From the total and interstitial aerosol particle size distributions the size resolved activation of the aerosol particles was calculated according to:
where FN is the scavenging ratio, DP the particle diameter, and Ntot, Nint are total and interstitial particle number concentration, respectively, at a given DP.
The determined scavenging curves were quite different for summer and winter measurements. In the summer clouds (temperature range -7°C < T < 5°C during summer campaign) all particles larger than a certain threshold became activated. The activation curve shows a typical S-shape. On average over the 3 weeks' summer campaign a diameter of 0.5 (50%) activation (D50) of about 100 nm was found for LWC > 0.15 g m-3.
The results from the winter measurements (temperature range -24°C < T < -5°C during winter campaign) were significantly different to those from the summer campaign. The scavenging curves for clouds in the temperature range -5°C > T > -12°C were similar to the summer observations, but without complete activation even for larger particles, i.e., the curve remained well below 1. For clouds in the lower temperature range (-12°C > T > -24°C) the activation curve became a rather flat line, indicating the absence of cloud droplets. Only for particle diameters larger than 500 nm an increase of the scavenging ratio was found, which might reflect the observation that aerosol particles larger than 200 nm are better suited as ice nuclei than smaller aerosol particles.
Combining the field observations with model calculations, we conclude that:
1. The summer clouds consisted only of water drops, because the temperatures were too warm for ice formation. Therefore, the scavenging curve can be assumed as typical for warm clouds.
2. According to the microphysical data, the winter clouds were mixed-phase clouds for temperatures between -5°C and -12°C, consisting of supercooled droplets, ice crystals, and rimed ice particles. Under these conditions, droplets may evaporate in order to compensate for the water vapor deposition to the ice crystals. This process (Bergeron-Findeisen-process) is due to a different saturation vapor pressure over ice and droplets and may result in an increase of the interstitial aerosol fraction, which in turn will yield a scavenging ratio below 1.
3. The winter clouds observed at temperatures below -12°C consisted nearly exclusively of ice crystals, as indicated by the microphysical data. This cloud can possibly be regarded as an aged mixed-phase cloud, where after sufficiently long time all the liquid water was evaporated, due to the Bergeron-Findeisen-process.
Supplementary URL: http://gaw.web.psi.ch/