Long lived dusts from low latitudes may dominate primary ice production in polar regions

For most regions of the world, the availability of mineral dust particles drives the production of primary ice in mixed-phase clouds. The mineral dust acts as an ice nucleating particle (INP), which facilitates the freezing of supercooled cloud droplets at higher temperatures than in its absence (~ -38C). Mixed-phase clouds are ubiquitous in the polar regions, but are far from the world’s primary sources of dust emissions in the tropics and subtropics. Secondary sources from high-latitude sources exist but do not fully explain observations of relatively high INP concentrations during the full annual cycle.

In this study we identify a previously overlooked source of low-latitude dust: those from long-lived dusts in smaller size modes that have been in the atmosphere for up to 5 months. Using simulations of the UK Earth System Model (UKESM), we find that although fresh dust contributes most of the dust mass in the polar regions, the INP contribution is weighted towards the older dust particles. In some regions, dust older than 90 days contributes over 50% of the total dust-sourced INP concentration. This occurs due to changes in the dust population size distribution, and has important implications. Close to primary dust sources, the INP concentration is dependent on the super-micron particles, whereas far from the source (i.e. the polar regions and remote oceans) the INP concentration is dependent on the smaller, older, sub-micron particles.