The ice nucleation activity of carbonaceous particles

Clouds are essential for Earth`s climate system, weather phenomena, and the hydrological cycle. Cloud microphysics determine cloud albedo in the visible and infrared spectral ranges and cloud lifetime, which are essential for the effect of clouds on the radiation balance of the Earth-atmosphere system and, together with the precipitation properties of clouds, global climate. The impact of ice clouds on the Earth`s radiation balance is still undetermined, as the higher albedo of ice clouds should result in a cooling effect, but as they are more likely to precipitate, they may reduce the total cloud albedo and lead to a warming effect. In all these processes, aerosol particles play a crucial role by acting as cloud condensation nuclei (CCN) for liquid droplets and as ice nuclei for the formation of ice particles. Although the formation of warm clouds containing liquid droplets has been studied extensively, much less is known about ice clouds. Several mechanisms of ice particle formation have been proposed, but even the chemistry and structure of ice nuclei is still under investigation. Ice nuclei can have quite different chemical composition. The ice nucleation ability of e.g. mineral dust particles has been studied fairly well, but very little is known about carbonaceous or biological particles and their ability to act as ice nuclei. In this project, we focus on the ice nucleation abilities of carbonaceous particles and especially on combustion soot and secondary organic aerosol particles. Current laboratory studies of their ice nucleation abilities often give contradictory results indicating that ice nucleation models may be too simplistic to adequately describe the complexity of heterogeneous ice nucleation. An adequate parameterization of particles` surface properties as well as the time dependence of the ice nucleation process is still under discussion. The concept of active sites, i.e. areas on particles where ice can form preferentially, is widely used, and we propose to study the structure and ice activation of particles with respect to their active sites in laboratory experiments under well-defined conditions. We will investigate ice nucleation on a large range of well characterized ice nuclei under conditions found in cirrus cloud formation under controlled temperature and cooling rates.

In the project The ice nucleation activity of carbonaceous particles, which has been a collaboration of the TU Wien, the University of Vienna, and the University of Innsbruck, we aimed to broaden our understanding of the role of carbonaceous particles in atmospheric ice nucleation events. These events affect cloud glaciation and therefore our weather and climate. Fresh soot is nonpolar and will therefore not mix with water. Soot gains polarity through aging (surface reactions with oxygen and other atmospheric trace gases). To examine its nucleation ability at all aging stages, a new, automated setup with a corresponding measurement method were introduced and the existing set-up was reviewed critically. As soot particles are based on graphene-like structures, graphenes can be used as a proxy for soot. We were able to show that the order of the graphene layers impacts their ice nucleation activity profoundly. Real soots were created in a laboratory of the University of Vienna, using a CAST burner. The soots can be differentiated in black and brown carbon. Black carbon shows a low content of organic material compared to brown carbon, a main constituent of wood smoke. To understand aging, we analysed the impact of heterogeneous gas reactions with NO2 and SO2 on the ice nucleation activity of soot. The results show that brown carbon exhibits a stronger ice nucleation activity than black carbon. This ability of brown carbon is strongly enhanced by reactions with both NO2 and SO2. This indicates that the ice nucleation activity of soot increases with atmospheric life time. Another area of interest has been biological particles, whereby the focus was set to plant materials. Many plants are known to contain ice nucleating particles, however research was in most cases motivated by biological questions and very little was done on the aspect of atmospheric processes. We focused mainly on birch trees and berries from perennial plants native to the boreal regions. In the case of birches, the ice nucleation activity of birch pollen is well documented. The rest of the tree however had never been tested. Our results clearly show that ice nucleation active material is present ubiquitously throughout the tree, adding importance to birches as source for ice nucleating particles in the atmosphere. Only two of the analysed berries were previously known to be ice nucleation active. Our results suggest that all of the berries analysed in the course of the presented project contain ice nucleation active substances. Further tests indicate that proteins play a vital role in this process.