Ice nucleation by soil dusts: relative importance of mineral dust and biogenic components (Invited Presentation)

Agricultural dust emissions have been estimated to contribute in the region of 20% to the global dust burden. In contrast to dusts from arid source regions, the ice-nucleating abilities of which have been relatively well studied, soil dusts from fertile sources often contain a substantial fraction of organic matter. However, the impacts of agricultural soil dusts on atmospheric ice formation remains poorly constrained, despite the fact that soil organic matter was first proposed as a source of highly active ice nuclei some four decades ago. Using an experimental methodology which is sensitive to a wide range of ice nucleation efficiencies, we have characterised the immersion mode ice-nucleating activities of dusts extracted from fertile soils collected at four locations around England. By controlling droplet sizes, which ranged in volume from pico- to micro-Liter, we have been able to determine the ice nucleation behaviour of soil dust particles at temperatures ranging from 267 K (-6 °C) down to the homogeneous limit of freezing at about 237 K (-36 °C). At temperatures above 258 K (-15 °C) we find that the ice-nucleating activity of soil dusts is diminished by heat treatment or digestion with hydrogen peroxide, suggesting that the ice nuclei stem from biogenic components in the soil. By comparison to the ice-nucleating behaviour of the most abundant components of soil organic matter, the humic substances, we show that the high temperature nucleating activities likely stem from other, minor components of soil organic matter. Conversely, at temperatures below 258 K, we find that the ice-nucleating activities tend towards those expected from the mineral components in the soils, suggesting that the inorganic fraction of soil dusts, in particular the K-feldspar fraction, becomes increasingly important in the initiation of the ice phase at lower temperatures. We conclude that although only a relatively minor contributor to the global atmospheric dust burden, the enhanced ice-nucleating activities of dusts generated from agricultural activities may play an important role in mixed-phase cloud glaciation, particularly at temperatures above 258 K.