Anthropogenic activities, in particular increasing energy use and production, and natural processes release large quantities of trace substances into the atmosphere influencing living conditions on Earth. Since the beginning of the industrialisation in the 19th century, a significant increase of atmospheric trace gases and pollutants has been observed world-wide. On one side, changing greenhouse gases and aerosols perturb the energy balance of the atmosphere and thereby the climate system. On the other side, increasing air pollution levels constitute a threat to human and ecosystem health especially in industrialised countries and large cities. The institute IEK-8 investigates the physical and chemical processes in the troposphere which have a major impact on the chemical composition of the atmosphere. The processes include (1) the natural and anthropogenic emissions of trace substances at the earth's surface, (2) the chemical transformation of compounds in the atmosphere, and (3) the distribution of pollutants by atmospheric transport. The research focusses on the longterm observation of atmospheric trace gases, the understanding of atmospheric self-cleaning, and the formation and aging of aerosols. Observations and experimental results form the basis for the development of improved atmospheric models that are being used for predictions of regional air quality, and atmospheric chemistry and climate interactions.