Field Studies of the Effect of Anthropogenic Aerosols on Ice Nucleation

It is now highly certain that anthropogenic activities have caused a warming of the Earth's atmosphere. The addition of small aerosol particles has offset, to some extent, the warming attributed to greenhouse gases via the so-called ‘direct effect'. Aerosol particles can also act as sites of condensation and lead to the formation of clouds and this is termed an ‘indirect effect'. Some specific particles, known as ice nuclei (IN), are highly efficient at the nucleation of water ice and thus the formation of ice and mixed-phase clouds. Whereas the vast majority of atmospheric particles require temperatures of 233 K and lower and saturations near that of liquid water, IN can form ice within a few degrees below the equilibrium freezing point of liquid water and at saturations near that of water ice. Studies using ‘ice chambers' which mimic cloud formation conditions in the laboratory and at remote field sites have recently allowed for analyses to determine the composition of IN and the role played by anthropogenic components.