Collection of small ice particles by means of a novel counterflow virtual impactor system for the determination of microphysical and chemical properties of tropospheric ice nuclei

对流层中低层云中的积冰对水循环和全球气候具有重要意义。冰粒的出现引发了降水过程，并可能改变云的辐射特性。在对流层中，由于由称为冰核（IN）的大气气溶胶粒子的子集诱导的非均匀冰形成机制，冰粒子在低于-5 ° C的温度下成核。到目前为止，有关大气颗粒物作为IN和不同的非均质冰成核机制的大气相关性的知识是边缘的，特别是对流层云。在这里，冰相出现在预先存在的过冷液滴之间，由于结冰问题和由于区分液体和冰相的实验必要性，使经验研究复杂化。在第一次可行性评估中，基于逆流虚拟撞击器（CVI）技术的专用采样系统已被证明可以专门收集混合相云内新形成的冰粒，并排除间隙颗粒、液滴和大的冰聚集体，这些冰聚集体应该包含凝结和成霜过程产生的清除气溶胶颗粒。在分离和取样时，小冰粒在所谓的ICE-CVI中蒸发，释放出干燥的残留颗粒。这些残留物被认为是冰成核的中心，因为在云中，新形成的冰粒仅通过水蒸气扩散而生长，并且在云处理的这段时间内不会使粒子凝结。因此，物理化学表征的冰粒残留物将提供识别的冰核，真正形成的对流层云的冰。特别是，在对流层云的冰形成的人为和沙漠尘埃粒子的作用，可以用这种方法来阐明。此外，它将有可能获得有关相关的异质成核过程的信息。云凝结核（CCN）的化学成分（高可溶性物质）与冰核（不溶性物质）不同，如果通过液滴冻结形成冰，则应作为残留颗粒存在。另一个过程，蒸发冻结，最近才成为焦点，将只产生CCN成分被测量的ICE-CVI，因为过冷液滴的蒸发冷却被认为是导致冻结没有冰核的参与。显然，对冰粒残留物的具体分析对于这种方法是绝对必要的。特别是，单粒子分析，如质谱和电子显微镜是必需的，这两者都可以通过参与联合冰相项目的新型ICE-CVI来实现。

Ice formation in lower and middle tropospheric clouds is of great importance for the water cycle and global climate. The appearance of ice particles initiates precipitation processes and might change cloud radiative properties. In the troposphere ice particles nucleate at temperatures below -5°C due to heterogeneous ice formation mechanisms induced by a subset of atmospheric aerosol particles, named ice nuclei (IN). Up to now the knowledge about atmospheric particles acting as IN and about the atmospheric relevance of different heterogeneous ice nucleation mechanisms is marginal especially for tropospheric clouds. Here the ice phase emerges between preexisting super-cooled droplets complicating the empirical investigation due to icing problems and due to the experimental necessity to distinguish between liquid and ice phase. A dedicated sampling system based on the counterflow virtual impactor (CVI) technique has been proven in a first feasibility evaluation to exclusively collect freshly formed ice particles within a mixed-phase cloud and rejecting interstitial particles, liquid droplets and large ice aggregates that are supposed to contain scavenged aerosol particles as a result of coagulation and riming processes. Upon separation and sampling, the small ice particles are evaporated in the so-called ICE-CVI releasing dry residual particles. These residuals are considered to be the centers of ice nucleation, because in the cloud the freshly formed ice particles solely grow by water vapor diffusion and do not scavenge particles during this period of cloud processing. Thus, a physico-chemical characterization of the ice particle residuals will offer the identification of ice nuclei that have truly formed ice in tropospheric clouds. Particularly, the role of anthropogenic and desert dust particles for ice formation in tropospheric clouds can be elucidated with this approach. Moreover it will be possible to obtain information about the associated heterogeneous nucleation processes. Cloud condensation nuclei (CCN), whose chemical composition (highly soluble matter) is different from ice nuclei (insoluble matter) should be present as residual particles if ice formation appears via droplet freezing. Another process, evaporative freezing, that came into focus just recently, would yield only CCN components to be measured by the ICE-CVI, because the evaporative cooling of super-cooled droplets is assumed to cause freezing without participation of ice nuclei. Obviously, a specific analysis of the ice particle residuals is absolutely necessary for this methodology. Particularly, single particle analysis, like mass spectrometry and electron microscopy is required, which both can be realized by a participation of the novel ICE-CVI in joint ice phase projects.