Atomic and molecular fluorescence as a stratospheric species monitor

The results of an extensive evaluation are presented assessing the potential of atomic and molecular fluorescence as a stratospheric monitor of the concentrations of any one of eighteen minor species. These include Cl, Cl₂, ClO, ClO₂, CO, H₂, HCHO, HCl, HNO₂, HNO₃, H₂S, NH₃, NO, NO₂, N₂O, O, OH and SO₂. All spectral regions from the vacuum u.v. through to the i.r. have been included. Where appropriate, detection limits (signal/noise ratio of unity) are presented for each species under various sample pressure conditions and are based on practical systems that could be constructed using current technology.The most promising systems, with typical detection limits indicated either as parts per million, billion or trillion by volume, are for CO(5ppb), NO₂(<1ppb), OH(0.2ppt) and O(50–200ppt). The fluorescence sensitivities for Cl(0.5–1 ppt), H₂(0.2 ppm at 10 torr sample pressure) and SO₂(1–10 ppb) are marginally insufficient at present for such a stratospheric application. Likewise HCHO(10 ppb) and NO(100 ppb) fluorescence detection may be of interest in other applications where sensitivity demands are not as severe. There are no promising analytical possibilities using direct fluorescence techniques for Cl₂, ClO₂, HCl, HNO₂, HNO₃, H₂S, NH₃ or N₂O. ClO fluorescence has not yet been characterized.It has been noted, for various reasons, that i.r. fluorescence techniques in general cannot be exploited in the development of sensitive analyzers. However, by far the most surprising outcome of this study has been the recognition of the analytical potential of vacuum u.v. fluorescence. For some species, under certain conditions, extremely high sensitivities are possible even with samples in air at atmospheric pressure.