Organic matter matters for ice nuclei of agricultural soil origin

Heterogeneous ice nucleation is a crucial process for forming ice-containing clouds and subsequent ice-induced precipitation. The importance for ice nucleation by airborne desert soil dusts composed predominantly of minerals is widely acknowledged. However, the potential influence of agricultural soil dusts on ice nucleation has been poorly recognized, despite recent estimates that they may account for up to 20-25% of the global atmospheric dust load. We have conducted freezing experiments with various dusts, including agricultural soil dusts derived from the largest dust-source region in North America. Here we show evidence for the significant role of soil organic matter (SOM) in particles acting as ice nuclei (IN) under mixed-phase cloud conditions. We find that the ice-nucleating ability of the agricultural soil dusts is similar to that of desert soil dusts, but is clearly reduced after either H2O2 digestion or dry heating to 300 degrees C. In addition, based on chemical composition analysis, we demonstrate that organic-rich particles are more important than mineral particles for the ice-nucleating ability of the agricultural soil dusts at temperatures warmer than about -36 degrees C. Finally, we suggest that such organic-rich particles of agricultural origin (namely, SOM particles) may contribute significantly to the ubiquity of organic-rich IN in the global atmosphere.