Experimental study on collision efficiencies and contact freezing with a new collision chamber

Experimental collision efficiency experiments for submicron aerosol particles scavenged by cloud drops were carried out with a new collision chamber. Pure liquid water drops with radii between 12.8 μm and 35 μm falling down collide with LiBO₂ particles with radii between 0.05 μm to 0.33 μm in a laminar flow. The droplets are produced with a piezo droplet generator in a chamber with a variable length. The droplets are then collected with a cup impactor. The collected solution is then analyzed for the scavenged aerosol mass by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and then collision efficiencies can be derived. Our experimental measurements were compared with theoretical values, using a combination of Tinsley et al. (2006) and Park et al. (2005) models and with literature data as well. We found the theoretical and expected behavior of the collision efficiencies as a function of the particle radius with a minimum between 0.2 μm and 0.3 μm.

At temperatures below 0ºC contact freezing experiments were run in the collision chamber. A big advantage with our experimental setup is that we can study this heterogeneous nucleation mode at more atmospherically relevant sizes as compared with previous studies. In a proof of concept study we used silver iodide (AgI) particles as ice nuclei (IN). Using a custom-made ice optical detector (IODE, Nicolet et al., 2010) we can distinguish between liquid drops and ice crystals to determine the frozen fraction at different temperatures down to 243K. We investigated the influence of the aerosol concentration on the measured frozen fraction of droplets. We found that the ice formation starts at higher temperatures (267 K) due to contact freezing as compared to immersion freezing (264 K) (Lüönd et al. 2010) for the same type of IN. However, we did not reach a frozen fraction of 100%. Further investigations are ongoing.