Measurements of zenith scattered sun light have been used for a long time for the analysis of atmospheric trace gas abundances: Since the 1920s the thickness of the stratospheric ozone layer has been measured in a world wide network of the Dobson photospectrometers. Since the 1970s also minor stratospheric constituents like NO 2 , OClO and BrO could be analysed in spectra of zenith scattered sunlight. These observations contributed substantially to the understanding of the mechanisms leading to the ozone hole.
In the recent decade, observations of scattered sun light are not only taken in zenith direction, but also at various, mostly slant, elevation angles. Such observations are usually referred to as Multi-AXis-(MAX-) DOAS measurements. For MAX-DOAS measurements made at low elevation angles the light paths close to the earth’s surface can become very large providing high sensitivity to tropospheric trace gases. In recent years MAX-DOAS observations have been widely used to study air quality, to investigate polar tropospheric chemistry, to quantify emission sources and to validate satellite observations. Also the retrieval algorithms for tropospheric trace gases and aerosols have been largely improved. The presentation will give an overview about the current status of MAX-DOAS instrumentation and data analysis.