Ice residual properties in free tropospheric mixed-phase clouds

One of the major sources of uncertainty in climate model projections is an insufficient understanding of aerosol-cloud interactions. A particularly poorly understood process is the formation of mixed-phase clouds, where ice nuclei (IN), a small but important subset of aerosol particles, facilitate heterogeneous ice nucleation. The abundance of IN influences the ice mass fraction, strongly affecting cloud radiative properties. Furthermore, cloud glaciation augments precipitation formation, resulting in decreased cloud cover and lifetime. Meanwhile, the physical and chemical properties of atmospherically relevant IN are not well known. One burning question in this context is whether anthropogenic emissions of black carbon (BC) contribute significantly to IN number, besides natural IN such as bacteria or dust. Should BC be an atmospherically important IN, it may cause a glaciation indirect effect, with an increase in absorption of shortwave radiation by the Earth-atmosphere system. Due to the high climate relevance of ice formation in mixed-phase clouds, investigation of the properties of ice nuclei is of great importance. Amongst the necessary measurements is field sampling of freshly nucleated ice crystals and analysis of the ice residuals (IR) contained within (the IR within such crystals can be deemed representative of the original IN). However, extraction of IR in mixed-phase clouds is a difficult task, requiring separation of the few small, freshly formed ice crystals not only from interstitial particles, but also from the numerous supercooled droplets which have aerodynamic diameters similar to those of the ice crystals.

In order to address the difficulties with ice crystal sampling and IR extraction in mixed-phase clouds, the new Ice Selective Inlet (ISI) has been designed and deployed in the field. The design is inspired by the Ice-CVI inlet (developed at the Leibniz Institute for Tropospheric Research, Germany), albeit with some key differences. Foremost amongst these is separation of ice crystals from supercooled droplets in the airborne state, as opposed to physical impaction on cool plates, thus limiting potential artifacts, e.g., from ice crystal break-up. The phase separation in the ISI is accomplished with the use of a droplet evaporation unit with ice-covered inner walls; the ice cover maintains saturation with respect to ice, resulting in removal of droplets using the Wegener-Bergeron-Findeisen process, and transmission of the ice crystals. Interstitial particles and cloud condensation nuclei released from the droplets in the droplet evaporation unit are removed from the sample flow with the use of a pumped counterflow virtual impactor. The ice crystals extracted with the PCVI are subsequently sublimated and the physical and chemical properties of the ice residuals are probed.

The hydrometeors sampled via the inlet are counted, sized and imaged by a set of Welas optical particle counters (OPC) and a Particle Phase Discriminator (PPD). The Welas sensors and the PPD enable a detailed monitoring of droplet and ice transmission through the inlet. Moreover, the scattering patterns of individual particles acquired by the PPD enable unambiguous distinction between droplets and ice crystals, as well as illuminating microphysical properties of mixed-phase clouds.

The ISI was successfully deployed in the field for the second time during winter 2014 at the High Alpine Jungfraujoch Research Station (3580 m.a.s.l) as part of the CLACE 2014 field campaign. The campaign included comprehensive measurements of both cloud microphysics and aerosol properties. Particular focus was placed on analysis of the physical and chemical characteristics of IR. A host of online aerosol instrumentation was deployed downstream of the ISI, including a Grimm OPC, a scanning mobility particle sizer (SMPS) and an Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) for number size distribution measurements and a single particle soot photometer (SP2) and Waveband Integrated Bioaerosol Sensor (WIBS-4) for analysis of the chemical composition, with particular focus on the content of black carbon (BC) and biological particles in IR. Furthermore, IR were collected using a single-stage impactor for scanning electron microscopy and scanning transmission x-ray microscopy analysis. Corresponding instrumentation sampled through a total aerosol inlet. By comparing observations from the ISI with those from the total inlet the characteristics of ice residuals relative to the total aerosol could be established. First results from these analyses will be presented.

Acknowledgements

This work was supported by the Swiss National Science Foundation, MeteoSwiss (GAW-CH program), the European Research Council, the German Research Foundation and the International Foundation High Altitude Research Station Jungfraujoch and Gornergrat.