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Abstract:
Cloud condensation nuclei counter (CCNC) measurements performed at 14
locations around the world within the European Integrated project on
Aerosol Cloud Climate and Air Quality interactions (EUCAARI) framework
have been analysed and discussed with respect to the cloud condensation
nuclei (CCN) activation and hygroscopic properties of the atmospheric
aerosol. The annual mean ratio of activated cloud condensation nuclei
(N-CCN) to the total number concentration of particles (N-CN), known as
the activated fraction A, shows a similar functional dependence on
supersaturation S at many locations - exceptions to this being certain
marine locations, a free troposphere site and background sites in
south-west Germany and northern Finland. The use of total number
concentration of particles above 50 and 100 nm diameter when calculating
the activated fractions (A(50) and A(100), respectively) renders a much
more stable dependence of A on S; A(50) and A(100) also reveal the
effect of the size distribution on CCN activation. With respect to
chemical composition, it was found that the hygroscopicity of aerosol
particles as a function of size differs among locations. The
hygroscopicity parameter kappa decreased with an increasing size at a
continental site in south-west Germany and fluctuated without any
particular size dependence across the observed size range in the remote
tropical North Atlantic and rural central Hungary. At all other
locations kappa increased with size. In fact, in Hyytiala, Vavihill,
Jungfraujoch and Pallas the difference in hygroscopicity between Aitken
and accumulation mode aerosol was statistically significant at the 5%
significance level. In a boreal environment the assumption of a
size-independent kappa can lead to a potentially substantial
overestimation of N-CCN at S levels above 0.6 %. The same is true for
other locations where kappa was found to increase with size. While
detailed information about aerosol hygroscopicity can significantly
improve the prediction of N-CCN, total aerosol number concentration and
aerosol size distribution remain more important parameters. The seasonal
and diurnal patterns of CCN activation and hygroscopic properties vary
among three long-term locations, highlighting the spatial and temporal
variability of potential aerosol-cloud interactions in various
environments.
