Size-dependent activation of aerosols into cloud droplets at a subarctic background site during the second Pallas Cloud Experiment (2nd PaCE): method development and data evaluation T. Anttila1, P. Vaattovaara2, M. Komppula3, A.-P. Hyvärinen1, H. Lihavainen1, V.-M. Kerminen1, and A. Laaksonen1,2 1Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland 2Department of Physics, University of Kuopio, P.O. Box 1672, 70211 Kuopio, Finland 3Finnish Meteorological Institute, Kuopio Unit, P.O. Box 1627, 70211 Kuopio, Finland
Abstract. In situ measurements of aerosol water uptake and
activation of aerosols into cloud droplets provide information on how
aerosols influence the microphysical properties of clouds. Here we present a
computational scheme that can be used in connection with such measurements
to assess the influence of the particle hygroscopicity and mixing state (in
terms of the water uptake) on the cloud nucleating ability of particles.
Additionally, it provides an estimate for the peak supersaturation of water
vapour reached during the formation of the observed cloud(s). The method was
applied in interpreting results of a measurement campaign that focused on
aerosol-cloud interactions taking place at a subarctic background site
located in Northern Finland (second Pallas Cloud Experiment, 2nd PaCE).
A set of case studies was conducted, and the observed activation behavior
could be successfully explained by a maximum supersaturation that varied
between 0.18 and 0.26% depending on the case. In these cases, the
diameter corresponding to the activated fraction of 50% was in the range
of 110–140 nm, and the particles were only moderately water soluble with
hygroscopic growth factors varying between 1.1 and 1.4. The conducted
analysis showed that the activated fractions and the total number of
particles acting as CCN are expected to be highly sensitive to the particle
hygroscopic growth properties. For example, the latter quantity varied over
a factor between 1.8 and 3.1, depending on the case, when the mean
hygroscopic growth factors were varied by 10%. Another important
conclusion is that size-dependent activation profiles carries information on
the mixing state of particles.
Citation: Anttila, T., Vaattovaara, P., Komppula, M., Hyvärinen, A.-P., Lihavainen, H., Kerminen, V.-M., and Laaksonen, A.: Size-dependent activation of aerosols into cloud droplets at a subarctic background site during the second Pallas Cloud Experiment (2nd PaCE): method development and data evaluation, Atmos. Chem. Phys., 9, 4841-4854, doi:10.5194/acp-9-4841-2009, 2009.