The Impact of Anthropogenic Lead on Atmospheric Ice Formation

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 of water ice. Some lead-containing substances, for example lead iodide, are highly effective IN and this material has been used in cloud seeding studies. We have now studied anthropogenic lead emitted to the atmosphere as it impacts the formation of ice. Field work conducted on two mountaintop sites and subsequent laboratory studies show that anthropogenic lead can lead to the enhancement of ice nucleation by pre-existing atmospheric particles and, thus, the formation of clouds.