studies the 50 partitioning diameters dropped significantly below 100 nm to roughly 70 nm. q 2000 Elsevier Science B.V. All rights reserved.
Ž .
Keywords: Phase partitioning in clouds; CCN cloud condensation nuclei ; Nucleation scavenging; Counter- Ž
. flow virtual impactor CVI
1. Introduction
A cloud is a complex system composed of three phases: gases, solid material, and liquids. In clouds, gases and aerosol particles can dissolve or become suspended inron
hydrometeors which themselves have polydisperse size distributions, ranging from micrometers to millimeters in diameter. Particles that activate at a given supersaturation
Ž . S of water vapor are denoted as cloud condensation nuclei CCN S . It has frequently
been assumed that most of the submicrometer mass is incorporated into cloud water Ž
. Ž
. e.g., Hegg, 1983 . Besides coarse mode aerosol diameter 1 mm , the accumulation
Ž .
Ž . mode aerosol
0.1–1 mm is the predominant source of CCN S
and contributes significantly to the content of dissociated species such as sulfate, ammonium, and
Ž .
hydrogen ions in hydrometeors Charlson et al., 1983 . This is conceivable in the simplest case of an aerosol population with uniform chemical composition where all
Ž . Ž
particles above a diameter D S become activated as a function of supersaturation cf.
act
. Fig. 1a . However, for aerosol populations where the chemical composition is size-de-
Ž . pendent the CCN S
concentration is a more complicated function of the particle Ž
. Ž
diameter D cf. Fig. 1b and even smaller particles may become activated adapted from Ž
. .
Seinfeld and Pandis 1998 , adding an arbitrary but reasonable x-axis . A size-dependent state of mixture of soluble and insoluble material will further complicate this picture
Ž .
Heintzenberg and Covert, 1990 . Thus, in the past years, the determination of the scavenging fraction as a function of
Ž .
particle size has been addressed in a number of studies. Noone et al. 1992b measured for a polluted ground fog near Bologna, Italy that aerosol particles between 0.4–0.7 mm
in diameter activated to form droplets. For a mountain site in central Sweden Heintzen- Ž
. Ž
. berg et al. 1989 found residual sizes between 0.08–1.6 mm. Hallberg et al. 1994
reported of particle diameters where 50 of the particles are incorporated in the cloud Ž
. phase D
of 0.11 mm to 0.31 mm for the Kleiner Feldberg, Germany. Martinsson et
50
Ž .
al. 1997 reported D in the range of 0.11–0.17 mm with the Droplet Aerosol
50
Ž .
Ž .
Analyzer DAA for the Great Dun Fell GDF , Great Britain, and values of 0.096 mm Ž
. to 0.15 mm during a subsequent GDF campaign Martinsson et al., 1999 .
Typical in-cloud size distributions of the interstitial aerosol as well as size spectra of cloud droplets and ice crystals are shown in Fig. 2. The interstitial size distributions of
uniform as well as of size-dependent chemical compositions are derived from Fig. 1a,b. Ž
. Droplet spectrum and crystal sizes were adapted from Arends et al.
1994 and
Ž .
Harrington et al. 1996 , respectively. These ‘‘typical’’ cloud size distributions were chosen in order to outline the objectives and experimental approach in the present study
of complementary sampling and analysis of interstitial and cloud phase with a Round Jet Ž
. Ž
. Impactor RJI and a Counterflow Virtual Impactor CVI , respectively. Fig. 2 illustrates
Ž . Ž .
Fig. 1. CCN S for a uniform chemical composition a of the aerosol population with particle activation Ž .
Ž . diameter D
S and for a multi-component aerosol population with size-dependent chemical composition b .
act
Ž .
Adapted from Seinfeld and Pandis 1998 .
schematically the case of a cloud where a CVI should operate at a lower cut size in the range of few microns to sample the entire number concentration of hydrometeors.
However, up to now 6.78 mm was the lowest cut size for a ground-based CVI where
Ž .
Fig. 2. Interstitial particle size distributions derived from Fig. 1 of uniform chemical composition and Ž .
size-dependent chemical composition. In addition, typical cloud droplet spectra, incorporating the CCN S , and ice crystal spectra are shown.
Ž .
calibration evidence is available Anderson et al., 1993 . Anderson et al. constructed a mini-CVI with minimum cut-size due to a variety of improvements compared to earlier
Ž .
CVI designs Ogren et al., 1985; Noone et al., 1988 and due to a superior dry calibration method having superior reliability than a former wet calibration method
Ž .
Noone et al., 1988 . Reported cut sizes in the range of 4–6 mm of ground-based CVIs Ž
. used in campaigns during late Eighties and early Nineties e.g., Noone et al., 1992b
were based on model estimates rather than experimental calibrations. A further advancement of this study, besides the development of an improved CVI,
Ž .
was the coupling of the CVI to a Differential Mobility Analyzer DMA for direct residual size measurements including the entire Aitken mode range. In earlier studies,
optical particle spectrometers were interfaced to CVIs that performed at lower detection Ž
. limits 100 nm e.g., Noone et al., 1992b; Hallberg et al., 1994 , and in a single
Ž .
experiment down to 80 nm Heintzenberg et al., 1989 . In the main part of the study, two cloud events will be presented, showing the combined data of the two samplers,
with novel results of small activated particles in moderately polluted airmasses.
2. Experimental approach for studying cloud processes
