Background concentrations of photochemically active trace constituents in the stratosphere and upper troposphere

Summary High resolution absorption spectroscopy can be used for obtaining very small background concentrations or very low upper limits of trace constituents, particularly from measurements inQ-branches of fundamental vibration-rotation bands. As examples, the results of a search for NH₃ and SO₂ in the Vermande and Dionne Mont-Louis solar spectra will be reported. The relative concentration of NH₃ is very much lower above the Pyrenees than the generally accepted minimum background concentrations near sea-level.