PNNL

The Science

Loess particles, silt-sized terrestrial sediment formed by the accumulation of wind-blown dust, can travel long distances and be lifted to high altitudes to form ice crystals within clouds. However, the distinct ice nucleation process involving loess particles in the formation of mixed-phase clouds is poorly understood. Pacific Northwest National Laboratory investigated the ice nucleating ability of wind-blown loess at three temperatures below and above liquid water saturation conditions. To understand the effect of interactions with acids in the atmosphere during long-range transport, similar experiments were also performed on acid-treated loess samples. The loess particles were shown to form primary ice crystals more efficiently than other types of mineral dust particles. However, their ability to form the ice was decreased when they were treated with acid.

The Impact

The results of this study provide an analysis of the ice nucleation efficiency of bare and acid coated loess from the Columbia Plateau region of the northwestern United States. The new ice nucleation data could be used in cloud models to more accurately parameterize loess particles.

Summary

To understand the implications of loess variability towards ice nucleation, bulk surface samples were collected from six sites distributed across the Columbia Plateau. These samples were size-selected based on their mobility diameter and investigated for their ice nucleation efficiency at various temperatures and humidity conditions. To understand the effect of atmospheric aging, where the condensation of acids may occur while particles are transported in the atmosphere, the samples were treated with acids, and the ice nucleation efficiency of these acid treated particles was investigated. Results showed that loess particles are efficient ice nucleating particles, and their ice nucleating efficiency is higher than natural and soil dust particles. During long-range transport, dust particles might get coated with various salts and sulfates. Experiments with acid coated loess particles were performed to characterize the aging effects. It was observed that acid treatment substantially decreased the ability of particles to nucleate ice at all saturation conditions. Ice nucleation parameterization for loess particles using active-site density approach is developed to model the loess type of dust particles.

PNNL Contacts

Jerome Fast, Pacific Northwest National Laboratory, Jerome.Fast@pnnl.gov  

Gourihar Kulkarni, Pacific Northwest National Laboratory, Gourihar.Kulkarni@pnnl.gov  

Funding

This research was supported by the Office of Science of the U.S. Department of Energy (DOE) as part of the Atmospheric Research Program.