Chapter 1: Overview of Ice Nucleating Particles

Abstract

ce particle formation in tropospheric clouds significantly changes cloud radiative and microphysical properties. Ice nucleation in the troposphere via homogeneous freezing occurs at temperatures lower than -38 °C and relative humidity with respect to ice above 140%. In the absence of these conditions, ice formation can proceed via heterogeneous nucleation aided by aerosol particles known as ice nucleating particles (INPs).In this paper we describe new developments in identifying the heterogeneous freezing mechanisms, atmospheric relevance, uncertainties and unknowns about INPs. We explain how conventional wisdom regarding the requirements of INPs is changing as new studies discover physical and chemical properties of these particles. INP sources and known reasons for their ice nucleating properties are presented. We also highlight where more studies are required to systematically identify particle properties that facilitate ice nucleation.The atmospheric relevance of long range transport, aerosol aging and coating studies (in the laboratory) of INPs are also presented. Possible mechanisms for processes that change the ice nucleating potential of INPs and the corresponding challenges in understanding and applying these in models are discussed. How primary ice nucleation affects total ice crystal number concentrations in clouds and the discrepancy between INP concentrations and ice crystal number concentrations are presented. Finally, we discuss limitations of parameterizing INPs and of models in representing known and unknown processes related to heterogeneous ice nucleation processes.