On the impacts of phytoplankton-derived organic matter on the properties of the primary marine aerosol – Part 2: Composition, hygroscopicity and cloud condensation activity 1Centre for Atmospheric Sciences, School of Earth, Atmospheric and Environmental Sciences, Manchester M13 9PL, UK
18 Mar 2011
2Scottish Association for Marine Science, Oban, UK
Received: 04 Oct 2010 – Published in Atmos. Chem. Phys. Discuss.: 04 Nov 2010Abstract. The effect of nanogel colloidal and dissolved organic matter
<0.2 μm, secreted by marine biota, on the hygroscopic growth and
droplet activation behaviour of the primary marine aerosol was studied.
Seawater proxies were prepared by the combination of artificial seawater
devoid of marine organics and natural seawater enriched in organic exudate
released by laboratory-grown phytoplankton cultures, as described in a
companion paper. The primary aerosol was produced by bubble bursting, using a
plunging multijet system as an aerosol generator.
Revised: 03 Mar 2011 – Accepted: 15 Mar 2011 – Published: 18 Mar 2011
The aerosol generated from seawater proxies enriched with marine exudate
presented organic volume fractions on the order of 8–37%, as derived by
applying a simple mixing rule. The hygroscopic growth and cloud condensation
nuclei (CCN) activity of the marine organics-enriched particles where 9–17%
and 5–24% lower, respectively, than those of the aerosol produced from
artificial seawater devoid of exudate. Experiments in a companion paper
indicated that the cloud nuclei formation could be enhanced in diatom bloom
areas because of the increase in the primary particle production induced by
marine organics. The experiments in the present study, however, indicate that
the impacts of such an enhancement would be counteracted by the reduction in
the CCN activity of the primary particles enriched in marine organics.
The extent of the effect of the biogenic matter on the particle behaviour was
dependent on the seawater organic concentration and type of algal exudate.
Aerosol produced from seawater proxies containing diatomaceous exudate
presented higher hydrophobicity and lower CCN activity than those enriched
with nanoplankton exudate. The organic fraction of the particles was found to
correlate with the seawater organic concentration, without observing
saturation of the particle organic mass fraction even for unrealistically
high organic matter concentration in seawater. These findings are indicative
that discrepancies on the composition of the primary aerosol between
different studies could partly be explained by the difference in the nature
and concentration of the organic matter in the source seawater employed.
Consistently across the experiments, theoretical analysis based on the
Köhler model predicted a reduction in the primary marine aerosol CCN
activity upon the incorporation of marine organics into the particle
composition. This effect is consequence of the replacement of small inorganic
sea salt molecules by large molar mass organic molecules, together with a
moderate suppression of the surface tension at the point of activation of
5–0.5%, which leads to a dominance of the reduction in the dissolved solute
in the Raoult term.
Citation: Fuentes, E., Coe, H., Green, D., and McFiggans, G.: On the impacts of phytoplankton-derived organic matter on the properties of the primary marine aerosol – Part 2: Composition, hygroscopicity and cloud condensation activity, Atmos. Chem. Phys., 11, 2585-2602, doi:10.5194/acp-11-2585-2011, 2011.