Cloud activation properties of aerosol particles in a continental Central European urban environment
Authors: Salma Imre , Thén Wanda , Vörösmarty Máté , Gyöngyösi András Zénó 
Published: July 2021

Abstract. Collocated measurements using a condensation particle counter,
differential mobility particle sizer and cloud condensation nuclei counter
were realised in parallel in central Budapest from 15 April 2019
to 14 April 2020 to gain insight into the cloud activation properties of
urban aerosol particles. The median total particle number concentration was
10.1 × 103 cm−3. The median concentrations of cloud
condensation nuclei (CCN) at water vapour supersaturation (S) values of 0.1 %, 0.2 %,
0.3 %, 0.5 % and 1.0 % were 0.59, 1.09, 1.39, 1.80 and 2.5 × 103 cm−3, respectively. The CCN concentrations represented 7–27 % of all
particles. The CCN concentrations were considerably larger but the
activation fractions were systematically substantially smaller than observed in
regional or remote locations. The effective critical dry particle diameters
(dc,eff) were derived utilising the CCN concentrations and particle
number size distributions. Their median values at the five supersaturation values considered were 207, 149, 126, 105 and 80 nm,
respectively; all of these diameters were positioned within the accumulation mode of the
typical particle number size distribution. Their frequency distributions
revealed a single peak for which the geometric standard deviation increased
monotonically with S. This broadening indicated high time variability in the
activating properties of smaller particles. The frequency distributions also
showed fine structure, with several compositional elements that seemed to reveal a consistent or monotonical tendency with S. The relationships between the critical S and
dc,eff suggest that urban aerosol particles in Budapest with
diameters larger than approximately 130 nm showed similar hydroscopicity to corresponding continental aerosol particles, whereas smaller particles in Budapest were less hygroscopic than corresponding continental aerosol particles. Only modest seasonal cycling in CCN concentrations and
activation fractions was seen, and only for large S values.
This cycling likely reflects changes in the number concentration,
chemical composition and mixing state of the particles. The seasonal
dependencies of dc,eff were featureless, indicating that the droplet activation properties of the urban
particles remained more or less the same throughout
the year. This is again different from what is seen in non-urban locations. Hygroscopicity parameters (κ values) were computed without
determining the time-dependent chemical composition of the particles. The median values for κ
were 0.15, 0.10, 0.07, 0.04 and 0.02, respectively, at the five supersaturation values considered. The averages suggested
that the larger particles were considerably more hygroscopic than
the smaller particles. We found that the κ values for the urban aerosol were substantially
smaller than those previously reported for aerosols in regional or remote locations. All of these characteristics can be linked to the specific source
composition of particles in cities. The relatively large variability in the
hygroscopicity parameters for a given S emphasises that the individual
values represent the CCN population in ambient air while the average hygroscopicity parameter mainly corresponds to particles with sizes close to the effective critical
dry particle diameter.