High-sensitivity detection of negative ions in the stratosphere.

Mass spectrometer composition measurements of stratospheric negative ions were first made by Arnold and Henschen using a balloon-borne instrument. These initial measurements, which were taken at an altitude of 36.5 km, revealed that the major ion species are NO(HNO) and HSO(HSO)(HNO). The suggestion that the unexpected HSO cores are formed from NO cores by a reaction involving stratospheric sulphuric acid vapour was strongly supported by laboratory measurements of the relevant ion-molecule reactions by Viggiano and colleagues. Meanwhile, major instrumental refinements in balloon-borne ion mass spectrometers, including increased mass range, sensitivity and mass resolution, have resulted in the detection of more massive HSO(HSO) ions and in the extension of the height range from a thin layer around 35 km to a region extending from 23 to 39 km (refs 3,4). Here we report in situ negative ion composition measurements of even higher sensitivity which led to the detection of previously unobserved ion species.