Nitrogen-containing compounds are detected by chemically converting (210) them to nitrogen dioxide, and detecting (10) the amount of nitrogen dioxide. A semiconductor laser or light emitting diode (132) provides a modulated light (131) in the blue- violet-green wavelength range and a narrow bandwidth photo-acoustic sensor (10) detects the standing waves produced by the absorption of the light by the nitrogen dioxide. The photo-acoustic sensor (10) uses a resonant cavity (161, 182a-b) with a resonant frequency that corresponds to the modulation frequency of the light (131). For detecting nitric oxide, a surface chemical oxidation unit (210) is preferably used to convert the nitric oxide to nitrogen dioxide, using, for example potassium permanganate (KMnO4) filter, or a platinum (Pt) catalyst unit (260).