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Cerina Wittbom. Photo.

Cerina Wittbom

Research coordinator

Cerina Wittbom. Photo.

Cloud Droplet Forming Potential of Ageing Soot and Surfactant Particles : Laboratory research and Köhler modelling

Molndroppsbildningspotential för åldrande soot och surfaktant partiklar : Forskning i laboratorium och Köhler-modellering


  • Cerina Wittbom

Summary, in English

Aerosol particles affect Earth’s climate system by scattering and absorbing light. The perturbation of the climate system caused by a change in aerosol configuration due to anthropogenic emissions has recently improved. However, the radiation balance and hydrological cycle of Earth are also highly influenced by clouds: a small change in cloud configuration can have a large effect on the climate. Cloud droplets form on preexisting particles. For instance, the ability of black carbon (BC) particles when co-emitted with organic carbon (OC) to form cloud droplets are less well understood. A large fraction of the ambient organic aerosol is formed via oxidation of volatile organic compounds from both biogenic and anthropogenic sources. During atmospheric ageing, the properties of freshly emitted soot nanoparticles will change as volatile organics oxidize and interact with the primary soot emissions. As the particles undergo photochemical processing in the atmosphere they progressively become more hygroscopic and will thereby influence the climate due to their improved ability to act as cloud condensation nuclei (CCN) as well as affecting the human health by altering the uptake in the respiratory system.
This work is part of the PhD work of the author of this report, with the aim to evaluate the ability of aerosol nanoparticles to form cloud droplets; of biogenic and anthropogenic origin. To gain better understanding of the cloud forming potential of organic compounds produced from living organisms as well as soot when co-emitted with organic compounds both laboratory research and Köhler modeling has been performed. Different biosurfactants was measured and compared with two different techniques in the first study. In the second study, soot from both a diesel vehicle and a flame soot generator was photochemically processed in a smog chamber, monitored with a comprehensive instrumental set-up. For both studies the measurement technique of the cloud activation properties was improved, gaining better resolution in data.
The results in the first study show that the biosurfactants have good cloud forming abilities, however not as good as previously believed. There are discrepancies in the results from the two measurement techniques (on- and off-line), which partly can be explained by surface partitioning. In the second study, the freshly emitted soot particles neither showed any hygroscopic growth (at 90 % relative humidity) nor activated into cloud droplets (at a supersaturation of 2 %). As the emissions are photochemically processed the properties of the particles change and they become progressively more hygroscopic. The enhanced cloud forming abilities of the soot particles are due to changes in the organic fraction, both regarding quantity and quality, as well as a change in size and shape of the particles. Experimental and modeled (κ-Köhler theory) results show good agreement for particles with higher organic content, and with a κ-value of ~0.13. The κ-value derived from the chemical composition and the CCN measurements are consistent. Due to the morphology of the soot particles, predictions of the cloud droplet activity and hygroscopic growth cannot be performed using the measured mobility diameter. Instead, the volume equivalent diameter is a better size measure. This parameter has in this study successfully been estimated from the mobility diameter and the organic aerosol fraction of the particle.
In summary, both the biosurfactants and aged anthropogenic particles show cloud droplet forming potential relevant in the ambient air, with the ability to affect the climate. The results from the anthropogenic experiment also imply that the sunlight affect the lifetime of soot in the atmosphere, due to changed cloud droplet activation as the UV radiation was turned on during experiments. Probably, the uptake in human lungs of larger particles increases as the soot particles age.


  • Nuclear physics
  • MERGE: ModElling the Regional and Global Earth system

Publishing year




Document type

Licentiate thesis


Fysiska institutionen, Lunds universitet


  • Natural Sciences


  • Hygroscopicity, CCN, BC




  • Birgitta Svenningsson