Good knowledge of the composition, and chemistry of the troposphere and stratosphere is the key to understanding global environmental problems such as stratospheric ozone depletion, air pollution or greenhouse warming. Trace constituents are emitted into the atmosphere by natural and anthropogenic sources. Physicochemical processes in the air eventually transfer these species back into the biosphere, lithosphere, or the oceans. Land use change (i.e. transferring natural ecosystems into agricultural areas or into cities and highways) and air pollution (fossil fuel burning, traffic and industrial exhaust) are the major human impacts on the natural system of atmospheric chemistry. Besides changing the total primary emission of trace gases these anthropogenic impacts also change chemical pathways in the air. Anthropogenic activity thus exhibits a strong climate forcing because it feeds back to concentrations of greenhouse gasses and the formation of aerosols and clouds.