Journal article

The analysis of size-segregated cloud condensation nuclei counter (CCNC) data and its implications for cloud droplet activation

Paramonov M, Aalto P, Asmi A, Prisle N, Kerminen V, Kulmala M, Petäjä T ...see all

Atmospheric Chemistry and Physics, vol. 13, issue 20 (2013) pp. 10285-10301

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Ambient aerosol, CCN (cloud condensation nuclei) and hygroscopic
properties were measured with a size-segregated CCNC (cloud condensation
nuclei counter) in a boreal environment of southern Finland at the SMEAR
(Station for Measuring Ecosystem-Atmosphere Relations) II station. The
instrumental setup operated at five levels of supersaturation S covering
a range from 0.1-1% and measured particles with a size range of 20-300
nm; a total of 29 nonconsecutive months of data are presented. The
median critical diameter D-c ranged from 150 nm at S of 0.1% to 46 nm
at S of 1.0 %. The median aerosol hygroscopicity parameter kappa ranged
from 0.41 at S of 0.1% to 0.14 at S of 1.0 %, indicating that ambient
aerosol in Hyytiala is less hygroscopic than the global continental or
European continental averages. It is, however, more hygroscopic than the
ambient aerosol in an Amazon rainforest, a European high Alpine site or
a forested mountainous site. A fairly low hygroscopicity in Hyytiala is
likely a result of a large organic fraction present in the aerosol mass
comparative to other locations within Europe. A considerable difference
in particle hygroscopicity was found between particles smaller and
larger than similar to 100 nm in diameter, possibly pointing out to the
effect of cloud processing increasing kappa of particles > 100 nm in
diameter. The hygroscopicity of the smaller, similar to 50 nm particles
did not change seasonally, whereas particles with a diameter of similar
to 150 nm showed a decreased hygroscopicity in the summer, likely
resulting from the increased VOC emissions of the surrounding boreal
forest and secondary organic aerosol (SOA) formation. For the most part,
no diurnal patterns of aerosol hygroscopic properties were found.
Exceptions to this were the weak diurnal patterns of small, similar to
50 nm particles in the spring and summer, when a peak in hygroscopicity
around noon was observed. No difference in CCN activation and
hygroscopic properties was found on days with or without atmospheric new
particle formation. During all seasons, except summer, a CCN-inactive
fraction was found to be present, rendering the aerosol of 75-300 nm in
diameter as internally mixed in the summer and not internally mixed for
the rest of the year.

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  • M. Paramonov

  • P. P. Aalto

  • A. Asmi

  • N. Prisle

  • V. M. Kerminen

  • M. Kulmala

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