Direct Observations of Atmospheric Aerosol Nucleation

Aerosol Formation Most atmospheric aerosol particles result from a growth process that begins with atmospheric molecules and clusters, progressing to larger and larger sizes as they acquire other molecules, clusters, and particles. The initial steps of this process involve very small entities—with diameters of less than 2 nanometers—which have been difficult to observe. Kulmala et al. (p. 943 ; see the Perspective by Andreae ) developed a sensitive observational protocol that allows these tiny seeds to be detected and counted, and they mapped out the process of aerosol formation in detail.