Analysis of insoluble particles contained in single cloud droplets and ice crystals

Different types of aerosol particles influence cloud formation in various ways. Hygroscopic particles like sulfate or sea salt are preferential cloud condensation nuclei (CCN). They completely dissolve inside cloud droplets. Insoluble material like mineral dust or soot remains solid and floats in the liquid. In addition to cloud droplets forming on CCN, particles can also enter droplets through collisions. Some insoluble particles can act as ice nuclei for heterogeneous freezing, a process which occurs in mixed-phase clouds in the temperature range between 0°C and -38°C. In laboratory studies, mainly large droplets containing many potential immersion ice nuclei have been examined. To transfer these laboratory results into parameterizations of heterogeneous freezing under atmospheric conditions (for example for a global climate model), they have to be scaled with realistic values of concentrations of insoluble particles inside cloud droplets. To date, no such information is available.

We present a simple experiment using a replication technique described in the literature (Ma et al., 2003). Single droplets are collected on a thin collodion film on a smooth polycarbonate membrane. The settling droplet forms a replica and then slowly evaporates. Both soluble and insoluble material is left behind. During the evaporation process, the soluble material agglutinates and adheres to the insoluble particles, complicating the analysis.

During the CLACE5-campaign on the Swiss high altitude measurement site at Jungfraujoch in March 2006, cloud droplets and ice crystals will be collected using the described method. The samples will be chemically analysed and characterized by electron microscopy, allowing one to count the number of residual particles and estimate the droplet or crystal diameter. From this an estimate of the concentration of insoluble particles inside the sampled hydrometeors can be derived, which is valuable information for understanding heterogeneous freezing.