Ice nucleating particle measurements during the Cloud, Aerosol, and Complex Terrain Interactions experiment.

Abstract:

Ice nucleating particles (INPs) play a significant role in climate. By facilitating the formation of ice and mixed-phase clouds at temperatures above homogeneous freezing (T>-36°C), they are central to the hydrological cycle by triggering the majority of precipitation. They also impact radiative forcing by modifying cloud optical properties. While several mechanisms cause the formation of ice crystals in clouds, immersion freezing is the most ubiquitous. Here we present a multi-season study of INPs active via the immersion freezing mechanism during the Cloud, Aerosol, and Complex Terrain Interactions experiment, which took place in north central Argentina (Sierras de Córdoba), a region where previous INP data is lacking. The Córdoba region is a convective storm region surrounded by agricultural lands, salt flats, mountains, scrubland, pine forest and one large city, and thus contains many potential sources of particles. INPs were collected with filters (mid-spring 2018 to late fall 2019) and processed as suspensions using the Colorado State University ice spectrometer. Freezing of droplet arrays were measured untreated, following exposure to heat (95 °C), and after hydrogen peroxide digestion, to differentiate between biological (denatured by heat), organic (decomposed by peroxide) and inorganic (remaining after peroxide) INPs. Measurements of aerosol distributions and meteorology were used to investigate the relation between INPs and environment. Unexpectedly, we observed no seasonal cycle of INP concentrations. Biological INPs dominated the population active at higher freezing temperatures (-5 to -20 °C), whereas organic and inorganic INPs dominated at lower temperatures (-20 to -28°C). Rain occurring during, and up to 24 h before, INP sampling enhanced high temperature (~-12°C) biological INPs while simultaneously decreasing concentrations of low temperature INPs (organic and inorganic). We also observed a generally constant ratio of organic to inorganic INPs (~4 at -24°C), with both linearly correlated with aerosol concentration, which suggested a soil source. These results update and expand the understanding of rain-INP relations found in previous studies.