A new photolysis laser-induced fluorescence instrument for the detection of H 2 O and HDO in the lower stratosphere Rev. Sci. Instrum. 79, 064101 (2008); Nitrogen dioxide (NO 2 ), an essential gas in atmospheric chemistry, is almost always measured indirectly with just one technique: photolysis of NO 2 to NO, followed by detection of resulting NO. However, the photolysis introduces artifact signals that complicate the extraction of the NO 2 signal at low NO 2 volume mixing ratios. A new prototype instrument uses laser induced fluorescence to measure tropospheric NO 2 directly. The atmospheric fluorescence spectra measured with this instrument match the fluorescence spectra of NO 2 in a reference cell over the wavelength range 565.1-565.2 nm. This match indicates that fluorescence from other chemical species is not interfering with the measurements. The prototype's current detection limit is 280 parts per trillion ͑pptv͒ in 2.5 min with an absolute uncertainty of Ϯ15% ͑2 confidence͒. This detection limit, which can be improved with better optical filtering and more laser power, is currently adequate for urban air conditions.

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