Atmospheric aerosols: From the Junge layer to the VOC Connection

The last half a century has seen an explosion of scientific results characterizing atmospheric aerosols. Much of the knowledge of airborne particles in the troposphere and the stratosphere through the 1950s was reviewed by Junge in 1963. Notable among the achievements in this field at the time were the beginnings of an understanding of the dynamics of size differentiated particles, the recognition of the ubiquity and importance of sulfate chemistry, and the discovery of a particle layer in the stratosphere. In the intervening years, these three elements and others have expanded to encompass a rich literature on the origins and nature of particles as they interact with chemically reactive gases in the atmosphere. Today, the aerosol particle dynamics is well understood in terms of a multi-modal size distribution. The physical and chemical properties of the particle suspension are described in terms of complex probability distributions. The evolution of these distributions can be calculated with some confidence using modern computers. The importance of sulfate chemistry as a feature of aerosol mass concentration has been expanded to include other components derived from the photochemically related oxidation of nitrogen oxides and volatile organic compounds in the troposphere. The richness of stratospheric particle chemistry has come full circle with its intimate linkage to the dynamics of the ozone layer. The gaps in knowledge about atmospheric aerosols continue to challenge the scientist. One of the more challenging areas is that of the tropospheric carbonaceous component, which is highly complex in its origins and chemistry. It is problematic to project where the paths of modern chemistry will lead in this field, but one can guess that major advances are on the horizon, given the continuing investment in studies from the atmospheric micro- to the macro-scale.