Ice nucleating particles above agricultural landscapes: linking sources with environment and land management

Ice nucleating particles (INPs) play a key role in the water cycle. By triggering the freezing of supercooled cloud droplets, they initiate precipitation and impact cloud behavior, albedo and lifetime. The agricultural landscape is awash with efficient, warm temperature INPs. At -10 °C, arable soils contain around 10⁸ INPs per gram, while cereal crops can release 10⁶-10^7.5 INPs per gram of leaf. Many of the INPs harbored in the soils and on plants are of biological and organic origin (e.g., plant and soil bacteria and fungi, and their cell-free INPs; pollen; a mass of as-yet unidentified INPs in the soil organic matter). Due to their abundance, biogenic INPs often predominate over mineral INPs to below -20 °C in the overlying boundary layer air.

The INP reservoirs are both large and patchy (in both INP concentration and activity), and their emissions are driven by all scales of weather (microscale, mesoscale and synoptic). The typical atmospheric INP background will also be punctuated by periodic mass releases, such as from regional harvesting. Land management practices that change bare soil exposure, soil organic matter levels, and crop type will also influence INP emissions. For example, conventional tillage involves several stages of land preparation (e.g. disking, cultivating, harrowing), which emit soil dust. By contrast, conservation tillage reduces both soil disturbance and wind erosion. Adoption of conservation tillage should therefore lead to a reduction in INP emissions.

To quantify the mechanisms that drive INP release and predict how land-use- and climate-change will impact them, we need to link atmospheric INPs with their sources. This first requires identifying INPs by their characteristics (e.g. proteins denatured by heat), followed by associating these different subsets with key emission events, such as rainfall, harvesting, or soil dust lofting. Using diverse techniques, including real-time and offline measures of INPs, physico-chemical tests, and DNA profiling, and drawing upon sampling studies ranging across the Central United States, France and Argentina, we present progress in classifying atmospheric INPs into characteristic populations, and subsequently linking them to landscapes, weather and agricultural practices.