Atmospheric Processing and Variability of Biological Ice Nucleating Particles in Precipitation at Opme, France

Atmospheric ice nucleating particles (INPs) contribute to initiate precipitation. In particular, biological INPs act at warmer temperatures than other types of particles (>−10 °C) therefore potentially defining precipitation distribution. Here, in order to identify potential environmental drivers in the distribution and fate of biological INPs in the atmosphere, we conducted a mid-term study of the freezing characteristics of precipitation. A total of 121 samples were collected during a period of >1.5 years at the rural site of Opme (680 m a.s.l. (above sea level), France). INP concentration ranged over two orders of magnitude at a given temperature depending on the sample; there were <1 INPs mL−1 at ≥−5 °C, ~0.1 to 10 mL−1 between −5 °C and −8 °C, and ~1 to 100 mL−1 at colder temperatures. The data support the existence of an intimate natural link between biological INPs and hydrological cycles. In addition, acidification was strongly correlated with a decrease of the freezing characteristics of the samples, suggesting that human activities impact the role of INPs as triggers of precipitation. Water isotope ratio measurements and statistical comparison with aerosol and cloud water data confirmed some extent of INP partitioning in the atmosphere, with the INPs active at the warmest temperatures tending to be more efficiently precipitated.