Over recent decades, the atmospheric chemistry community has devoted great effort to quantify the gas phase processes that control the formation and destruction of photooxidants in the troposphere. The focus has been on ozone and hydroxyl radical formation and destruction, with emphasis on the role played by precursors such as the nitrogen oxides, carbon monoxide, methane and nonmethane hydrocarbons. This approach has highlighted the importance of biogenic emissions from oceans, soils, and vegetation (including biomass burning), as well as anthropogenic perturbations.

More recently, it has become evident that atmospheric aerosols could affect the processes that control the budget of photooxidants. Several studies have shown, for example, that conversion of nitrogen oxides into nitric acid on sulfate aerosols could be a very efficient mechanism, especially during nighttime. Other mechanisms affecting ozone and other photooxidants in the troposphere have been proposed, including the uptake of peroxy radicals on sulfate particles, the destruction of ozone on soot particles, the release of active forms of halogens from seasalt, and the heterogeneous formation of nitrous acid in highly polluted areas. The role of such multiphase reactions remains controversial, making them deserving of considerable attention.

It is clear today that the simple representation of sulfate aerosols in global and regional models has become insufficient to account for important chemical and climate effects. Modern approaches must consider the effects not only of sulfate and nitrate aerosols, but also of mineral dust, soot, organic particles, sea-salt, and probably other types of particles. Challenging work to measure the chemical composition of particles in the boundary layer and in the free troposphere is underway, often under IGAC sponsorship. More detailed representations of aerosols are being implemented in the most advanced models, including climate models.

The present issue of IGACtivities focuses on microphysical processes that lead to the formation of new particles in the atmosphere. Several studies sponsored by the European Commission are summarized that have been conducted to understand nucleation processes in different environments. New and exciting science has resulted from these studies.