Diurnal cycle of bioaerosols is a key driver of ice nucleating particle variability for Eastern Mediterranean orographic clouds

Abstract

We report the drivers of diurnal cycles of ice nucleating particles (INPs) for mixed-phase orographic clouds in the Eastern Mediterranean. In the planetary boundary layer, pronounced INP diurnal periodicity is observed, which is mainly driven by coarse-sized particles. Based on particle fluorescence, we estimate at least 20% of the INPs in the PBL are of biological origin and a major driver of the diurnal cycle. The presence of Saharan dust increases INPs during nighttime but not for daytime compared to non-dust days, thus reducing the INP range throughout the day. Bioaerosols also are important contributors to INP in the free troposphere without any evident diurnal cycle. Given the ubiquity of forests in mountainous regions, the unique ability of bioaerosol to form ice at warm temperatures (effects of which is further amplified by secondary ice processes) and the lack of such cycles in models points to an overlooked glaciation cycle and an important source of bias in precipitation and extreme events in continental and mountainous regions.