Investigating Ice Nucleating Particles from Agricultural and Natural Landscapes

Ice nucleation in clouds greatly affects microphysical cloud properties and is a key process for precipitation. It is well known that mineral dust is a major source of ice nucleating particles (INPs), but other potential sources – in particular, biogenic INPs – are still being researched. The goals of this study were to compare INP concentrations at agricultural and natural (non-agricultural) sites and to identify whether the INPs were primarily organic, biological, or mineral in origin. Aerosol samples were collected on filters at locations in the Western U.S. (CO, KS, WY, and UT), then rinsed and distributed into liquid aliquots to analyze for INPs using an instrument that allows for ice nucleation via immersion freezing down to -28˚C. To measure the amount of biological material in INPs, a heat treatment was used on the aerosol samples; to measure organic material, a peroxide oxidation treatment was developed. Bulk soil samples from the same locations were analyzed to study native sources of INPs and to relate these INPs to those found in ambient air. Results indicated that agricultural land emits higher concentrations of INPs than natural ecotypes. Harvesting on the agricultural lands further increases the total INP concentrations and generates large emissions of biological and organic INPs.  Additionally, for all landscapes tested, organic material was shown to have ice nucleating activity until about -22˚C or colder, which is colder than in previously published studies. These results suggest that human activity, by altering landscapes, can increase aerosol and INP concentrations in the atmosphere, thereby influencing cloud properties, weather, and climate.