CONTRAIL MICROPHYSICS

This article reviews the current state of understanding of the science of contrails: 1) how they are formed, 2) their microphysical properties as they evolve into contrail cirrus and whether their microphysical properties can be distinguished from natural cirrus, and 3) the ice-nucleating properties of soot aerosols and whether these aerosols can nucleate cirrus crystals. Key gaps and underlying uncertainties in our understanding of contrails and their effect on local, regional, and global climate are identified. These include 1) better quantification of the fraction of ice number and mass that survives the vortex phase and the aircraft-specific influences on the vortex dynamics, 2) more accurate measurements of the ice crystal size distributions of contrail cirrus and cirrus in general, which are uncertain because of instrument limitations, and 3) more measurements of the ice-nucleating properties of aircraft exhaust and other ambient ice nuclei in situ under cirrus-forming conditions. Future field campaigns aimed at satisfying measurement needs are proposed