  • Production Engineering, Human Work Science and Ergonomics
Original languageEnglish
Pages (from-to)9831-9854
JournalAtmospheric Chemistry and Physics
Volume14
Issue number18
Publication statusPublished - 2014
Publication categoryResearch
Peer-reviewedYes

Bibliographic note

The information about affiliations in this record was updated in December 2015. The record was previously connected to the following departments: Ergonomics and Aerosol Technology (011025002), Nuclear Physics (Faculty of Technology) (011013007)

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Cerina Wittbom, 2014 Jan 24, LUTFD2 / (TFKF-3105) / 1 - 35/ 2014 ed. Lund: Fysiska institutionen, Lunds universitet. 35 p.

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