How Well Do We Understand and Predict Ice-Nucleating Particle Sources and Concentrations around the World?

Ice nucleating particles (INPs), rare among total atmospheric aerosols, exert an unresolved yet oversized potential impact on cloud phase (liquid versus ice), precipitation and climate. Measurements and efforts to incorporate INPs of different global sources have expanded in recent years. In this talk, we describe studies into the many, yet disparate, source contributions to INP concentrations in the atmosphere and the ability of regional and global aerosol models to predict these. We show that for many locations, measurements demonstrate that a consideration of contributions of mineral dust INPs and marine (ocean sea spray-sourced) INPs can provide a minimal representation of the budget of such particles affecting mixed-phase clouds. Due to the much higher ice nucleation efficiency and long-range transport of dust INPs throughout the troposphere compared to marine INPs, dust INPs can dominate in many scenarios, or lead to complex layered situations over cold remote ocean regions. We will show that this is both observed and predicted by global aerosol transport models. Empirical evidence supports the fact that many of the remaining potentially important (at least regionally), but poorly quantified INP emission sources for mixed-phase cloud ice formation are organic in nature, and these emanate from biomass burning, arable soil and high latitude glacial dust emissions, and biological particles from land and ocean sources. Urban pollution remains an uncertain, though non-general source. It seems now especially clear that fossil fuel combustion and biomass burning sources of black carbon do not provide a significant global contribution to INPs, as demonstrated by extending measurements to chemical transport model simulations.

We will also touch on the present difficulty in separating the influence of biases in representation of INPs due to the specifics of different aerosol models versus implementation of INP parameterizations, and on the important need to investigate and consider the role of atmospheric processing of INPs on their behaviors following emission, with a few examples given.

Finally, we demonstrate how knowledge of INPs impact cloud microphysical and radiative properties in aerosol-aware and interactive Earth system model simulations, using the sensitive Southern Ocean region as an example. This allows to also point out the great need for understanding and representing ice formation processes that include not only primary nucleation, but also secondary ice formation processes.