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Composition analysis of ice particle residuals combining aerosol mass spectrometry and counterflow virtual impactor technique

Subject Area Atmospheric Science
Term from 2011 to 2018
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 170852269
 
Final Report Year 2019

Final Report Abstract

In this project, we studied the chemical and microphysical properties of atmospheric ice nucleating particles (INP). Such INP are important for the freezing of cloud droplets at temperatures higher than -38°C, a process that it essential for the formation of rain at mid-latitudes and cloud climate forcing. We used two different approaches: One part focused on real ambient clouds that exist between zero and -38°C, the so-called mixed phase clouds, because ice crystals and supercooled cloud droplets can co-exist in this temperature regime. We studied this kind of clouds at the high Alpine research station Jungfraujoch where we operated a specially designed inlet system (Ice-CVI) that extracts the ice crystals from the mixed-phase clouds, evaporates the ice, and delivers the residual aerosol particles to the connected measurement instruments. The resulting composition measured with an aerosol mass spectrometer partly confirmed previous findings that mineral dust and metal-containing particles are enriched in ice residuals compared to the out-of-cloud aerosol. However, during the field campaign in 2017, we observed also sea-spray containing particles in the residuals, a finding that had not been expected because this particle type is typically not acting as an INP. A speculative explanation may be the presence of biological compound from the sea surface microlayer that initiated the freezing. A problem that occurred during these measurements was the large presence of aluminum oxide particle detected in the residuals by electron microscopy that are thought to are artefact particles generated by the inlet system. On the other hand, since all surfaces of the inlet had been coated with Nickel to avoid such contamination, this issue remains unclear. The second approach was to combine an ice nucleus counter with a pumped CVI in order to extract the ice crystals from the IN counter. An IN counter creates an artificial ice cloud by cooling the sampled air and thereby producing supersaturation with respect to ice, such that the INP contained in the air form ice crystals. By the combination with a pumped CVI that extracts only the ice crystals and leads them to the analysis instruments, the properties of potential INP in ambient air can be studied, also when no natural cloud is present. This development turned out to be extremely challenging such that we could demonstrate a proof-of-concept only in the last project year. For this, we created a particle mixture consisting of a “good” INP and a “bad” INP with different pre-selected sizes. This mixture was then led through the IN counter and the pumped CVI to the analysis instruments. Both the sizing instrumentation and the chemical composition measurements clearly show that only the “good” INP were found in the residuals when the temperature was above -38°. Only below -38°, where homogeneous freezing occurs, the “bad” INP was also observed in the residuals. Since the success of this development was achieved so late in the project phase, no field experiments with this combination could be conducted. However, the technology and the expertise is now available and can be used for further INP studies.

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