The atmosphere contains a large variety of particles, ranging in size from near molecular (~1 nm) to larger than 10,000 nm. The total number concentration N of particles is dominated by nanoparticles ≤ 100 nm in diameter. Discovery of atmospheric nanoparticles dates back to M. Coulier's observations in 1875. A new field of atmospheric science opened up 13 yr later, beginning with J. Aitken's pioneering work investigating airborne particles generated “by the action of sunlight” and essentially confirmed their origin as postulated centuries before by da Vinci. “Nanoparticles” is a contemporary term for the aerosol community's identification with Aitken particles [condensation nuclei (CN)] and Langevin ions. Nineteenth- and twentieth-century research has provided a “classical” foundation for the recent, popular attention to nanoparticles, including extensive observations of concentrations in urban and rural environments and their formation chemistry. The capability for measuring the particle size distributions of nanometer size, combined with improved knowledge of their chemistry, has created major opportunities for characterizing particle distributions from ~1 to >10,000 nm in diameter. The formation and dynamics of nanopartricles is considered important as a unique part of contemporary atmospheric chemistry. Their potential for creating adverse air pollution effects on humans, alteration of cloud condensation microphysics, and perhaps (indirectly) climate forcing has stimulated new, broadened interest beyond simply source identification and evolution in the atmosphere.
