Pollen Ice Nucleating Particles and Their Response to Atmospheric Processing

Abstract

Clouds are crucial to life on Earth as they help regulate temperature and distribute water resources. Yet, large uncertainties exist around cloud development and evolution with many open questions about processes spanning from micro- to macro-scales. On the micro-scale, ice formation contributes to precipitation formation and cloud radiative properties; yet our understanding of this process is limited. This thesis assesses the ice nucleating (IN) ability of pollen and subpollen particles (SPPs) under mixed-phase and cold cloud conditions. In a first step, pollen IN activity was investigated under mixed-phase cloud conditions where IN activity was found to vary between pollen types and that submicron SPPs drive pollen IN activity. Additionally, one pollen type, grey alder, was identified to initiate freezing above −10°C, thus contributing to a small set of highly active ice nucleating particles (INPs). In a second step, atmospheric processing of SPPs was investigated, as SPPs can experience atmospheric transport and exposure to varying environmental conditions. Several processing pathways were found to compromise pollen IN activity. The largest impacts were observed for the most IN active pollen by exposure to simulated solar radiation. Under cold cloud conditions, chemical oxidation negatively impacts the IN activity of silver birch and grey alder pollen, occurring on a similar timescale as changes to the molecular structure were observed. The findings from laboratory INP measurements were translated to parametrize INP concentrations from SPPs, indicating INP concentrations below 10-2 m-3 in one modeling scenario. To overcome potential barriers in detecting such low INP concentrations by commonly used INP instruments, a Portable Fine Particle Concentrator was evaluated for INP measurements in the field. Enrichment factors are found to be particle size dependent while an INP enrichment factor of 16±5 was achieved in a mountain-top field study. The insights gained from this thesis will help assess the importance of pollen as INPs and reduce uncertainties that cloud processes currently impose on weather and climate predictions.